Vitamin D supplementation is the single most effective way to achieve and maintain target blood levels of vitamin D throughout pregnancy according to a study published in the Journal of Nutrition. Commenting on the study data, Dr Nisa Aslam a GP from the Health & Food Supplements Information Service – www.hsis.org notes:
“This was a study in 79 pregnant women recruited during the first trimester (average 9.3 weeks pregnant) which measured vitamin D blood levels in all the women in each of their three trimesters of pregnancy. The researchers also measured vitamin D intake from diet and supplements and Body Mass Index (BMI).
“Vitamin D blood concentrations are lower in people with obesity than in healthy weight people. This is due to dilution of vitamin D in the larger volume of fat tissue, muscle and liver. The researchers were interested to evaluate vitamin D levels in relation to BMI and to look at associations between BMI, vitamin D blood levels and vitamin D intake.
“Blood levels of vitamin D increased across trimesters, a finding that is common in studies of pregnant women. However, the key finding in the current study was that vitamin D intake from supplements, and not vitamin D intake diet, was the main influence on vitamin D blood levels in these 79 pregnant women. This finding was true for each of the three trimesters but was most significant during the third trimester. More than three quarters (78%) of the 79 women were taking a supplement containing vitamin D, either from a multivitamin supplement or a single vitamin D supplement. A total of 22% took no supplement containing vitamin D.
“Evaluation of this finding in the context of other studies is difficult because few other studies have measured vitamin D intake. Increase in vitamin D levels observed in pregnancy may be due to hormonal changes and depending on the time of year the study is conducted, vitamin D levels may increase according to the season. But accounting for season in this study made no difference to the findings.
“Women who reported taking vitamin D supplements of 600IU (15 micrograms) or more had higher blood vitamin D than those with lower intakes. All of the women taking the higher doses had blood vitamin levels of 50nmol/litre or more. The target vitamin D level in Canada is 40 to 50nmol/litre compared to 25nmol/litre in the UK.
“Very few women in the Canadian study had blood levels below the threshold for Canada or above the safe limit of 125 nmol/litre. Women with obesity, of whom there were 43% in the study, had lower levels of vitamin D in the first and third trimesters but 93% of the overweight or obese women had vitamin D levels in the target range (over 40 nmol/litre) in the second and third trimesters.
“The average intake of vitamin D from supplements in this study was 400IU (10 micrograms) daily. This is the recommended intake for vitamin D supplementation in pregnant women in the UK. It is striking in the Canadian study that this level of supplementation (ie 10 micrograms daily) maintained vitamin D levels within the Canadian target range of 40-50nmol/litre.
“Different thresholds for vitamin D blood levels are set in different countries. In the UK the threshold is lower than that in Canada and is set at 25nmol/litre. The most recent NDNS data show that 18 per cent of adult women (19-64 years) have a vitamin D blood level below the 25nmol/litre threshold and are at risk of deficiency. It is therefore essential that women follow the government recommendation to take a supplement containing 10 micrograms vitamin D. This is important for bone and muscle health and in pregnancy this dose appears to be capable of ensuring that vitamin D blood levels are maintained above the UK threshold.”
Savard C, Bielecki A, Plante A-S, Lemieux S, Gagnon C, Weiler HA, Morisset A-S, Longitudinal Assessment of Vitamin D Status across Trimesters of Pregnancy, The Journal of Nutrition, Volume 151, Issue 7, July 2021, Pages 1937–1946, https://doi.org/10.1093/jn/nxab060