Another reason to skip the Fruit Loops and have an omelette - sugary foods can cause testosterone levels to drop by as much as 25 percent, regardless of whether a man has diabetes or normal glucose tolerance. The new findings mean that men with low testosterone should have their hormone levels retested after they fast overnight, as eating may transiently lower them, say the researchers.

The findings were presented at The Endocrine Society’s 91st Annual Meeting in Washington, D.C. “Both the incidence of low testosterone [hypogonadism] in men and the annual number of testosterone prescriptions are increasing, likely as a result of the obesity epidemic and our aging population,” said researcher Frances Hayes, an endocrinologist at St. Vincent’s University Hospital in Dublin, Ireland. “The decision to prescribe testosterone therapy is based on the result of a blood sample, so obtaining an accurate measurement of testosterone is key to making a correct diagnosis of hypogonadism.”

Current guidelines for evaluating men with hypogonadism recommend measuring blood levels of testosterone on two or more occasions in the morning, when testosterone is highest. However, no guidelines exist on when to draw a testosterone sample in relation to food intake.

Previous research showed that a high level of insulin, the hormone primarily secreted after eating, is related to low testosterone levels. Like eating, glucose intake causes blood glucose (sugar) levels to rise, which stimulates the secretion of insulin. In the new study, the researchers examined the impact of a standard dose of glucose on testosterone levels in 74 men.

The authors found that the glucose solution decreased blood levels of testosterone by as much as 25 percent, regardless of whether the men had diabetes, prediabetes or normal glucose tolerance.

Two hours after glucose administration, the testosterone level remained much lower than before the test in 73 of the 74 men, a statistically significant difference, the authors say. Of the 66 men who had normal testosterone levels before the test, 15 percent became hypogonadal at one or more time points during the test.

The results did not differ by changes in insulin levels, according to the study abstract. Other hormones that could change testosterone measurements also did not appear to affect results. “More research is needed to find the factor or factors responsible for this drop in testosterone,” Hayes noted