The following is a guest post by Laura Zenni, who is currently spending a two-week internship in Ann Arbor with Melissa Gerharter.

Did you know that cold, winter months can hurt more than just your tan? You may be one of the three in four Americans who suffer from a vitamin D deficiency. Vitamin D, also known as the sunshine vitamin, is a fat-soluble vitamin than can be produced by the body with the assistance of sunlight. From October to April, sunlight is insufficient to trigger the production of vitamin D in locations at or above 35 degrees north latitude, which means here in Ann Arbor.

Vitamin D is important for many functions in the body, including adequate calcium absorption for the bones and teeth, immune function and regulation of the nervous system and skeletal muscle. Additionally, new research shows that vitamin D may be important for cognitive function. Vitamin D deficiency is very common in the US, causing loss of bone mass, decreased muscular strength, depression, fatigue, cognitive difficulties and poor balance. It can also increase risk for diabetes, cardiovascular disease, infectious disease and different types of cancer. 

In November of 2010, the Institute of Medicine increased the recommendation for daily intake of vitamin D to 600 IU (15 μg) every day for children and most adults, and 800 IU (20 μg) every day for adults older than 70. Intakes about 4,000 IU per day are not recommended because of an increased risk for potential negative side effects.

During the winter months, it is important to have adequate vitamin D intake through foods and supplements. Natural sources of vitamin D are very limited and include liver, egg yolk and fatty fishes such as salmon, mackerel, sardines and tuna. Luckily, many foods are fortified with vitamin D to help prevent deficiency. Examples of these foods include margarine, ready to eat cereal, orange juice and milk, which is the most significant source of vitamin D in the U.S., providing 100 IU of vitamin D in one eight-ounce serving. 

So, focus your winter season on consuming foods that are high in vitamin D. Supplements are also available that can provide 800-1,000 IU per supplement. Adequate vitamin D in the blood can help improve your mental ability, strengthen your muscles and decrease your risk for bone fractures, which can lead you to have a happier winter season!

Laura Zenni is currently a dietetic intern at Emory University Hospital in Atlanta, Ga. She graduated in May of 2010 from Miami University of Ohio with a bachelor's degree in dietetics and exercise science. She hopes to complete her master's degree in exercise physiology next year and eventually work in the health and wellness field. She is currently spending a two-week internship here in Ann Arbor with Melissa Gerharter, MS, RD.