Does eating too little fat lower testosterone?

Short answer

It does. In a 6-week trial that cut fat from 40% of daily calories to 25%, testosterone fell 15%, and in another 8-week low-fat trial it fell 12%. In the 6-week trial, it recovered once people went back to their usual diet.

Does eating too little fat lower testosterone?
How much of a cut, and how big a drop?
Many people cut fat first when dieting. That brings the claim that testosterone will drop. We checked trials that cut fat: how far fat was cut, how much testosterone fell, and whether it came back.
Cutting fat from 40% to 25%
Testosterone fell 15% in 6 weeks
30 people cut fat from 40% of daily calories to 25% and lived on that for 6 weeks. Total calories were kept nearly the same. Testosterone fell about as much as 100 becoming 85, and free testosterone, the part the body actually uses, also dropped.
Other trials pointed the same way
12% in 8 weeks, 13% when swapping diets for 10 weeks each
39 people kept calories the same and ate a lower-fat, higher-fiber diet for 8 weeks, and blood androgens (male hormones) fell 12%. In a trial where 43 people took turns eating a 41% fat diet and an 18.8% fat diet for 10 weeks each, testosterone passed in urine was 13% higher on the high-fat diet.
Eating fat again brought it back
When the diet went back, the numbers went back
The 30 people who had cut fat from 40% to 25% went back to their usual diet, and all the lowered testosterone levels returned to where they were. The effect of cutting fat showed up while people were eating that diet.
Real life showed the same direction
Low-fat eaters were 7% lower
A survey of 3,128 people found that the 457 on a low-fat diet had testosterone about 7% lower than those on a regular diet. It was still lower after matching for age, body weight and activity. In 12 people with 17 days of food records, those getting a larger share of calories from fat also had higher resting testosterone.
Does the type of fat matter?
Looked at alone, type showed no link
The trial diets cut the amount of fat while also raising the share of unsaturated fat and fiber. A 2024 study of the diets of 2,546 people found more saturated fat went with higher testosterone, but the link disappeared after matching other lifestyle factors. In this survey, fat type had no separate link. The drops in the trials came from diets that changed fat amount, fat type and fiber together.
Summary: trials that cut fat saw 12~15% drops
The low-fat diets were about 42~56 g at 2,000 kcal a day
In trials that cut fat, testosterone fell 12~15%. In the trials that reported fat shares, the low-fat diets got 18.8~25% of daily calories from fat, and the comparison diets 40~41%. At 2,000 kcal a day, that is about 42~56 g versus 89~91 g. In the 6-week trial, levels recovered once people went back to their usual diet.