What does a low-fat diet do to your testosterone?
Studies suggest that a low-fat diet can slightly lower your testosterone, though the results are mixed. What comes through more consistently is that eating too few calories has a greater impact on your hormones than the amount of fat in your diet.
Two large summaries of the available studies paint a contradictory picture. One summary of six small studies found that men on a low-fat diet had, on average, slightly lower testosterone levels than men on a higher-fat diet. This applied to total testosterone, free testosterone and testosterone measured in urine. In men of European descent the difference was somewhat larger. A later and more thorough summary of eleven studies in nearly 900 people, however, found no meaningful effect whatsoever of a low-fat diet on testosterone or other sex hormones. The two findings directly contradict each other.
An important distinction: this is not simply a matter of fat versus less fat. When total calorie intake is too low, testosterone falls regardless. Eating more carbohydrates within that same low intake offered some protection against hormone declines, but the effect on testosterone itself fell just short of being statistically detectable. This suggests that an energy deficit plays a stronger role than the proportion of fat in your diet.
A ketogenic diet, in which very few carbohydrates and a lot of fat are eaten, shows mixed effects in men who exercise. The outcome depends on the type of sport, how much muscle mass a person has, and how many calories are consumed. In women with polycystic ovary syndrome, a ketogenic diet lowers testosterone, which is actually a beneficial effect in that group.
Genetic research turned up an unexpected signal: people who naturally eat more sugar appear to have higher levels of biologically available testosterone. The researchers themselves were surprised by this and are calling for further study. A link between higher fat intake and abnormal sperm cells was found, but that figure was surrounded by so much uncertainty that it is barely useful.
This answer draws on two large summaries of 6 and 11 studies respectively, supplemented by a small study, a review of ketogenic diets covering 7 studies, and genetic research. The researchers behind the largest and most recent summary themselves emphasise that the number of studies is small and rate the certainty of their findings as low. That rating applies to all sub-analyses. The two main studies also contradict each other, which further increases the uncertainty.