Metabolic consequences of a high dietary-protein intake in adulthood: assessment of the available evidence.

In Western Europe and the United States, protein consumption amounts to about 1.5 to 2 times (Adolf et al. 1994) the recommended intakes (WHO 1985), which is currently considered to be harmless and, according to public opinion, may be even beneficial. Usually relatively short-term experimental studies on the effects of high-protein intakes have been performed, and so the consequences of longer-term or chronic nutrient intake are difficult to judge. Epidemiologic evidence, on the other hand, which aims to circumvent this difficulty, can be flawed by various biases, confounding effects or even limitations of the data which make it difficult to obtain clearcut cause-effect relationships (Taubes 1995). It was stated (WHO 1985) that “there are no functional indicators that can usefully be applied in experimental situations to detect protein inadequacy before clinically detectable changes occur.” This statement is equally pertinent for the determination of the tolerable upper intake level (TUL) particularly, if adequacy of protein intake in various physiological situations (i.e., maintenance, growth, pregnancy, lactation) is to be estimated. Moreover, no extensive body of data exists covering the issue. However, there are bits and pieces of information suggesting that there is no benefit from increasing the dietary-protein intake far above the recommended intake level. Below we summarize the relevant available literature and attempt, by making prudent assumptions, to estimate a TUL which is defined as the highest level of daily intake that is likely to pose no risk of adverse health effects (Institute of Medicine, Food and Nutrition Board 1999).