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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/LInterestingly, 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 Vitamin D Tied to Testosterone Dip in Healthy Men

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."

Even though you may want to avoid phthalates, it is very hard to avoid them because they are commonly found in plastic food and beverage containers, perfume, hair spray, deodorants, almost anything fragranced (shampoo, air fresheners,etc.), insect repellent, carpeting, vinyl flooring, plastic toys, the steering wheel in cars, soft tubing in medical devices, etc. From Science Daily:

Reduced testosterone tied to endocrine-disrupting chemical exposure

Men, women and children exposed to high levels of phthalates -- endocrine-disrupting chemicals found in plastics and some personal care products -– tended to have reduced levels of testosterone in their blood compared to those with lower chemical exposure, according to a new study.

Testosterone is the main sex hormone in men. It contributes to a variety of functions in both sexes, including physical growth and strength, brain function, bone density and cardiovascular health. In the last 50 years, research has identified a trend of declining testosterone in men and a rise in related health conditions, including reduced semen quality in men and genital malformations in newborn boys.

Animal and cellular studies have found that some phthalates block the effects of testosterone on the body's organs and tissues. Researchers set out to examine whether these chemicals, which are widely used in flexible PVC plastics and personal care products, had a similar effect in humans.

"We found evidence reduced levels of circulating testosterone were associated with increased phthalate exposure in several key populations, including boys ages 6-12, and men and women ages 40-60," said one of the study's authors, John D. Meeker, MS, ScD, of the University of Michigan School of Public Health in Ann Arbor, MI. "This may have important public health implications, since low testosterone levels in young boys can negatively impact reproductive development, and in middle age can impair sexual function, libido, energy, cognitive function and bone health in men and women."

The cross-sectional study examined phthalate exposure and testosterone levels in 2,208 people who participated in the U.S. National Health and Nutrition Examination Survey, 2011-2012. Researchers analyzed urine samples to measure concentrations of 13 substances left after the body metabolizes phthalates. Each participant's testosterone level was measured using a blood sample.

Researchers found an inverse relationship between phthalate exposure and testosterone levels at various life stages. In women ages 40-60, for example, increased phthalate concentrations were associated with a 10.8 to 24 percent decline in testosterone levels. Among boys ages 6-12, increased concentrations of metabolites of a phthalate called di-(2-ethylhexyl) phthalate, or DEHP, was linked to a 24 to 34.1 percent drop in testosterone levels.