The nature and significance of nutritional adaptation.

Possible adaptations to a low protein intake are: a decrease in the obligatory nitrogen loss, which would be too small to detect in short-term studies, but would be significant over a longer term; an increase in the efficiency of protein utilization, which has been demonstrated in depleted subjects; and a decrease in lean body mass, mainly at the expense of muscle. However, we do not know the extent to which this last mechanism may really be an adaptation without significant functional loss. In the case of energy there is controversy about the extent to which the gross efficiency of muscular work can be improved. One mechanism might be an alteration in the distribution of fibre types, with a shift from fast to slow fibres. A possible way of reducing the cost of both muscular work and basal metabolism would be a reduction in the mitochondrial proton leak. Both these mechanisms are at least partially under the control of the thyroid gland, which therefore may play an important role in economizing energy expenditure.