Scientists from the University of Aberdeen and the Chinese Academy of Sciences have undertaken the largest study of its kind looking at what components of diet – fat, carbohydrates or protein – caused mice to gain weight.
Since food consists of fat, protein and carbs, it was difficult to identify exactly which aspect of the typical diet leads to weight gain.
Why studies were done on mice
It is very difficult to do studies on humans where what they eat is controlled for long enough periods to determine what the most important factors are. Studies on animals that are similar to humans can however help point in the right direction.
The study, which was published July in the journal Cell Metabolism, included 30 different diets that vary in their fat, carbohydrate (sugar) and protein contents.
The mice were fed these diets for three months, which is equivalent to nine years in humans. In total over 100,000 measurements were made of body weight changes and their body fat was measured using a micro MRI machine.
Result of study unequivocal
Professor John Speakman, who led the study, said: “The result of this enormous study was unequivocal – the only thing that made the mice get fat was eating more fat in their diets.
“Carbohydrates including up to 30% of calories coming from sugar had no effect. Combining sugar with fat had no more impact than fat alone. There was no evidence that low protein (down to 5%) stimulated greater intake, suggesting there is no protein target. These effects of dietary fat seemed to be because uniquely fat in the diet stimulated the reward centres in the brain, stimulating greater intake.
“A clear limitation of this study is that it is based on mice rather than humans. However, mice have lots of similarities to humans in their physiology and metabolism, and we are never going to do studies where the diets of humans are controlled in the same way for such long periods. So the evidence it provides is a good clue to what the effects of different diets are likely to be in humans.”
To summarize how the research was done
- A total of 30 different diets were given to mice over a period of three months – the equivalent of nine human years.
- Each varied in their fat, carbohydrate (sugar) and protein content.
- More than 100,000 measurements of body weight changes of mice were made, including a micro MRI machine to assess body fat.
- Mice fed fat-heavy diets consumed the most calories because fat stimulated the reward centres in their brains.
- Mice fed carb-loaded diets, including up to 30% of their calories coming from sugar, gained no significant weight.
- Combining sugar with fat had no bigger impact than fat alone.
- There is no evidence that low protein diets can stimulate a greater intake of food.
Past fads and fallacies
During the 1980s and 1990s it was widely accepted that fat content was the most important factor in weight gain.
In the early 2000s it was suggested this focus on fat was misplaced, and the main factor driving obesity was refined carbs. Several hugely popular books were published suggesting that eating fat might actually protect us from obesity.
In recent years attention has turned to protein, with the hypothesis that people eat food mostly to obtain protein rather than energy. According to this idea, when the protein content falls, humans eat more to meet the target protein intake – causing them to consume too many calories.
WATCH: Fat consumption is the only cause of weight gain in mice