Vitamin D deficiency may damage the brain, according to a new study published in Free Radical Biology and Medicine. Approximately 40% of Americans are vitamin D deficient.

The study showed that middle-aged rats with low vitamin D intake for several months developed free radical damage in the brain and had higher levels of certain harmful proteins potentially linked to cognitive decline. The rats also performed poorly on cognitive tests of learning and memory compared to counterparts fed a normal diet.

The elderly and people with dark skin are at particular risk for low vitamin D levels. The body is able to produce vitamin D from sun exposure, but the vitamin can also be found in foods such as oily fish, eggs and fortified milk as well as in supplement form. Ten to 15 minutes of sun exposure without sunscreen daily helps the body generate sufficient vitamin D, but is not a wise choice for those who burn easily. As excess vitamin D may not be safe, those who are concerned they might be deficient should see their doctors to have their levels checked.

Whether this study’s results translate to humans is not yet known, but prior studies have also demonstrated a potential link between vitamin D deficiency and cognitive and memory decline with age, including Alzheimer’s disease.