12 January - 20 June 2016

Mice study reports fat not carbohydrates causes weight gain

The impacts of different macronutrients on body weight regulation remain unresolved, with previous studies suggesting increased dietary fat, increased carbohydrates or reduced protein may all stimulate overconsumption and drive obesity. A mice study by researchers from the Institute of Genetics and Developmental Biology at the Chinese Academy of Sciences in Beijing and the University of Aberdeen, UK, have investigated the effect of different macronutrients on body fat gain.

The impacts of different macronutrients on body weight regulation remain unresolved, with previous studies suggesting increased dietary fat, increased carbohydrates or reduced protein may all stimulate overconsumption and drive obesity. A mice study by researchers from the Institute of Genetics and Developmental Biology at the Chinese Academy of Sciences in Beijing and the University of Aberdeen, UK, have investigated the effect of different macronutrients on body fat gain.

The study by Speakman et al. published in Cell Metabolism, used 30 different diets that contain different amounts of protein (source was casein), carbohydrates (source was corn starch and maltodextrose) and fats (was a mixture of cocoa butter, coconut oil, menhaden oil, palm oil and sunflower oil).  The diets were grouped into 3 different series. Series 1, fat was fixed at 60% of dietary energy content and protein varied from 5% to 30% with the rest 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%. 

The researchers randomly split nearly 1000, different genetically engineered strain mice into one of 30 diets.  The mice were fed their respective diets for a period of 12 weeks, which is thought to be the equivalent of 9 human years. Before and throughout intervention, Speakman et al. measured food intake and changes to body weight and body fat.  Towards the end of the experiment, the researchers also measured the energy expenditure and physical activity of the mice. 

The study found that changing protein intake made no difference to how much the mice consumed, however when fat content was 60%, body weight and adiposity both increased as protein increased. When fat energy content was 20%, body measures increased when protein increased from 5–20%, but then decreased as protein increased from 20-30%. A dietary fat content of 50-60% was associated with the highest level of adiposity.   Changing the carbohydrate content made no difference to food adiposity and food intake.

Speakman et al. also investigated the hunger signalling pathways in the hypothalamus of the mice exposed to the different diets.  Diets higher in fat were associated with stimulated brain chemical activity associated with addiction, pleasure, and craving in humans.  Speakman et al. report increase levels of dopamine and opioid activity in the hypothalamus. 

The researchers discuss their findings noting that strong public health messages report that sugar intake should be limited to 1% of energy, however they found “ no stimulation of energy intake or elevated adiposity by varying levels of sucrose in the diet (up to 30% by calories) independent of other macronutrient changes”.  They do state that “one interpretation of why we did not find these effects in mice might be because mice have different food intake and macronutrient regulations from humans, and that such systems are not strongly conserved across species.”

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