Low levels of vitamin D are associated with low levels of testosterone in healthy middle-aged men. Note that hypovitaminosis D means low levels of vitamin D in the blood, defined by a lab test as "25-hydroxyvitamin D level below 30.0 ng/L. Interestingly, being overweight also seems to lower the testosterone level, and losing weight raises the level, and supplementing with vitamin D may raise the testosterone level in those with low levels (the German study mentioned in the article). And research with mice suggests that there is something about testosterone synthesis that needs vitamin D. From Medscape:
Low levels of vitamin D are significantly and independently associated with low levels of testosterone in otherwise healthy middle-aged men, according to a study presented at the American Urological Association 2015 Annual Meeting in New Orleans.
In this new analysis of data from of the World Trade Center CHEST program, blood samples from 824 men were analyzed for various parameters, such as 25-hydroxyvitamin D and total testosterone. Hypovitaminosis D was defined as a 25-hydroxyvitamin D level below 30.0 ng/L.
Level of 25-hydroxyvitamin D were insufficient in 68% of the samples....Total testosterone was higher in men with normal levels of 25-hydroxyvitamin D than in men with lower levels...."subjects with hypovitaminosis D still had significantly lower total testosterone than those with normal total testosterone (P = .019)," Dr McLaughlin and her colleagues report. When levels of 25-hydroxyvitamin D were lower, body mass index was higher than when levels were normal (30.8 vs 29.12 kg/m²; P < .001), waist circumference was greater (41.3 vs 39.8 inches; P < .001), and lipid profiles were less favorable.
In previous studies, testosterone levels were shown to be lower in mice who had the vitamin D receptor genetically deleted, said Dr McLaughlin."This suggests that there is something about testosterone synthesis that needs vitamin D," she explained.
In a small German study of healthy overweight men with a low baseline level of 25-hydroxyvitamin D and testosterone levels at the lower end of the reference range, there was a significant increase in total testosterone levels after 12 months of vitamin D 3000 IU daily (Horm Metab Res. 2011;43:223-225)...the German study was primarily a weight-loss study, and weight loss in and of itself causes increases in testosterone."We see that in patients who undergo bariatric surgery to remove visceral fat," he explained. "Once the weight loss has taken place, their testosterone levels normalize independent of anything else."