Why Some People Can Eat What They Want and You Can’t: Metabolism Explained

Suzie Glassman

Learn how metabolism works and what (if anything) you can do

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It’s easy to talk about metabolism like it’s some mysterious code you can crack. Drink more green tea! Take this pill! Boost your metabolism with these five fat-burning foods! Buy my multi-level-marketing coffee, and you’ll see pounds melt away!

The truth is metabolism is far more complicated, and there’s still a lot scientists don’t know about the biological process of producing and burning energy. Most products intended to “boost” your metabolism are the modern equivalent of snake oil.

Here, I’ll break down what scientists know and don’t know about metabolism and how that knowledge translates to the world of weight gain and loss.

Metabolism 101

According to the Mayo Clinic,

Metabolism is the process by which your body converts what you eat and drink into energy. During this complex process, calories in food and beverages are combined with oxygen to release the energy your body needs to function.

Note your metabolism is in every living cell of your body. This fact is why we can’t stop eating and drinking entirely for more than a few weeks. We need food and water to provide energy to our cells to keep the lights on.

By far, most of the energy we burn is from our resting metabolic rate (aka basal metabolic rate). We also burn calories through the energy required to digest food (thermic effect of food) and our physical activity (including exercise and non-exercise).

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Most people are surprised to learn our resting metabolic rate (RMR) accounts for 60–80% of the calories we burn each day. This rate is the amount of energy required to keep us alive. Think if you were required to stay in bed for an entire day.

The body’s major organs — the brain, liver, kidneys, and heart — account for about half of the energy burned at rest, while fat, the digestive system, and the body’s muscles account for the rest. Various calculators exist to help you estimate your RMR. Here is one that comes with a great explanation as to how these estimates are derived and how they work.

Digesting food counts for anywhere between 5 and 10% of your daily calorie burn. I’ll explain later how some foods require more energy to digest.

Next, your physical activity accounts for the rest. It’s important to know there are two components to physical activity. The first is called non-exercising activity thermogenesis (NEAT), and the second is formal exercise.

NEAT comprises all the activities you do in a day outside of exercise (walking, cooking, doing chores, standing up, etc.) By far, you burn more calories with NEAT than you do from exercise. You may think that’s not possible, but if you’re awake 16 hours in the day, you may spend around an hour performing formal exercise. That leaves 15 other hours in the day for everything else.

Why Does Metabolism Vary So Much Between Individuals

The short answer is no one knows why some people seem to be able to eat and drink whatever they want without gaining weight, and others can hardly touch a french fry without having it forever stuck to their waist.

Whether we gain, lose or maintain weight is a function of how many calories we eat and how many burn. Of course, not all calories are created equal for health purposes. But how is it some people seem to eat thousands of calories while remaining thin?

Will Wong, a researcher and professor at the Johns Hopkins Center for Metabolism and Obesity Research, told Vox,

Why this is remains a “black box.” We don’t understand the mechanism that controls a person’s metabolism.

While large variations remain a mystery, researchers have found a few predictors of how fast a person’s metabolism will be. Age, genetics, and the amount of lean body mass to fat tissue play a role. Gender also seems to be a factor, as women burn fewer calories than men of comparable age and body composition.

Another possible explanation could be some people naturally move more in response to the added energy food provides. One review of over-feeding studies found that some people respond to over-eating by subconsciously increasing their NEAT — meaning they up their activity without realizing it.

The author explains,

Individuals in whom overeating effectively activates NEAT dissipate as much as 69% of the excess energy as heat. Those less able to activate NEAT store a higher proportion of the excess calories as fat.

Basically, when some people eat too much, they move more to offset the additional calories.

What Causes Metabolism to Slow

Research points to two main factors that cause metabolism to slow: age and crash dieting. If you’ve ever said, “I just can’t eat like I used to,” you weren’t fooling yourself. Metabolism slowly declines from the time we turn 18 until we die.

Michael Jensen, a researcher who studies obesity and metabolism at the Mayo Clinic, explains,

Why 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 we have.

Some people claim they can’t lose weight after turning 40. Theoretically, this isn’t true, but what you did to lose weight in your 20s may have to change due to a slower metabolism when you’re older.

Next, multiple studies show (the most famous of which is the Biggest Loser study published in Obesity) how crash dieting can lower your metabolism to the point where you can have the same muscle mass and fat tissue as someone else your same gender and age but burn fewer calories.

Vox writes,

For years, researchers have been documenting a phenomenon called “metabolic adaptation” or “adaptive thermogenesis”: As people lose weight, their basal metabolic rate — the energy used for basic functioning when the body is at rest — actually slows down to a greater degree than would be expected from the weight loss.

Unfortunately, when you lose a lot of weight (especially quickly), the body fights much harder to keep your weight from dropping. Essentially, your brain is incredible at adapting for survival. Our ancestors often went long periods without a reliable food supply, so we are hard-wired to conserve energy when food is scarce (like during a diet). Our metabolism becomes more efficient, allowing the body to survive on less energy than similar-sized bodies that were not calorie deprived.

Even gradual weight loss can cause your RMR to slow, mainly because you have less body mass to carry around, which requires less energy. You also may lose some muscle as a result of weight loss.

However, Jensen notes,

We don’t really see that much of a drop in resting metabolism [as seen in the Biggest Loser study]. With slow, gradual weight loss, the metabolic rate holds out really well.

What You Can do to Raise Your Metabolism

While the promise of eating hot peppers, popping a supplement, and drinking green tea sounds alluring, the truth is they may raise your metabolism a small amount, but the effect is short-lived and won’t mean anything for your weight.

The only thing we can do to increase our RMR is to have as much muscle as possible, as we have no control over the other components that make up this large portion of daily calorie burn. More muscle means we’ll burn more calories at rest, which is great. The side effect is we may feel hungrier from the training required to gain muscle.

Engaging in more NEAT and beginning an exercise regimen (or varying your current routine) will increase the calories you burn. As long as you remain in a deficit, you should be able to lose weight slowly. Choose lifestyle changes you can make permanent, as long-term research on keeping weight off proves you may have to practice things like exercising regularly and being mindful of what you eat indefinitely.

Last, protein requires more energy to digest. One study showed subjects burned 80–100 more daily calories on a high protein diet. Yes, you can easily consume 100 calories of something else (mmm, peanut butter) to make up for it, but technically eating more protein does cause your body to burn a few more calories than a low-protein diet. Protein is also highly satiating, so you may end up eating fewer calories overall with more protein on your plate.

The Myth of the Slow Metabolism

It’s easy to blame middle-aged weight gain on a slow metabolism, but the excess pounds are more likely from a less active lifestyle, filled with more stress and less sleep. As we age, we are tied down with responsibilities like an office job, daily commute, parenting commitments, stress, etc. We stop playing for fun as we did in our youth.

Here’s an example from Breaking Muscle:

NEAT can account for as little as 15% of energy expenditure in the very sedentary and up to 50% in very active individuals. If a woman has a BMR of around 1,000 calories (we’ll use that nice even number for simplicity’s sake), she’ll burn about 150 calories digesting the food she eats each day. She may also burn anywhere from 150 to 500 calories more per day depending on whether she has a day full of walking around, shopping, and cleaning or if she spends the day sitting and working on the computer.

We also overestimate how many calories we burn exercising. Breaking Muscle argues that the average calories burned during an hour of intentional exercise is about 328 calories for every 100lbs of body weight (on average, as the amount varies based on age, lean body mass, and RMR). If you weigh 150 pounds, that’s about 250 calories for a 30-minute cardio workout. I can down 250 calories in a small snack if I’m not careful.

Last, we are terrible at estimating the number of calories we eat. Most people underestimate. We may be able to pick out which foods have more calories than others, but we aren’t good at determining portion sizes, especially when it comes to high-fat, high-sugar foods. You may be blaming your metabolism when in fact you’re eating more than your daily energy requirement.

Final Thoughts

Jensen argues,

It may be tempting to blame your metabolism for weight gain. But because metabolism is a natural process, your body has many mechanisms that regulate it to meet your individual needs.
Only in rare cases do you get excessive weight gain from a medical problem that slows metabolism, such as Cushing’s syndrome or having an underactive thyroid gland (hypothyroidism).

Metabolism is dynamic and complicated — meaning it can and will change throughout your lifetime. Be wary of any hack promising to turn you into a fat-burning machine overnight. There is still a wealth of information scientists don’t know about metabolism and why it can vary so much.

It’s easy to look with envy at your skinny friend across the room who’s downing her third slice of pizza and second beer and question if your metabolism is broken. The truth is she may have won the genetic lottery.

Put that aside and take the cards you were dealt. Focus on long-term solutions that can allow you to eat well, exercise regularly, and remain in good health for as long as possible. Your metabolism will be right there with you.

Photo credit: By Kotin

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I write about health and fitness with the goal to help you live a healthier, happier life.

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