A low-carb diet may boost exercise performance and health

Dr. Tim Noakes discusses the benefits of a low-carb, high-fat diet for athletes, performance, and overall health.


Article highlights

  • Recent research shows high-fat, low-carb diets do not reduce performance in recreational athletes and the body can shift to fat oxidation even during high-intensity exercise.
  • The brain requires some minimum level of glucose but the body can produce glucose from fat sources, allowing prolonged athletic feats without carbohydrate intake.
  • High-carb diets may increase inflammation, risk of injury, and onset of insulin resistance over a season compared to high-fat diets for some athletes.
  • Continuous tracking of glucose and fat oxidation could reveal performance limitations when glycogen is depleted in carb-adapted athletes versus sustained energy from fat.
  • Promoting high-carb intake without regard for insulin resistance has harmed the long-term metabolic health of many athletes, which lower-carb diets may improve.

Professor Tim Noakes is a South African scientist, author, emeritus professor, and runner. He’s focused on how a low-carbohydrate, high-fat diet relates to performance. He’s no longer registered as a medical doctor, and now focuses on spreading scientific information. 

Dr. Noakes founded the Noakes Foundation, whose goal is to fix the future of human health by changing food policies and the way people eat. Dr. Noakes sat down with Josh Clemente, co-founder of Levels, to discuss this future, how Dr. Noakes’ perspective has evolved, and how low-carbohydrate diets impact performance.

Debunking the Carbohydrate Myth 

Josh Clemente: Professor Tim Noakes, thanks so much for joining us. I’m really excited to dig into metabolism and metabolic fitness, including our research collaboration on a lot of those things.

Prof. Tim Noakes: My pleasure, Josh. Lovely to be with you. Thank you.

Josh Clemente: Your career has spanned many decades and covers the whole spectrum of metabolic health, as well as fitness and fitness adaptation under different metabolic circumstances. I’m really excited to speak about your recent work, which Levels had an opportunity to be a part of, which focused on adapting to low-carb, high-fat diets for recreational male athletes.

Prof. Tim Noakes: I started doing sports science in 1981. That’s when I really became committed. The first thing we studied was hypoglycemia during exercise, because I was sure that hypoglycemia developed during exercise and at that time, people were being encouraged even not to drink water during marathons. It was quite a long time ago, and quite different. 

We started experimenting. Our focus was on the Comrades Marathon, which is 90 kilometers. It became very clear that some elite athletes developed hypoglycemia and really struggled. Then there were examples where they took carbohydrates and were able to finish the race comfortably. Bruce Fordice, probably the greatest Comrades runner in history, and I developed a product called FRN.

I was a marathon runner myself. FRN was the first goo people used during exercise. I was totally committed to the belief that carbohydrates were essential for exercise, and I bought into the idea that muscle glycogen depletion causes fatigue during prolonged exercise. I went along with this and wrote the book, Lore of Running, published in 2002. It’s all full of carbohydrates: You must eat every single carbohydrate you can see. 

My career took a major turn in 2010 because I learned about low-carb, and I read Eric Westman, Stephen Phinney, and Jeff Volek’s book. In two hours, that convinced me I was completely and utterly wrong. I changed that day. Within four months, my health had improved dramatically. I’d lost a lot of weight, my running was improving, and I started telling people I’d gone on this low-carb diet. 

Here’s a guy who’s promoted the high-carb diet, and I’ve now gone to a low-carb diet. I wrote some articles, and I almost immediately lost all my funding. I was killed. I was dead in the water. Then my university kicked me out. Well, they didn’t kick me out. I just retired and they exposed me in the press and said I was no longer believable because I was promoting a diet on the grounds that it had reversed Type 2 diabetes, which they thought was a ridiculous claim. Then I had to go to court to fight for four years because of what they’d done. They tried to destroy my career. 

During this time, my Type 2 diabetes reversed. In 2018, we won the case and it was all over. For once I’d actually learned a bit about nutrition. I had thought I knew about human nutrition, but I really didn’t because in medical school, we don’t learn about it. I’d maybe studied carbohydrates during exercise, but I didn’t understand whole-body nutrition. 

Then Philip Prins, a fellow exercise science and physiology researcher, phoned me and said, “Tim, I’d really like to do some research with you.” I met him in Ohio, at Jeff Volek’s conference. He said, “What should we do?” I said, “Let’s do a 5K time trial, because your team is really good at doing 5K time trials on the treadmills. Let’s do that and see what they burn.” 

We’d also done a VO2 max test before. I looked at Jeff Volek’s research. He’d also done VO2 max testing. I wondered if he’d ever looked at the so-called crossover point, the point at which you start to burn more carbohydrate than fat. He hadn’t done it. I said, “Why don’t we look at those data?” 

Philip did that and produced a really good paper showing that the crossover point shifted far to the right to about 85% VO2 max, which is when you’re not meant to be burning any fat. We published that one. 

I thought, If it works at five kilometers, maybe it’ll work at one kilometer, because that’s a real test. We did one-kilometer time trials and found no difference between athletes on a high-carb, low-fat diet and those on a high-fat, low-carb diet. But I warned him, “If you find no difference, people will say it’s because the athletes had lots of glycogen, even though a low-carb diet had enough glycogen for the 1K mark.”

The textbooks say that at 85% VO2 max, you burn no fat. We saw high rates of fat oxidation, exactly at the point where they were not meant to be burning any fat. That shows that muscle glycogen doesn’t have an obligatory role during exercise performance. 

Then I said, “Why don’t you do six 800-meter repetitions?” He decided to measure oxygen consumption and metabolism, which was the key thing. Surprise, surprise: there was no difference. According to the traditional hypothesis, after about three 800-meter sprints, the athletes’ performance should have gone down in the low-carb group because, theoretically, they’ve got no glycogen left. That hypothesis predicted they’d run out of glycogen after about three 800-meter repetitions. It didn’t happen. They stayed exactly the same, regardless of diet.

Using oxygen consumption and the respiratory quotient, we can calculate carbohydrate and fat oxidation. The carb-fat oxidation just went up and up. The more repetitions they did, the higher the fat oxidation. And by chance, they were exercising at 86% VO2 max. The textbooks say that at 85% VO2 max, you burn no fat. We saw high rates of fat oxidation, exactly at the point where they were not meant to be burning any fat. That shows that muscle glycogen doesn’t have an obligatory role during exercise performance. 

Josh Clemente: Even at these high intensities, which is really the focal point here.

Prof. Tim Noakes: Correct. The original studies claim there’s this obligatory role for carbohydrates and muscle glycogen. In those original trials done by the Scandinavians in 1967, they showed that at the point of exhaustion, even though the guys had low muscle glycogen, their blood glucose levels were incredibly low. They were all hypoglycemic. 

But I looked at those same studies. I looked at the metabolism at the point of exhaustion in the people who were eating a high-carbohydrate diet, a mixed-carbohydrate diet, and a low-carbohydrate diet. There were big differences in muscle glycogen and exhaustion, which doesn’t fit the model. There were big differences in fat oxidation and exhaustion, which doesn’t fit the model. There were also big differences in carbohydrate oxidation. The one thing that wasn’t different was the blood-glucose concentration.

You need to look at the variable that’s not different. That’s going to be the limiting factor. It was very clear that it was blood glucose. I then went and looked at a whole bunch of other studies, and I now know that when you take in carbohydrates during exercise, or load up on carbohydrates before exercise, all that happens is you substitute a little bit more carbohydrate for a little bit less fat when exercising. 

The kilojoules absolutely matched. If you burn an extra, let’s say, five or eight kilojoules per minute, which is a trivial amount of carbohydrate, you burn eight less kilojoules of fat. Since the 1980s (which is when the sports-drink industry got involved) we’ve been told that’s the difference: It’s this extra carbohydrate you’re burning in the muscles, which makes you perform better. 

Burning eight kilojoules per minute more of carbohydrate is only 10, 15% of the total energy you’re expending at that time. Why is that so special that you couldn’t make it up with fat? We’ve now proven that you could make it up with fat, but these people were hypoglycemic. If you take in carbs, you can go longer because your blood glucose is higher and your brain still functions. That’s what I know. I’m working very hard on a couple of articles, which really show that blood glucose is the obligatory fuel. It’s not muscle glycogen.

Josh Clemente: It’s fascinating. We went pretty deep there and I’m excited to break that apart a bit more. These assumptions have been around from the beginning for many of us. Since our exposure to sports, these assumptions about what the body needs in order to perform were ingrained like law. 

Having better tools, better techniques, and better questions is really interesting because it’s overturning many of these laws. 

Essentially, there are two fuels we’re running on: carbohydrates and fats. Through this research, we are seeing that the body can shift from a carbohydrate reliance toward a fat reliance at all intensities of exercise. It used to be believed that above a certain threshold of effort, you had to run on carbohydrates. You’ve now shown that is not true. 

Glucose, Glycogen, and Fat: How the Body Fuels and Regulates Itself

Josh Clemente: You brought up how blood glucose is the limiting factor. A lot of us think of glycogen and blood glucose almost as proxies for one another. Can you break this down for us? What’s the difference here, and what’s happening?

Prof. Tim Noakes: When I started looking through this literature, I believed exactly the same thing. As physiologists, we say it doesn’t matter if it’s the carbohydrates coming from the muscle or from the bloodstream—the regulation is the same. That’s totally false, and it took me a long time to realize it. 

During exercise, your muscle glycogen drops as a linear function of the duration, which tells you largely that the exercise is regulating how much glycogen you are using. But it’s always going down. I did not know until six months ago that the glucose coming out of the liver and into the bloodstream is then circulating to the muscles and getting used by the muscles, but the regulation is totally different. Why? Because blood glucose oxidation goes up. 

That’s paradoxical because you don’t really want that to happen—the poor liver is becoming more and more depleted of glucose and glycogen. It’s having more and more difficulty producing glucose. The muscles are saying, “I don’t care about you. I want that glucose.” And ultimately you will always reach the state where the muscle demands more glucose than the liver can provide, and your blood glucose will fall. 

As physiologists, we say it doesn’t matter if it’s the carbohydrates coming from the muscle or from the bloodstream—the regulation is the same. That’s totally false, and it took me a long time to realize it. 

The brain’s not so stupid that it says, “Okay, the blood glucose is falling, we must just continue until you die because the glucose in the bloodstream is crucial for the brain to function.” The brain has a protective mechanism. In all these studies, you can see how, as the glucose starts to drop, the power output of the athlete starts to drop as well, and they start to get fatigue symptoms. But if you give them glucose after 10 minutes, the glucose starts to rise and they feel fantastic and they can go on for a long time.

It’s very clear to me that the regulation is different. The next question is, Why is the regulation different? I’m now speculating. What we’ve shown—and you’ve been crucial to this as well—is that the blood glucose is crucial and that the body will burn the glycogen. But it could burn fat for everything. Why doesn’t it? Why doesn’t the body just burn fat? 

One of the key things I observed was that in the studies where people are studied at rest, 50% of the energy is coming from carbohydrates. That does not make sense because this is the jet fuel the body’s trying to conserve. That’s why you’re filling your mouth with carbs—to provide the muscles with carbs and glycogen—and the body’s wasting it by burning it at rest and when you’re sleeping. Why? The answer is very simple, and it’s provided by George Cahill, who wrote this in 1971.

He gave one of his famous lectures in 1971 and explained how the first rule in metabolism is that the body regulates the blood glucose concentration and keeps it within a narrow band. Everything’s focused on that. As soon as the glucose goes out of sync, the body responds dramatically to try and get it back into range. 

The one way you can get the glucose back into range very quickly is to dump the glucose somewhere—namely, in the muscles. The body’s so clever that it says, “Okay, we’ve got too much glucose or glycogen in the muscles. I know you’re going to go out and have another Coca-Cola and you’re going to have some chips and bananas in three hours. I’ve got to anticipate that. I’ve got to get rid of this glucose in the muscles. When the next load comes in, I can store it.”

That’s what’s happening. The only reason you burn glucose is to regulate your blood glucose concentration. The muscles respond, and if they’ve got lots of glucose, they will burn glucose. They have to. The only way you can stop that is by not eating carbs. Then your muscles are full of fat and have very little carbohydrate, and then you will burn fat. 

Two years ago, we studied a low-carb athlete. He was a really good athlete and could cycle at a very high rate from the moment he started a ride. We had him do a 100K time trial, and from the instant he got on the bike, he was burning 1.7 grams of fat per minute, which, normally, if you’re carbohydrate adapted, you would never get anywhere near there. If you’re carb adapted, you start at 0.4, 0.5, and it takes you hours to get anywhere near where he was. We subsequently did studies to prove that the content of muscle glycogen determines how much fat and carbohydrates you burn. 

I went back to those original studies, the 1967 studies, and looked at the group on the low-carb diet. Although they could only exercise for about an hour, they had a rate of fat oxidation higher than anything that had ever been reported. But they didn’t notice it because they were focusing on the carbs. From the word “go,” they were burning a whole lot of fat. They were not fat adapted, which is the other interesting point. 

These people were burning fat even without being fat adapted. I interpreted it to mean that the body’s designed to burn fat. The only way you don’t burn fat is if you’re eating a high-carb diet. The natural state is to burn fat. You don’t need to train for it. It develops.

Louise Burke took some Olympic athletes who were high carb, and within five days they were burning 1.6, 1.7 grams per minute. They didn’t need to go out and train.

Josh Clemente: The theory is kind of evolving—we are actually seeing an adaptation to carbohydrates we’ve induced. We think it is the natural state, but that might be upside down, and in fact, the natural state would be fat oxidation.

Even when we are really fat adapted, we will never go to zero glucose in the bloodstream. We’ll always be producing glucose and maintaining it in that tightly regulated band George Cahill was describing. Why doesn’t the body just burn fat, even when it’s fat adapted? Why is it that the body will never get to zero circulating glucose?

Prof. Tim Noakes: Because glucose is obligatory for the brain. That’s why blood glucose oxidation goes up during exercise. That shows it’s obligatory. You can do what you like, but you can’t stop it; you can take in all the carbs you like, but it’s going to go up. 

The only way you don’t burn fat is if you’re eating a high-carb diet. The natural state is to burn fat. You don’t need to train for it. It develops.

You can start exercise carbohydrate depleted. The rate of blood glucose oxidation is the same. It’s fulfilling some obligatory role, and obviously part of it’s brain—some of it may be kidneys, some may be other tissues, and it may also be obligatory for muscle. I’m not going to exclude that. But it’s such a tiny amount. It’s not the predominant source, as we used to think. It’s a tiny, tiny amount, and I’m not excluding the possibility that you need a little bit of carbohydrate to keep the muscles working properly.

Josh Clemente: Thank you for clarifying that, because that’s a really crucial point—we don’t want to get hedged in one direction or the other. The fact is that we have been in one direction. I’ve seen this, as somebody who has tried to train in various ways over my lifetime and tried to replicate what I see elite athletes doing, assuming that’s what’s best. It leads you to a very carbohydrate-heavy fueling strategy. You see somebody in the gym training for the Tour de France and the way they’re consuming calories—the vast majority in those training cycles are consuming carbohydrates.

The whole culture is oriented around that. It’s not that we should overcorrect in the other direction, it’s that the science probably shows we’ve adapted to operate in that environment and we need to understand what the natural adaptation is. We know, however, that glucose will never go away; it’s always going to be a critical element for brain fuel. We think about the brain and how it has a critical fuel dependency on glucose, but the heart has a critical fueling dependency on lipids, on fats. I believe the muscle tissue in the heart almost exclusively oxidizes fat. Is that correct?

Prof. Tim Noakes: It’s really interesting because my PhD was actually on little rat hearts, which we got to contract. We had them pumping as if they were in the animal, and we’d stressed them. I developed the system for testing them to their maximum output, and I measured what fuel they preferred. Glucose and insulin were actually the best. But ketones also worked very well. 

It’s difficult to study fat metabolism for various reasons. We have to bind fat albumin, and that changes the whole characteristics of the fluid the heart is pumping. And it changes the dynamics. But the heart loves glucose and lactate and ketones. It just depends on what you give it. It’ll burn.

What We’ve Learned From Studying Athletes

Josh Clemente: I’d love to talk a little more about these findings and how you would say they apply to various groups of people. The study cohort was a group of recreational male athletes. How would these findings, in your opinion, apply to elite athletes? I’d also like to hear how it would apply to female athletes.

Prof. Tim Noakes: You’re quite correct because you saw the athletes and they were recreational, but they were actually better than 88% of all American runners: although they were recreational, they were elite recreational. They didn’t need to eat a high-carb diet. There was no difference in performance. 

Now you say, “Okay, what about the other 12%?” Eliud Kipchoge runs a marathon in two hours. People usually talk about his VO2 max, but you really want to know how many kilojoules per minute is he expending, and where those kilojoules are coming from.

It turns out that because he’s only 52 kilograms, he’s actually only burning 78 kilojoules per minute, which is two grams of carbohydrate—two grams of fat per minute. Technically he could run the marathon on fat if our model is true. The problem is he’s been raised on a high-carbohydrate diet. The Kenyans run on high-carbohydrate diets because that’s what’s available to them. 

But I will guarantee you that their diet has changed in the last 100 or 200 years. Many of today’s African runners came from generations who were cattle wrestlers, who were eating animal-based diets, I would guess. It’s only more recently they’ve moved to high-carbohydrate diets.

Josh Clemente: It demonstrates the amazing flexibility of the body to be able to perform correctly, whatever the environmental circumstances provide.

Prof. Tim Noakes: Another study I looked at in detail was the first study to show that carbohydrates ingested during exercise could improve performance. It was written by Ed Coyle in 1986. We published another paper in 1986 trying to prove that hypoglycemia could be reversed in performance, and we couldn’t find it because our model was wrong. 

Coyle got into the laboratory, and starved the people for 12 hours before exercise. That was the key. I went back and worked out the metabolic state of those athletes as well, and I noticed they didn’t report the actual fat oxidation rates. They reported the carbohydrate oxidation rates, and they had a picture showing carbohydrates, fats, and muscle glycogen oxidation. 

When I made the calculations, I showed that when these people took in carbohydrates, they were burning fat at 1.2 grams per minute, which was the highest value ever reported at that time. And no one noticed it. Who were these athletes in 1986 who were burning so much fat? They were Olympic-class, elite cyclists in Austin, Texas, where Lance Armstrong comes from. 

They were obviously very competitive. Ed Coyle got some of the best cyclists, and they were fat adapted in 1986. Why? Because that was before the carbohydrate craze hit. That was one reason. Secondly, because they would go out on long cycles. I’ll bet if you studied the Tour de France cyclists, although they’re eating high-carbohydrate diets, because they cycle for five or six hours frequently, they’re fat adapted as well. 

That study showed how people eating a high-carbohydrate diet could be fat adapted. Clearly they were doing some training that wasn’t being done by everyone. A runner can’t run for six hours. A runner is less likely to be fat adapted than a cyclist, in my opinion.

Josh Clemente: When you say a runner can’t run for six hours, is that at a certain intensity level?

Prof. Tim Noakes: I’m talking about training. The Tour de France cyclists spend the most time on the bicycle. They’re going to burn through carbohydrates and get into that fat-burning zone much more frequently than runners.

Josh Clemente: Given this certain work-exercise domain, you can essentially outpace the dietary substrate you are working from, and then your body kicks over into the adapted state depending on which. On a bike you can produce 350 watts of power continuously. You don’t need to, and most people do not do that when running. 

Like you said with Eliud Kipchoge, his actual power output is quite low given that he does not weigh a lot, and he’s moving very efficiently. It’s really interesting.

Prof. Tim Noakes: One of the first elite athletes to contact me was Dave Scott, who won the Ironman five or six times. He said, “Tim, if I’d followed your diet in the 1970s, I would’ve gone 40 minutes faster in the Ironman.” He said that was the worst mistake he ever made, to eat a high-carb diet. 

There’s another lovely story about Paula Newby-Fraser, who won the Ironman eight times and won 28 Ironman triathlons. She’s actually from South Africa, and she was such a talent. She went to America after one year of training, and came in third in her first Ironman. That was in the early 1980s, around when Steve Phinney’s first paper came out saying, you burn more, eat more fat. She phoned me and said, “Tim, I’ve read this paper. What do you think? Should I eat more fat?” I said, “Yes, Paula, eat more fat.”

But at the time I was promoting the high-carb diet. She believed she should go on a low-carb diet. She went on the low-carb diet, and won all these on Ironmen. When she retired, she said, “The best piece of advice I got was to eat the low-carb diet.” And I said, “But I never taught you that.”

Josh Clemente: What a funny coincidence. Let’s take the recent results of the research and discuss how these results, which were entirely based on male recreational athletes, apply to female athletes. Do the same sort of biochemical principles hold for both sexes?

Prof. Tim Noakes: When I was running marathons more competitively in the 1970s, there was a German doctor who came up with a theory. He said women were going to outrun men. Why was that? Because women burn more fat when they’re running, and will have better endurance. That was the theory in the 1970s.

He was probably studying women who were eating more fat in their diets. I can’t see any reason why women’s muscles would be different and would metabolize any differently than men, as far as the energy metabolism goes. I would think it applies to women. 

But what I would like to say is that having developed Type 2 diabetes as a result of following a high-carb diet, even after running 70 marathons or ultramarathons, I’m a bit more suspicious about the health effects of the diet.

The study we did also showed that 30% of these recreational athletes became pre-diabetic on the high-carbohydrate, low-fat diet, whereas when they were eating the high-fat, low-carbohydrate diet, the control was absolutely perfect. 

Andrew Kutnik, who was crucial in this analysis, showed that the athletes who burned the most fat were the ones whose glucose control improved the most when they went on the high-fat diet. That was the first time linking fat oxidation and muscle with more resistance to Type 2 diabetes or pre-diabetes. That was the first study showing pre-diabetes developing without people putting on weight or changing their exercise habits. The only thing they changed was the diet.

Josh Clemente: This is shown through the continuous glucose data. It was a crossover trial. Essentially, we had a low-carb, high-fat group and a high-carb, low-fat group. Then they crossed over at the midpoint. They each did that diet for four weeks, then they switched and did the other diet for four weeks. 

Ingesting Carbohydrates in a Way That Supports Metabolic Health

Josh Clemente:  What you’re describing is the onset of these abnormal glucose dynamics as tracked by a continuous glucose monitor during the high-carb, low-fat portion. Is that right?

Prof. Tim Noakes: That’s correct. I now spend my life telling people that the key to being healthy for metabolic health is to titrate how much carbohydrates are in your diet and how much you can cope with. Those three athletes of the 10 on the standard diet were eating too many carbohydrates, and the body couldn’t regulate their blood glucose under those circumstances. That’s going to harm them. If they stay on that diet for 10 or 20 years, they will become diabetic. 

Do they represent all runners in the world? I would say that 30% of runners eating this high-carbohydrate diet in their thirties or forties, if we tested them properly, would be pre-diabetic. When we started running in the 1970s, you trained really hard, and at the end of three hours, that was the end of the race. If you hadn’t finished within three hours, no one hung around to see you finish. They were gone after three hours, and everyone was lean—but really lean, because they trained hard, and they didn’t eat so much carbs. 

Now you go to these marathons and see that people finish in six hours. They’re a metabolic disaster. They’re carrying all this extra fat, but they believe that because they’re running, they’re going to be healthy. That’s not true. Others are telling them to eat high-carbohydrate diets to run faster. 

The only people who can eat high-carbohydrate diets as runners are those who prove they’re not pre-diabetic or not going to become pre-diabetic. You have to earn your carbs by being healthy if you’re not healthy.

The only people who can eat high-carbohydrate diets as runners are those who prove they’re not pre-diabetic or not going to become pre-diabetic. You have to earn your carbs by being healthy if you’re not healthy.

It’s simple: if you’ve got visceral obesity, if you’ve got a tummy and your waist is too wide, carbs are killing you. People have to understand that. The reason I’m so vocal about this is I watched my dad die from Type 2 diabetes, and I couldn’t help him because I didn’t understand. It is an awful, awful death. 

In a sense, that’s why I came out and said I was so wrong about the diet, because I knew a lot of people will become diabetic if they follow the dietary advice I was giving. I’m terribly sorry I was wrong. I don’t want you to die. I don’t want you to be sick as a result of this.

Josh Clemente: I want to draw a distinction between all carbohydrates and the ultra-processed stuff. At my fittest point in my life, in my mid- to late twenties, I was working as a CrossFit trainer. I had a lot more muscle than I do even now, and I’m in decent shape now. When I first used a continuous glucose monitor, I immediately recognized I was pre-diabetic. My blood sugar was quite chaotic. I was following, again, the best practices based on the sports science around me, which was a lot of very processed carbohydrates before workouts to carb load, and a lot of processed carbohydrates after workouts to replenish glycogen. I was using a lot of sports drinks loaded up in very fast-metabolizing glucose and fructose.

I was constantly bombarding my system, and my body simply couldn’t keep up. Even though I was not showing the visceral adiposity, my blood glucose dynamics were horrendous. I’ve done a bunch of things personally. And again, this is an n-of-one, but I’ve gone all the way to the other direction—ketogenic diet, lots of running, lots of zone two, lots of fasted exercise. 

Now I’ve sort of balanced out. I’m titrating the carbohydrates and picking the carbohydrates I know I can trust—the ones not going to be an assault on my bloodstream, and which are going to break down in the context of fiber in a whole food. These tend to be berries or sweet potatoes or something like that. Every once in a while I’ll make some rice or even some pasta when I want to indulge, but I’ll be very careful about what grain varieties I pick. 

I also pick the portion size based on how active I’ve been, how much I’m expending, how much my body is sort of depleted. 

When we’re talking about carbohydrates, how much are you indexing on the type of carbohydrate, the processing versus the macronutrient itself?

Prof. Tim Noakes: You’ve made all the valid points you need to make. You have to be very cautious and not eat ultra-processed foods. Again: very important advice. The addiction to ultra-processed foods is also a problem. To me, obesity is based on addictive foods, and the ultra-processed foods are designed to addict you.

When I was running, I was clearly pre-diabetic, because the only time I would start to feel good was after running for about four or five hours. Then I think I went into ketosis and my glucose finally stabilized and my body felt much better. Doing a little exercise didn’t help. It had to be a really long exercise session. 

When I trained really hard—up to 150 kilometers a week—I felt good because now I was burning all those carbs. They were just going out of the body. I was burning them immediately. The minute I stopped, I couldn’t believe how quickly I would put on weight. Within a month, I would’ve put on three, four kilograms. I couldn’t believe it. And now I understand why: I couldn’t regulate my blood glucose. My body was just packing the carbohydrates into fat.

Josh Clemente: It’s interesting how quickly these things change. 

How Exercise Impacts Glucose and Insulin

Josh Clemente: We’ve talked about how your research showed that with a high-fat, low-carbohydrate diet, high-intensity performance is not reduced relative to a high-carbohydrate, low-fat diet. We saw how glucose glycogen reduces in the muscle, and how glucose trends in the bloodstream.

Some people using CGM see varying results. Speaking for myself, at a certain intensity level, I start to see my blood sugar dramatically increase, in some cases over 200 milligrams per deciliter. What is that? That’s like over 10 millimoles of glucose in my bloodstream just from a workout, fasted. And that’s at a very high intensity. What’s going on there? How should people think about those glucose spikes in the context of their overall health?

Prof. Tim Noakes: People who are pre-diabetic are the ones who will raise their glucose during high-intensity exercise. I noticed that on myself, when I was a 28-year-old. We were doing studies on low-carb diets. On a high-carb diet, my glucose shot up during exercise, and it shouldn’t do that, even during prolonged endurance. My glucose rose, and I didn’t spot it because all the data I had seen up to that point said that the glucose stays normal during prolonged exercise. I knew that I was insulin resistant. I’ve interpreted that to be a marker of the pre-diabetic state. Even more reason why you need to keep your carbs down if you see that spike during high-intensity exercise. Do I think it’s serious? Probably not. It’s the sustained glucose with the high insulin levels. 

One of the other things I’ve learned in the last few months looking at the data is how quickly your insulin drops during exercise. You can start with an insulin of 60 units, which is extremely high, because we are trying to get down to six units. Insulin can be 10 times what’s normal, but it’ll be back to normal within 10 minutes of exercise. 

The best way to lower your insulin, apart from not eating carbs, is to exercise. I would think that, although your glucose is going up, your insulin’s coming down, and probably the insulin is more of the problem. Also, high-intensity exercise doesn’t last too long, so you probably won’t have high glucose for too long. 

To summarize, if you’re seeing high glucose during exercise, you’re probably pre-diabetic. But we’re talking about 20 years in the future; we’re not talking about tomorrow. You’re just an insulin resistant person, and you need to interpret that appropriately.

Josh Clemente: If they were to adapt to fat oxidation, and start to eat a higher fat-based diet, should they expect those glucose spikes to reduce over time, given the same intensity?

Prof. Tim Noakes: That’s what one would assume. And again, from the data we have, we saw normalization even during exercise. There was a greater return to normal. I would expect that. I can’t see why, if you’re not eating too many carbs, you should have these abnormal glucose spikes.

The best way to lower your insulin, apart from not eating carbs, is to exercise.

Josh Clemente: From my own kind of research on myself, I will say I’ve seen a reduction over time in the absolute peak of those high-intensity spikes. I’m going to have to dig into this a bit more and see how that trend is looking. We’re short on research on this matter, but we know fundamentally it’s totally different to be experiencing elevated glucose from sitting on the couch drinking a Coca-Cola versus pushing weights around or running at a high intensity. What’s happening biochemically in the body is totally different. The insulin level is a great way to think about it—it’s just your insulin also being elevated.

Prof. Tim Noakes: The guy who set the new world record for the Ironman completed it in under seven hours. He showed his blood glucose concentrations. They were unbelievably elevated; they were pre-diabetic. He said he was never going to become hypoglycemic. I realized that actually his problem was hyperglycemia. He must have been loading up on the carbs to great excess—completely unnecessary.

Josh Clemente: A lot of people now, myself included, incorporate fasted exercise into their routines. I personally feel really good when I’m going to do prolonged zone-two exercise fasted versus doing it after any type of meal—high-fat, low-fat, doesn’t really matter. 

How do you think about fasted exercise in the context of adaptation and improving metabolic function and metabolic fitness?

Prof. Tim Noakes: Fasting is really healthy, and combining it with exercise would suggest to me that you’ve got a double benefit. I don’t have any evidence that’s the case, but I do know that fasting is one really good way to help reverse metabolic syndrome. Stressing your body in those two ways is very helpful. 

The irony is that the greatest threat to the body is not starvation or fasting, it’s high-carbohydrate diets. People don’t understand that. And by starvation, I mean a few days of not eating. The thing the body can’t cope with is a high-carbohydrate diet. It can cope with fasting. The world record for fasting is like 380 days, but that guy had a lot of fat on him to burn off.

Josh Clemente: It’s a fascinating story. His name was Angus Barbieri. He just took in minerals and water for 380-something days. The evidence is very clear: Fasting is not an acute danger, considering, of course, the individual circumstances, and everybody is at a different starting point. Hormones also really play a role here. 

Fat as Fuel, and the Need for Further Research 

Josh Clemente: Did evolution select for the most efficient fuel by default? For each gram of fat, you get nine calories of energy, and for each gram of carbohydrate, you get four calories of energy. That means that if I’m the body, I’m evolution trying to figure out what fuel this body should carry around with it. What’s going to be most efficient? 

Well, I’m getting 2.2 times more energy per gram of fat that I carry. We now know that when you look at an average person—let’s say someone who weighs 150 pounds and is 20% body fat—that person is carrying over a hundred thousand calories of fat energy. If they were carrying that same body weight in glucose or glycogen, they would only be carrying about 30,000 to 40,000 calories. It would be much lower efficiency, in terms of how much weight you’re carrying around and how much energy it contains. 

Does that factor in at all? How do you think about the evolutionary context for why the body prefers which substrate?

Prof. Tim Noakes: That’s a great question you’re asking: Why do we store so much as fat, as opposed to carbohydrates? What you didn’t mention is when you store carbohydrates, you store water with it as well. I used to think that was the reason you can’t store as much energy as carbohydrate, because it’s full of water. Evolution adapted us to eat high-fat, high-protein diets. 

Scientists recently looked at 120,000-year-old elephant fossils. They looked at the elephant foot bone. The toe is down there, and below that the elephant had a huge pad of fat. Humans obviously liked fat because if you looked at the bones, they had cut marks. Humans had specifically gone and cut to get the fat away. Three million years ago, Paranthropus, one of our predecessors, was killing rhinoceroses. Rhinoceros bones with these cut marks have been found, showing that humans had taken the meat off these bones. 

That’s what we grew up on. We grew up on fat and protein diets, and that’s why we developed the gut and our brain. We’ve been adapting to this for three million years. We’ve been adapted to burn fats, not carbohydrates. Carbohydrates, as you know, came into the picture 12,000 years ago, with the agricultural revolution. They came in because we ran out of fat animals on all the continents. We just didn’t get enough fat. Now we had to find something else, and we couldn’t eat lots of protein because our bodies are not designed to cope with too much protein. It was obvious we had to get some carbohydrate. That became cereals and grains.

Josh Clemente: It was very seasonal to be able to come across high-carbohydrate foods. Even if we just look at carbohydrates as a basis of energy for tens of thousands of years, maybe even longer, the past 100 years is still a black-swan event in terms of how much processed sugar and carbohydrate we can get into the bloodstream in a relatively short amount of time. 

In 15 minutes I can consume more sugar in a liquid or powdered form—which goes directly to the bloodstream—than most of our predecessors would come across in months or maybe even years. We have exponentially increased the load on our systems.

Prof. Tim Noakes: The populations in Africa that started eating sugar and refined carbohydrates destroyed their teeth. They didn’t have dentists, so they couldn’t survive. If your diet kills your teeth, you’re finished, because you get brain abscesses and so on. Going back far enough, they couldn’t have been eating a lot of carbohydrates, or they would’ve had terrible teeth.

Josh Clemente: What’s the most important question you think researchers need to answer next, in terms of fuel adaptation, low carb, low fat? What is the question burning in your mind on the research side?

Prof. Tim Noakes: What’s burning on my mind is to get people to accept the data. The only way we can do that is by having lots of people who previously promoted high-carb diets doing research and proving they were wrong. That’s what we need to do. That’s probably not going to happen because the industry funds people to study carbohydrates. That’s partly the reason why carbohydrates are so dominant. 

I’m pointing to that because we were funded by the carbohydrate industry for 15, 20 years, and we did fabulous studies, which I now realize were really amazing. But they were all looking at carbohydrate metabolism. Only right at the end did we do some low-carb studies, and they were critical. They were absolutely critical because they showed where the controls were: If you have high muscle glycogen, you burn carbohydrates. If you have low muscle glycogen, you burn fat. 

Even insulin isn’t the main driver. It’s what’s in the muscle at the start of exercise. Insulin helps and glucagon helps, but the driver is in the muscle itself. That’s what I’d like to see, because runners are getting the wrong advice and their health is being affected, as my health was affected. 

I just wish I’d known about this high-fat diet when I was 20 or so, because my career would’ve been very different. I would’ve run many more ultra-marathons successfully. The more carbs I ate, the more quickly I tailed off, the more pre-diabetic I became and the worse my running went. 

I’ve become a great friend of one of the Indian diplomats in Cape Town. Indian people, as you know, eat vegetarian diets. He was a runner, and he read about this low-carb diet. He wondered, What happens if I switch from being a vegetarian? I said, “Well, it’s not going to harm you. It might help.” 

If you have high muscle glycogen, you burn carbohydrates. If you have low muscle glycogen, you burn fat. 

He switched to become a carnivore, an Indian carnivore. He said, “It’s terribly difficult because whenever I have all my friends from India come out to meet me, or other important people from India visit, we have to provide them with the conventional vegetarian diet.” 

He’s converted to this diet, his performances are just going up and up and up. He sends me an email every few months after he’s done another race. The funny thing is, the people he trains with eat high-carb diets. When they race, he just leaves them all behind. They can’t understand. They said, “But how can you run like that if you’re not taking in carbs?” They just don’t get it.

A study I would love to do is to see what happens in high-carb adapters after three hours of exercise. If you’re going along burning, you suddenly run out of glycogen. You can’t burn fat because your body’s not designed to burn fat. You have to slow down. You absolutely have to slow down. But if you’re burning fat from the start, you’ll find that one to two grams of fat are going to get you through three or four or five hours. 

At the moment, I don’t believe your diet plays any role in your performance. That’s an important point. We’ve been made to believe that carbs make a difference, and we’ve shown they don’t. However, if you could get athletes to do four or five hours of exercise, I would like to see what happens in a carb-adapted athlete. They’re going along and then they run out of carbs. What do they do? 

If you’re burning fat, you shouldn’t have a crisis. You should be able to go on. The problem is no one will run on a treadmill for four or five hours. We tried it. We tried a cycling exercise for up to eight hours, and we actually found evidence that the high-fat group were doing better, but we only had a few numbers. 

The reason I want to do that is because I think you’re healthier if you’re eating a high-fat diet. I would also make the point that all the studies and laboratories are one-off races. What happens over a season? That’s the other question that needs to be answered. Your body’s more inflamed if you’re burning carbohydrates. Dave Scott said that. He will not tell any of his athletes to eat a high-carbohydrate diet, because their bodies are always inflamed and they’re more likely to get injured.

Josh Clemente: I’m sure there are some athletes who may be interested in an eight-hour treadmill endeavor. But we have people in our audience like Mike McKnight, who ran a 100-mile race completely fasted, and showed with glucose data that his body was able to super effectively manage that situation. Clearly he was doing that entirely on, or almost entirely on fat. 

We’re seeing the ability to take the lab equipment measurement systems and put them on the body directly. And now they’re mobile and we can start to log data in much more interesting environments. The continuous glucose monitor is an example of this. 

What molecules, in addition to continuous glucose data, would you want to see tracked continuously? If you could wave a magic wand and measure anything about the body and you wanted to do so to demonstrate the metabolic conditions for best metabolic health and also how they affect performance, what other molecules would you be looking at?

Prof. Tim Noakes: Fat oxidation is the key. It’s a very important factor. The more fat you can oxidize, the healthier you are. That’s something I would look at. In the past, we’ve measured the gas exchange in athletes completing a 56-kilometer race. But we used such archaic equipment that we didn’t get proper data. 

There’s not an individual molecule you can look at that’s as helpful as glucose at the moment. But if you could measure your glucose and your fat oxidation at the same time, that would be really helpful. You could then classify people into different levels of carbohydrate oxidation and fat oxidation during exercise and see what happens, and also see how it tracks over time. 

My prediction is that if you’re a carb-adapted athlete and we ask you to run for four or five hours, you’re going to hit some sort of plateau when you run out of glycogen. If you’re not fat adapted, you’re not going to be able to burn the fat as effectively as if you’re fat adapted. That’s where the difference will become apparent. Then, of course, it extends further. 

We have an athlete in Cape Town who cycles about 150 kilometers a day and eats zero carbohydrates. Biologists will tell you that’s impossible, that you can’t convert fat to glucose in the liver. I’m convinced you can. That’s where you can generate your glucose from fat. I suspect that’s happening with these athletes who do not need to eat anything during a hundred-mile race. They have to become hypoglycemic. If they don’t, it’s because they’re converting fat into blood glucose. If you’re doing that, you’ve solved the problem. You no longer have a metabolic issue during exercise. You can go on forever, because you’ve got lots of fat and you’ve got enough glucose to keep you going.

Josh Clemente: That’s as interesting a topic of research as any. Hopefully we’ll be able to have another conversation and discuss some results in this area sometime soon. This has been amazing. I’m really excited to continue to follow along with your work and I appreciate you coming and joining us. Thanks for making this happen and thanks for continuing to push forward on the science. It’s really amazing, and it’s informative. I love that it’s cutting edge right now.

Prof. Tim Noakes: Thanks Josh.  It’s been lovely, and thank you for thinking it through and asking all the relevant questions I could answer as best I could. We got a message out there, and a lot of people are saying they’re actually going to begin to accept what we found. There’s going to be resistance, because people’s careers have been built on saying carbohydrates are important. 

But be like me. They need to be like me and just say, “Okay, I got it wrong. Let’s move on.” That’s the way to do it. The more quickly we get the message out to athletes that they don’t need to eat all this carbohydrate, and that such a high-carbohydrate diet is going to harm their long-term health, the better. That’s the message we need to get out.