What we get wrong about our metabolism

Changing your weight is more difficult than you think.


A lot of people talk about their metabolism like it’s a muscle or organ they can somehow control. But in reality, the term refers to a series of chemical processes in each cell that turn the calories you eat into fuel to keep you alive.

“It’s the culmination of different tissues with different needs and how many calories it takes to keep them functioning,” says Michael D. Jensen, physician-scientist who studies obesity and metabolism at the Mayo Clinic in Rochester, Minn.

The body’s major organs—the brain, liver, kidneys and heart—account for over half of the energy burned at rest, while fat, the digestive system and especially the body’s muscles account for the remainder.


There are three main ways you burn energy: a) the basal metabolism, which is the energy used for your body’s basic functioning while at rest; b) the energy used to break down food, also known as the thermic effect of food; and c) the energy used in physical activity. One very underappreciated fact about the body is that your resting metabolism accounts for a huge amount of the total calories you burn each day. Physical activity, on the other hand, accounts for a tiny part of your total energy expenditure—about 10 to 30 per cent (unless you’re a professional athlete or have a highly physically demanding job). Digesting food accounts for about 10 per cent.

‘It’s generally accepted that for most people, the basal metabolic rate accounts for 60 to 80 per cent of total energy expenditure,’ says Alexxai Kravitz, a neuroscientist and obesity researcher at Washington University in St. Louis.

“It’s not nothing, but it’s not nearly equal to food intake,” Kravitz added. ‘This is why it’s not so surprising that exercise leads to [statistically] significant, but small, changes in weight:’


It’s true that two people with the same size and body composition can have different metabolic rates. One can consume a huge meal and gain no weight, while the other has to count calories. But why this is remains a “black box,” says Will Wong, a researcher and professor. at the Johns Hopkins University Center for Metabolism and Obesity Research, in Baltimore. “We don’t understand the mechanism that controls a person’s metabolic rate.” Researchers have been able to find some predictors of how fast a person’s metabolism will be. These include: the amount of lean muscle and fat tissue in the body, age and genetics (though researchers don’t know why some families have higher or lower metabolic rates). Sex also matters, since women with any given body composition and age burn fewer calories than comparable men.

You can’t easily measure your resting metabolic rate in a precise way. There are some commercially available tests, but the best measurements come from research studies that use expensive equipment. However, you can get a rough estimate of your resting metabolic rate by plugging some basic variables, like age, height and weight, into online calculators. These will tell you how many calories you’re expected to burn each day, and if you eat that many and your weight stays the same, it’s probably correct.


The effect happens gradually, even if you have the same amount of fat and muscle tissue. So if you’re 60, you’ll burn fewer calories at rest than you did when you were 20.

Jensen said this continual decline starts in young adulthood—and why this happens is another metabolism question researchers haven’t answered. “Why do your energy needs go down as you age, even if you keep everything else pretty much the same? That’s one of the bigger mysteries.”


There’s a lot of hype around “speeding up your metabolism” and losing weight by exercising more to build muscle, eating different foods or taking supplements. But it’s a myth.

While there are certain foods—like coffee, chili and other spices—that may increase the basal metabolic rate just a little, Jensen says the change is so negligible and short-lived, it would never have an impact on your waistline.

Building more muscles, however, can be more helpful. Here’s why: one of the variables that affects your resting metabolic rate is the amount of lean body mass you have. At any given weight, the more muscle on your body, the higher your metabolic rate. That’s because muscle uses a lot more energy than fat while at rest.

So the logic is, if you can build up your muscle, you’ll have a higher resting metabolism and will burn the fuel in your body more quickly. But there’s a caveat.

According to Michael Rosenbaum, professor of pediatrics and medicine at Columbia University’s Irving Medical Center and a researcher in New York who studies weight loss and metabolism: “If you have more muscle, it burns fuel more rapidly. But that’s only half the question.” If you do gain more muscle and effectively speed up your metabolism, “You have to fight the natural tendency to [want to) eat more as a result of your higher metabolism:’

Jensen also notes that it’s difficult for people to sustain the workouts required to keep the muscle mass they gained. “For most, it’s kind of impractical;’ he added.

Overall, he says, “There’s not any part of the resting metabolism that you have a huge amount of control over.”


While it’s extremely hard to speed up the metabolic rate, researchers have found there are things that can slow it down—like drastic weight-loss. “[Crash diets] probably have the biggest effect on resting metabolism;’ says Jensen. But not in a good way. For years, researchers have been documenting a phenomenon called “metabolic adaptation:” As people lose weight, their basal metabolic rate actually slows down to a greater degree than would be expected from the weight loss. To be clear, it makes sense that losing weight will slow down metabolism. Slimming down generally involves muscle loss, which, in turn, means the body doesn’t have to work as hard to keep running. But the slowdown after weight loss, researchers have found, often appears to be substantially greater than makes sense for a person’s new weight.

Sandra Aamodt, a neuroscientist and author of the book Why Diets Make Us Fat, believes this may be the body’s way of vigorously defending a certain weight range, called the set point.

Once you gain weight and keep the weight on for a period of time, the body can get used to its new, larger size. When that weight drops, a bunch of subtle changes kick in—to the hormone levels, the brain—slowing the resting metabolism and having the effect of increasing hunger and decreasing satiety from food, all in a seeming conspiracy to get the body back up to that set point of weight.

“I don’t think most people appreciate how big these metabolic changes can be when they lose a lot of weight,” Aamodt said. “Weight gain and loss are not symmetrical: the body fights much more strongly to keep weight from dropping than it does to keep weight from increasing.”


There are some interesting hypotheses, however. One of the most persistent is an evolutionary explanation.

“Over hundreds of millennia, we evolved in an environment where we had to confront frequent periods of undernutrition,” Columbia’s Rosenbaum says. “So you would predict that human DNA would be full of genes that favour the storage of extra calories as fat. That ability would to some extent increase our ability to survive during periods of undernutrition and increase our ability to reproduce—genetic survival.”

Today, the thinking goes, this inability to lose weight is our body defending against periods of undernutrition, even though those are now much rarer. But not all researchers agree with this so-called “thrifty gene” hypothesis. As epigeneticist John Speakman wrote in a 2013 Annual Review of Nutrition analysis, one issue with the hypothesis is that not everybody in modern society is overweight.

And, Rosenbaum added, “The evolution of our genetic predisposition to store fat is complex. It involves a frequently changing environment, interactions of specific genes with that environment, and even interactions between genes.” This interplay of fac tors is still a mystery.


In trials, “15 per cent of people, on average, manage to lose 10 per cent of their weight or more and keep it off,” Rosenbaum said. So weight loss is possible.

For any would-be weight loser, Rosenbaum said the key is finding

lifestyle changes you can stick to over a long period of time.

He pointed to the National Weight Control Registry, a study that has parsed the traits, habits and behaviours of adults who have lost at least 13 kilo grams and kept it off for a minimum of one year—as an example of how they do that. The registry currently has more than 10,000 members enrolled in the study, and these folks respond to annual questionnaires about how they’ve managed to keep their weight down.

The people who have had success in losing weight have a few things in com mon: They weigh themselves at least once a week. They exercise regularly at varying degrees of intensity, with the most common exercise being walking. They restrict their calorie intake, stay away from high-fat foods and watch their portion sizes. They also tend to eat breakfast.

But there’s a ton of diversity as to what makes up their meals. (So there is no “best” diet or fad diet that did the trick.) And they count calories.

“They made huge changes to their diet and exercise plans to keep it off,”

Rosenbaum says. “It’s hard.”
Source: Reader’s Digest, Sept, 2019

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