Two years ago, I wrote an article for Freelap on whether antioxidant supplementation was a good idea for athletes. I presented information that illustrated how long-term supplementation with high doses of antioxidants likely has a negative effect on training outcomes, reducing exercise-induced adaptations, and potentially negatively impacting recovery speed. My conclusion was that, on the whole, athletes should probably avoid antioxidant supplementation during training phases.
A few weeks ago, I was at a high-level sports club, discussing with them changes they could make in their training. One of the things I was asked about was antioxidant supplementation, and my advice was that, generally, it was best to stay clear from both a health and performance perspective. As I left that meeting, I felt a bit uneasy. I hadn’t presented the information with all the correct context and, perhaps more worryingly, I was incredibly sure that I was correct. I decided a few years ago that whenever I’m sure I’m correct, I need to revisit the research to ensure that biases haven’t crept in. This article is the result of me revisiting this information, and it therefore acts as an update to my article from 2015.
What Are Antioxidants?
When we exercise, we produce reactive oxygen species (ROS)—often referred to as free radicals—as a consequence of metabolism within the mitochondria, and also muscle contraction. These ROS are highly reactive, so they can damage structures they come into contact with, such as the walls of our cells, and also interfere with normal cellular function. This, in turn, contributes to fatigue, immune dysfunction, and muscle damage, all of which are harmful to sports performance. Antioxidants help to buffer these ROS, reducing the damage they can cause and mitigating the increases seen in fatigue and immune dysfunction after exercise.
There are two main types of antioxidants: endogenous and exogenous. The endogenous antioxidants are produced by our body, which has evolved over the years a very competent system to mitigate oxidative stress. Exogenous antioxidants come from the diet. They include, but aren’t limited to: vitamins A, C and E; selenium; and two classes of nutrients known as polyphenols and flavonoids. Exogenous antioxidants can come from food sources, or from dietary supplements.
A ‘Health First’ Approach
Having discussed what antioxidants are, let’s take a look at whether supplementation with antioxidants is a good idea. To begin with, I’ll approach this from a health perspective: Although it’s easy to forget, athletes are also people, and their lives will last far longer (hopefully) than their sporting career. To that end, it’s important (in my opinion), to take a “health first” perspective. An unhealthy athlete won’t perform to their potential, and an unhealthy athlete might have 70 years after the end of their career for their health to further suffer. My first question, then, is: Do antioxidant supplements improve our health?
To answer this question, I will mostly look at meta-analyses and review articles. Meta-analyses combine the data from a number of studies, and so give a good idea of the overall research in a particular area. The first meta-analysis I want to introduce comes from 2007, and is comprised of 232,606 subjects, which is a lot of participants. The aim of the study was to see whether antioxidant supplements had any effect on mortality. Overall, the results indicated that these supplements had no effect on mortality—i.e., they were neither positive nor negative. Further analysis of the trials allowed the researchers to separate those at low risk of bias. In doing so, the data indicated that beta carotene, vitamin A, and vitamin E, either together or individually, increased the risk of death. Vitamin C had no significant effect.
An earlier study looking specifically at gastro-intestinal cancers found that antioxidant supplementation did not prevent these cancers, but instead appeared to increase their incidence. A 2005 paper found that high-dose vitamin E supplementation might increase risk of death from all causes, and should be avoided. This more or less replicated the results from a 2003 meta-analysis. Another meta-analysis found no effect of antioxidant supplementation on cancer incidence, unless you were a smoker—in which case beta-carotene supplementation increased your cancer risk.
So antioxidant supplementation appears to be at best neutral, and quite possibly negative in regards to health. However, it is also well-established that low levels of antioxidants within the blood are associated with increased risks of death. For example, an older study from 1991 conducted on almost 3,000 men in Switzerland found that overall cancer mortality was associated with lower plasma levels of carotenes and vitamin C, such that being in the lowest quartile for antioxidant intake could increase disease risk by almost a factor of three. Higher plasma levels of lycopene, an antioxidant found in tomatoes, are associated with a decreased risk of prostate cancer.
This is curious; we clearly need antioxidants within our bloodstream to keep us healthy, but antioxidant supplementation appears to be unhealthy, or at best neutral. Why is this? Well, as with most things, the dose makes the poison. Antioxidant supplements tend to contain doses of antioxidants far higher than what would naturally be found. For example, in one of the vitamin E studies above, it was high-dose vitamin E supplementation that was unhealthy. The dose in this case was 400IU of vitamin E, which is the equivalent of around 1kg of almonds or spinach—I’m guessing you don’t eat that much in one sitting, or even in one day.
Supra-physiological doses of antioxidants, in the form of supplements, appear to be at best neutral in terms of health. However, antioxidants from natural sources appear to be healthful, in part because the doses are kept low. In addition to this, antioxidants from food often come with complementary nutrients, which can synergistically work to improve health. High-dose antioxidant supplements often come with few additional nutrients, which in turn can increase the amount of ROS present in the body, causing further damage—as illustrated by the potential increase in mortality seen in the high-dose antioxidant supplementation trials. This is further evidenced by the protective effect of higher intakes of vegetables and fruits (foods that contain the greatest amount of antioxidants) on both cancer and all-cause mortality risk. (Studies one, two, three, four, five, six, and seven.)
Supra-physiological doses of antioxidants, in the form of supplements, appear to be at best neutral in terms of health. However, antioxidants from natural sources appear to be healthful, in part because the doses are kept low.
Bringing this section to a close, we can conclude the following:
- Antioxidant supplementation, particularly at high doses, appears to be at best neutral, and at worst negative for health.
- High vegetable and fruit intake appears to be at worst neutral, and very likely beneficial for health.
- Therefore, it seems appropriate to recommend that, for most people, antioxidant supplementation should be avoided, and higher intakes of fruits and vegetables should be recommended.
Do Athletes Need a Greater Intake of Antioxidants?
Having looked at general health, the next step is to examine antioxidant requirements of athletes, who are engaged in regular physical activity. It would be tempting to assume that, because exercise increases the amount of oxidative stress, athletes require a greater amount of antioxidants to buffer this. But is that correct?
The evidence tends to suggest that exercise, both through skeletal muscle contraction and also cellular respiration, does increase the amount of ROS that form. Whether this is positive or negative isn’t all that clear. If the body cannot buffer these ROS, they will cause damage, and the more prolonged or intense the exercise, the greater the damage that occurs. However, ROS also serve as important signals for adaptation. They signal for an increase in gene expression, for example, and mediate many of the adaptations following exercise, particularly those that occur within the mitochondria. Exercise itself, and the ROS that form during it, also increase the capacity of antioxidant enzymes, such that individuals who are engaged in regular exercise are better at dealing with oxidative stress than sedentary individuals.
We can consider that, while exercise promotes oxidative stress, this oxidative stress is crucial for adaptation, and one of the adaptations that occurs following exercise is a greater ability to buffer oxidative stress. It’s not entirely clear, therefore, whether athletes do need an increased amount of antioxidants in order to support exercise training. It does appear logical, given that exercise increases oxidative stress, that a greater intake of antioxidant nutrients would support exercise recovery. Indeed, that’s what a number of different papers have found, such as this one.
So Athletes Should Take Antioxidant Supplements, Right?
This is where things start to get interesting. Remember that I previously mentioned that oxidative stress is an important stimulator of exercise adaptations. Well, it follows that when taking in high doses of antioxidants, these signals will be blunted. This is why a number of research papers find that high-dose antioxidant supplementation decreases exercise-induced adaptations.
For example, one study showed that 1g per day of vitamin C (the equivalent of around 14 oranges), reduced some of the beneficial effects of aerobic training. High-dose vitamin C and E supplementation has also been shown to reduce the health benefits of exercise. It’s worth pointing out that this isn’t always what is found; for example this study found that antioxidant supplementation altered muscle signalling pathways after training, but had no effect on actual exercise performance. Note, however, that antioxidant supplementation did not improve training adaptations, again indicating that it is perhaps, at best, neutral.
The two biggest review articles on the subject shed some further light on the evidence. The first of these, published in 2011, concludes that the consistent research finding is that antioxidant supplementation reduces exercise-induced oxidative stress, but that high-dose supplementation blunts exercise-induced adaptations. A second review, from 2014, adds further context. Here, the authors examined 12 studies published between 2006 and 2013. Of these 12, seven reported no effect (positive or negative) on exercise adaptation. Two reported that antioxidant supplementation reduced exercise adaptation, with two showing the opposite—that antioxidant supplementation improved exercise adaptation. The last study only contained partial results.
The interesting aspect from this second review article was the different dosages between the trials. When antioxidant supplementation occurred at high doses (e.g., 1000mg vitamin C, 400IU vitamin E), exercise adaptation was more likely to be reduced. When it was at low doses (e.g., 200mg of vitamin C and 30mg vitamin E), it was more likely to support exercise adaptations. The key thing here, for me, is that these low dosages are easily achievable from food. For example, 200mg vitamin C is around 100g of sweet pepper and 100g of broccoli.
Based on the findings of these research papers, and others like them, we can conclude that:
- High-dose antioxidant supplementation is at best neutral, and potentially negative, when it comes to training induced adaptations.
- Low doses of antioxidants, such as those found in food, appear to have a potentially positive, and at worst neutral, effect on training induced adaptations.
- Therefore, it seems appropriate to recommend that athletes consume the vast majority of their antioxidants from food, which will protect against high-dose intakes of isolated antioxidants.
The Importance of Context
And that was more or less where I left my previous article on this subject—stating that high-dose antioxidant supplementation probably wasn’t a good idea for athletes during training. Based on what I’ve written about so far, I could reasonably conclude that high-dose antioxidant supplementation probably isn’t a good idea for any healthy individual. However, that ignores the context of that person. While chronic antioxidant supplementation potentially reduces exercise adaptation, sometimes in sport we’re not looking to adapt to exercise. For example, in competition, the only goal is performance, not adaptation. And research tends to indicate that, in a competitive setting, antioxidant supplementation likely does have a beneficial effect on performance.
To demonstrate this, a review article from 2015 found that supplementation with antioxidants such as vitamin E, quercetin, resveratrol, beetroot juice, polyphenols, and n-acetyl-cysteine all potentially have performance-enhancing effects on endurance exercise performance. A second review from 2017 found that polyphenol supplements, especially quercetin, could improve performance by almost 3% when taken for seven days before competition, which is far from trivial. This all makes sense: Exercise does increase ROS, and they can cause damage that can reduce exercise performance. While this damage is useful in terms of exercise adaptation, it is a negative if it reduces exercise performance acutely.
This nicely demonstrates the importance of context. When looking to adapt, antioxidant supplements are likely unnecessary, but when looking to compete—when absolute performance is important—antioxidant supplementation may have a role to play. When taken for short periods of time, it seems unlikely that these supplements will have a negative effect on health.
While chronic antioxidant supplementation potentially reduces exercise adaptation, sometimes in sport we’re not looking to adapt to exercise. For example, in competition, the only goal is performance, not adaptation.
During my athletic career, there were other occasions when I was directed to supplement with antioxidants. One was before the Beijing Olympics in 2008, when the medical team was concerned that the high levels of atmospheric pollutants and smog might cause us negative health consequences. Research does tend to suggest that acute antioxidant supplementation might protect against environmental pollutants, and given the pre-competition timing and short duration of supplementation, this might be a beneficial practice.
Other occasions were during high risk periods of immune-suppression, such as post-competition, or during air travel (the two often come together). Here, the research is perhaps a bit less clear: A recent consensus statement seems to indicate that antioxidant supplements have no effect on immune function, and this is supported by research in non-athletic populations. However, being pragmatic, there once again appear to be no negative side effects to this if it occurs infrequently and for a short time period.
Another situation in which there might be a role to play for antioxidant supplementation is when an athlete is restricting food in an attempt to lose weight, as they may be unable to consume sufficient nutrients from the foods they are eating. The same is true when athletes travel outside their normal environment, where nutrient-dense foods may be scarce.
We’ve examined research looking at the effects of chronic antioxidant supplementation on health and exercise adaptations, where the conclusion appears to be that these supplements are at best neutral, and potentially negative. We’ve also looked at acute supplementation of antioxidants, and we’ve seen that there is a potentially beneficial effect of these supplements on performance. There are also a few special cases, such as travel, where there might be a role to play for these supplements.
While the nutrient class of antioxidants has an important role to play in health, it is not a case of more is better. It appears that the doses found naturally within a diet high in fruits and vegetables are sufficient for most athletes, most of the time. Athletes should focus on consuming a wide range of fruits and vegetables of many different colors. These foods often contain complementary nutrients that work synergistically to improve health and performance, and don’t occur at doses high enough to cause issues. By following a diet high in the food sources of these nutrients, along with targeted periods of antioxidant supplementation, athletes can enhance both training adaptation and performance, without sacrificing health.