The primary source of vitamin D for the human body is sun exposure, so people who don’t see enough daylight are often deficient in this vitamin. When sunlight hits the skin, a series of chemical reactions take place that result in the formation of vitamin D in the body.

Older people who live in care facilities or don’t get out of the house often, people who work in an office most of the day, and those who live in northern latitudes where it is dark more hours than it is light may not be able to take advantage of the sun’s rays enough to meet their vitamin D needs.

Less likely to come to mind are Navy personnel who live in submarines, but a recent study looked at the vitamin D status of submariners and found that up to 40 percent may experience vitamin D deficiency.

After their submarine tour of duty, the occurrence of vitamin D deficiency increased from 13 percent to 30 percent. Vitamin D insufficiency increased from 27 to 40 percent.

Vitamin D deficiency is diagnosed when a person’s blood levels are below 20 ng/ml or nanograms per milliliter; 20-30 ng/ml is considered an insufficient level. A level of 30 ng/ml or above is considered an adequate level of vitamin D.

Diet usually has little to do with vitamin D deficiency since there are few foods that naturally provide substantial amounts of the vitamin other than fish. Certain medical conditions can interfere with the absorption of it though.

Portuguese researchers reviewed past research studies involving the vitamin D status of in active-duty Navy personnel. After finding 48 such studies, 13 were chosen to be included in the review. Nine of those studies provided information on a total of 305 men living on submarines for anywhere from 30 to 92 days.

Before submarine deployment, the average vitamin D level of sailors ranged from 13.7 to 42.7 ng/ml. After their submarine tour of duty, those levels had dropped to a range of 7.9 to 30 mg/ml.

One study the team reviewed tracked vitamin D levels before and after submarine patrol and found that the occurrence of deficiency increased from 13 percent to 30 percent, while vitamin D insufficiency increased from 27 to 40 percent. Vitamin D sufficiency fell by half for those on submarine duty in that study.

Overall, the review of research on sailors’ vitamin D levels determined that 29 to 37 percent of navy personnel experienced vitamin D deficiency after submarine patrol.

There are other factors that also put naval personnel at risk for D deficiency. These include the protective military clothing they wear, shift work and assignments to high latitude regions of the world.

As the researchers advise in their paper, “It seems appropriate to implement a vitamin D food fortification policy in the Navy, assess vitamin D levels periodically, implement a vitamin D supplementation policy for vitamin D-deficient cases to ensure adequate levels before deployment, and provide a vitamin D-rich diet while on board.”

Studies of vitamin D levels among sailors on submarine deployments of less than 30 days, as well as the long-term effects of repeated underwater patrols on bone health — one of the negative effects of vitamin D deficiency — of naval personnel are possible topics for future research.

The study is published in Occupational & Environmental Medicine.