Resistance to lean mass gain in constitutional thinness in free-living conditions is not overpassed by overfeeding.

The blunted muscle energy mechanism, previously described in CTs in free-living state, is associated with basal saturated protein turn over suggested by the concordance of positive nitrogen balance and an increased urine excretion of several essential amino acids. This saturation cannot be overpassed by increasing this spontaneous high-protein intake suggesting a resistance to lean mass gain in CT phenotype.