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Dr. Fuhrman warns:

DO NOT take multivitamins
that contain folic acid.
If you are pregnant,
DO NOT take prenatal vitamins
that contain folic acid.

Folic acid supplementation is dangerous—and may increase breast cancer risk.

Women who take supplemental folic acid may increase their breast cancer risk by 20-30 percent, according to a study in the American Journal of Clinical Nutrition. Researchers collected data on women's folic acid intake from multivitamins over a 10-year period—they found that the women who took multivitamins containing folic acid were more likely to be diagnosed with breast cancer than those that did not.1,2

Another study found that women who took multivitamins regularly were actually morelikely to be diagnosed with breast cancer than those that didn’t take multivitamins after a follow-up of 9.5 years—this observation was attributed to folic acid.3

Also alarming was a study that compared women who took a large dose of folic acid (5 mg) during their pregnancy to those that did not. Thirty years later those women who were given the hefty dose of folic acid during pregnancy were twice as likely to die from breast cancer.4

Folic acid is the synthetic form of folate, a member of the family of B vitamins that is involved with DNA synthesis and DNA methylation, which essentially turns genes on and off. Because of these crucial functions, folate plays important roles in fetal development and nerve tissue health as well as cancer initiation and progression. One study investigating both food folate and synthetic folic acid found that only supplemental folic acid increased breast cancer risk. In other studies, women with low levels of food folate intake were more likely to be diagnosed with breast cancer.2,5,6 Real folate from food is protective, fake folate from supplements is dangerous.

Some studies have shown no effect when examining the relationship between total folate (food plus supplemental folate) and breast cancer risk,7 but results from studies on supplemental folic acid have suggested significant cancer risks.1,3,4,8,9 Folate is considered a “double-edged sword” by many scientists: preventing deficiency is beneficial, but excess folic acid from supplements could be risky.10-12 This information is very important for women, especially since folate is so readily available in cancer-preventive foods, such as green vegetables and beans.

Men should be cautious about folic acid too. A 2011 meta-analysis of six folic acid supplementation trials (including both men and women) found that the incidence of all cancers was 21% higher in the folic acid supplementation groups.8 Results of a randomized controlled trial on folic acid supplements reported a 163 percent increase in prostate cancer risk in the group of men taking folic acid supplements.13

A meta-analysis of folic acid supplementation and colorectal cancer risk found that those who took folic acid for more than three years increased their risk of having a colorectal adenoma by 35%.14 In the U.S., Canada, and most recently Chile, colorectal cancer rates have climbed since the introduction of mandatory fortification of flour with folic acid.15,16

There have also been reports of potential negative effects on children due to folic acid supplementation during pregnancy. One study found that folic acid supplementation by pregnant women was linked to a 26 percent increase in the risk of childhood asthma.17 A second study linked folic acid supplementation during pregnancy to increased incidence of respiratory tract infections in infants, especially those resulting in hospitalization.18

In Norway, where there is no fortification of flour with folic acid, researchers conducting a six-year study on the homocysteine-lowering effects of B vitamins in patients with heart disease made an unexpected finding: the patients whose supplement included folic acid had a greater risk of cancer incidence and cancer mortality. These patients were 43% more likely to die from cancer.9

If folic acid might have these dangerous effects, why is it included in most multivitamins, prenatal vitamins and fortified grain products?

The protective effects of folate against neural tube defects (NTDs) have received much attention in the past. Unfortunately, this knowledge and public attention did not result in a campaign by the U.S. government encouraging women to get plenty of natural dietary folate from vegetables—instead, pregnant women are pushed to take folic acid supplements, and white flour is fortified with folic acid.

Folic acid is chemically different from dietary folate, which results in differences in uptake and processing of these two substances by the cells in the intestinal wall. Some folic acid is chemically modified to be more similar to natural folate, but the intestinal cells are limited in how much folic acid they can modify—excess folic acid often enters the circulation unmodified. Scientists do not yet know the implications of circulating synthetic folic acid. Many Americans, through multivitamin use and consumption of fortified foods, are taking in excessive amounts of folic acid, and thus may have unmodified folic acid circulating in their blood—this could contribute to the cancer-promoting effects.15,16,19

The recommendation that pregnant women take folic acid supplements is especially troubling—these women could safely increase their folate status and prevent neural tube defects (NTDs) by eating green vegetables and other healthy produce regularly before pregnancy, but instead they are instructed to take folic acid supplements, after they find out they are pregnant, which is not only not very effective at preventing birth defects, but could also put them at risk for breast cancer later in life. The children are also put at risk—women who take folic acid supplements as a substitute for good nutrition fail to provide their unborn children with other critically protective nutrients in folate-containing foods. Maternal nutrition is a critical determinant of childhood health—there are inverse associations between maternal vegetable intake and childhood cancers.20-23 That means that informing and insisting on the consumption of adequate intake of folate from vegetables, is where the attention should be focused, thus preventing not just birth defects, but also decreasing the occurrence of childhood cancers. Plus, the protection against neural tube defects is related to the folate intake before pregnancy. Folic acid supplement have to be taken before pregnancy and during the first 4 weeks (before most women even know they are pregnant) to be effective. Taking a folic acid supplement once you find out you are pregnant is almost worthless to prevent birth defects anyway. The point is there is no substitute for eating your vegetables all through life, and if you are eating healthfully, your body will store enough folate, even during a period of weeks with morning sickness and minimal food intake. Eating healthfully before pregnancy decreases the occurrence of vomiting during early pregnancy. Morning sickness usually occurs after the first 6 weeks of pregnancy, after the most vulnerable point for neural tube formation, in other words, eating right is the answer; the only sensible option for the informed women of child-bearing age and interest.

Unlike synthetic folic acid, folate obtained from food sources—especially green vegetables—protects against cancer.

Paradoxically, in people who do not take folic acid supplements there is inverse relationship between dietary folate intake and breast and prostate cancer.5,13 Folate is an essential nutrient with vital functions. It is probable that folate levels need to be tightly regulated by the body—that the timing and dose of folate is an important determinant of whether folate has positive or negative effects. Folate's actions on DNA may prevent cancer from initiating, but may also promote the proliferation of tumor cells that may already present.10,24 Even supplementing with real folate does not make sense for healthy eaters. Luckily, getting our folate exclusively from food ensures that we do not get too much. It comes naturally packaged in balance with other micronutrients and the body regulates its absorption.19

Rich sources of food folate

As a reference point, the U.S. RDI for folate is 400μg. Below is the approximate folate content of some common plant foods.8

Asparagus, 2 cups cooked

486 μg


Black beans, ½ cup cooked

156 μg

Mustard greens, 2 cups cooked

402 μg

Summer squash, 3 cups

100 μg

Edamame, 1 cup cooked and shelled

358 μg

Avocado, 1

121 μg

Collards, 2 cups cooked

354 μg

Pomegranate, 1

107 μg

Broccoli, 2 cups cooked

337 μg

Artichoke, 1

107 μg

Adzuki beans, ½ cup cooked

139 μg

Papaya, 1 cup

53 μg

Chickpeas, 1 cup cooked

282 μg

Orange, 1

39 μg

Lentils, 1 cup cooked

178 μg

Hazelnuts, ¼ cup

38 μg

Cauliflower, 1 small head

225 μg

Blackberries, 1 cup

36 μg

Romaine lettuce, 3 cups raw

192 μg

Walnuts, ¼ cup

29 μg

Spinach, 3 cups raw

175 μg

Sunflower seeds, ¼ cup

26 μg25

All produce contains folate. Clearly, we do not need folate or folic acid supplements to meet our folate needs. If you eat healthfully, you get plenty, and if you don’t, then taking supplements cannot protect you from diseases caused by the lack of produce in your diet. 

To learn more about folate, folic acid and cancer risk, read Dr. Fuhrman’s position paper “Folate vs. Folic Acid.

1. Stolzenberg-Solomon RZ, Chang SC, Leitzmann MF, et al. Folate intake, alcohol use, and postmenopausal breast cancer risk in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. Am J Clin Nutr 2006, 83:895-904.
2. Kim YI. Does a high folate intake increase the risk of breast cancer? Nutr Rev 2006, 64:468-475.
3. Larsson SC, Akesson A, Bergkvist L, Wolk A. Multivitamin use and breast cancer incidence in a prospective cohort of Swedish women. Am J Clin Nutr 2010, 91:1268-1272.
4. Charles D, Ness AR, Campbell D, et al. Taking folate in pregnancy and risk of maternal breast cancer. BMJ 2004, 329:1375-1376.
5. Sellers TA, Kushi LH, Cerhan JR, et al. Dietary folate intake, alcohol, and risk of breast cancer in a prospective study of postmenopausal women. Epidemiology 2001, 12:420-428.
6. Gong Z, Ambrosone CB, McCann SE, et al. Associations of dietary folate, Vitamins B6 and B12 and methionine intake with risk of breast cancer among African American and European American women. Int J Cancer 2014, 134:1422-1435.
7. Zhang YF, Shi WW, Gao HF, et al. Folate intake and the risk of breast cancer: a dose-response meta-analysis of prospective studies. PLoS One 2014, 9:e100044.
8. Baggott JE, Oster RA, Tamura T. Meta-analysis of cancer risk in folic acid supplementation trials. Cancer Epidemiol 2011.
9. Ebbing M, Bonaa KH, Nygard O, et al. Cancer incidence and mortality after treatment with folic acid and vitamin B12. JAMA 2009, 302:2119-2126.
10. Smith AD, Kim YI, Refsum H. Is folic acid good for everyone? Am J Clin Nutr 2008, 87:517-533.
11. Kim YI. Folate: a magic bullet or a double edged sword for colorectal cancer prevention? Gut 2006, 55:1387-1389.
12. Ulrich CM. Folate and cancer prevention: a closer look at a complex picture. Am J Clin Nutr 2007, 86:271-273.
13. Figueiredo JC, Grau MV, Haile RW, et al. Folic acid and risk of prostate cancer: results from a randomized clinical trial. J Natl Cancer Inst 2009, 101:432-435.
14. Fife J, Raniga S, Hider PN, Frizelle FA. Folic Acid Supplementation and Colorectal Cancer Risk; A Meta-analysis. Colorectal Dis 2009.
15. Hirsch S, Sanchez H, Albala C, et al. Colon cancer in Chile before and after the start of the flour fortification program with folic acid. Eur J Gastroenterol Hepatol 2009, 21:436-439.
16. Folic-Acid Fortification of Flour and Increased Rates of Colon Cancer [http://www.medscape.com/viewarticle/591111 ]
17. Whitrow MJ, Moore VM, Rumbold AR, Davies MJ. Effect of supplemental folic acid in pregnancy on childhood asthma: a prospective birth cohort study. Am J Epidemiol 2009, 170:1486-1493.
18. Haberg SE, London SJ, Stigum H, et al. Folic acid supplements in pregnancy and early childhood respiratory health. Arch Dis Child 2009, 94:180-184.
19. Harvard School of Public Health; The Nutrition Source: Keep the Multi, Skip the Heavily Fortified Foods [www.hsph.harvard.edu/nutritionsource/what-should-you-eat/folicacid/ ]
20. Kwan ML, Jensen CD, Block G, et al. Maternal diet and risk of childhood acute lymphoblastic leukemia. Public Health Rep 2009, 124:503-514.
21. Tower RL, Spector LG. The epidemiology of childhood leukemia with a focus on birth weight and diet. Crit Rev Clin Lab Sci 2007, 44:203-242.
22. Petridou E, Ntouvelis E, Dessypris N, et al. Maternal diet and acute lymphoblastic leukemia in young children. Cancer Epidemiol Biomarkers Prev 2005, 14:1935-1939.
23. Jensen CD, Block G, Buffler P, et al. Maternal dietary risk factors in childhood acute lymphoblastic leukemia (United States). Cancer Causes Control 2004, 15:559-570.
24. Kim YI. Folic acid fortification and supplementation--good for some but not so good for others. Nutr Rev 2007, 65:504-511.
25. NutritionData.com: Nutrient Search Tool [http://www.nutritiondata.com/tools/nutrient-search ]

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