Many Americans believe in the disease-preventive, life-extending powers of supplemental vitamins.
Their faith flows from the many studies linking diets high in vitamin- and mineral-rich foods to lower rates of heart disease and some major cancers.
However, epidemiological studies cannot prove a cause-effect relation between diet and health.
And recent years witnessed release of several epidemiological studies and evidence reviews that found vitamins unhelpful … even harmful in certain contexts.
These studies focused mostly on the body's antioxidant vitamins – C, E, A, beta-carotene – and the essential antioxidant mineral, selenium.
Tara Parker-Pope of The New York Times offered a succinct summary of those findings in “News Keeps Getting Worse for Vitamins> http://well.blogs.nytimes.com/2008/11/20/news-keeps-getting-worse-for-vitamins/<.”
The evidentiary pendulum swings back to positive
Just one year after the string of negative studies, researchers reported generally positive outcomes from four studies, as described in “Vitamin Studies Paint a More Positive Prevention Picture>/shop/pc/articlesView_old.asp?id=861<”.
While one study found no evidence that that multivitamins protect women from cancers and heart disease, the others found that multivitamins may lower heart disease death risk, that multivitamin users gain “younger” cells, and that supplement users may have better lung-cancer survival rates.
Since then, the pendulum has continued to swing back and forth, with the publication of evidence for (Qiao YL et al. 2009) or against (Bjelakovic G et al. 2012) the presumed benefits of antioxidant vitamins and minerals.
And it just took another swing in favor of the idea that antioxidant vitamins and minerals serve as potent health-protectors.
Antioxidant vitamins linked to less cancer, longer life
Analysts who compared the diets of 16,008 Americans to their health outcomes report more good news about diets rich in antioxidant vitamins (Goyal A et al. 2013).
The news comes from the Columbia University's School of Public Health, and appears in a journal of the American Association for Cancer Research (Cancer Epidemiology, Biomarkers and Prevention).
The Columbia team examined more than 14 years of blood and health data collected from 16,008 Americans participating in the Third National Health and Nutrition Examination Survey (NHANES III).
They divided the participants into “quintiles” (fifths), according to their blood levels of antioxidant vitamins – C, E, beta-carotene, selenium – and vitamin A.
For each nutrient, the fifth of participants with the lowest blood levels were placed in quintile 1 (Q1).
Likewise, for each nutrient, the fifth of participants with the highest blood levels were placed in quintile 5 (Q5).
The remaining three-fifths fell into Q2, Q3, or Q4 of a given nutrient, depending on their blood level.
The Columbia scientists compared each quintile to its' members' health outcomes over an average of 14.2 years.
Echoing the findings of many prior analyses of epidemiological data, the new study linked medium-to-high blood levels of vitamins A, C, and E and selenium to reduced risk of cancer or death.
The researchers noted that the beneficial effect seen with each nutrient was not necessarily a “linear” or “dose-response” effect.
That is, some nutrients seemed beneficial at low or medium blood levels, while some looked healthier at higher levels:
- Vitamin C reduced cancer and overall death risks more as the levels grew higher, in a linear, dose-response fashion.
- Vitamin A was linked to reduced risk of cancer death only at the lower blood levels of people in quintiles 1 and 2, with no further decline in risk seen at higher levels.
- Vitamin E yielded the lowest risk of cancer at the high blood levels seen in Q4.
- Vitamins A and E seemed to reduce death risk at moderate levels, but not at low or high levels.
- The risk of dying from cancer dropped in the lower, Q1-Q2 blood levels of beta-carotene and at low-to-high (Q1-Q4) levels of selenium.
- Beta-carotene and selenium were lined to lower death risk at levels Q1-Q2 but did not change significantly at higher levels (i.e., Q3-5).
These findings may seem confusing, but the results generally support three key concepts.
First, foods remain the best and most fully healthful sources of vitamins and minerals.
Second, the majority of evidence supports the idea that supplemental vitamins and minerals are beneficial … in ample but moderate doses.
Finally, few Americans get enough of every essential nutrient they need – vitamins, minerals, and omega-3 fatty acids – from foods alone.
So it makes sense to take supplemental vitamins, minerals, and omega-3s in amounts justified by the evidence – which are generally reflected in the U.S. RDAs – just don't go overboard!
Reasons for skepticism about bad vitamin news
Some critics of negative study results noted that supplemental nutrients often come in forms slightly different from those found in foods (e.g., synthetic vitamin E).
Other critics noted that people who take multivitamins are generally healthier and more likely to engage in healthy habits … therefore likely to benefit, compared with people eating nutrient-poor diets.
And many of the extant clinical and epidemiological studies involved people already suffering from major diseases.
Sadly, we still lack the credibly strong and lengthy clinical trials needed to reliably prove or disprove the presumed preventive health benefits of supplemental vitamins and minerals.
Finally, faulting antioxidant vitamins for failing to cure existing disease misses two key points:
Vitamins aren't drugs, and their anti-aging, disease preventing effects accumulate over time.
- Bjelakovic G, Nikolova D, Gluud LL, Simonetti RG, Gluud C. Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database Syst Rev. 2012 Mar 14;3:CD007176. doi: 10.1002/14651858.CD007176.pub2. Review.
- Eichholzer M, Stähelin HB, Gey KF. Inverse correlation between essential antioxidants in plasma and subsequent risk to develop cancer, ischemic heart disease and stroke respectively: 12-year follow-up of the Prospective Basel Study. EXS. 1992;62:398-410.
- Goyal A, Terry MB, Siegel AB. Serum Antioxidant Nutrients, Vitamin A, and Mortality in US Adults. Cancer Epidemiol Biomarkers Prev. 2013 Aug 8. [Epub ahead of print]
- Lin J, Cook NR, Albert C, Zaharris E, Gaziano JM, Van Denburgh M, Buring JE, Manson JE. Vitamins C and E and beta carotene supplementation and cancer risk: a randomized controlled trial. J Natl Cancer Inst. 2009 Jan 7;101(1):14-23. doi: 10.1093/jnci/djn438. Epub 2008 Dec 30.
- Qiao YL, Dawsey SM, Kamangar F, Fan JH, Abnet CC, Sun XD, Johnson LL, Gail MH, Dong ZW, Yu B, Mark SD, Taylor PR. Total and cancer mortality after supplementation with vitamins and minerals: follow-up of the Linxian General Population Nutrition Intervention Trial. J Natl Cancer Inst. 2009 Apr 1;101(7):507-18. doi: 10.1093/jnci/djp037. Epub 2009 Mar 24. Erratum in: J Natl Cancer Inst. 2010 Jan 20;102(2):140.