Polyunsaturated fatty acids make you slimmer and more muscular

For a few days now we, the unworldly compilers of this free web blog, have been immersed in studies on the effect of fatty acid composition in the diet on muscle tissue and body fat. Recently we wrote about a human study which has shown that a high intake of omega-6 fatty acids inhibits fat accumulation and boosts muscle increase. Today we read that all plant-based polyunsaturated fatty acids probably have this effect.

High fat

We draw this cautious conclusion from an animal study published by researchers at the University of Fribourg in Switzerland in 2011. In that study the researchers gave rats a high-fat diet, in which almost 60 percent of the calories were derived from fat.


To start with the researchers gave their rats half of the amount of food they would normally eat for a few weeks. The animals therefore lost weight. Then the rats were given the calorie-rich high-fat diet for four weeks, and put on weight.

The type of fat the researchers gave the rats varied. The figure below shows what happened to the animals’ fat and lean body mass when fed with lard, rapeseed oil, safflower oil, grapeseed oil, sunflower oil or maize oil.

As you can see, the rats that were given vegetable oil all built up more lean body mass and less fat than the rats that were given lard. The best results were achieved with safflower oil.

If you look at the composition of the fats that were used you can see that it’s above all the amount of polyunsaturated fatty acids [PUFA] that determines the effect on body composition: the more polyunsaturated fatty acids, the more muscle and the less fat the rats built up.



N-6 fatty acids

The polyunsaturated fatty acids in the oils mentioned above are mainly of the n-6 type. The researchers were curious to know whether n-3 fatty acids would have a different effect, so they repeated their experiments, this time using linseed oil. Linseed oil contains large amounts of the n-3 fatty acid alpha linolenic acid.



“Taken together, the possibility therefore arises that diets enriched with more modest amounts of PUFA as linoleic acid and/or alpha linolenic acid than shown here to be effective in improving body composition in rats may have relevance for improving body composition […] in humans”, the researchers conclude.

“Whether diets providing such high PUFA intake, exceeding 25% of total energy intake, are safe and effective in modulating body composition and metabolic health in humans are issues that must be considered with caution.”

Dietary modulation of body composition and insulin sensitivity during catch-up growth in rats: effects of oils rich in n-6 or n-3 PUFA.


The present study investigates whether excessive fat accumulation and hyperinsulinaemia during catch-up growth on high-fat diets are altered by n-6 and n-3 PUFA derived from oils rich in either linoleic acid (LA), ?-linolenic acid (ALA), arachidonic acid (AA) or DHA. It has been shown that, compared with food-restricted rats refed a high-fat (lard) diet low in PUFA, those refed isoenergetically on diets enriched in LA or ALA, independently of the n-6:n-3 ratio, show improved insulin sensitivity, lower fat mass and higher lean mass, the magnitude of which is related to the proportion of total PUFA precursors (LA+ALA) consumed. These relationships are best fitted by quadratic regression models (r2>0·8, P < 0·001), with threshold values for an impact on body composition corresponding to PUFA precursors contributing 25-30 % of energy intake. Isoenergetic refeeding on high-fat diets enriched in AA or DHA also led to improved body composition, with increases in lean mass as predicted by the quadratic model for PUFA precursors, but decreases in fat mass, which are disproportionately greater than predicted values; insulin sensitivity, however, was not improved. These findings pertaining to the impact of dietary intake of PUFA precursors (LA and ALA) and their elongated-desaturated products (AA and DHA), on body composition and insulin sensitivity, provide important insights into the search for diets aimed at counteracting the pathophysiological consequences of catch-up growth. In particular, diets enriched in essential fatty acids (LA and/or ALA) markedly improve insulin sensitivity and composition of weight regained, independently of the n-6:n-3 fatty acid ratio. PMID: 21281526 [PubMed - indexed for MEDLINE] Source: http://www.ncbi.nlm.nih.gov/pubmed/21281526