Low fat diets are a popular form of weight loss, which is unfortunate as their efficacy is questionable when compared to non restrictive alternatives. For example it has been shown that non-restrictive low carbohydrate diets cause superior weight loss compared to restrictive low fat diets in humans1. Low fat diets are traditionally used by those who believe that calorie counting is the only way to lose weight. In this regard, it is assumed that because fat contains the most energy, removing it from diet will cause the most significant lowering of energy intake and cause the most weight loss. While the first part of this statement is factually correct, the latter assumptive part is erroneous. Let us forget for a moment that forced caloric restriction does not lead to weight loss and humour the concept. If it were true, would a low fat diet be the best way to restrict energy? Probably not, but anyhow the deleterious effect of a low fat diet and the deterioration in health caused would likely outweigh any health benefits gained from the weight loss.
Low fat diets are detrimental to weight loss and the health because they cause a number of metabolic changes to the body. Firstly, low fat diets interfere with sex hormone production. Reducing fat content of the diet from around 40 % to around 20 % of energy intake will cause reductions in testosterone of around 15 to 20 % in a man. One of the problems associated with forced calorie restriction is a loss of skeletal muscle mass, and decreases in testosterone levels can only promote this effect further. High cortisol levels brought about through the stress of dieting in the presence of low testosterone levels will cause immune suppression, deleterious mood changes and a loss of motivation and confidence. In fact weight training might be beneficial to fat loss because it increases testosterone output. Low fat diets are therefore problematic in the short-term as they will considerably increase the likelihood of weight loss from skeletal muscle.
The second major problem with low fat diets is that they must exclude sources of fat that are essential to the health and are required for metabolic function. A 10 % fat diet will require the elimination of all extraneous sources of fat from the diet as well as the consumption of low fat foods, because even foods with relatively low fat contents can still provide 10 % of their energy from fat. This 10 % of fat in the diet will comprise mainly of the trace amounts of mainly saturated fat that is found in foods such as low fat milk, bread and other staple dietary items. Long-term consumption of such food without additional sources of fat has a high probability of failing to supply the dieter with the required amount of the essential fatty acids alpha linolenic acid and linoleic acid. Further to this, low fat diets have also been shown to provide inadequate fat soluble vitamins making insufficiencies more likely to develop. Low fat diets are therefore deleterious to the health irrespective of their questionable ability to cause weight loss.
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