New research has found that women who are taking oestrogen-based birth control pills have higher quantities of circulating vitamin D and that women who stop taking these contraceptives face a significant drop in vitamin D levels.
Around 90 percent of vitamin D is produced in the skin after exposure to sunlight and a deficiency can lead to rickets and softening of the bones. As vitamin D is vital in the formation of bones, it is particularly important during pregnancy when it is required to enable the bones of the foetus to develop properly. Therefore, pregnant women have an increased risk of vitamin D deficiency and as a result, a heightened risk of developing bone problems.
The research investigated whether there were any changes in vitamin D levels associated with taking oral contraceptives and was carried out by the National Institutes of Health's National Institute of Environmental Health Sciences in North Carolina, USA.
Vitamin D and contraception
The researchers carried out a cross-sectional analysis of data from the Study of Environment, Lifestyle, and Fibroids (SELF), an investigation of reproductive health. The project included almost 1,700 African-American women living in and around Detroit who were aged between 23 and 34 years.
The study asked women about their contraceptive use and included questions about the amount of time they spent outside and any vitamin D supplements they took. Blood tests ascertained levels of the most common circulating form of vitamin D, called 25-hydroxy vitamin D. The study found that women who were using contraception containing oestrogen tended to have higher vitamin D levels than other women. Even after controlling for confounding factors, such as seasonal exposure to light, the effect remained significant.
The study was led by Dr Quaker E. Harmon who said, "We could not find any behavioural differences such as increased time spent outdoors to explain the increase. Our findings suggest that contraceptives containing oestrogen tend to boost vitamin D levels, and those levels are likely to fall when women cease using contraception."
Dr Harmon advises that women attempting to become pregnant should take steps to ensure that vitamin D levels are adequate while trying to conceive and during pregnancy in order to avoid the risk of any deficiency.
The findings were published in the Journal of Clinical Endocrinology & Metabolism.