Do vitamin C and E supplements slow muscle growth?

Short answer

No. Pooling 9 studies showed no difference in muscle or strength. A high dose of 1,000 mg a day only weakened post-workout signals, and healthy people got no benefit.

Do vitamin C and E supplements slow muscle growth?
'Antioxidants → weaker training signals → less muscle' — does this formula hold?
Exercise creates free radicals in the body, and one idea is that these act as a signal telling muscle to grow. So people say that taking vitamins C and E, which mop up free radicals, makes muscle grow less. We checked real training studies.
Signals right after training did get weaker
With 1,000 mg of vitamin C and 235 mg of vitamin E a day
32 people trained hard 4 times a week for 10 weeks while taking 1,000 mg of vitamin C and 235 mg of vitamin E, or a placebo, every day. Right after a session, several muscle-building signal proteins switched on less in the vitamin group. The first link of the formula held.
But muscle did not grow less
Signals were weaker, yet muscle gain showed no difference
In the same 32 people, the muscle gained over 10 weeks and the rate of muscle protein building right after training showed no difference between groups. Biceps curl strength, though, rose less in the vitamin group.
Pooling 9 studies: no difference
Not in muscle, not in strength, and age did not change it
An analysis that pooled 9 strength-training studies with vitamins C and E found no difference from placebo in lean mass (muscle and everything except fat) or strength. Age did not change the result. In 9 endurance studies, fitness gains were not reduced either.
Less gain showed up in two small studies
9% vs 1.4% in 34 people, with no strength difference
34 people trained for 12 weeks, taking 500 mg of vitamin C and 117.5 mg of vitamin E before and after each session. Lean mass rose 3.9% on placebo and 1.4% on vitamins. Strength rose 15-21% in both groups. In a 10-week study of 23 people, upper-body muscle also grew only on placebo.
Endurance training showed the same pattern
Fitness rose 8% in both groups; only cell changes shrank
54 people did 11 weeks of running training on the same dose (1,000 mg of vitamin C and 235 mg of vitamin E). Maximal oxygen uptake rose 8% in both groups. But a marker of the muscle's energy factories (mitochondria) rose 59% on placebo and did not rise on vitamins.
The dose used was about 13 times what food gives
A benefit appeared only in 60 people with muscle loss
The 1,000 mg of vitamin C used in these studies is about 13 times the 78 mg a day that one study's participants got from food. The only benefit came in 60 people with age-related muscle loss (sarcopenia) who trained for 12 weeks: arm muscle grew 0.96 kg vs 0.59 kg.
Bottom line: they didn't block muscle, but there's no reason to take them
The formula breaks at the 'signal → muscle' link
High-dose vitamins C and E weakened training signals, but pooling 9 studies showed no difference in muscle or strength. In the studies reviewed here, healthy people never gained more muscle with them. For building muscle, training without the pills works.