Women who stop taking the pill could experience a drop in vitamin D levels, according to a new US study which looked at levels of the vitamin in African-American women.

Published on Thursday, August 4 in the Endocrine Society's Journal of Clinical Endocrinology & Metabolism, the study found that levels of vitamin D fell when women stopped using birth control pills or other contraceptives containing estrogen, which could be of particular importantance to those who stop their birth control in order to become pregnant.

The researchers looked at a total of 1662 African-American women aged 23-34 years, all living in the Detroit area in Michigan, USA.

Data on the women was taken from the Study of Environment, Lifestyle & Fibroids (SELF), which looks at reproductive health and in particular fibroids.

Participants were asked questions about their use of contraceptives, any vitamin D supplements they took, as as well as the amount of time they spent outdoors, an important factor in the production of important vitamin D.

Blood samples were also taken to analyze levels of 25-hydroxy vitamin D, a good indicator of how much vitamin D is in the body.

After taking into account seasonal exposure to sunlight, the researchers found that using a contraceptive pill, patch or ring containing estrogen was associated with a 20 percent higher level of 25-hydroxy vitamin D.

As the researchers were unable to explain the increase by any behavioral differences, including an increased amount of time spent outdoors, they concluded that it was the use of contraceptives containing estrogen that boosted vitamin D levels, and therefore vitamin D levels are also more likely to fall when women stop using contraception.

The study's first author, Quaker E. Harmon, from the National Institutes of Health's (NIH's) National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina, commented on the results saying, "Our findings indicate women may run the risk of developing vitamin D deficiency just when they want to become pregnant. For women who are planning to stop using birth control, it is worth taking steps to ensure that vitamin D levels are adequate while trying to conceive and during pregnancy."

For women looking to increase their levels of vitamin D, one of the main sources of this vitamin is sunlight; Individuals can also up their levels by consuming foods rich in the vitamin including oily fish, beef or calf liver, eggs and cod liver oil.