Zitat:
Experiments with rats show that greater concentrations of vitamin A in the testes increase basal testosterone secretion, as well as transferrin, which is responsible for the transport of iron; and a variety of growth factors including IGF-binding protein 4 (which transports IGF), androgen-binding protein (which transports androgens), transforming growth factor-beta (which causes cell growth but suppresses cancer) and steroidogenic acute regulatory protein (which is responsible for the transport of cholesterol into the mitochondria for its conversion to steroids). Vitamin A also decreases estrogen production in the male testes. Rats that are deficient in vitamin A experience decreased testosterone until the accessory sex organs atrophy, indicating that vitamin A not only aids in, but is essential to, testosterone production.
Zitat:
Vitamin A is not only depleted by a high intake of protein, but it is also necessary for the synthesis of new protein, which is the goal of the bodybuilder. Rats fed diets deficient in vitamin A synthesize protein at a lower rate than rats fed adequate vitamin A.7 Cultured skeletal muscle cells increase the amount of protein per cell when exposed to vitamin A and D, but not when exposed to vitamin D alone.
Vitamin A/ Retinol, dass ausschliesslich in tierischen, gesättigten Fetten vorkommt (nicht zu verwechseln mit Provitamin A/ Beta Carotin in Pflanzen) steigert die Testosteronproduktion.