New research suggests that taking a contraceptive pill could increase levels of vitamin D in the body.

The study, reported in the Journal of Clinical Endocrinology & Metabolism, showed that women using estrogen-containing contraceptives - such as birth control pills, patches, or rings - may have significantly higher vitamin D levels than those who do not.

Dr Quaker E Harmon led the study that examined the reproductive health in nearly 1,700 African-American women between the ages of 23 and 34. The participants answered questions about contraceptive use, as well as the amount of time they spent outdoors and any vitamin D supplements they took.

1,662 women provided blood samples, which were analyzed to measure levels of 25-hydroxy vitamin D - the primary circulating form of vitamin D.

After adjusting for seasonal exposure to sunlight, the researchers found the use of contraceptive pills, patch or ring containing estrogen was associated with a 20% higher vitamin D level. They also discovered that current birth control users tended to have higher levels of vitamin D in the blood and past contraceptive users had average levels of Vitamin D.

"Our study found that women who were using contraception containing estrogen tended to have higher vitamin D levels than other women," said Dr Harmon.

"We could not find any behavioral differences such as increased time spent outdoors to explain the increase. Our findings suggest that contraceptives containing estrogen tend to boost vitamin D levels, and those levels are likely to fall when women cease using contraception."

Vitamin D is a hormone involved in the immune system and managing calcium levels in the blood. Proper calcium levels are necessary for bone health. 

The body produces vitamin D in a chemical reaction when the skin is exposed to sunlight. A smaller portion of the body's vitamin D supply comes from food, including fortified milk, fish and eggs. 

During pregnancy, women produce increased amounts of the active form of vitamin D to support formation of the fetal skeleton.

Dr Quaker said that although they are still unsure what causes the increased levels, the research suggests that women who stop taking their contraceptive and then become pregnant may face an increased risk of developing vitamin D deficiency.

"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."