Why Do You Need Vitamin D, and How Much Do You Need?

Our evolution and existence are dependent on exposure to sunlight. The sun’s ultraviolet rays should provide us more than 80 percent of our daily vitamin D requirement. However, over the past few decades, our exposure to sunlight has declined. This has led to an increasing incidence of vitamin D deficiency and the disorders associated with it. Consequently, the role of diet and dietary supplements has become more important. Vitamin D is not only needed to regulate calcium and phosphorus [FOSfor-russ] in the body, it also plays an important role in maintaining bone structure and general health.


Sun exposure is the most reliable and economical way to get vitamin D. More than 80 percent of the vitamin D needed by humans comes through exposure to sunlight. For a person with lighter skin, exposure of the hands, face, arms and legs to sunlight (without sunscreen) for about 20 minutes, three to four times a week, makes the vitamin D needed by the body. However, longer sun exposure is required for persons with darker skin and the elderly. The necessary exposure time varies with age, skin type, season and time of day.

Vitamin D is stored in the liver and in body fat and is present in small amounts in some foods, including fatty fish such as herring, mackerel, sardines and tuna. Sunexposed or irradiated [ih-RAY-dee-ay-ted] mushrooms are the only vegetable sources with higher amounts of vitamin D. In North America, vitamin D is added to dairy products, some juices and cereals. Vitamin D is made in laboratories using ultraviolet ray exposures and is given as a supplement to treat vitamin D deficiency. For a variety of reasons, most experts think vitamin D3 is superior to D2.

Vitamin D deficiency is becoming more widespread worldwide and is nearly epidemic. This worsens several existing medical disorders. Yet most people have undiagnosed and untreated vitamin D deficiency.


Vitamin D deficiency is very common; half of the people in the United States are affected. It is common even in sunny states, such as Florida, possibly because people are staying indoors more, and when they are outside, they are covered up in clothes or sunscreens that prevent the skin from making adequate amounts of vitamin D. Those who are not getting sufficient sunlight exposure are at higher risk. More than 80 percent of those residing in assisted living, nursing homes and disability centers are deficient in vitamin D, and, thus, need to be supplemented with adequate doses.

The risk for vitamin D deficiency and consequent complications increases after 60 years of age. This is in part because people spend less time in the sun, are less able to generate vitamin D in their skin, do not get vitamin D in their diet, are less able to absorb vitamin D from their diet, and may have more trouble converting vitamin D to its active form because of liver or kidney damage. Thus, for better health, vitamin D supplements are necessary for many older people and people with dark skin who live in northern areas.

Measurement of the blood’s vitamin D level is the only reliable way to diagnose vitamin D deficiency. Most professional societies recommend a blood/serum vitamin D concentration of 30 to 50 ng/mL, with an average level of 40 ng/mL (100 nmol/L), as the healthy range. For most, this can be achieved by taking approximately 1,000 IU (25 μg) daily in adults and 2,000 IU (50 μg) for people older than 65.


Rickets is a disease caused by deficiency of vitamin D. More than 100 years ago, children with rickets were exposed to sunlight to cure it. Later, they were successfully treated with cod liver oil. Today, rickets is much less common in the United States. Vitamin D deficiency causes softer bones (osteomalacia[ah-stee-oh-mah-LAY-she-uh]), weaker bones (osteoporosis [ah-stee-oh-pore-OH-sis]), and increased falls and fractures. Vitamin D deficiency also increases the activity of the parathyroid [pah-ruh-THIGH-roid] glands located in the neck. This condition, known as secondary hyperparathyroidism [hie-per-pahruh-THIGH-roid-is-m], further weakens bones. Vitamin D needs to be activated in the kidney to become its active, hormonal form. Thus, another highly vulnerable group is those with kidney failure. Unless treated with vitamin D, they are likely to have bone problems and fractures, and blood vessel calcification, which leads to premature death.

Recent evidence suggests that vitamin D may help prevent many disorders, such as diabetes, multiple sclerosis, rheumatoid arthritis, chronic obstructive pulmonary disease, asthma, bronchitis, premenstrual syndrome, increased blood pressure, strokes and heart attacks, and even cancer. Low serum vitamin D levels are also associated with being overweight, abdominal obesity, metabolic syndrome, stroke and diabetes. In addition, having lower blood vitamin D levels for a long period is associated with increased heart attacks and all-cause mortality.

Community-based and countrywide preventative strategies are necessary to ensure adequate vitamin D in relation to where people live, seasonal variations, skin pigmentation, and culture and habits.


The current recommendation for total daily calcium intake is 1,200 mg, but no more than 1,500 mg (that is, diet + supplements). Virtually all of our diets contain about 300- 400 mg of calcium, even those devoid of dairy products. This means for a vast number of people, supplemental calcium should not exceed 800 mg per day. Consumption of one or two tablets containing 400 to 800 mg of calcium a day is adequate for most patients. When in doubt, talk with your physician, endocrinologist, or registered dietitian for assistance with choosing dietary supplements. Extra vitamin D is necessary for premature infants and those who are exclusively breast-fed to ensure proper growth and optimal health. People older than 65 years are likely to need 2,000 international units (IU) per day of vitamin D, whereas 1,000 IU per day may be enough for people younger than 65 years. Those with low serum vitamin D levels are also treated with high doses (e.g., 50,000 IU capsules) for a shorter duration; however, this should be done under medical supervision. Daily consumption of vitamin D-fortified foods, such as milk and other dairy products, is encouraged.

Vitamin D supplementation is safe and cost-effective in improving a variety of disorders.

In certain groups of patients, including those with granulomatous [gran-u-LO-muh-tus] diseases (chronic conditions with granulomas, which are types of nodules or lumps due to inflammation), such as sarcoidosis [sar-koy-DOH-sis]), lymphoma, or overactive parathyroid glands, vitamin D needs to be replaced cautiously under medical supervision to prevent increasing their serum and urine calcium levels. There are no known interactions of vitamin D with food and no serious drug interactions of vitamin D with other medications. In addition to calcium and vitamin D, a balanced diet, good eating habits and moderate weight-bearing physical activity are recommended.


Vitamin D is safe when taken by mouth, in recommended amounts. National scientific organizations indicate taking as much as 4,000 IU vitamin D per day is safe for longerterm supplementation. However, one needs to be cautious, as taking larger quantities of vitamin D for a long duration may lead to adverse effects related to increased calcium in the blood and urine. These adverse effects include weakness, irritability, fatigue, sleepiness, headache, loss of appetite, dry mouth, metallic taste, and nausea and vomiting. In general, vitamin D supplementation is safe and cost-effective in improving a variety of disorders.

Dr. Sunil Wimalawansa is a Professor of Medicine and former chief of the Division of Endocrinology, Metabolism and Nutrition at the Robert Wood Johnson Medical School and Robert Wood Johnson University Hospital. He is an international authority on bone disorders and is currently a member of the AACE board of directors.