Adaptation of protein metabolism in relation to limits to high dietary protein intake

Studies of the effects of dietary protein level on human metabolism have us ually concentrated on the effects of protein deprivation and on establishin g a minimum dietary requirement. By contrast, less is known about the effec ts of very high protein diets, although general levels of protein intake in the developed world are increasing, and high protein diets have been advoc ated for maintaining or increasing muscle mass in certain groups of the pop ulation. This article, therefore, examines the response of protein metaboli sm to high dietary protein, studied in adults by nitrogen balance and isoto pic tracer techniques. and concentrating on the evidence for increased lean body mass. It is concluded that high protein feeding initially results in protein retention, with greater cycling of body protein in response to meal s, but that neither N-balance nor isotopic tracer methods possess sufficien t sensitivity to detect whether a long term increase ill functional lean ti ssue ensues. Improved methods of body composition measurement will be neede d to establish this. Moreover, the absence of strong evidence that high pro tein diets confer any advantage in terms of strength or health must be weig hed against potentially injurious consequences.