Short answer
No. Cutting fat can lower testosterone by about 7%, but that's not enough to change muscle growth.
Does cutting dietary fat slow down muscle growth?
'Less fat → less testosterone → less muscle' — does this formula hold?
A common claim: cut fat while dieting and your testosterone drops, so your muscles won't grow. We checked the formula one step at a time.
Does cutting fat lower testosterone?
It does, but by about 7%
A survey of 3,128 American men found those on low-fat diets had testosterone levels about 7% lower on average than those on regular diets. That's like 100 becoming 93. Another analysis that pooled 888 people found no difference at all.
So does testosterone decide muscle?
It does with big changes, like doubling it or more with injections
When 61 people got different testosterone injection doses for 20 weeks, those whose levels went way up gained 3.4–7.9 kg of lean mass (muscle and everything except fat). Hormones do affect muscle. But that's when levels change twofold or more.
Ups and downs this small didn't change muscle
Even with bigger hormone rises, muscles grew the same
In a study of 49 people training for 12 weeks, those whose hormones rose more after exercise didn't grow more muscle. When 12 people trained for 15 weeks with one arm under conditions that sharply raised hormones and the other arm under conditions that didn't, both arms grew similarly, 10% and 12%.
Flip side: does eating lots of fat grow more muscle?
No — keto didn't add more muscle either
An analysis that pooled 111 people training on a keto diet, where most calories come from fat, found lean mass no different from a regular diet. Another analysis that pooled 244 people found the keto group actually had 1.26 kg less.
Real natural bodybuilders ate little fat
Top finishers ate more carbs
51 athletes who made a UK natural bodybuilding final ate lots of protein and carbs and little fat throughout contest prep. Men who placed in the top 5 ate about 40% more carbs early in prep than those who didn't place. For a 70 kg (154 lb) person, that's about 360 g vs 260 g a day.
Bottom line: you can cut fat and keep your muscle
The formula breaks at the 'hormone → muscle' link
Cutting fat can lower testosterone by about 7%, but that didn't change muscle. Eating lots of fat didn't grow more muscle either. Keeping fat at 15–30% of your daily calories (about 33–67 g of fat on 2,000 kcal a day) and eating enough protein and carbs is enough.
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