This site uses cookies to store information on your computer. Learn more x


Providing perspectives on recent research into vitamins and nutritionals


How Low Can You Go? Vitamin D Levels through the Seasons in Pregnant Women

By Julia Bird

From a strict nutrition point of view, vitamin D is not an essential nutrient. The term “essential” is normally reserved for nutrients that are required for life, and must be supplied from an external source because they cannot be produced by our own bodies. In the presence of sunlight, humans can produce enough vitamin D to support life, which makes vitamin D conditionally essential.

The problem with this strategy is that for people who have low exposure to sunlight, vitamin D is essential and must be provided by the diet or supplements. Vitamin D is only produced after exposure to strong sunlight. For areas of the world that are closer to the poles than 37 degrees north or south of the equator, there is a strong seasonal effect as the sun is not strong enough to produce vitamin D in the colder months. This applies to people living in most parts of Europe, northern USA and Canada (including New York, Boston, San Francisco, Richmond, Denver and St Louis), some parts of the Middle East including Afghanistan, Iran and part of Turkey, northern China, southern Chile and Argentina, and Tasmania in Australia. There is a strong seasonal effect of vitamin D levels, and this can mean that deficiency is more likely at the end of winter.

A recent publication looked at the seasonal effects of vitamin D production in the skin in a large group of pregnant women (Luque-Fernandez et al.). Pregnant women should make sure that their vitamin D levels are adequate as it is linked to lower risk of pre-eclampsia, gestational diabetes, and caesarean section. This study used a high proportion of African American women, as they are at increased risk of deficiency due to lower vitamin D production in the skin. The authors used this large sample to create a statistical model showing the effect of seasons on vitamin D levels in both White and Black women living in the USA.

The authors found that there is a clear seasonal cycle, with a trough in February/March and a peak in August/September. Compared to January, levels of circulating vitamin D were 16% higher during the peak and 14% lower in the trough. Although the shape of the curve in Black and White women was the same, Black women had vitamin D levels around 12 ng/ml lower than White women. This markedly increases their risk of vitamin D deficiency, as has been found previously (see the CDC’s Second National Report on Biochemical Indicators of Diet and Nutrition for nationally representative data).

These results can help shape public health interventions to prevent vitamin D deficiency. For example, season-based cut-off points for defining deficiency can be developed to avoid that women have adequate levels when measured in the summer, that follow a predictable decline leading to deficiency in winter. Interventions that provide supplemental vitamin D in appropriate seasons can also be developed. Information on ethnic differences can help avoid deficiency in minorities that appear to be most at risk of vitamin D deficiency.  

Main citation:

Luque-Fernandez MA, Gelaye B, Vanderweele T, Ferre C, Siega-Riz AM, Holzman C, Enquobahrie DA, Dole N, Williams MA. Seasonal Variation of 25-Hydroxyvitamin D among non-Hispanic Black and White Pregnant Women from Three US Pregnancy Cohorts. Paediatr Perinat Epidemiol. 2013 Dec 20. doi: 10.1111/ppe.12103. [Epub ahead of print]   

Supporting citations:

Bodnar LM, Catov JM, Simhan HN, Holick MF, Powers RW, Roberts JM. Maternal vitamin D deficiency increases the risk of preeclampsia. J Clin Endocrinol Metab. 2007 Sep;92(9):3517-22. Epub 2007 May 29.

Scholl TO, Chen X, Stein P. Maternal vitamin D status and delivery by cesarean. Nutrients. 2012 Apr;4(4):319-30. doi: 10.3390/nu4040319. Epub 2012 Apr 20.

Zhang C, Qiu C, Hu FB, David RM, van Dam RM, Bralley A, Williams MA. Maternal plasma 25-hydroxyvitamin D concentrations and the risk for gestational diabetes mellitus. PLoS One. 2008;3(11):e3753. doi: 10.1371/journal.pone.0003753. Epub 2008 Nov 18.