The many roles of vitamin D in our body

Brenda Braslow, MS, RDN, LDN, CDCES - Registered Dietitian Nutritionist and Certified Diabetes Care and Education Specialist (CDCES)

Do you know your serum vitamin D level? Vitamin D plays a significant role in body functions, from the major organs down to the cellular level.

Vitamin D, a fat-soluble vitamin, has always been considered an important vitamin. It is essential for adequate calcium absorption and also helps maintain sufficient blood levels of calcium and phosphate. This is critical for good bone and teeth growth, density and strength. Without adequate vitamin D, bones can become thin, mishapen, and easily broken.

If preventing osteoporosis is not enough incentive for getting adequate vitamin D, consider that this vitamin plays an important role in our immune function, reducing inflammation, maintaining good muscle strength, and for blood sugar control, high blood pressure and heart disease. Low vitamin D levels have been associated with higher rates of depression. Researchers are also looking at vitamin D's role in type 2 diabetes prevention. Vitamin D is essential for function of the pancreas, the organ that produces insulin. Insulin is the hormone that carries sugar out of the blood into the body tissues. The exact mechanism is not clearly understood, but it is being earnestly explored.

The human diet is usually a poor source of vitamin D because very few foods naturally contain it. The best sources are fatty fish (tuna, salmon, and herring) and fish liver oil. Vitamin D is also found in smaller amounts in liver, cheese, and egg yolks. Most of the vitamin D consumed in foods is found in foods fortified with the vitamin. In the 1930s, the U.S. started a national milk fortification program to prevent rickets, a common condition at that time caused by vitamin D deficiency. Today, other commonly fortified foods include yogurt, ready-to-eat cereal, and orange juice.

The National Health and Nutrition Examination Survey, 2015-2016, showed that the estimated vitamin D intake from food was adequate for most people in the US, aged 1 and older. The Federal Food and Drug Administration's Food and Nutrition Board established a Recommended Dietary Allowance (RDA) for vitamin D based on minimal sun exposure. This RDA is sufficient to maintain bone health (preventing osteoporosis) and normal calcium function. The RDA for ages 1 through 70 is 15 mcg (600 IU)/day. See this page for a breakdown for all age groups, pregnancy and lactation. The UL recommended for supplemental vitamin D is 100 mcg (=4000 IU). Do not exceed the UL unless under the advice of a doctor.

It is thought that most people obtain at least half of their vitamin D needs through sunlight exposure. Vitamin D is synthesized when the skin absorbs UV rays from sunlight. Vitamin D researchers suggest that approximately 5-15 minutes of sun exposure on about 6% of the body (face, arms, legs or back), 2-3 days per week, without sunscreen should lead to adequate vitamin D synthesis. In other words, there is no need to get excessive sun exposure that can increase skin cancer risk. Individuals with limited sun exposure should consider taking a Vitamin D supplement. Most daily multivitamin-mineral supplements contain 400 IU vitamin D which is a nice supplemental daily dose.

Know your vitamin D level

Serum concentration of 25(OH)D is the best indicator of vitamin D status. It reflects the amount of vitamin D stored in the body. There is on-going debate about levels needed to prevent deficiency, promote bone health and for optimal overall health. The Institutes of Medicine state that vitamin D deficiency risk occurs at <30 nmol/L (equal to <12 ng/mL) and that all people are sufficient at 50 nmol/L (equal to 20 mg/mL). In 2011, the Endocrine Society stated that the desirable serum 25(OH)D is >75 nmol/L (equal to > 30 ng/mL) for maximizing calcium, bone and muscle metabolism. Vitamin D toxicity occurs most likely from high-dose supplementation and can occur at levels of 125 nmol/L. Symptoms of vitamin D toxicity include loss of appetite, weight loss and excessive urination.

Why not ask your doctor to check your serum vitamin D level? It is a non-fasting blood test, and if it is low, you can take supplements to fill up your stores. Not only can this benefit your bone health, but you may be strengthening your immune system and building your prevention arsenal against type 1 and type 2 diabetes, high blood pressure and heart disease.

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