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31.10.2012

Overweight halves teenagers' testosterone levels

If you're carrying around more kilos of fat than is good for you, you also have less testosterone circulating in your blood than Mother Nature intended. Obesity in males aged between 14 and 20 can actually halve their testosterone levels, write researchers at State University of New York at Buffalo in Clinical Endocrinology.

Body weight & testosterone
In an earlier study, the researchers looked at the effect of overweight and diabetes on the testosterone levels of men aged over 45. They discovered that 40 percent of the men with serious overweight – BMI higher than 30 – but no diabetes had low testosterone levels. For the seriously overweight men with diabetes the figure was 50 percent. [Diabetes Care. 2010 Jun;33(6):1186-92.]

Study
Because serious weight problems are increasing among young people, the researchers wanted to know whether testosterone levels were low in overweight teenagers. That's why they studied a group of 25 slim and 25 overweight teenagers.

Results
And indeed, the higher the teenagers' BMI, the lower the concentration of free testosterone in their blood.

The overweight subjects had an average BMI of 36; that of the slim subjects was 20. The testosterone [TT] and free testosterone [cFT] levels in the overweight teenagers were about half those of the slim teenagers, as the table below shows.

The accepted theory is that fat tissue converts testosterone into estradiol. The researchers are not sure whether this is the case for this group, however. Although the overweight teenagers had a slightly raised estradiol level, this did not correlate with the reduced testosterone level.

The researchers have put forward a different theory: the overweight teenagers' insulin sensitivity is lower. One measure of insulin sensitivity is the HOMA-IR (which is calculated by multiplying the fasting glucose and insulin levels with each other). The higher the HOMA-IR, the lower the amount of free testosterone.

The researchers suspect that reduced sensitivity to insulin prompts the hypothalamus to secrete less GnRH. Normally the hypothalamus increases its GnRH secretion if the insulin level rises. So GnRH indirectly shuts off the body’s testosterone production.

Conclusion
"In view of the rising prevalence of obesity, the association of significantly lower testosterone concentrations with obesity in males is alarming and points to major public health problem", the researchers conclude.

Source:
Clinical Endocrinology doi: 10.1111/cen.12018.

More:
Weight loss makes fat men more masculine 12.10.2011
Fat belly neutralises DHT 29.12.2010