EVOLUTION, BIOLOGY AND HEALTH

Why Humans Are Known as the Fat Ape and What We Can Do About It

How our evolutionary history set us up for weight problems in the age of fast food

We humans have a problem with our weight. We usually blame this on the explosion of delicious and calorie-dense foods that have occurred in the last hundred years. That’s undoubtedly true, a diabolical case of evolutionary mismatch, but that’s not why we’re known as the fat ape.

Metabolism and lifespan among the apes

We wear many biological hats, as we belong to a variety of categories in the hierarchy of taxonomy, but our closest living relatives are the apes (technically, great apes, which includes us).

We’re still separated from them by millions of years of evolution, but since we’re the only species of human left standing, the apes are the closest family we’ve got.

Genetically speaking, among the great apes, we’re most closely related to the chimps and bonobos (equally close, it turns out), then gorillas, and orangutans. Gibbons are also apes, but not great apes. Strangely, though, our ape cousins don’t seem to share our weight problems.

For example, even highly sedentary chimps at zoos rarely get above 10% body fat. By contrast, 10% body fat is considered an achievement for regular people today, given our constant exposure to the treacherous and tempting world of modern Western diets.

So, what makes humans different?

Measuring metabolism by counting heartbeats

Metabolism seems to be a vital piece of the puzzle. One fascinating aspect of metabolism is that animals of very different sizes appear to be united by common scaling principles.

A mouse and an elephant differ in size by orders of magnitude, but they have roughly the same number of heartbeats in their lifetimes. This indicates a tradeoff between the speed and length of life: it slows down when you stretch it out and speeds up as you condense it down.

But there are always exceptions. Birds are one, as they can have remarkably long lives for an animal with such a fast-paced metabolism.

We’re another, as we have a fast metabolism by the standards of our ape cousins and yet also live longer than they do.

But hold on: if our metabolism is fast, why do we have trouble losing weight? Shouldn’t we have trouble gaining weight? The answer lies in the fact that the brain is prioritised over all other organs, and it’s an extremely picky eater.

Your brain is the glucose junkie.

The brain is energetically expensive, consuming roughly 20% of our metabolic budget despite only representing around 2% of our body mass. It’s also at the top of the metabolic pecking order and demands to be fed in the way it prefers, even at the expense of the rest of the body.

You can see this when we undergo periods of starvation and/or malnourishment, and we’re forced to make decisions about where to spend our available calories.

This is a kind of metabolic triage, and the brain is always our first priority. For example, studies of people who’ve experienced starvation due to famine and war show that the brain is the last organ to waste away, as the body is sacrificed to keep the brain going. In extreme cases of deprivation, even heart tissue is eventually sacrificed to feed the brain.

Brain cells can metabolise other fuels, like fats, that power the metabolism of heart cells. However, the brain is a hopeless sugar junkie, as metabolising glucose provides the largest payout in terms of life’s universal energy currency: ATP.

As a result, we’re forced to maintain a healthy supply of circulating blood sugar to feed our hungry brains.

Because the brain has priority, this puts considerable pressure on us to consume any available sugars. Across evolutionary time, when sugars and calories were mostly scarce, this was an effective strategy.

In fact, this scarcity is thought to have driven our species’ unusually strong tendency to cooperate, at least with people within our ‘in-group’.

However, now that we’re drowning in affordable sugar and the many other calories that come with it, our old evolutionary strategy is making us fat.

Diet, exercise, and the future of the fat ape

The invention of delicious, sugar-packed, calorie-dense foods is badly matched to our evolutionary strategy. That’s a massive bummer, but biology is not destiny, and some recent discoveries in the last 20 years give advice for what we can do about it.

One surprising finding sets the tone: the amount of calories you burn is mostly independent of how much exercise you do. Evolutionary biologist and metabolic scientist Herman Pontzer discusses this in his book Burn: New Research Blows the Lid Off How We Really Burn Calories, Lose Weight, and Stay Healthy.

Our daily energy use remains the same no matter how active we are

It turns out that our daily energy budget is actually constrained within fairly tight limits, regardless of how active we are. Whether you get 10,000 steps every day or embrace the sedentary life of a couch potato, the amount of calories you spend each day will largely remain the same. For example, studies have found that office workers in Chicago burn the same number of calories as farmers in West Africa.

Things change somewhat when people push to the margins of what’s physically possible, like in contests such as ultramarathons and the Tour de France. But even that doesn’t increase our overall energy use anywhere close to what our intuitions predict.

Moreover, the evidence suggests that if we maintain extreme levels of activity for long enough, our bodies find ways to compensate and bring us back into our desired metabolic range.

Pregnancy and metabolism

Interestingly, metabolic pressures are also at play during pregnancy, which is one of the most extreme things that a person can experience. This is evident in the timing of childbirth, which usually occurs around the point at which the baby’s metabolic demands begin to exceed the mother’s ability to provide enough energy to meet both of their needs.

Implications for diet, exercise, and health

What does this mean for diet, exercise, and health for a fat ape like us? A few things.

First, we can’t exercise our way out of a bad diet. No matter how active we are, our body has a limit to the number of calories it will try to spend on a daily basis. This means that managing our weight is mostly about regulating the food we put in our bodies, not the energy we burn through physical activity.

At this point, you may be thinking, if exercise isn’t good for losing weight, then why bother exercising at all? This brings us to the second point: your level of activity determines how you spend your daily calorie budget, even though it mostly doesn’t change the size of that budget.

If you’re more active, more of your daily energy budget will be spent on the costs of movement and repair. If you’re less active, more of that budget will be spent elsewhere. Where exactly, you ask? Here’s where problems begin to creep in.

Much remains to be learned, but the evidence suggests that inactive people spend more calories on things like inflammation and immune activity, as those calories have to be spent somewhere in order to keep our overall energy balance in our desired range.

The implication is that less active people spend more calories on bodily responses that lead to a number of nasty conditions. This includes cardiometabolic disease (e.g., heart disease and diabetes), immune dysfunction (e.g., autoimmune disorders and chronic inflammation), and a greater risk of cancer.

Summing up

Where does this leave us?

By shaping how you spend your daily energy budget, physical activity can make you healthier, even if it isn’t very helpful for losing weight.

Unfortunately, it seems that we can’t simply eat whatever we want and burn it off later with some extra exercise. We need to learn how to consume fewer calories than we spend if we want to maintain a healthy weight.

Given the huge variety of delicious and affordable junk foods that most of us are surrounded by daily, that’s a tall order for a fat ape.

Still, if we can put our big, metabolically expensive brains to work, hopefully, just by understanding the metabolic and evolutionary pressures that guide us, we can use that knowledge to make healthier choices.