Purpose of review: Protein requirement in healthy young and old individuals is traditionally defined as the lowest protein intake sufficient to achieve neutral body protein balance. This concept, however, cannot be applied to those conditions characterized by unavoidable protein catabolism despite optimal nutrition, such as inactivity and diseases associated with systemic inflammation.
Recent findings: The ability of dietary proteins to promote protein anabolism is reduced by inactivity and inflammatory mediators, whereas physical exercise ameliorates the efficiency in using dietary proteins. Consequently, the protein intake level associated with the lowest rate of catabolism in inactivity and/or inflammation is greater than the minimum protein intake required to achieve neutral protein balance in healthy, physically active individuals. A protein intake of 1.2 g.kg.day is currently recommended for inactive healthy individuals, whereas guidelines recommend up to 1.5 g.kg.day in patients with severe systemic inflammation, such as those affected by critical illness or cancer. High protein intake accelerates progression of renal insufficiency but does not affect renal function in healthy individuals.
Summary: In inflammation and/or inactivity a relatively high protein intake may be required to promote synthesis of specific proteins, prevent depletion of selected amino acids (e.g., glutamine or arginine), modulate immune functions, counteract insulin resistance and redox unbalance. Thus, an optimal protein/amino acid intake may be greater than that required to decrease whole body protein wasting.