Antioxidants After Exercise: Why Muscles Need Their Free Radicals

At the end of September, a team led from the University of Copenhagen published a paper in Nature Aging about a fat you have probably never heard of. It is called cardiolipin. It makes up less than 2% of the fats in muscle membranes, and it sits in the inner wall of every mitochondrion, the small engines that turn food and oxygen into movement. In ageing muscle, the paper reports, there is less of it.

The headlines that followed this week went for the mechanism: ageing muscle may give up some strength to protect itself. That is a good story. But the part I keep thinking about is a small experiment near the end. The researchers gave an antioxidant to mice whose muscles were already struggling. The muscles got worse, not better.

If you have ever taken a vitamin C tablet after a hard session to "help recovery", that result is about you. It is also not the first time science has said this, and the honest version of the story is more interesting than either the supplement aisle or the backlash against it.

A mouse study with an awkward result

The study, from Fabian Finger, Zachary Gerhart-Hines and colleagues, started with a known puzzle. As muscle ages, its mitochondria work less well. Yet ageing muscle also shifts towards slow-twitch fibres, the type that is richest in mitochondria. The power fibres fade first, and the endurance fibres take a bigger share.

To see why, the team switched off cardiolipin production in the muscle of young mice. The young muscle began to look old. It shifted towards slow fibres, it wasted, and the mice died early. The signal ran from the mitochondria to the cell nucleus through a protein called ERRγ, and part of that signal was carried by reactive oxygen species, the molecules many of us still call free radicals.

The muscle was using those molecules as a message. It rerouted glucose to build up its own antioxidant defences. So when the researchers added N-acetylcysteine, an antioxidant, to the drinking water for eight weeks, they did not rescue anything. In their words, it made the effects of the cardiolipin loss worse. When they let the mice make cardiolipin again, the muscle began to rebuild and the early deaths stopped.

Two cautions before we go further. These were young mice with a gene switched off, not naturally old ones. And the groups in the antioxidant experiment were small, a handful of animals each. This paper does not tell you what an antioxidant does to your muscle. What it does is add one more voice to an argument that has been running in human trials for almost twenty years.

Free radicals are messages, not only damage

The popular idea of ageing is rust. Oxygen burns, sparks fly off, and the sparks slowly damage everything they touch. In that story, antioxidants are the fire brigade, and more is always better.

Exercise makes this story hard to keep. A hard session produces a burst of reactive oxygen species, far more than sitting still. If those molecules were only damage, training would be harmful. It is not. Part of the reason is that the burst is also a signal. It tells the muscle that it has been stressed and that it should build more mitochondria and more of its own antioxidant enzymes before the next time.

Biologists call this hormesis: a small dose of a stressor that leaves you stronger than before. I prefer a plainer image. The soreness and the oxidative burst are the body taking notes. Mop up the notes too fast, and the body may not get the message.

Four studies, read together

Where antioxidants blunted training, and where they did not

What the human trials found

In 2009, Michael Ristow and colleagues gave 39 healthy young men four weeks of training, five days a week. Half took 1,000 mg of vitamin C and 400 IU of vitamin E a day; half took a placebo. In the placebo group, training made the body more responsive to insulin, and the muscle made more of its own antioxidant enzymes. In the vitamin group, both of those gains failed to appear. The paper’s title said the antioxidants had blocked the health-promoting effects of exercise, and it became one of the most quoted papers in the field.

Five years later, Gøran Paulsen and colleagues ran a longer test: 54 adults, 11 weeks of mostly running, 1,000 mg of vitamin C and 235 mg of vitamin E a day, or placebo. The vitamins blunted the rise in the proteins muscles use to build new mitochondria. But fitness, measured as VO₂max and running performance, improved about the same in both groups.

That split matters. The cells showed a smaller response. The runners, by the tests the study used, did not.

The backlash went too far as well

Here is where the contrarian take needs its own correction. After 2009, "antioxidants cancel your workout" became a fitness rule. The evidence never said that.

In 2020, Tom Clifford and colleagues pooled the randomised trials that gave vitamin C, vitamin E or both alongside at least four weeks of supervised training. Across nine endurance trials, the vitamins did not reduce gains in VO₂max or endurance performance. Across nine strength trials, they did not change gains in lean mass or strength.

So we have two true statements that sound like they disagree. High-dose antioxidants can quiet the cell signals that training produces. And in trials lasting weeks to months, that quieter signal has not shown up as less fitness or less strength. Both can be right. The body has more than one way to get a message through, and a few months may be too short to see a cost that builds slowly, if there is one. We do not yet know which.

Why this matters more as muscle ages

The new paper adds a twist for anyone past middle age. It suggests that some of what we call muscle ageing is a defence. When the mitochondria weaken, the muscle uses oxidative signals to rebuild itself around slower, more resilient fibres. The price is power. We have written before about why power fades faster than strength with age, and this is one possible piece of the mechanism.

If that is right, then blanket suppression of oxidative signals is the wrong instinct in older muscle too. The muscle is not only being damaged. It is negotiating. It needs to hear its own signals to do that well.

That is still a hypothesis built on mice. But it fits the human picture: the body runs its own antioxidant system, it adjusts that system in response to stress, and a flood of outside antioxidants at the wrong time can turn down the adjustment.

What to do with this

None of this means you should fear a bowl of berries. Food carries antioxidants at doses far below the trials above, and none of these studies tested a normal diet. Here is what the evidence does support.

First, stop treating soreness and the post-session burn as something to clean up. They are part of how training works. We made a similar point about cold-water immersion after training, which can also blunt some adaptations to strength work. Not every recovery tool helps you adapt. Some only help you feel better sooner.

Second, if you take high-dose vitamin C or E for other reasons, the meta-analysis suggests your fitness gains are not at serious risk. If you want to hedge anyway, the simple option is to keep high doses away from the hours around your training. That is a precaution based on how the trials dosed, not a tested protocol. Vitamin C contributes to the normal function of the immune system, and at an extra 200 mg a day above the usual intake it contributes to maintaining that function during and after intense physical exercise. Our review of what the vitamin C trials show covers dose and form in more detail.

Third, judge your training by what changes over weeks, not by how you feel the morning after. A slowly falling resting heart rate, and a lower heart rate at the same easy pace, are signs that adaptation is happening. An Agen Band can show those trends. We explain how to read one of them in heart rate recovery after exercise.

The bottom line

This week's paper is a mouse study, and it should be read as one. But it points the same way as two decades of human research: the molecules we were taught to fear are also how a muscle learns. High-dose antioxidants can quiet that learning at the cell level. So far, the trials have not shown that the quieter signal costs you fitness or strength.

The useful lesson is calmer than either camp. Train hard enough to send the message, eat real food, keep the megadoses away from the session if you want to be careful, and measure the result over weeks. If you have a medical condition or take medication, talk to your doctor before you change your supplements. For how this fits a wider routine, start with how to build a longevity protocol.