S-testosterone decrease after a mixed meal in healthy men independent of SHBG and gonadotrophin levels.

Reproducible and accurate assessment of serum testosterone (S-T), S-LH and S-SHBG is of crucial importance for assessment of testicular endocrine function and diagnosis of hypogonadism and investigating male health in a broader sense. Testosterone secretion has a circadian rhythm with the highest component in the morning and is influenced by a series of factors including physical activity, mental stress and nutrition. For diagnostic purposes, analysis of morning samples is recommended and reference values are generally based on samples drawn between 7 and 10 am. In the literature, there are also indications that food intake can influence serum levels but fasting has not been a standard procedure. To carefully address the influence of food intake, we analysed S-testosterone, S-LH and S-SHBG after an overnight fasting compared to samples taken after a standard meal of 550 kcal. We found no change in S-LH or S-SHBG but a decline of S-T of 30% from 60 to 120 min after food intake compared to samples taken in the fasting state. This decline may give false low S-T values and overestimate the number of men with suspected hypogonadism. Until the mechanism behind this effect has been explored, we suggest that assessment of S-T for diagnostic purposes should be collected in the morning after an overnight fasting.