Muscle function and nutrition.

Wasting of muscle and a negative nitrogen balance are obvious effects of malnutrition and have led to the use of anthropometric measurements and nitrogen balance for assessing the extent of malnutrition. A positive nitrogen balance and an increase in limb muscle circumference are believed to be indices of the beneficial effects of nutritional support. In experiments with growing rats and young children nitrogen retention and growth are recognised to be the desirable effects of optimal nutritional intake. This concept has been applied to malnourished adult humans (non-growing) who have been considered to be potentially able to "regrow" the lost tissue. While it is true that patients receiving long term (more than six months) home total parenteral nutrition gain body weight and nitrogen over many months and years of observation, this process is not seen during shorter (less than 40 days) nutritional intervention given in hospital.1 Despite adequate intakes of nitrogen and calories little or no increase in total body nitrogen is seen in a variety of patients receiving total parenteral nutrition in hospital over several weeks.2-7 Despite the absent or very modest gain in nitrogen nutritional support does seem to improve outcome by reducing complications and mortality after a period of support so short that body composition is hardly changed. Young et al showed that although amino acids and amino acids plus calories both resulted in equivalent sparing of body nitrogen, amino acids plus calories was associated with ruicker wound healing and fewer complications.Thus the outcome and body composition data suggest that the reversal of the adverse effects of malnutrition is not based on improvement of the traditional variables of nutrition, such as gain in body nitrogen, or a demonstrable increase in muscle mass or plasma proteins.8 This discrepancy is further supported by the observation that global clinical assessment is at least equivalent to, and in some respects better than, individual objective traditional measurements of nutritional state in predicting outcome.9