Conversely, researchers from the University of Newcastle upon Tyne, UK, who examined nearly 1,000 women and their newborn babies, found that higher levels of folate (found in some vegetables, fruits and cereals, and also known as vitamin B9, or folic acid) were linked with increased birth weights.
Expectant mothers who smoked were more likely to have lower levels of folate in their blood, which might explain why women who smoke often have lighter babies.
The health benefits of folic acid for babies are already known but this is the first time that folate levels commonly seen in UK mothers in early pregnancy have been linked with birth weight. The study is published in the British Journal of Nutrition.
Seven per cent of all babies born are low birth weight (less than 2.4kg or 5.5lbs) and they have a 50 per cent chance of having a severely disabling condition as a result of being too light. Healthy birth weights are a marker for good health in infancy and later life.
Researchers say that current NHS policy is to advise women to take folic acid at their first pregnancy check-up but this means that many mothers-to-be miss out on taking the vitamin in the crucial, early stages of gestation.
They also say the findings have implications for health promotion policies, adding weight to arguments in favour of fortifying everyday foods like bread and cereals with folic acid.
For the study, Dr Caroline Relton, of Newcastle University's School of Clinical Medical Sciences, and colleagues studied 998 expectant mothers attending a hospital in North West England (West Cumberland Hospital, Whitehaven, Cumbria) and their newborn babies.
The team analysed blood samples that the women gave during their routine antenatal visits (averaging 11.5 weeks gestation) and recorded the amount of folate in their red blood cells - these cells show activity over the previous three months. They gathered lifestyle information, such as smoking habits, from a questionnaire, and also recorded the weight of each baby when born.
Medical evidence suggests that folic acid influences birth weight because it is an essential nutrient for growth and gene expression in the foetus. Smoking is likely to reduce a women's folate levels because it is a significant source of oxidative stress and may alter the ability of the cell to metabolise and ultimately store the vitamin.
The best way to increase folic acid levels is by taking supplements but it is also advisable to eat folate-rich foods like green vegetables (broccoli, spinach), citrus fruits (oranges, grapefruits) or wholemeal products like bread or cereals. The current recommended intake is 200 micrograms per day and women are advised to take an additional 400 micrograms a day before conception and during the first 12 weeks of pregnancy.
Dr Relton said: "Folic acid is highly important in preventing birth defects which affect a small number of pregnancies. This study suggests that it is also important in every pregnancy to help the developing baby reach a healthy birth weight. However, many women are missing this critical window in the first few weeks of gestation during which their baby really needs folic acid to grow and develop.
"The evidence from this study strengthens the argument for fortifying everyday foods like bread and cereals with folic acid. For example, in the United States, all wheat products have been fortified since 1998, with the intention of reaching women of childbearing age.
"Fortifying a range of foods promises to be a more effective solution than a campaign to encourage women to take folic acid supplements. Our previous research shows younger women and those from deprived backgrounds are less likely to take these supplements, and although some cereals are currently fortified in the UK, they tend to be the more expensive, brand-name products."
The research was funded by the Westlakes Research Institute and the Minnie Henderson Trust.
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