January 7, 2020
Physical exercise, including walking, jogging, high intensity interval training, and resistance training with weights, can change the ability of our cells to take up and utilize glucose, and in turn increase our metabolic fitness.
The positive effects of exercise on metabolic fitness include an increase in glucose transporters traveling to the lining of cells (GLUT4 channels), allowing more glucose to enter which lowers circulating glucose - without additional insulin. Exercise may also improve pancreatic beta cell function (the cells that produce insulin) and insulin activity, increase the amount of fat we burn in between meals, and increase the number of mitochondria we have in our cells (the part of the cell that burns fat and glucose to make energy).
A large body of research shows that the amount of exercise required to produce these benefits may be surprisingly low: moderate aerobic activity for just 30 minutes, at least 3 times per week over 8 weeks improves insulin resistance and glycemic control, including fasting glucose levels.
These adaptations are just part of the story though. Tracking glucose levels can also help you optimize “metabolic flexibility,” your body’s ability to utilize different fuel sources. By understanding when and how these fuel sources are accessed, we can make choices that lead to greater endurance and better performance - in the gym and in everyday life.
Our bodies have two major sources of energy: glucose (sugar) and fat. There are advantages to each “fuel”, but ideally we are able to easily switch between the two without major mental or physical disruption. The term “metabolic flexibility” refers to this efficient switching process.
Plenty of factors affect this capability, including the intensity and duration of exercise routines, our physical state, and what we eat, however there is evidence that low carbohydrate diets may be best for promoting metabolic flexibility.
An important difference between glucose and fat as a primary fuel is the storage capacity of each in the body. A 155lb person with 10% body fat can store the equivalent of about 1,800 calories of glucose (in the form of glycogen), enough to fuel about 90-120 minutes of continuous exercise, but has about 63,000 calories available in the form of fat.
You might be familiar with the term “bonking,” when an athlete suddenly depletes their glycogen stores and experiences severe fatigue and possibly even hypoglycemia, a sometimes dangerous condition caused when glucose levels in the body fall too low. The mechanism is simple: the athlete has simply run out of the finite amount of glucose that their body can store.
Conventional wisdom once held that high-carbohydrate, glucose-spiking diets were necessary to optimize exercise performance, and “training tables” full of pasta and bread were a regular feature for athletes. This made sense at the time because it was well known that depleting stored glucose would lead to fatigue, and so the goal was to replenish glucose with high carbohydrate meals.
As the science progressed however, we learned that activating and optimizing fat burning pathways could be a more successful strategy than loading up on carbohydrates. The ease with which our bodies can convert food to body fat means that calories stored as fat are a nearly unlimited source of energy for long-duration exertion. If we can shift our metabolic processes to efficiently use this fat instead of sugar, we may find that that we can optimize athletic performance and endurance.
A diet low in carbohydrates can help promote this metabolic shift: athletes who follow these diets adapt to burn fat (fatty acid oxidation) at significantly higher rates during prolonged exercise. This has important implications particularly for long duration fitness events, and the ability to oxidize fat has been correlated with performance in Ironman competitions (>8hr events).
In fact, physical training alone can shift the body toward higher rates of fat oxidation, a signal that it may be the preferred energy pathway when the human body is pushed for higher performance. A study of endurance athletes showed that even without a change in diet, their bodies adapted to favor usage of fat rather than glucose. These highly-trained athletes showed a threefold increase in fatty acid oxidation, perhaps explaining their improved capacity to perform high intensity activities when compared to recreational athletes.
In contrast, insulin resistant, obese individuals don’t utilize fatty acid oxidation pathways as efficiently as lean individuals. Indeed, emerging research suggests that glycemic control, insulin sensitivity, and metabolic flexibility are all important determinants of an athlete’s ability to efficiently utilize stored fat as energy.
When athletes consume a low carbohydrate diet, keeping glucose (and insulin) levels low, they develop an enhanced cellular ability to utilize fats. The subsequent production of ketones and glucose precursors offers an abundant source of fuel. Fat burning byproducts like beta-hydroxybutyrate have also been shown to increase gene expression of health-promoting antioxidants and reducing tissue-damaging reactive oxygen species in the body, potentially making exercise recovery speedier as well.
Whether your body uses primarily glucose or fat is also affected by how intensely you’re exercising. In most people, the switch in energy use from glucose to fats happens when endurance exercise is moderate, or below about 60% VO2max. However, in high intensity anaerobic activities, or those above 80% VO2max, glucose generally becomes the predominant source of fuel.
Interestingly, circulating glucose tends to rise during brief bouts of intense exercise (>80% VO2max), and even more so for the hour after exercise ends. This is thought to be because high intensity exercise stimulates secretion of specific hormones (catecholamines, including “the stress hormone” epinephrine), which stimulate glucose production in the liver to increase up to 8-fold. The muscles, however, only increase their use of glucose by about 3-4 fold, and the result is a supply/demand mismatch where there is excess glucose in the blood. This elevated glucose signals a rise in insulin, at which point muscles take up the excess glucose to replenish stored glucose (glycogen), and we see a concurrent fall in glucose back to baseline values.
If you’re tracking your glucose using CGM, you might be worried by this apparent glucose rise during high intensity exercise. You shouldn’t be. Despite the acute rise in glucose, high intensity training actually improves both fasting glucose and insulin sensitivity in as little as two weeks. Both of these adaptations lead to better metabolic flexibility and glucose control.
We know that post-meal glucose spikes are a risk factor for heart disease, stroke, and vascular damage. Glucose oscillations also cause “oxidative stress,” the process through which free radicals in the body cause tissue damage. Tracking glucose gives you the tools to reduce these spikes by helping you determine the best type of exercise and timing of exercise to support stable glucose levels.
You might be a committed daily runner, which clearly benefits your health and fitness. But after tracking your post-meal glucose levels, you might find that you’re not getting the results you’d like. Consider the timing and duration of those runs.
A study compared 3 exercise timing regimens (20 minutes of jogging before each meal, versus 20 minutes of jogging after each meal, versus short bursts of jogging for 3 minutes repeated 20 times a day), with all regimens adding up to 60 minutes of activity per day. The study found that the scenario with 20 short bursts of jogging throughout the day were significantly more effective in reducing post-meal glucose spikes.
It’s not just jogging; the superior effect of short, frequent bouts of activity on metabolic control has been shown in other studies as well. Another study looked at walking for a discrete 30 minute period once per day versus walking for just 1 minute 40 seconds every 30 minutes during waking hours. While both groups walked a grand total of 30 minutes, the study showed that the frequent, short walks were significantly more effective at reducing post-meal glucose peaks and insulin levels.
While you likely don’t have a schedule that allows for 20 jogs every day, armed with this information and a means of easily measuring your glucose, you might shift your schedule to get the most benefit in the time you have, and in particular, try to fit activity all throughout your day, as opposed to one big chunk.
Additionally, exercising in a fasted state can also promote metabolic flexibility and fat burning capacity. A study from 2019 in overweight and obese men showed that exercising before eating breakfast leads to increased use of fat for energy during the workout, reduces post-meal insulin elevation, and increases insulin sensitivity over 6 weeks.
Research suggests that real-time, individualized feedback on glucose levels can inspire people to exercise more. A study in a diabetic population has shown that wearing a CGM as part of an individualized counseling program reduces average blood glucose and weight levels significantly, perhaps due to increased focus and motivation brought on by seeing the improvements as they happen. Additionally, CGM is associated with individuals upping the intensity level of their exercise, from “sedentary” and “light activity” to more “moderate intensity,” and can lead to significant increases in total exercise time per week.”
The relationship between glucose and exercise is complex, but research shows us that there are steps we can take to optimize the impact of our activity on our metabolic health. Even if you’re not an elite athlete, these choices may help you burn more fat, increase endurance, lower post-meal glucose levels, and even inspire more intense and frequent workouts.
A metabolic fitness tool like Levels can unlock the power of biometric data to give you visibility into how specific behaviors, foods, and routines are affecting your glucose levels, giving you the biofeedback to understand and optimize your exercise routine and gain more benefit from your hard work.
Disclaimer: The information on this site is intended to provide general educational information only, and does not constitute, nor is it a substitute for, medical advice.
Photo credit: Victor Xok
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