Food and diet

'Proof' it's fats, not carbs, that cause weight gain – but only in mice

"Fat consumption is the only cause of weight gain!" declares the Mail Online, reporting on a study where mice were exposed to different diets and monitored for weight gain and increased energy intake.

Around 30 different diets, all highly controlled, were given to mice over a period of 12 weeks.

The diets varied in the amount of carbohydrate, fat and protein they contained. The mice's body composition and food intake was measured every day.

Mice who consumed a steady amount of protein but an increasing amount of dietary fat gained the most body fat during the study.

There was no change in body fat gain when the researchers gave mice increasing amounts of carbohydrates while they ate steady amounts of fat and protein.

This study gives us possible insight into the effect that making small changes to specific dietary components may have on body fat gain.

One point noted by the researchers is fat stimulates the so-called "reward pathways" in the brain, which leads to a strong desire to consume more of it, as seen with addictive substances such as alcohol and cocaine.

It may be the case that the more fat you eat, the more fat you want to eat (at least in mice).

But it's unclear whether these findings would apply to people, or what role physical activity might have in modifying the results.

Current guidelines recommend that:

  • men shouldn't have more than 30g of saturated fat a day
  • women shouldn't have more than 20g of saturated fat a day
  • children should have less

Where did the story come from?

The study was carried out by researchers from the Chinese Academy of Sciences, the University of Aberdeen, the Guangdong Institute of Applied Biological Resources, the University of Dali, and the Center for Excellence in Animal Evolution and Genetics in Beijing.

It was funded by the Chinese Academy of Sciences Strategic Program, the 1000 Talents program, a Wolfson merit award, the National Natural Science Foundation of China, and funds from Guangdong Academy of Sciences.

It was published in the peer-reviewed journal Cell Metabolism.

The UK media covered the details of the research well, but overstated the strength of the conclusions given that this was a study in mice rather than humans.

Also, the Daily Mirror claimed the study provided " 'unequivocal' findings" that fats were solely responsible for weight gain.

But the "fats versus carbs versus sugar" debate has been ongoing for decades, so we doubt this is the last we'll hear about the issue.

What kind of research was this?

This was an animal study in which researchers aimed to measure the effect of different macronutrients like fat, carbohydrate and protein by exposing mice to different diets.

This type of study has the benefit of being able to precisely control the kind of nutrients that the mice had access to in a way that wouldn't be possible in a human study.

But we can't guarantee that the same results would be seen in people, as sometimes animal study results don't directly translate.

What did the research involve?

Researchers identified 30 different diets that had different amounts of protein, carbohydrates and fats.

These were organised into 5 different series:

  • series 1: fat was fixed at 60% of dietary energy content, and protein varied from 5% to 30%, with the rest as carbohydrate
  • series 2: fat was fixed at 20% of dietary energy content, and protein varied from 5% to 30%, with the rest as carbohydrate
  • series 3: protein was fixed at 10% of dietary energy content, and fat varied from 10% to 80%, with the rest as carbohydrate
  • series 4: protein was fixed at 25% of dietary energy content, and fat varied from 8.3% to 66.6%, with the rest as carbohydrate
  • series 5: fat was fixed at 41.7% of dietary energy content, protein was fixed at 25%, and carbohydrate varied from 5% to 30% (it appears the full details about this last series aren't included in the study)

The protein source was casein (found in animal milk), the carbohydrate source was corn starch and maltodextrose, and the fat source was a mix of cocoa butter, coconut oil, menhaden oil, palm oil and sunflower oil.

The researchers randomly allocated mice to one of the 30 diets, given for 12 weeks.

They looked at different "strains" of mice, where members of a particular strain have the same genetic features.

For the main strain used, they allocated 20 mice per diet.

The main results the researchers were interested in were food intake and changes to body weight and body fat (adiposity).

These were measured daily, firstly for a 2-week period before the diets began, and then throughout each diet.

Adiposity was measured using a body composition analyser, while food intake was measured by weighing how much food was left to deduce how much each mouse had eaten during that day.

What were the basic results?

For the diets where protein content was varied, altering the protein made no difference to how much the mice ate.

When fat energy content was 60%, body weight and adiposity both increased as protein content increased.

When fat energy content was 20%, body measures increased when protein increased from 5 to 20%, but then decreased as protein went from 20 to 30%.

For the diets where fat content was varied, the highest level of adiposity occurred when the dietary fat content was 50 to 60%.

Altering carbohydrate content while mice ate fixed fat and protein made no notable difference to food intake and adiposity.

Researchers also found that diets higher in fat stimulated brain chemical activity associated with pleasure, craving and addiction in humans, similar to the effects of dopamine and serotonin.

How did the researchers interpret the results?

The researchers said their results had some "important translational implications" [may have implications for humans] but acknowledged that their study had some limitations.

They only looked at male mice during early adulthood for a period equivalent to around a decade in human terms.

They speculated that they might find different results in female mice, and if the diets were followed for a longer period of time to cover a larger stretch of the lifespan.


This study, which explored the effects of different precisely controlled diets on body fat gain and food intake in mice, may be of interest to scientists and nutritionists.

But it doesn't have direct implications for the general public.

Whether the human body would respond to these changes in nutrients in the same way is less clear.

The protein, fat and carbohydrate sources used in these test diets were also far narrower than the range of sources that most people eat.

This means they might not be completely representative of "typical" carbohydrates, proteins and fats in a human diet.

Also, the complex and varied role of various human factors, such as our individual genetic make-up, health status and lifestyle factors like physical activity, hasn't been considered here.

There are many reasons why a healthy and balanced diet, and not just a single food group, is beneficial to your health and wellbeing.

Read more about the benefits of a balanced diet.

NHS Attribution