Many women are familiar with folate, a B vitamin that's vital to a healthy pregnancy. Getting enough of the nutrient – found in spinach, broccoli, lentils and oranges – reduces the risk of neural tube defects, birth defects of the brain and spinal cord.
But it's not only moms-to-be who should be minding their folate. According to a new study, a folate-rich diet also helps guard against colorectal cancer.
What's more, the study did not find that very high intakes of folate spiked colorectal cancer risk as some experts have worried over the past 10 years.
There's compelling evidence from observational studies that a habitually low folate intake increases colorectal cancer risk. People with higher folate intakes are 20 to 40 per cent less likely to develop the cancer. The vitamin ensures that cells grow properly; it's used to synthesize and repair DNA, the genetic material inside cells.
In 2007, however, a randomized trial reported that participants given folic acid supplements had an increased risk of adenomas, colorectal polyps that have the potential to become cancerous. Lab studies have also suggested a link between high folic acid intake and colon cancer risk.
Folic acid is often used interchangeable with folate. Folate refers to the B vitamin found naturally in foods; folic acid is the synthetic version added to vitamin supplements and fortified foods. Folic acid is more readily absorbed than folate.
Researchers speculate that high doses of folic acid could accelerate the growth of precancerous polyps and increase the risk of colon cancer in some individuals, in particular older adults. (The likelihood of having polyps increases with age.)
Concerns have been raised about the safety of being exposed to high levels of folic acid through a combination of vitamin supplements and fortified foods. Since 1998, adding folic acid to white flour, enriched pasta and enriched corn meal has been mandatory in Canada and the United States in order to reduce the risk of neural tube defects.
The majority of studies linking greater folate intake to reduced cancer risk were conducted before food fortification began.
The current study, published online in The American Journal of Clinical Nutrition, set out to determine the link between folate and colorectal cancer risk after fortification started.
The study followed 525,488 men and women, aged 50 to 71, from 1995 to 2007. (This included almost nine years of follow-up after fortification began.)
After accounting for factors such as weight, physical activity, smoking and other aspects of diet, a higher total folate intake – from diet and supplements – was linked with a lower risk of colorectal cancer. (Supplemental folic acid came from multivitamins.)
Individuals who consumed at least 900 micrograms a day – compared with less than 200 – were 30 per cent less likely to develop the cancer over the nine-year period.
Interestingly, the beneficial effect of folate plateaued at a daily intake of 400 to 500 micrograms. In other words, increasing your intake beyond 400 micrograms may not confer additional protection.
The findings from this large study suggest that fortifying grains with folic acid does not seem to lead to an increased risk of colorectal cancer.
The study found no evidence of a detrimental effect of a higher folate intake – from foods and multivitamins – even among those participants who reported a history of colorectal polyps. Rather, folate was associated with a substantial decrease in colorectal cancer risk.
While these findings are reassuring, there is still concern that taking very high levels of folic acid could be harmful, particularly in people with existing adenomatous polyps.
More studies are needed to look at the risk of colorectal cancer over longer periods of time. (It takes at least 10 years for adenomas to progress to cancer.)
In the meantime, how much folate do you need? Adults need 400 micrograms of folate each day. Women who are pregnant and those who are breastfeeding need 600 and 500 micrograms daily, respectively. The safe upper daily limit of folic acid(e.g. from supplements) is 1,000 micrograms.
The best foods sources of folate include cooked spinach, collard greens, broccoli, asparagus, artichoke, avocado, orange juice, lentils, black beans, peanuts and seeds.
You'll also find the B vitamin in white flour products (e.g. bread, crackers, pastries, pizza crusts), fortified breakfast cereals, enriched pasta and enriched cornmeal. It's estimated that fortified foods add 100 to 200 micrograms of folic acid to your daily diet.
Multivitamins generally provide 0.4 milligrams (400 micrograms) of folic acid. The Society of Obstetricians and Gynaecologists of Canada advises healthy women to take a multivitamin with 0.4 to one milligram of folic acid for at least two to three months before becoming pregnant and throughout pregnancy and breastfeeding.
Since one-half of all pregnancies in Canada are unplanned, all women of childbearing age are urged to take a multivitamin providing 0.4 milligrams of folic acid.
Other folks don't need to overdo folic acid. Include folate rich foods in your diet and if you supplement, a multivitamin with 0.4 milligrams of folic acid is enough. There's no need to add more from a separate folic acid pill or a B complex supplement.
If you have colon cancer, avoid taking supplemental folic acid. Get what you need from diet alone.
Chicken liver, 3.5 oz 770
Black beans, cooked, 1/2 cup 135
Chickpeas, cooked, 1/2 cup 85
Kidney beans, cooked, 1/2 cup 120
Lentils, cooked, 1/2 cup 189
Peanuts, 1/2 cup 96
Sunflower seeds, 1/3 cup 96
Artichoke, 1 medium 64
Asparagus, 5 spears110
Avocado, California, 1/2 medium113
Avocado, Florida, 1/2 medium 81
Bean sprouts, 1 cup 91
Beets, 1/2 cup 72
Broccoli, cooked, 1/2 cup 84
Brussels sprouts, cooked, 1/2 cup 83
Romaine lettuce, 1 cup 80
Spinach, raw, 1 cup 115
Spinach, cooked, 1/2 cup 139
Orange, 1 medium40
Orange juice, freshly squeezed, 1 cup 79
Orange juice, frozen, reconstituted, 1 cup 115
Source: The Complete A to Z Nutrition Encyclopedia © Leslie Beck, 2010. Reprinted by permission from Penguin Group (Canada), a division of Pearson Canada Inc.
Leslie Beck, a Toronto-based dietitian at the Medcan Clinic, is on CTV's Canada AM every Wednesday. Her website is lesliebeck.com.