Folic Acid

Folic Acid

What is it?

Folic acid is also known as vitamin B9. It is found naturally in leafy green vegetables, where it is known as folate, and it is also an additive in breakfast cereals and granary bread.


Women who are trying to conceive, or are in the first trimester of pregnancy are advised to take folic acid supplements daily.

The general advice is that women should take 400 micrograms of folic acid per day, but the RCOG recommend that this is increased to a whopping 5 milligrams daily for obese women (women with a BMI greater than 30).


Folic acid supplements help the growing fetus’ spine develop properly and reduce the likelihood that a baby will have a neural tube defect (NTD) like spina bifida.

It is very unlikely that diet alone would contain enough folic acid to prevent neural tube defects, even at the relatively low level of 400μg, and so a supplement is recommended for all pregnant women and women trying to conceive.

Overweight women (see the page on BMI for explanation of the classifications) are more likely (1.22 times according to one study) than normal weight women to have a baby with NTD. Obese women were 1.7 times more likely, and ‘severely obese’ women were 3.11 times more likely to have a baby with NTD.

Women with a BMI of 27 or above have also been found to have lower levels of folate in their blood.

But a study of over 1000 women in California in 1996 recorded that while obese women have an increased likelihood of a baby developing a NTD, taking folic acid supplements did not reduce that risk.

There are no studies yet on the efficacy of increased doses of folic acid for obese women to prevent NTD.

The RCOG, in suggesting doses of 5mg are extrapolating results from a study of a different group of women who were more likely to have a baby with NTD; women who had previously had a baby with a NTD. That study found that giving a higher dose of folic acid (4mg) daily to those women reduced their risk of the baby developing a NTD by 72%.

There is nothing to prove that increased doses of 5mg of folic acid for obese women in pregnancy will reduce the incidence of NTD in babies born to those women. At best, the advice to take 5mg seems to be a de facto trial of higher dose folic acid. Once there is enough data, we will be able to determine whether it works or not.

Anyone else uncomfortable with being a guinea pig?

Fortunately folic acid seems relatively harmless, and cheap, which may explain the RCOG’s keenness to recommend its use without any strong evidence about its efficacy.


If you would like to try higher dose folic acid to protect against NTD, you can ask for a prescription from your GP. Of course, once you are pregnant, prescriptions become free, but when you are trying to conceive you will have to pay for the prescription (unless you are exempt from prescription charges for other reasons).

You could buy over-the-counter folic acid tablets which are relatively inexpensive, but they only come in 400μg tablet form.

To get the prescription you may have to provide a copy of the RCOG guidance (below) to explain why, as the recommendation does not appear to be widely known; a 2010 CMACE national clinical audit showed that just 2.1% of obese women were taking the 5mg supplement recommended.

Why not?

Reported side effects of higher doses of folic acid include nausea, abdominal distension and flatulence, allergic reactions, including skin redness, rashes, itching, hives, and shortness of breath, although these are all rare, occurring in less than 1 in a 1000 cases.


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