The study of obesity is the study of mysteries.

Mystery 1: The Obesity Epidemic 

The first mystery is the obesity epidemic itself. It’s hard for a modern person to appreciate just how thin we all were for most of human history. A century ago, the average man in the US weighed around 155 lbs. Today, he weighs about 195 lbs. About 1% of the population was obese back then. Now it’s about 36%.

Back in the 1890s, the federal government had a board of surgeons examine several thousand Union Army veterans who fought in the Civil War. This was several decades after the end of the war, so by this point the veterans were all in their 40’s or older. This gives us a snapshot of what middle-aged white men looked like in the 1890s. When we look at their data, we find that they had an average BMI of about 23 (overweight is a BMI of 25 and obese is a BMI of 30 or more). Only about 3% of them were obese. In comparison, middle-aged white men in the year 2000 had an average BMI of around 28. About 24% were obese in early middle age, increasing to 41% by the time the men were in their 60s.

(Most experts consider measures like body fat percentage to be better measures of adiposity than BMI, and we agree. Unfortunately, nearly every source reports BMI, and most don’t report body fat percentage. Here, we use BMI so that we can compare different sources to one another.)

It’s not just that we’re a little fatter than our great-grandparents — the entire picture is different.

People in the 1800s did have diets that were very different from ours. But by conventional wisdom, their diets were worse, not better. They ate more bread and almost four times more butter than we do today. They also consumed more cream, milk, and lard. This seems closely related to observations like the French Paradox — the French eat a lot of fatty cheese and butter, so why aren’t they fatter and sicker?

Our great-grandparents (and the French) were able to maintain these weights effortlessly. They weren’t all on weird starvation diets or crazy fasting routines. And while they probably exercised more on average than we do, the minor difference in exercise isn’t enough to explain the enormous difference in weight. Many of them were farmers or laborers, of course, but plenty of people in 1900 had cushy desk jobs, and those people weren’t obese either.

Something seems to have changed. But surprisingly, we don’t seem to have any idea what that thing was.

Mystery 2: An Abrupt Shift 

Another thing that many people are not aware of is just how abrupt this change was. Between 1890 and 1976, people got a little heavier. The average BMI went from about 23 to about 26. This corresponds with rates of obesity going from about 3% to about 10%. The rate of obesity in most developed countries was steady at around 10% until 1980, when it suddenly began to rise.

Today the rate of obesity in Italy, France, and Sweden is around 20%. In 1975, there was no country in the world that had an obesity rate higher than 15%.

This wasn’t a steady, gentle trend as food got better, or diets got worse. People had access to plenty of delicious, high-calorie foods back in 1965. Doritos were invented in 1966, Twinkies in 1930, Oreos in 1912, and Coca-Cola all the way back in 1886. So what changed in 1980?

Common wisdom today tells us that we get heavier as we get older. But historically, this wasn’t true. In the past, most people got slightly leaner as they got older. Those Civil War veterans we mentioned above had an average BMI of 23.2 in their 40s and 22.9 in their 60’s. In their 40’s, 3.7% were obese, compared to 2.9% in their 60s. We see the same pattern in data from 1976-1980: people in their 60s had slightly lower BMIs and were slightly less likely to be obese than people in their 40s (See the table below). It isn’t until the 1980s that we start to see this trend reverse. Something fundamental about the nature of obesity has changed.

Mystery 3: The Ongoing Crisis 

Things don’t seem to be getting any better. A couple decades ago, rising obesity rates were a frequent topic of discussion, debate, and concern. But recently it has received much less attention; from the lack of press and popular coverage, you might reasonably assume that if we aren’t winning the fight against obesity, we’ve gotten at least to a stalemate.

But this simply isn’t the case. Americans have actually gotten more obese over the last decade. In fact, obesity increased more than twice as much between 2010 and 2018 than it did between 2000 and 2008.

Rates of obesity are also increasing worldwide. As The Lancet notes, “unlike other major causes of preventable death and disability, such as tobacco use, injuries, and infectious diseases, there are no exemplar populations in which the obesity epidemic has been reversed by public health measures.”

All of this is, to say the least, very mysterious.

1.1    Weird Mysteries

Then there are the weird mysteries.

Mystery 4: Hunter-Gatherers 

A common assumption is that humans evolved eating a highly varied diet of wild plants and animals, that our bodies still crave variety, and that we would be better off with a more varied diet. But when we look at modern hunter-gatherers, we see this isn’t true. The !Kung San of Tanzania get about 40% of their calories from a single food source, the mongongo nut, with another 40% coming from meat. But the !Kung are extremely lean (about 110lbs on average) and have excellent cardiovascular health.

Of course, variety isn’t everything. You would also expect that people need to eat the right diet. A balanced diet, with the right mix of macronutrients. But again, this doesn’t seem to be the case. Hunter-gatherer societies around the world have incredibly different diets, some of them very extreme, and almost never suffer from obesity.

Historically, different cultures had wildly different diets — some hunter-gatherers ate diets very high in sugar, some very high in fat, some very high in starch, etc. Some had diets that were extremely varied, while others survived largely off of just two or three foods. Yet all of these different groups remained lean. This is strong evidence against the idea that a high-fat, high-sugar, high-starch, low-variety, high-variety, etc. diet could cause obesity.

A Tanzanian hunter-gatherer society called the Hadza get about 15 percent of their calories from honey. Combined with all the sugar they get from eating fruit, they end up eating about the same amount of sugar as Americans do. Despite this, the Hadza do not exhibit obesity. Another group, the Mbuti of the Congo, eat almost nothing but honey during the rainy season, when honey can provide up to 80% of the calories in their diet. These are all unrefined sugars, of course, but the Kuna of Panama, though mostly hunter-gatherers, also obtain white sugar and some sugar-containing foods from trade. Their diet is 65% carbohydrate and 17% sugar, which is more sugar than the average American currently consumes. Despite this the Kuna are lean, with average BMIs around 22-23.

The Inuit, by contrast, traditionally ate a diet consisting primarily of seal meat and blubber, with approximately 50% of their calories coming from fat. This diet is quite low in fruits and vegetables, but obesity was virtually unknown until the arrival of western culture. The Maasai are an even more extreme example, subsisting on a diet composed “almost exclusively of milk, blood, and meat”. They drink “an average of 3 to 5 quarts/day of their staple: milk supplemented with cow’s blood and meat“. This adds up to about 3000 calories per day, 66% of those calories being from fat. (They also sometimes eat honey and tree bark.) But the Maasai are also quite lean, with the average BMI for both men and women being again in the range of 22-23, increasing very slightly over age.

Kitava is a Melanesian island largely isolated from the outside world. In 1990, Staffan Lindeberg went to the island to study the diet, lifestyle, and health of its people. He found a diet based on starchy tubers and roots like yam, sweet potato, and taro, supplemented by fruit, vegetables, seafood, and coconut. Food was abundant and easy to come by, and the Kitavans ate as much as they wanted. “It is obvious from our investigations,” wrote Lindeberg, “that lack of food is an unknown concept, and that the surplus of fruits and vegetables regularly rots or is eaten by dogs.”

About 70% of the calories in the Kitavan diet came from carbohydrates. For comparison, the modern American diet is about 50% carbohydrates. Despite this, none of the Kitavans were obese. Instead they were in excellent health. Below, you’ll see a photo of a Kitavan man being examined by Lindeberg.

Kitavans didn’t even seem to gain weight in middle age. In fact, BMI was found to decrease with age. Many lived into their 80s or 90s, and Lindeberg even observed one man who he estimated to be 100 years old. None of the elderly Kitavans showed signs of dementia or memory loss. The Kitavans also had no incidence of diabetes, heart attacks, stroke, or cardiovascular disease, and were unfamiliar with the symptoms of these diseases. “The only cases of sudden death they could recall,” he reports, “were accidents such as drowning or falling from a coconut tree.”

Mystery 5: Lab Animals and Wild Animals 

Humans aren’t the only ones who are growing more obese — lab animals and even wild animals are becoming more obese as well. Primates and rodents living in research colonies, feral rodents living in our cities, and domestic pets like dogs and cats are all steadily getting fatter and fatter. This can’t be attributed to changes in what they eat, because lab animals live in contained environments with highly controlled diets. They’re being fed the same foods as always, but for some reason, they’re getting fatter.

This seems to be true everywhere you look. Our pets may eat scraps from the table, but why would zoo animals, being fed by professionals, also be getting fatter? Even horses are becoming more obese. This is all very strange, and none of it fits with the normal explanations for the obesity epidemic.

Mystery 6: Palatable Human Food 

Lab rats gain some weight on high-fat diets, but they gain much more weight on a “cafeteria diet” of human foods like Froot Loops [sic] and salami (see also here).

It used to be that if researchers needed obese rats for a study, they would just add fat to normal rodent chow. But it turns out that it takes a long time for rats to become obese on this diet. A breakthrough occurred one day when a graduate student happened to put a rat onto a bench where another student had left a half-finished bowl of Froot Loops. Rats are usually cautious around new foods, but in this case the rat wandered over and began scarfing down the brightly-colored cereal. The graduate student was inspired to try putting the rats on a diet of “palatable supermarket food”; not only Froot Loops, but foods like Doritos, pork rinds, and wedding cake. Today, researchers call these “cafeteria diets”.

Sure enough, on this diet the rats gained weight at unprecedented speed. All this despite the fact that the high-fat and cafeteria diets have similar nutritional profiles, including very similar fat/kcal percentages, around 45%. In both diets, rats were allowed to eat as much as they wanted. When you give a rat a high-fat diet, it eats the right amount and then stops eating, and maintains a healthy weight. But when you give a rat the cafeteria diet, it just keeps eating, and quickly becomes overweight. Something is making them eat more. “Palatable human food is the most effective way to cause a normal rat to spontaneously overeat and become obese,” says neuroscientist Stephan Guyenet in The Hungry Brain, “and its fattening effect cannot be attributed solely to its fat or sugar content.”

Rodents eating diets that are only high in fat or only high in carbohydrates don’t gain nearly as much weight as rodents eating the cafeteria diet. And this isn’t limited to lab rats. Raccoons and monkeys quickly grow fat on human food as well.

We see a similar pattern of results in humans. With access to lots of calorie-dense, tasty foods, people reliably overeat and rapidly gain weight. But again, it’s not just the contents. For some reason, eating more fat or sugar by itself isn’t as fattening as the cafeteria diet. Why is “palatable human food” so much worse for your waistline than its fat and sugar alone would suggest?

Mystery 7: Altitude 

People who live at higher altitudes have lower rates of obesity. This is the case in the US, and also seems to be the case in other countries, for example Spain and Tibet. When US Army and Air Force service members are assigned to different geographic areas, they are more at risk of developing obesity in low-altitude areas than in high-altitude ones. Colorado is the highest-altitude US state and also has the lowest incidence of obesity.

If you look at a map of county-level obesity data in the United States, the Rockies, the Sierra Mountains, and the Appalachians stand out quite clearly: 

Similarly, there is a condition called “altitude anorexia” where individuals who move to a high-altitude location sometimes lose a lot of weight all at once (see also here, here, and weight loss results here). This effect also seems to apply to lab rats who are moved to labs at higher altitudes.

In addition, there is some evidence for a similar relationship between altitude and the rate of diabetes, with people living at a higher elevation having lower rates of diabetes than those living near sea level, even when statistically adjusting for variables like age, BMI, and physical activity.

We know that oxygen and carbon dioxide vary with elevation, so you might expect that this is attributable to these differences. But the evidence is pretty thin. Combined with a low-calorie diet, exercise in a low-oxygen environment does seem to reduce weight more than exercise in normal atmospheric conditions, but not by much. Submarines have CO₂ levels about 10 times higher than usual, but a US Navy study didn’t find evidence of consistent weight gain. The atmosphere itself can’t explain this.

One paper, Hypobaric Hypoxia Causes Body Weight Reduction in Obese Subjects from Lippl et al. (2012), claims to show a reduction in weight at high altitude and suggests that this weight loss is attributable to differences in oxygen levels. However, there are a number of problems with this paper and its conclusions. To begin with, there isn’t a control group, so this isn’t an experiment. Without an appropriate control, it’s hard to infer a causal relationship. What they actually show is that people brought to 2,650 meters lost a small amount of weight and had lower blood oxygen saturation, but this is unsurprising. Obviously if you bring people to 2,650 meters they will have lower blood oxygen, and there’s no evidence linking that to the reported weight loss. They don’t even report a correlation between blood oxygen saturation and weight loss, even though that would be the relevant test given the data they have. Presumably they don’t report it because it’s not significant. In addition there are major issues with multiple comparisons, which make their few significant findings hard to interpret (for more detail, see our full analysis of the paper).

Mystery 8: Diets Don’t Work 

There’s a lot of disagreement about which diet is best for weight loss. People spend a lot of time arguing over how to diet, and about which diet is best. I’m sure people have come to blows over whether you lose more weight on keto or on the Mediterranean diet, but meta-analysis consistently finds that there is little difference between different diets.

Some people do lose weight on diets. Some of them even lose a lot of weight. But the best research finds that diets just don’t work very well in general, and that no one diet seems to be better than any other. For example, a 2013 review of 4 meta-analyses said:

Numerous randomized trials comparing diets differing in macronutrient compositions (eg, low-carbohydrate, low-fat, Mediterranean) have demonstrated differences in weight loss and metabolic risk factors that are small (ie, a mean difference of <1 kg) and inconsistent.

Most diets lead to weight loss of around 5-20 lbs, with minimal differences between them. Now, 20 lbs isn’t nothing, but it’s also not much compared to the overall size of the obesity epidemic. And even if someone does lose 20 lbs, in general they will gain most of it back within a year.

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