Vitamin D is unique among vitamins. Because there are few foods that provide it and because the body can make it when skin is exposed to sunlight. That also means, however, that the farther from the equator you live, the less vitamin D your body will produce, especially in the winter months.

Since you can't eat your way out of a vitamin D deficiency, a large number of older adults are deficient in the vitamin, according to a new study. This is especially concerning given the evidence that vitamin D makes a difference when it comes to problems associated with aging such as loss of muscle strength, weak bones, diabetes prevention and memory loss.

People who took vitamin D supplements were not as likely to be deficient, but only about four percent of the people in the study took them.

Researchers from Trinity College Dublin studied the vitamin D levels of over 6,000 middle-aged and older adults, using data from the English Longitudinal Study of Ageing, to create a profile of older people who were more likely to experience vitamin D deficiency. Study participants lived in Northern latitudes where UVB radiation — sunlight — is not as intense.

More than half of the participants had low levels of vitamin D in their blood, and over a quarter of those were classified as deficient.

Factors associated with vitamin D deficiency included being female, 80 years of age or over, smoking and obesity. Non-white ethnicity and poor self-reported health also added to a person's likelihood of being deficient in Vitamin D. On the other hand, healthy weight, regular vigorous exercise, taking a vitamin D supplement and travel to sunny places once a year were associated with less risk of deficiency.

Geography was a big factor. Vitamin D deficiency was more prevalent among northern England residents than those living in the South, even after adjusting for other predictors of vitamin D status.

People who took vitamin D supplements were not as likely to be deficient, but only about four percent of the people in the study took them.

Are you getting enough vitamin D? There's a good chance you're not, especially if you live far from the equator. The only good food sources of the vitamin are fatty fish (tuna, mackerel, salmon) and fish liver oils with small amounts found in beef liver, egg yolks and cheese. Vitamin D is added to milk, margarine and yogurt.

Supplements can help, but it's best not to depend solely on them. Eat foods that contain vitamin D and spend time outdoors with your skin exposed to the sun.

The study is published in Nutrients.