Fumonisin also interferes with the cellular uptake of folic acid, a vitamin that is known to reduce the risk of neural tube defects in developing fetuses.

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The group also found that folic acid, a vitamin known to help reduce the chances of birth defects, did not decrease that risk among heavier women.

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Autism risk might be cut nearly in half by making sure that mothers are taking supplements containing folic acid, a B vitamin, at the time of conception and in the early months of pregnancy.

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Health experts now recommend that women who are planning to become pregnant take a supplement of vitamin D as well as folic acid.

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Taking them with juice is a better idea, as the acid and vitamin C in juice may help with the absorption.

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Professors Brian Wharton and Ian Booth, from the Institutes of Child Health in London and Birmingham, warned that in people with vitamin B12 deficiency, consumption of folic acid may mask a condition called megaloblastic anaemia - which if not picked up can lead to degeneration of the spinal cord.

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Taking too much folic acid - over 1mg a day - can mask signs of vitamin B12 deficiency.

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