Evidence suggests that high protein diets are effective at aiding weight loss. The mechanisms by which increased dietary protein may decrease body weight include improved postprandial satiety and better control of insulin and glucose levels. During periods of intense exercise, high protein diets may also allow better nitrogen retention and therefore allow greater retention of lean mass. The weight loss experienced during consumption of high protein diets suggests that in the short term, high protein diets improve general health and reduce the risk of developing a number of diseases associated with being overweight. However, care should be taken that reductions in carbohydrate content are carefully controlled to provide adequate fibre and beneficial phytonutrients. This is because fibre and some phytonutrients are essential for the health of the microflora of the gut, and high protein low carbohydrate diets that do not maintain adequate fibre levels, may produce detrimental changes in colonic health.
Researchers1 have assessed the effects of high protein low carbohydrate diets on the health of the colon by analysing the metabolites of colonic microflora that are considered to be beneficial for colonic health. In a crossover design study, 17 obese men were fed a weight maintenance diet containing 85g protein, 116g fat and 360g carbohydrate for 7 days to create baseline conditions. The subjects were then switched to a high protein moderate carbohydrate diet (HPMC; 139g protein, 82g fat, and 181g carbohydrate) for 4 weeks, followed by a high protein low carbohydrate diet (HPLC; 137g protein, 143g fat, and 22g carbohydrate) for a further 4 weeks. Analysis of faecal samples showed that both treatments resulted in increased production of branched chain fatty acids and phenylacetic acid indicative of protein fermentation. Phenylacetic acid and branched chain fatty acids are known to be metabolised further to toxic products.
The concentration of N-nitroso compounds also increased compared to the baseline diet. N-nitroso compounds were inversely associated with carbohydrate intake and are known to be carcinogenic. The high protein diets also resulted in a reduction in the amount of short chain fatty acids, in particular reductions in the amount of butyrate, which resulted in an increase in the colonic pH. The faecal matter also showed a decrease in the populations of the butyrate producing Eubacterial rectale, suggesting that the colonic floral populations had been altered. Short chain fatty acids are important compounds that are known to be essential for gut health and immunity and their reductions suggest detrimental changes to the gut health. Phenolic compounds from plants were also reduced in the faecal matter in both high protein diets, many of these compounds having the potential to improve gut health.
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