Almost all of the women in the study had blood levels of the vitamin below 80 nmol/l, a level which is widely considered to be the cut-off for vitamin D sufficiency, showing that current recommendations may be insufficient.
Currently in the UK, pregnant women are recommended by the Food Standards Agency (FSA) to take supplements with a daily dose of 10 micrograms vitamin D. In the US and Canada, where no specific recommendations exist for pregnant women, adequate intakes are inline with the general population and set at 5 micrograms per day.
Dr Maria Barnes from the University of Ulster, Coleraine, told NutraIngredients.com that, as their study was observational, it was impossible to determine the dose of vitamin D required to improve maternal vitamin D status.
“Clearly such recommendations can only be established following a number of well designed double-blinded, randomised placebo controlled vitamin D intervention studies during pregnancy,” said Dr Barnes.
“However, from our study it is evident that general dietary supplementation given to pregnant women may be inadequate to maintain or improve vitamin D status, particularly during wintertime,” she added.
The researchers, from Queen's University, Belfast, the University of Ulster, and Belfast City Hospital report their findings in the British Journal of Nutrition.
Bigger doses needed
A wider debate is ongoing in nutrition circles regarding vitamin D intakes. Late last year the American Academy of Pediatrics (AAP) said children should be consuming 400 International Units (IUs), or 10 micrograms per day. This would mean doubling the current US recommendations. Others have recommended increasing levels to 2,000 IUs.
The US National Academy of Sciences' Institute of Medicine (IOM) said it will be reviewing its recommendations “in the not too distant future”.
According to Dr Barnes and her co-workers, children born to vitamin D-deficient mothers are at an increased risk of rickets, while maternal insufficiency may detrimentally affect bone build-up, and increase the risk of type-1 diabetes and asthma.
The researchers recruited 99 pregnant women at 12, 20 and 35 weeks of gestation, and 38 non-pregnant controls living at a latitude of 54 to 55 °N. S
Vitamin D levels, calculated using serum concentrations of 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form of the vitamin in the body, showed that 35, 44 and 16 per cent were vitamin D deficient at 12, 20 and 35 weeks of gestation. Furthermore, 96, 96 and 75 per cent were vitamin D insufficient at the same gestation stage.
While women taking supplements did have higher vitamin D levels, “vitamin D insufficiency was still evident even in the face of supplement use”, wrote Dr Barnes and her co-workers.
“To our knowledge, the present study is the first to measure vitamin D status in free-living Caucasian women with uncomplicated pregnancies, which collected samples longitudinally throughout pregnancy whilst concurrently sampling non-pregnant age-matched controls,” stated the researchers.
“Given the potential consequences of hypovitaminosis D on health outcomes, vitamin D supplementation, perhaps at higher doses than currently available, is needed to improve maternal vitamin D nutriture,” they concluded.
Data on D
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. The former, produced in the skin on exposure to UVB radiation (290 to 320 nm), is said to be more bioactive.
While our bodies do manufacture vitamin D on exposure to sunshine, the levels in some northern countries are so weak during the winter months that our body makes no vitamin D at all, meaning that dietary supplements and fortified foods are seen by many as the best way to boost intakes of vitamin D.
Source: British Journal of Nutrition
September 2009, Volume 102, Issue 06, Pages 876-881, doi:10.1017/S0007114509297236
“Vitamin D deficiency and insufficiency in pregnant women: a longitudinal study”
Authors: V.A. Holmes, M.S. Barnes, H.D. Alexander, P. McFaul, J.M.W. Wallace