• Eating a low-fat (less than 30 percent of calories from fat), calorie-restricted diet may cause a decline in testosterone levels in men, according to a study published in The Journal of Urology.
• Normal weight, active men may be better off following a moderate fat diet to maintain hormonal health.

Your testosterone levels – which are critical not just in cycling, but in ‘normal’ life – will thank you for eating a little more fat.  Thanks to diet trends like the keto diet, fat is back in nutrition “fashion,” but there’s a legit reason why men may want to add more healthy fats to their plates: Men who follow a low-calorie, low-fat diet may be at risk for lowering their testosterone levels, especially if they’re already at a healthy weight, according to new research. That’s especially important for endurance enthusiasts like us cyclists, who are already at risk for slightly lower levels of this important hormone.

The study, which was published in The Journal of Urology, examined the relationship between popular diets and testosterone levels among 3,128 men between the ages of 18 and 80 from a nationwide health study (the National Health and Nutrition Examination Survey, or NHANES), which included two-day dietary histories and testosterone testing.

“We found that men who adhered to a fat-restrictive diet had lower serum testosterone than men on a nonrestrictive diet,” said lead researcher Jake Fantus, MD, from the University of Chicago in the study.

Men’s testosterone levels drop by about 1 to 2 percent a year after about age 40. While some decline is natural, if levels fall too low, it can lead to symptoms such as fatigue, muscle loss, depression, erectile dysfunction, and low sex drive, among other health issues.

The normal adult range for testosterone is 300 ng/dL to 1,000 ng/dL, according to the FDA. When levels fall below 300 ng/dL, they’re considered deficient, according to the American Urology Association.

Being overweight raises the risk of low testosterone. Research shows men who are more than 20 percent heavier than their ideal weight have 30 percent lower testosterone levels than lean men.

Exercise and weight loss, for those who are overweight, generally help boost testosterone levels. But the role of diet is less clear.

The Journal of Urology study was originally designed to examine the effects of four diets—low fat (<30 percent), low carbohydrate (<20 grams), Mediterranean (40 percent fat), and a non-restrictive diet—on testosterone levels, but there weren’t enough men in the low-carb group to base findings on (the last year in the study period was 2012, so keto had not yet become mainstream), so that diet wasn’t included.

The average testosterone level of the men in the study was 435.5 ng/dL. Testosterone was notably lower in men on the two restrictive diets: average 411 ng/dL for those on a low-fat diet and 413 ng/dL for those on the Mediterranean diet.

Once the researchers adjusted the data for factors that can affect testosterone, including age, body mass index (BMI), physical activity, and medical conditions, the low-fat diet was significantly associated with reduced testosterone, though the Mediterranean diet was not.

The researchers aren’t yet clear how significant these small testosterone differences across diets are, and it’s important to remember that the testosterone-raising benefits of weight loss might outweigh the small dip associated with following a low-fat diet. However, if you’re already active and aren’t trying to lose weight, you may be better off steering toward a more moderate-fat diet for hormonal health, Fantus told Bicycling.

“In normal-weight men, especially runners and cyclists, eating a fat-restricted diet (without the benefit of losing weight) might lead to lower testosterone levels,” he says. “While the decrease is not substantial, in order to enhance testosterone levels and improve cardiovascular fitness, finding a less fat-restrictive diet might be optimal.”