Can you change your metabolism?
Don’t believe the hype, marketing and celebrity testimonials, necessarily
Diet and exercise are all well and good, but what if you could also control your weight just by reading this article in a comfortable chair?
That’s the promise of dietary supplements and lifestyle hacks that claim to speed up your metabolism. These products and processes, it’s said, will increase your resting metabolic rate, and voilà, you can lose weight with less calorie counting and exercise. Unfortunately, despite the hype, marketing and celebrity testimonials, ramping up your metabolism is mostly a myth. “There is very little hope of changing your resting metabolic rate, because you’re fighting your biology,” says Eric Ravussin, director of the Nutrition Obesity Research Center at the Pennington Biomedical Research Center in Baton Rouge, La.
To understand why trying to speed up your metabolism is mostly a waste of time and money, let’s start with some physiological facts.
Your resting metabolic rate is expressed as the number of calories your body would need if you were to do nothing for the next 24 hours. (Your basal metabolic rate is a slightly different measure, though the terms are mistakenly used interchangeably.) A resting metabolic rate (RMR) is calculated by measuring oxygen consumption and carbon dioxide exhalation after the subject has been seated or lying down for at least 15 minutes and hasn’t exercised in the previous 12 hours. RMR plays an obvious role in weight: If the sum of someone’s daily calories consumed minus calories burned is greater than that person’s RMR, weight will increase.
How do you figure out your RMR, short of enrolling in a medical study? There are several online calculators, including the one by the National Institutes of Health, that estimate your resting metabolic rate in terms of number of calories per day. But that’s only an estimate.
People of the same sex, age, height, weight and body composition can have inherently different resting metabolic rates. Susan Roberts, director of the energy metabolism lab at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, says the difference can be about 10 per cent in each direction. A typical 35-year-old woman who is five-foot-six and weighs 140 pounds will typically have a resting metabolic rate equal to about 1,500 calories a day, while other similar women might need only 1,350 calories (10 per cent less) or 1,650 calories (10 per cent more).
Kevin Hall, section chief of the National Institute of Diabetes and Digestive and Kidney Diseases’ integrative physiology section, says part of the reason for different rates can be attributed to internal variations. “Some organs use more energy than others,” he says. “A person with a large liver can have a higher metabolic rate.” Differences in brain size can also play a role.
Other significant contributors to RMR include body composition — a 140-pound person with 15 per cent body fat will have a higher metabolic rate than a 140-pound person with 25 per cent body fat — and age. Roberts says that metabolism slows by one to two per cent per decade from a combination of factors that includes brain shrinkage and muscle loss.
Finally, there’s the issue of genetics. Ravussin, who has conducted extensive research on Pima Indigenous people, found family membership to be a significant factor in explaining differences in resting metabolism among people of similar size and body composition.
These factors explain why “revving up” your metabolism is, mostly, a doomed quest, akin to striving to be taller or to have greener eyes than what’s in your genetically prescribed range.
Not only is speeding up metabolism unlikely, but the methods that claim to do so also either don’t work or won’t create lasting results. For example, you might read that you can pump up your metabolism by getting enough sleep to keep your appetite hormones in check or by lowering your stress level so that your body doesn’t produce too much cortisol, which can lead to overeating. But those hormonal levels relate to how much you feel like eating, not how many calories a day your body burns for basic functioning.
Similarly, while higher-intensity workouts might result in a slight post-workout afterburn (research conflicts on this issue), those short-term results don’t affect what your metabolism will be the following day.
Supplement makers tout ingredients such as green tea, caffeine, capsaicin, selenium and more, either individually or in, as one company puts it, a “thermogenic fat-burning complex,” as metabolism boosters. Some have been shown to slightly increase the rate at which people burn calories, but not to an extent that’s going to make a significant difference over time.
“Might help you lose a small amount of weight” is the most a National Institutes of Health ingredient overview will say about such ingredients.
Drinking a lot of water has long been a staple of weightloss programs, in part because doing so makes you feel fuller. Some research has found that extra water consumption can also increase your resting metabolic rate. A study involving 50 overweight young women found that when they added three half-litre servings of water per day to their normal fluid intake, they burned an average of an additional 50 calories per day. That’s not an insignificant amount, but it’s equal to about half a banana.
Roberts says there are two dietary tweaks that can increase metabolism because they increase the body’s energy needs for digestion: eating more fibre and protein. She advises a diet that includes 25 to 35 grams of fibre per day (the average U.S. adult consumes about half that) and in which protein constitutes 25 to 30 per cent of calories. As with drinking water, the potential payoff is modest — fewer than 100 extra calories burned per day for most people.
As for exercise, increasing your muscle mass will slightly boost your resting metabolism. Note that this is a different — and much more difficult — undertaking than getting stronger; you can improve your performance at bench presses without necessarily adding pounds of muscle. “To increase muscle mass, you need very heavy-duty resistance training,” Ravussin says, “and you still might not see real results. I’ve worked with people who have said, ‘I cannot gain a pound of muscle.’ ”
A better approach: Do heavy resistance training to slow the rate that you lose muscle mass beginning in your late 30s or early 40s. Holding on to as much muscle as you can with age will keep your metabolism higher.
While you can’t really speed up metabolism, you can, unfortunately, slow it. An extreme example can be found in a study Hall published in 2016, involving 14 contestants from the reality show “The Biggest Loser.” The study made headlines not only because of the show’s popularity, but also because of Hall’s startling results.
Six years after “The Biggest Loser,” the average contestant had regained more than twothirds of the weight lost during the show. But that wasn’t the startling part. What surprised Hall and his colleagues was that the contestants’ bodies were mostly acting as if they were still their much-slimmer versions. Their resting metabolic rate was an average of about 500 calories per day lower than expected for people of their age and body composition.
These results were an illustration of what is called adaptive thermogenesis — a seemingly permanent reduced resting metabolism in people who have lost a large amount of weight. According to a 2010 review of the topic, maintaining a 10 per cent or greater weight loss is accompanied by a 20 to 25 per cent reduction in daily caloric needs. Hall’s findings suggest that the greater the initial loss, and the quicker the loss, the more that resting metabolism will be lowered long-term.
So you can change your metabolism, but not in the way you want. (Why can we only slow but not speed our metabolism? Perhaps because there was an evolutionary benefit to having a slow metabolism during times of famine, while there’s little corresponding need for a fast metabolism.)
A more realistic and healthy goal than trying to accelerate your metabolism is trying to keep it as high as possible. The steps you can take in pursuit of that goal align with practices that are good for both health and weight maintenance: Avoid large swings in weight, stay active, drink enough water, eat enough fibre and protein, build muscle when young and maintain the muscle as you age. You can’t control your biology, but you can control your choices.
“There is very little hope of changing your resting metabolic rate, because you’re fighting your biology.” ERIC RAVUSSIN NUTRITION OBESITY RESEARCH CENTER