What Is Vitamin D?
Vitamin D is called the “sunshine vitamin” because it is made in your body with the help of ultraviolet (UV) rays from sunlight.
In fact, most healthy people can synthesize all the vitamin D they need as long as they receive adequate sun exposure.
People who don’t obtain enough sun exposure must meet their needs through their diets.
Whether from food or sunlight, vitamin D enters your body in an inactive form. The ultraviolet rays of the sun convert a cholesterol-containing compound in your skin to previtamin D, which is then converted to an inactive form of vitamin D in your blood. The vitamin D in your foods is also in this inactive form.
This inactive form travels in your blood to your liver, where it is changed into a circulating form of vitamin D and is released back into your blood. Once in your kidneys, it is converted to an active form of vitamin D.
Functions of Vitamin D
Vitamin D Helps Bone Health by Regulating Calcium and Phosphorus
Once in an active form, vitamin D acts as a hormone and regulates two important bone minerals, calcium and phosphorus.
Vitamin D stimulates the absorption of calcium and phosphorus in the intestinal tract, helping to keep the levels of these minerals within a healthy range in your blood. Because of its role in regulating these minerals, vitamin D helps to build and maintain your bones.
Although phosphorus deficiency is very rare, dietary calcium deficiencies do occur, causing blood levels of calcium to drop. When this happens, vitamin D and parathyroid hormone cause calcium to leave your bones to maintain the necessary levels in your blood. Vitamin D then signals your kidneys to decrease the amount of calcium excreted in the urine.
All of these actions help to regulate the amount of calcium in your blood.
Vitamin D May Prevent Some Cancers and Diabetes
Research studies have shown that breast, colon, and prostate cancers are more prominent in individuals living in sun-poor areas of the world than in those living in sunny regions. Vitamin D helps regulate the growth and differentiation of certain cells. Researchers speculate that an inadequate amount of vitamin D in the body may reduce the proliferation of the healthy cells, and allow cancer cells to flourish.
Vitamin D may also help prevent diabetes mellitus. Many individuals with type 2 diabetes mellitus have low blood levels of vitamin D. One study revealed that insulin resistance, the inability of the cells to effectively use insulin in the blood, was greater among those with low blood levels of vitamin D, suggesting that the vitamin plays a role in insulin sensitivity.
Vitamin D May Help Regulate the Immune System and Blood Pressure
Vitamin D may also reduce the risk of developing certain autoimmune disorders, such as inflammatory bowel syndrome.
Most cells in the immune system have a receptor for vitamin D. The role of vitamin D is still not understood, but some researchers suggest that it may affect the function of the immune system and inhibit the development of autoimmune diseases.
Vitamin D may help reduce high blood pressure by acting on the gene that is involved in regulating blood pressure.
Blood pressure readings tend to be higher during the winter, when people are exposed to less sunlight, than in the summer.
People with mild hypertension may be able to lower their blood pressure by consuming adequate vitamin D.
Daily Needs
Not everyone can rely on the sun to meet their daily vitamin D needs. During the winter months in areas above latitudes of approximately 40 degrees north (Boston, Toronto, Salt Lake City) and below approximately 40 degrees south (Melbourne, Australia), sun exposure isn’t strong enough to synthesize vitamin D in the skin.
Individuals with darker skin, such as African-Americans, have a higher amount of the skin pigment melanin, which reduces vitamin D production from sunlight.
These individuals need a longer period of sun exposure, compared to a person with less melanin, to derive the same amount of vitamin D. The use of sunscreen can also block the body’s ability to synthesize vitamin D by more than 95 percent.
Because of these variables involving sun exposure, your daily vitamin D needs are based on the amount you would need to eat in foods and are not based on the synthesis of vitamin D in your skin from sunlight.
Based on the important roles that vitamin D plays in your body, it is currently recommended that adults ages 19 to 70 consume 15 micrograms, or 600 IU, of vitamin D daily. This is a significant increase from previously recommended daily amounts. Also based on revised Dietary Reference Intakes, adults over the age of 70 should incorporate 20 micrograms, or 800 IU, into their daily consumption.
When you are reading labels to assess the amount of vitamin D in your foods, keep in mind that the Daily Value (DV) on the Nutrition Facts panel is set at 400 IU, twice the current amount recommended for many adults.
Food Sources
One of the easiest ways to get your vitamin D from food is to drink fortified milk, which provides 100 IU, or 21⁄2 micrograms, of vitamin D per cup. Other than fatty fish (such as sardines and salmon) and fortified milk, breakfast cereals, juice, and yogurt, very few foods provide ample amounts of vitamin D. With this scarcity of naturally occurring vitamin D–rich sources, it isn’t surprising that many Americans are not meeting their daily dietary vitamin D needs.34
Too Much or Too Little
Consuming too much vitamin D can cause loss of appetite, nausea, vomiting, and constipation. The upper level for vitamin D has been set at 2,000 IU (50 micrograms), which is three to ten times higher than recommended daily.
As with the other fat-soluble vitamins, excess amounts of vitamin D are stored in the fat cells, and an accumulation can reach toxic levels, causing hypervitaminosis D. This condition causes over absorption of calcium from the intestines as well as calcium loss from bones. When both of these symptoms occur, blood calcium levels can become dangerously high.
A chronically high amount of calcium in the blood, or hypercalcemia (hyper = over, calc = calcium, emia = blood), can cause damaging calcium deposits in the tissues of your kidneys, lungs, blood vessels, and heart. Excess vitamin D can also affect your nervous system and cause severe depression.
The good news is that it is highly unlikely that you will get hypervitaminosis D from foods, even fortified foods. The only exception is fish oils, specifically cod-liver oil, which provides 1,360 IU of vitamin D per tablespoon. Luckily, the less-than pleasant taste of this oil is a safeguard against overconsumption. A more likely culprit behind hypervitaminosis D is the overuse of vitamin D supplements.
Sun worshippers don’t have to worry about getting hypervitaminosis D from the sun (although they should be concerned about the risk of skin cancer). Overexposing the skin to UV rays will eventually destroy the inactive form of vitamin D in the skin, causing the body to shut down production of vitamin D.
Rickets on the Rise
Rickets is a vitamin D deficiency disease that occurs in children. The bones of children with rickets aren’t adequately mineralized with calcium and phosphorus, and this causes them to weaken. Because of their “soft bones,” these children develop bowed legs, as they are unable to hold up their own body weight when they are standing upright. Since milk became fortified with vitamin D in the 1930s, rickets has been considered a rare disease among children in the United States. However, the disease has once again become a public health concern. In the late 1990s, a review of hospital records in Georgia suggested that as many as five out of every 1 million children between 6 months and 5 years of age were hospitalized with rickets associated with a vitamin D deficiency. This probably underestimates the prevalence of rickets in the state, as only hospitalized children were investigated. Similarly, more than 20 percent of more than 300 adolescents at a Boston-based hospital clinic were recently found to be deficient in vitamin D.
Changes in the diets and lifestyles of children provide clues as to why rickets is on the rise in America. One factor may be the increased consumption of soft drinks.
A U.S. Department of Agriculture (USDA) report found that the number of children, who drank soft drinks, in and outside of school, has more than doubled over a 20-year period. This displacement of milk (a good source of vitamin D) with soft drinks (a poor source) is causing many children to come up short in their vitamin D intake.
Increased concern over skin cancer may be another factor. Skin cancer is the most common form of cancer in the United States, and childhood sun exposure appears to increase the risk of skin cancer in later years. Because of this, organizations such as the Centers for Disease Control and the American Cancer Society have run campaigns that recommend limiting exposure to ultraviolet light.
People are encouraged to use sunscreen, wear protective clothing when outdoors, and minimize activities in the sun. The American Association of Pediatricians also recommends that infants younger than 6 months not be exposed to direct sunlight and that children use sunscreen before going outside. With less exposure to UV light, many children aren’t able to synthesize vitamin D in adequate amounts to meet their needs, thereby increasing their risk of developing rickets.
The increased use of child day-care facilities, which may limit outdoor activities during the day, may also play a role in this increased prevalence of rickets.
Finally, air pollution reduces the ultraviolet rays of the sun by as much as 60 percent-another factor limiting the production of vitamin D in the skin. In fact, children living in an industrial, polluted region of India were shown to have less vitamin D in their blood than children living in a less polluted area of the country.
Other Vitamin D Deficiency Disorders
Osteomalacia is the adult equivalent of rickets and can cause muscle and bone weakness and pain. The bones can’t mineralize properly because there isn’t enough calcium and phosphorus available in the blood. Although there may be adequate amounts of these minerals in the diet, the deficiency of vitamin D hampers their absorption.
Vitamin D deficiency and its subsequent effect on decreased calcium absorption can lead to osteoporosis, a condition in which the bones can mineralize properly, but there isn’t enough calcium in the diet to maximize the bone density, or mass.
Table Tips
Dynamite Ways to Get Vitamin D
Use low-fat milk, not cream, in your hot or iced coffee.
Buy vitamin-D–fortified yogurts and have one daily as a snack. Top it with a vitamin-D–fortified cereal for another boost of “D.”
Start your morning with cereal, and douse it with plenty of low-fat or skim milk.
Flake canned salmon over your lunchtime salad.
Make instant hot cocoa with hot milk rather than water.
