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December 26, 2019

There’s a line in Stephan Guyenet’s book The Hungry Brain that you may not want to think about right now.

In the US, he says, “most of our annual weight gain occurs during the six-week holiday feasting period between Thanksgiving and the new year, and … this extra weight tends to stick with us after the holidays are over.” We don’t have to worry about Thanksgiving over here, but there are three other days — Christmas Eve, Christmas Day and New Year’s Eve — that are, Guyenet says, “the definition of overeating”. And that’s true in the UK, just as much as in the States.

Guyenet’s observations about the festive season are consistent with the fundamental thesis of his book: that “repeated bouts of overeating don’t just make us fat: they make our bodies want to stay fat.”

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We’ve all heard the statistics of the obesity crisis. But Guyenet reminds his reader of some standouts. In 1890, about 4% of white men of retirement age were obese, as measured by BMI; in 2000, 41% of them were. The leap has been especially dramatic since 1960 when just 13% of US adults were obese; by 2010, that figure was 35%.

Most of us also think we know what’s causing it. But it’s not as simple as received wisdom would have us believe.

For instance, says Guyenet, in 1909 Americans ate more calories per head than they did in 1960. But there was no obesity crisis in 1909, at least as far as we are aware. That’s nice and easy to explain: in 1909, far more Americans had jobs involving physical labour; by 1960, most people had desk jobs. Also, by 1960, far fewer people walked to work; instead, they drove. So in 1909, people expended far more calories.

This raises a puzzle, though. When, between 1909 and 1960, our calorie expenditure began to drop, we didn’t get fatter. Instead, we started eating less food.

This implies two things. One, there must be some sort of mechanism which regulates our calorie consumption — a system of homeostasis like that which maintains our body core temperature within 0.5°C despite wild temperature fluctuations in our environment. Our bodies “knew” in some way that we were using less energy, so started demanding less. People, in essence, became less hungry.

And two, since 1960, something has interfered with that regulatory system. We know that because our exercise levels have not increased significantly since 1960, and yet the calories we take in — and the levels of obesity in our societies — have gone up significantly.

What’s changed? Do we eat more fat? Do we eat more sugar? More carbohydrates? Is it that certain foods affect our metabolic rate?

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Guyenet’s argument is that we’re looking at it the wrong way. First, it’s not that some foods are more fattening because of how they affect our bodies — they’re more fattening because of how they affect our brains. And second, it’s not a particular food group; it’s not fat or carbs or sugar. It’s that we have become extraordinarily good at developing foods which are hard to resist, are full of calories, and which don’t fill us up. Essentially, through the simple workings of market capitalism, the food industry has been steadily optimising for the most obesogenic, “hyper-palatable” foods it is possible to make.

You can see it in rats. Until the 1970s, when researchers wanted to study obesity in rats, they added fat to the normal rodent food they gave them. But it didn’t work very well — the rats tended to simply eat less. It would take months to get appropriately fat rats. But then a researcher called Anthony Sclafani noticed that if he gave rats human junk food, they would eat past the point of satiety. They neglected their normal foods and gained weight at an incredible rate; in a few weeks, they were obese. Simple high-fat and high-sugar diets didn’t work, only the stuff that human ingenuity has designed to be as attractive as possible.

It also works on humans, unsurprisingly. In the 1990s, Eric Ravussin put people through an experiment in which high-calorie hyper-palatable foods were available all the time through free vending machines; they ate 173% of their calorie requirements, and put on 5lb in a week. Another study found that when given “palatable” food, people ate 44% more than if it was bland.

So far, so shocking: people eat more food if they like that food. But how does that break the regulatory system that stops us from getting overweight?

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Here is how the regulatory system works. All the fat cells in your body release a hormone, leptin. The more fat there is in your body, the more leptin there is in your bloodstream; it’s also released when you eat large amounts of high-calorie food.

Your body tries to keep its leptin levels constant — it ‘defends’ a certain set point. If you eat too much or gain weight, then your body will try to reduce either its calorie intake or its calorie expenditure, i.e. make you less hungry or make you more physically active. (One way it does the latter is by encouraging you to fidget! “Non-exercise activity thermogenesis”, NEAT, is the scientific term for regular posture changes, fidgeting, and jiggling around; one study found it can burn up to 700 calories a day, a third to a quarter of our required intake. Some people fidget much more when they’ve overeaten; those people are much less likely to put on weight.) If you lose weight, it’ll make you hungrier and more sluggish until you put the pounds back on.

But if you overeat regularly enough, your body will start to acclimate to its new leptin levels; just as loud music slowly damages your ears, your leptin receptors will become inured. Extremely attractive foodstuffs such as those we face in the modern environment, which attract us even when we’re not hungry — and which are calorie-dense but unsatisfying, so we don’t feel full even when we’ve eaten more calories than we need — are the equivalent of that loud music.

If your body finds itself defending a higher weight, and you lose weight, it will react as though you are literally starving. People who are obese but lose significant weight report vastly increased hunger and obsession with food — to the point of hoarding pictures of food, or collecting food utensils — in just the same way that healthy volunteers in a starvation experiment in the Second World War reacted to losing a quarter of their body weight.

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This is less of a problem for some people than for others. Obesity is highly hereditary: about 70% of the variance within a modern country like the US is genetic. If one identical twin is obese, the other is very likely to be; non-identical twins and siblings are more different; adopted siblings are essentially no more similar than total strangers would be.

The difference in NEAT-fidgeting appears to be heavily genetic. People simply have different levels of willpower and attraction to food. But for some people, it is almost impossible to resist the extremely attractive foods with which we are surrounded; they eat large amounts; their weight goes up, and their bodies acclimate to the new leptin levels; and then the same thing happens again. Hence the getting fatter at Christmas and never quite losing it, and then the same next year.

Luckily, says Guyenet, there is a way around it. But it’s a bit bleak. The problem is not fat, or carbs, or sugar. It’s not ultra-processed foods or crisps or sweets. It’s the fact that all this tasty stuff is being combined in increasingly tasty ways; the problem is that tasty food is really hard to stop eating. “What do we do,” asked Scott Alexander in his excellent review of Guyenet’s book, “when the enemy is deliciousness itself?”

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We eat bland food — nutrient-rich but flavourless, with low calorie density so you get full quickly. Experimenters in one small 1965 study found that grossly obese people ate hugely reduced amounts — 10% or less of their usual intake. They lost nearly two stone in less than a fortnight. One volunteer stuck to it for 255 days and lost 200lb — half his bodyweight.

The answer to obesity, then, appears to be limiting yourself only to extremely bland foods that don’t make you want to eat more. One guy put himself on a 20-potato-a-day diet, just plain potatoes with a little cooking oil. He was trying to show that potatoes are healthy; he worked for the Washington State Potato Commission. He didn’t want to lose weight, but did: 21lb in two months.

Guyenet himself put out a press release as an April Fools for the “Bland Food Cookbook” (sample recipe: “Potato. Ingredient: One large potato”) which was so on brand and in line with the logical implications of his food that a lot of people fell for it. It may be that the extremely depressing answer to obesity is “you should only eat food you don’t actually like”.

But that’s easy to say. I wish I could tell you that no mince pies were harmed in the writing of this article, but I just polished off my second. Merry Christmas; may the cheeseboard be fully loaded.

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