Burn: New Research Blows the Lid Off How We Really Burn Calories, Lose Weight, and Stay Healthy

by Herman Pontzer

Our brains form, strengthen, and prune synapses throughout our lives (it’s happening in your brain right now as you form new memories from reading this book) but by far the most active period is in childhood, when we’re soaking up the world around us. Work by Christopher Kuzawa and colleagues has shown that in children three to seven years old, the brain accounts for over 60 percent of BMR, three times more than in adults. So much energy is channeled to the brain during these early critical years that it actually slows down growth in the rest of the body.

As the Covid-19 pandemic made clear to all of us, the world is full of nasty pathogens. But easy access to effective medical care—one of the triumphs of modernization—has led to a sort of cultural amnesia. We tend to forget how scary infectious disease is. Among the Hadza, acute infections kill four out of ten children before their fifteenth birthday.

The energy demand of fighting infection steals calories away from growth. When our immune system responds to an infection, it makes a number of molecules (immunoglobulins, antibodies, and other proteins) that circulate in the blood—telltale signs of the battles fought against bacteria, viruses, and parasites.

Adding a higher proportion of fat costs more, whereas adding a higher proportion of lean tissue (like muscle) costs less, because the energy content of fat is more than twice that of protein. One way to see that difference is to look at the energy burned when we lose weight—the mirror image of growth. The energy we expend to lose weight has to be equal to the energy content of the tissue that’s been lost. Since the tissue we lose during weight loss is mostly fat, the general rule is that it takes about 3,500 kcal to burn off one pound.

The free radical theory of aging has its problems. For one, studies of antioxidant consumption in humans and other animals don’t always show the expected effects on life span. The difficulties in finding clear, strong links between metabolism and longevity have left some researchers lamenting whether such links exist at all.

The simple armchair engineer view of metabolism dates back to the postwar era in the United States and Europe. With the widespread starvation and other atrocities of World War II fresh in their minds, researchers were interested in figuring out humans’ daily nutritional requirements.

Using surveys from the massive National Health and Nutrition Examination Survey, they found that women reported food intakes of about 1,600 to 2,200 kilocalories per day, while men reported intakes of 2,000 to 3,000. That would leave a rough average for all adults of somewhere between 2,000 and 2,500 kilocalories per day. To discourage overconsumption, and to have a nice round number to work with, they rounded down to 2,000, and that’s the number that stuck. Now you know who to blame if you thought that the typical American eats a 2,000-kilocalorie diet.

Our metabolic engines shift and change to make room for increased activity costs, ultimately keeping daily energy expenditure within a narrow window. As a result, physically active people—whether it’s hunter-gatherers living today or in our collective past, or people in the industrialized world who exercise regularly—burn the same amount of energy as people who are much more sedentary.

If the energy burned is really difficult to budge no matter how much we exercise, we’d be better off battling obesity by focusing on the amount of energy we eat.

First, the good news: the contestants all lost a lot of weight. By week 6 of the competition, they’d lost an average of thirty pounds. By week 13, those who hadn’t been sent home had lost another thirty to forty pounds. And by the big TV finale, a homecoming of sorts at week 30, where the contestants are flown back to the ranch for a final weigh-in after four more months of self-policed dieting and exercise, the contestants had lost an average of 127 pounds.

Now the not-so-great news: their bodies were in starvation mode. By week 30, their BMRs had dropped nearly 700 kcal per day, or about 25 percent. The reduction in BMR wasn’t just a function of weighing less; it was far greater than expected from weight loss alone. The change was deeper. Their cells had reduced their metabolic rate, working and burning energy more slowly. And the changes weren’t temporary.

When our bodies sense that we’re not eating enough to meet our daily energy requirements, we start throttling things down. The body struggles mightily to balance its energy budget so that expenditure doesn’t exceed intake. Our thyroid gland, the body’s master controller of metabolic rate, reduces the amount of thyroid hormone produced, which is like taking your foot off the gas pedal. Our cells slow down, which lowers BMR and daily energy expenditure. At the same time, the hormones and brain circuitry that control hunger increase our drive for food. We become ravenous, fixated on food as our body directs our mental energies toward finding something—anything—to eat. This is the evolved starvation response, also known as dieting.

Experiencing a period of starvation is a reasonably good indicator that you’re in a poor, unpredictable environment. In that case, storing a bit more fuel for next time is probably a good idea. Still, it’s impressive that their bodies “knew” what their normal weight was supposed to be and returned them to it, more or less, calling off the alarm when they’d hit their pre-study size. Clearly, the mechanisms that shape our metabolism and hunger are quite specific about the body weight and composition they work to defend.

BMR and daily energy expenditure don’t dictate weight change, they respond to weight change. The Biggest Loser contestants were in starvation mode during and after the competition. Their lower BMRs and daily expenditures were a desperate, evolved strategy to keep expenditures in line with their severely reduced intake. In the years following the show, contestants who ate the most and regained the most weight gave their bodies the strongest signals that the danger of starvation had passed. Their BMR and daily expenditures rebounded along with their body weight.

In concert with your brain stem, the hypothalamus senses energy coming in by monitoring the blood for factors like glucose and leptin (a hormone secreted by fat cells when they’re storing energy from a recent meal), and neural signals from the taste buds, stomach, and small intestine that relay information on the size and macronutrient content of a meal. The hypothalamus can also sense when we’re in negative energy balance, monitoring levels of ghrelin (a hormone produced by the stomach when it’s empty), leptin (which decreases when fat cells are depleted), and other cues. In response, the hypothalamus can crank our metabolism up or down by controlling the activity of the thyroid gland and the production of thyroid hormone. It can also change our hunger levels, adjusting the amount of food we need to eat to feel full.

If food restriction is severe and lasts for a long time, our organs will actually shrink. But not all organ systems are hit equally hard. We know from careful studies of the bodies of victims starved to death in war and famine that the brain is spared. The spleen, on the other hand, shrinks dramatically.

With the muscles demanding a much larger share of the business’s energy and draining fat reserves, the Darwinian manager acts to rebalance the budget. In the immediate term, hunger is increased in order to match intake to expenditure. If high levels of daily activity persist for weeks or months, though, other changes are made. Other systems, including reproduction, immune function, and stress response, are suppressed, making room in the budget for greater activity costs.