Your metabolism isn’t just about burning off calories from the food you eat; it’s actually the sum of all the biochemical reactions that take place in your body at any given moment. Metabolic rate refers to the speed at which all of these processes occur.
Many metabolic processes require energy, such as repairing damaged tissues and creating new cells. But metabolism also includes energy-producing reactions, such as those that occur when large molecules like carbohydrates and fat are broken down, and energy is released.
Given these complexities, some popular notions about what makes a “fast” and “slow” metabolism aren’t entirely accurate. Here’s what you need to know.
Myth #1: Smaller people have faster metabolic rates
One common misconception is those thin people who can seemingly eat whatever they want without gaining weight have a fast metabolic rate, whereas those who struggle to lose weight have a slow one. In reality, the opposite is true. In fact, physical size—which includes height and weight—is the number-one factor determining metabolic rate.
Metabolic rate comprises three main components, the largest of which, by far, is resting (basal) metabolic rate. Resting metabolic rate is the total number of calories burned when your body is at rest, and the larger you are, the higher this number will be.
Imagine you have a big semi-truck parked next to a little sedan. Even when idling, the semi uses more energy. And once the vehicles start going down the road, that semi, which has so much more mass, requires far more energy to move. Similarly, the more muscle, fat, and bone a person has, the more energy they need to function. And just as a smaller car would require much less fuel than a semi, thinner people tend to eat less than larger-bodied people to maintain their weight and overall energy needs.
Surprisingly, your organs significantly factor into your body mass, with the size of your kidneys, muscles, brain, and liver playing the most significant role in your metabolic rate. Research has found that up to 43% of the variation in metabolic rate from one person to the next is due to differences in the size of these organs.
Though often overlooked, even fat tissue has a metabolic rate, accounting for up to 6% of whole-body metabolic rate. Of course, this number goes even higher in a body with more adipose tissue.
Myth #2: The older you are, the slower your metabolic rate
Another common myth is that your metabolism slows with age. Although middle-aged folks often complain about gaining weight, recent evidence has disproven the notion that a metabolic slump is to blame. It’s true that children and teens—who grow and change rapidly—have the highest metabolisms, but from your 20s to about age 60, your metabolic rate remains relatively stable. Afterward, it starts to dip by about 0.7% per year.
So why might people gain weight if their metabolism hasn’t slowed? Lifestyle habits, especially how many calories you consume and how active or sedentary you are, are one major factor. Another is genes: One paper estimates that genetics accounts for between 30 and 70 percent of obesity factors.
Myth #3: Insulin doesn’t impact metabolic rate
One hormone that doesn’t get much attention with regard to metabolism is insulin. We’ve known for over 100 years that any state with low insulin is a state with elevated energy expenditure. But the moment insulin goes up, energy expenditure starts to drop.
Some people inherit a genetic predisposition to being insulin-resistant; others consume a diet that results in chronically elevated insulin levels. In either event, when you have high levels of insulin circulating in your bloodstream, it makes it easier for your body to store fat and harder to burn it.
Insulin prompts tissues to take more glucose from the blood, prevents fatty acids from being released from fat tissue, and blocks the production of ketones (alternative fuel) in the liver. It also promotes the storage of fat and glycogen. But it’s incredibly stingy when using energy to do these jobs. And when you’re miserly with energy, metabolic rate goes down. Glucagon, a hormone that promotes the breakdown of glycogen in the liver to release stored glucose, is the opposite: It’s good at wasting energy, so when it’s released as a result of blood sugar levels falling low, your metabolic rate goes up.
Myth #4: You can’t change your metabolic rate
Drastically increasing your metabolic rate is difficult because you can’t change the size of your organs or your overall frame. But there are absolutely ways to speed it up.
Physical activity helps because you’re using energy by moving around. Strength training has the biggest impact. When you lift weights or do resistance work, you increase muscle mass, and building muscle takes energy. What’s more, even at rest, a muscle cell has a metabolic rate that’s about five to 10 times higher than that of a fat cell. Once you start moving, that rate increases even more, and suddenly it’s 50 to 100 times higher than that of a fat cell.
Following a low-carb diet also raises your metabolic rate. Research from Harvard found that people who followed a low-carbohydrate diet (20% carbs) burned an extra 200 calories per day compared to those in a high-carbohydrate group (60% carbs)—even though both groups consumed the same total amount of calories. The impact was even greater in the case of people who started the study with elevated insulin levels.
Eating a low-carb diet–or anything else you do to keep your blood glucose relatively low and steady throughout the day–likely boosts metabolism because it keeps insulin down, and insulin has a direct, significant effect on metabolism.
Levels advisor Benjamin Bikman, PhD, is the author of Why We Get Sick. A scientist and associate professor at Brigham Young University, he studies the role of elevated insulin in obesity and diabetes.