Skeptics Stack Exchange is a question and answer site for scientific skepticism. It only takes a minute to sign up.
Sign up to join this community
I recently read an article on Yahoo (but have seen them other places) that claim antioxidants are very important and have health benefits. One article even talked about antioxidants ability to prevent strokes (which I find hard to believe). I've even heard about the mixing of certain antioxidants providing greater benefits.
Is there any real scientific research that support these claims?
This is a very interesting question, that has been extensively studied and has received great attention. As such, it has often been vandalized by generic press, leaving people quite confused on the matter.
That antioxidants have physiological effects I would say needs no proof: our body utilises antioxidants such as ascorbate (vitamin C) to keep down the amount of free radicals, as can be confirmed by any basic physiology textbook (I can provide references if you need).
To get to the actual point of your question: do antioxidants have the "ability to prevent strokes"? I don't know if the article you read specifically used those words, but this is the type of bad "generic press talk" I was referring to... Antioxidants do not prevent stroke. You can live a super-healthy life, take tons of antioxidants, exercise every day and still have a stroke.
What certain antioxidants can do is to reduce the possibility of having a stroke, which is a very different thing. This is mostly due to the fact that oxidative stress is a risk factor for different cardiovascular diseases.
In the same line of thought, certain antioxidants, in certain regimes and certain situations can help fighting cancer. They DO NOT CURE cancer by themselves, I cannot stress this enough. Again, the idea is that free radicals can damage DNA increasing the risk of cancer. Free radical do not provoke cancer, but they can be a risk factor.
Same for neurodegenerative diseases and many other illnesses where antioxidants can help in certain cases.
It is difficult to generalize because stroke, cancer, neurodegenerative diseases are very broad category of illnesses, that can have the most disparate etiologies. So free radicals are not always the issue and antioxidants are not always the solution. Also, free radicals are actively used by our bodies (e.g. during immune responses) so excess antioxidants is not necessarily good.
The scientific literature at this regard is simply huge and it would be difficult to have a complete bibliography here. Here's a sample of papers and reviews at this regard, if someone who is more in the field knows better papers/reviews please feel free to update my answer (sorry most of these are limited access papers).
Air pollution: mechanisms of neuroinflammation and CNS disease. - Block and Calderón-Garcidueñas, Trends Neurosci., 2009
Oxidative stress and endothelial dysfunction in cerebrovascular disease. - Chrissobolis et al, Front Biosci., 2011
Oxidative DNA damage in mild cognitive impairment and late-stage Alzheimer's disease. - Lovell and Markesbery, Nucleic Acids Res., 2007
Mitogenic signaling mediated by oxidants in Ras-transformed fibroblasts. - Irani et al, Science 1997
c-Myc can induce DNA damage, increase reactive oxygen species, and mitigate p53 function: a mechanism for oncogene-induced genetic instability. - Vafa et al, Mol Cell 2002
and there is many many more.
Not all antioxidants are good, however. For instance, this paper talks about the role of the transcription factor Nrf2, a factor which is involved in the physiological control of free radicals (so we can, by extension, call it an antioxidant inducing factor). The authors show that excess activation of Nrf2 does actually induce tumorigenesis rather then prevent it.
Oncogene-induced Nrf2 transcription promotes ROS detoxification and tumorigenesis. - DeNicola et al., Nature 2011
Antioxidants is a fairly broad class of substances. Depending on what type of stroke and what type of Antioxidant you will get different answers:
In terms of absolute risk, this translates into one additional haemorrhagic stroke for every 1250 individuals taking vitamin E, in contrast to one ischaemic stroke prevented per 476 individuals taking vitamin E.
On the matter of finding it hard to believe that a random substance has a physiological effect: Adjust your priors. A lot of substances that physiological effects that change how the body reacts to various illnesses.
Antioxidant supplements have either no measurable effect or actually increase mortality
Vitamin C and selenium seem harmless but don't reduce mortality; Vitamins A, E and beta-carotene increase mortality
There have been many studies of the effects of antioxidant supplements on health. Some of these studies use fairly dubious techniques and study designs but others are better. The best summary of the collective literature is the Cochrane review published in 2012.
We have found no convincing evidence that antioxidant supplements decrease mortality. Even more, beta‐carotene and perhaps, vitamin E and vitamin A seem to increase mortality. Therefore, we cannot recommend the use of antioxidant supplements as a primary and secondary preventive measure in the population groups studied in the present review.
The result comes from about 75 trials with good designs that reported all cause mortality selected from nearly 600 with bias, poor design or inadequate reporting of the mortality. The studies covered:
We considered for inclusion trials that compared antioxidant supplements (that is, beta-carotene, vitamin A, vitamin C, vitamin E, and selenium) at any dose, duration of treatment, and route of administration versus placebo or no intervention.
The results:
Our systematic review contains a number of findings. Beta-carotene, vitamin A, and vitamin E given singly or combined with other antioxidant supplements significantly increased mortality. There is no evidence that vitamin C or selenium may increase longevity. We confirmed that trials with inadequate bias control significantly overestimate intervention effects. The detrimental effect of antioxidant supplements became significantly more pronounced when we excluded all trials with potential confounding. Our findings support and extend our previous findings...
So the trials done well suggest either no obvious effect for some supplements or a small bad effect for others. And as more evidence arrives the results become clearer.
Clarification for context
The normal context for claims about supplements and for trials is normal people eating a normal diet. These people do not, mostly, have any deficiencies in their dietary intake of key vitamins, mineral or antioxidants (these categories overlap). The claims made for supplements usually argue that extra intake of the compounds over and above the normal dietary levels provide extra benefits. There is no argument that supplements are useful if your diet is deficient. So the referenced studies don't address the benefits of the compounds in general, just the benefits over and above the normal intake.
And the high quality studies are clear: extra intake of some supplements seems harmless but some are actively harmful.
Also note that the accepted answer for this question provides some theoretical justification for why some supplements might work but completely fails to address the question of whether they do work which makes the answer not just useless but actively misleading.
Of course, antioxidants have important health benefits and some of them (vitamins A, C and E and selenium) are actually essential nutrients for humans. But when you have normal levels of antioxidants in your body, taking additional antioxidants won't likely help in prevention or treatment of cardiovascular or other diseases.
IN SUMMARY:
NO EVIDENCE OF BENEFITS:
1) According to National Center of Complementary and Integrative Health, 2016):
Rigorous scientific studies involving more than 100,000 people combined have tested whether antioxidant supplements can help prevent chronic diseases, such as cardiovascular diseases, cancer, and cataracts. In most instances, antioxidants did not reduce the risks of developing these diseases.
There is no evidence to support the use of vitamin and antioxidant supplements for prevention of cardiovascular diseases.
Eight trials with a total of 107874 participants met the inclusion criteria...Pooled results from single antioxidant comparison indicated that vitamin E had no significant effect on the development and progression of nuclear (RR, 0.95; 95% CI, 0.87-1.02; p = 0.18) and cortical (RR, 0.93; 95% 0.81 - 1.06; P = 0.24) age related cataract. Also, the result was not in favor of beta carotene except among current smokers (RR, 0.75; 95% CI, 0.60-0.94; p = 0.01). Broad spectrum antioxidant (Centrum) was effective only on nuclear age related cataract. The combined antioxidant comparison of riboflavin plus niacin versus no vitamins resulted in highly protective and significant effect (OR, 0.59; CI, 0.45 - 0.79; p = <0.001). However, other combination of any of the following: retinol, zinc, ascorbic acid, molybdenum, selenium, alpha-tocopherol, beta carotene from the Sperduto and AREDS trials yielded no effect on the development and deceleration of age related cataract.
4) The already mentioned review in Cochrane, 2012 has not found any association between antioxidant supplements and disease prevention.
5) According to Cochrane, 2017, antioxidants do not likely help to prevent or reduce muscle soreness after exercise.
SOME EVIDENCE OF HARMS:
1) According to National Center of Complementary and Integrative Health, 2016, supplementing with high doses of beta-carotene may increase the risk of lung cancer in smokers and supplementing with high doses of vitamin E may increase risks of prostate cancer and one type of stroke.
2) According to Lancet, 2004, high intake of antioxidant supplements was associated with increased overall mortality.
3) According to a review of 47 trials in JAMA, 2007, high intake of vitamin A, beta carotene and vitamin E, but not vitamin C and selenium, supplements was associated with increased mortality.
An observation from the above studies: Water soluble antioxidants (vitamin C and selenium) that are readily excreted when consumed in excess, were not associated with harmful effects, while lipid-soluble ones (vitamin A and E and beta carotene) that accumulate in the body, were.
SOME EVIDENCE OF BENEFITS FROM FOODS (NOT SUPPLEMENTS):
According to Advances in Nutrition, 2018, diet (not supplements) high in antioxidants can reduce the risk of overall mortality:
The present study indicates that adherence to a diet with high antioxidant properties may reduce the risk of all-cause mortality. Our results confirm current recommendations that promote higher intake of antioxidant-rich foods such as fruit and vegetables.
It is not clear if vegetables and fruits are benefical due to their high antioxidant content or due to other substances.
