The concentrations of serum total and free testosterone were studied in 30 healthy, middle-aged men during a dietary intervention program. When men were transferred from their customary diet to an experimental diet, which contained less fat with a higher polyunsaturated/saturated ratio (P/S-ratio) and more fibre, there was a significant decrease in serum total testosterone concentrations ( +/- vs +/- nmol/l SEM, P less than ). Furthermore, serum free, unbound testosterone fell from +/- to +/- nmol/l SEM (P less than ). The hormonal changes were reversible. This observation suggests that testosterone activity in plasma can at least partly be modified by changing the composition of the diet.
In a 2011 randomized controlled study, researchers recruited 54 healthy men whose mean 25(OH)D levels were in the deficiency range for a year-long intervention. They divided the subjects into two groups. The first group of 23 men had an average serum 25(OH)D of nmol/L and took a daily placebo. The second group of 31 men had an average serum 25(OH)D of nmol/L and took a daily 3332-IU vitamin D supplement. After the trial was finished, the researchers observed a significant increase in total testosterone from nmol/L to nmol/L in the supplement group. 19 In contrast, there were hardly any changes in testosterone concentrations in the placebo group. 19 These findings suggest that men deficient in vitamin D who take a proper vitamin D supplement may fix low levels of low testosterone.