One of the basic and unsolved questions in obesity research is what kind of food causes the most obesity. Experts put forward different reasons, such as foods that contain fat or sugar or foods that lack protein, because they can cause us to eat too much without even knowing it. There is plenty of anecdotal evidence against any of the culprits, but there have been few long-term, large-scale studies comparing eating habits. It is neither ethical nor realistic to ask healthy subjects to binge on a certain food for years until they become obese.
But it is possible to do this in mice. In a diet study published this summer in Cell Metabolism, researchers randomly assigned one of 29 different diets to hundreds of adult male mice (the scientists hope to include females later in the experiment). Some foods provide up to 80 percent of calories in the form of saturated and unsaturated fats and very few carbohydrates. Others have less fat and are mostly made up of refined carbohydrates, mainly from grains and corn syrup, although some versions of these carbohydrates come from sugar. Another diet is characterized by a very high or very low percentage of protein. The mice were kept on the same diet for three months -- about nine human years, by some estimates -- while being allowed to eat and move around the cage as they liked. The researchers then measured the mice's weight and body composition and examined their brain tissue for evidence of altered gene activity.
Only some mice became obese -- almost entirely on a high-fat diet. The mice also showed signs of altered gene activity in brain regions associated with processing rewards; Apparently, fat food makes them happy. Other diets, including those rich in sugar, did not result in significant weight gain or similar changes in gene expression. Even a super-high-fat diet containing more than 60 percent fat did not significantly increase weight, and the mice on this diet consumed less food than their peers, probably because they could not eat so much fat. These findings also emerged in subsequent experiments with four other mouse species. Male mice on a relatively high-fat diet became obese, while others did not.
It looks like if you're a mouse and you're on a high-fat diet, and it's not extremely high-fat, it's going to lead to weight gain, said John Speakman, a professor at the Chinese Academy of Sciences in Beijing and at the university of Aberdeen in Scotland, who led the study. Spiekman and his co-authors believe that a fatty diet stimulates and alters certain areas of the brain, causing the mice to crave fatty foods so much that they ignore other body signals indicating they have consumed enough energy.
The study focused on weight gain, not loss, and its subjects were apparently mice, not humans. But the results are suggestive. Sugar did not make the mice fat, nor did a lack of protein. Only fat makes them fat.