Why your body wants to be fat

Anti-starvation system, meet the modern world

Humans are biological wonders. When we look at the fattest animals in the world, we find walruses whose bodies are 35% fat, seal pups that can reach 40%, and yearling beluga whales that tip the scales at 50% fat.

But only humans have broken the 50% barrier — in fact, we smashed it. The fattest documented humans are in a whole different ballpark, with bodies that are a staggering 70% fat by weight. Pound for pound, we’re the fattest creatures that have ever existed on this planet.

© Matthew MacDonald

Trying to find fat in the wild

What makes humans so much better at gaining fat than other animals? Look around the natural world, and you’ll have a hard time finding an animal that gets — and stays — obese. Yes, squirrels and pigeons bulk up for the winter. (They shed the weight before the warm weather returns.) And mice that stumble on a treasure trove of food do gorge themselves mightily. (Then they have dozens of babies, and shrink back to their original dimensions.) But if you want to find permanently fat animals, you’re looking in the wrong spot.

One place you can find fat animals is in our care. Recent studies suggest that the majority of our companionship animals — mostly dogs and cats — are creeping toward obesity or already there.¹ The same problem often affects the horses on our farms, and the gorillas we keep in our zoos.

Clearly, it’s not them. It’s us.

Credit: Andreas Almstedt / Pixabay

Forget the traits that we once thought made humans special: language, abstract thought, morality, the ability to use tools, the ability to deceive others. One by one, we’ve found crafty animals that could do all these things. But obesity, now that’s a wonder that just might be uniquely human.

Your anti-starvation system

It’s impossible to look at other animals without wondering what makes us so special. In other words — why are we so impressively accomplished at putting on pounds of fat?

To get the answer to this question, we first need to look back before human history, before the dawn of industrial farming and four-wheel drive. Because if we take a big perspective, the last four thousand years of recorded human history are a relatively minor footnote.

If our understanding of the fossil record is accurate, humans in the form we know them — the kind we call homo sapiens — have been kicking around the Earth for roughly 100,000 years. In other words, the lifestyle that we know today in the industrialized world is only a tiny sliver in the life story of our species.

For tens of thousands of years, humans were roaming restlessly. Your ancestors spent most of their waking hours in an endless hunt for food. The reason we crave sugar and salt today is because these nutrients are vital for life and weren’t easy to find in the natural world. The reason babies are born pleasingly plump isn’t because chubby is cute, but because fat is life — the difference between starving one particularly scarce night and making it to the next morning.

And the reason our body never learn how to deal with an unending avalanche of calorie-rich foods is because it never needed to. Look at our half-starved, wandering ancestors. Who could have dreamed that they would one day build an environment that provided their every need, itched their every craving, and showered them in limitless calories?

Can you fight nature?

As you’ve already guessed, the news isn’t good for the average dieter. It’s too late to be raised by wolves. If our old-school bodies don’t fit the modern world, what hope is there for people struggling with the health problems caused by severe obesity?

One good bit of advice is to approach diets with caution. If you severely cut down your food intake, your body’s anti-starvation system switches into high alert and fights extra hard to hold onto fat — effects that can last years longer than the diet itself.² And if you’re carrying a bit of extra weight, many nutritionists suggest you work not to lose it, but to avoid gaining any more. That goal is easier to achieve and less likely to throw you into a pitched battle against your body’s ancient habits.

Another good idea is to get as active as you possibly can. In recent years, dieters have flocked to the Paleo diet in an attempt to eat like our ancestors. But what’s more likely to keep you healthy is the Paleo lifestyle of constant activity, and that’s a harder sell.

Happily, modern studies suggest that the benefit of exercise is independent of weight — in other words, whether you are thin or fat to begin with, you improve the odds of living longer and staying healthier when you exercise.³ In fact, the benefit of exercise is probably more than the benefit of not being obese. And the human body likes to move. Exercise and you aren’t working against years of human history. You’re acting in concert with it.


Enjoyed this read? Check out the other articles in my Science of Eating series:

¹ German, AJ., Woods, GRT., Holden, SL., Brennan, L., Burke, C. (2018) Dangerous trends in pet obesity Veterinary Record 182, 25. https://veterinaryrecord.bmj.com/content/182/1/25.1.info

² Camps, S. G., Verhoef, S. P., & Westerterp, K. R. (2013). Weight loss, weight maintenance, and adaptive thermogenesis. The American Journal of Clinical Nutrition, 97(5), 990–994. https://doi.org/10.3945/ajcn.112.050310

³ Kathy Do, Ruth E. Brown, Sean Wharton, Chris I. Ardern, Jennifer L. Kuk. Association between cardiorespiratory fitness and metabolic risk factors in a population with mild to severe obesity. BMC Obesity, 2018; 5 (1) https://bmcobes.biomedcentral.com/articles/10.1186/s40608-018-0183-7