By Julie Redfern, R.D., L.D.N.
When you step out into sunlight, you're getting more than just a tan. When the ultraviolet B rays from the sun come in contact with your skin, they activate a series of reactions whose end product is a hormone. This hormone, 1,25 dihydroxyvitamin D3, is more commonly known as "vitamin D." In addition to sunlight, vitamin D is found naturally in some food sources such as egg yolks, herring and cod liver oil, and it is added to other foods such as milk.
Many years ago, when humans lived near the equator, they spent a great deal of time outside with much of their skin exposed to the sun. They did not use sunscreen, and vitamin D deficiency was not a problem. A fair-skinned person whose face and hands are exposed to sunlight, could produce 10,000 to 12,000 international units (IU) of vitamin D. However, many factors are making it harder for people to get adequate vitamin D from sunlight. Some live in parts of the world where the sun doesn't shine brightly. For example in Boston, from November to February, it's almost impossible to get adequate vitamin D from sunlight alone. In addition, concerns about sun damage and skin cancer have led to increased sunscreen use. Just an SPF of 8 decreases vitamin D production by more than 95%. Also if you are older than 70 or are dark- skinned, you will need longer sun exposure to convert vitamin D in the skin to its active form.
Vitamin D has long been known to be crucial for bone health, helping the body to better absorb calcium and thereby preventing bone diseases such as rickets in children and osteoporosis in adults. New research is suggesting that vitamin D may do much more than help with healthy bones. It likely aids in protecting muscle strength and preventing falls as we age. In addition newer studies show it has a positive role in preventing cancer, diabetes and autoimmune diseases such as multiple sclerosis, lupus and inflammatory bowel disease.
The Institute of Medicine recommends that people 70 and younger get 600 IU a day, and those 71 and older get 800 IU per day.
This level might not be so hard to achieve if you consume foods high in vitamin D. For example, a glass of milk or vitamin-D-fortified orange juice contains 100 IU, and 1 teaspoon of cod liver oil contains 450 IU. Sometimes labels will have vitamin D in micrograms (mcg) instead of IUs. To convert, 1micorgram (mcg) = 40 IUs. For example: 5 mcg= 200 IUs of vitamin D.
Vitamin D is a fat-soluble vitamin, which means it is stored in the body's adipose, or fat tissue, until the fat is metabolized. In very high doses, vitamin D can be toxic and can lead to serious side effects such as calcium deposits in the heart and other organs. The recommendation for the highest safe level (the "UL," or upper limit) set by the Institute of Medicine is 4,000 IU per day.
Milk is a common source of vitamin D in the American diet. However, many people don't drink milk because they are lactose-intolerant, vegan or they just don't like it. Vitamin D is found in other foods, but not many that are found in the daily diets of most Americans. Atlantic herring, cod liver oil, salmon, mackerel and canned tuna in oil all have appreciable amounts of vitamin D (see below).
So for some people, getting a daily dose of 600 to 800 IU of vitamin D can be difficult to achieve from diet alone.
Breast milk has very low amounts of vitamin D, so babies need supplemental vitamin D if they are exclusively breastfed.
Vitamin D is also found in supplements. For those who feel they may not be getting enough vitamin D from foods or sunlight, and you are in one of the high risk groups for deficiencies, you may consider taking vitamin-D supplements. When choosing a supplement, look for one that contains "cholecalciferol" (vitamin D3) rather than "ergocalciferol" (vitamin D2). The D3 or "cholecalciferol" form of Vitamin D has been shown to be better absorbed and available to the body.
Vitamin D has long been known to be important for health. It's now attracting attention as we find new health reasons to support our intake; and we realize this vitamin is deficient in the diets of many people.
Julie Redfern, R.D., L.D.N. is a registered dietitian in the Nutrition Consultation Service at Brigham and Women’s Hospital. She specializes in nutrition counseling for the obstetrics and gynecology department. She is a graduate of the University of Vermont and completed her dietetic internship at the University of Cincinnati Medical Center. Angela Illing is a dietetic intern at the Brigham and Women’s Hospital.