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6 things you should know about vitamin D
Figuring out all the factors that can affect your vitamin D level is complicated. Your body makes vitamin D when sunlight hits the skin. You can also get the vitamin from food (mainly because it’s been added; few foods are natural sources of vitamin D) or by taking a supplement.
The process by which the body makes vitamin D is complex. It starts when the skin absorbs rays in the invisible ultraviolet B (UVB) part of the light spectrum. The liver and the kidneys also participate to make a form of the vitamin that the body can use.
Get your copy of Vitamins and Minerals
About half of all Americans routinely take dietary supplements. The most common ones are multivitamin and multimineral supplements. This report explains the evidence behind the benefits and safety profiles of various vitamins and minerals. It also includes the recommended minimum and maximum amounts you should consume, as well as good food sources of each.
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A number of factors influence a person’s vitamin D levels. Here are six important ones.
Where you live. The further away from the Equator you live, the less vitamin D–producing UVB light reaches the earth’s surface during the winter. Residents of Boston, for example, make little if any of the vitamin from November through February. Short days and clothing that covers legs and arms also limit UVB exposure.
Air quality. Carbon particles in the air from the burning of fossil fuels, wood, and other materials scatter and absorb UVB rays, diminishing vitamin D production. In contrast, ozone absorbs UVB radiation, so pollution-caused holes in the ozone layer could end up enhancing vitamin D levels.
Use of sunscreen. Sunscreen prevents sunburn by blocking UVB light. Theoretically, that means sunscreen use lowers vitamin D levels. But as a practical matter, very few people put on enough sunscreen to block all UVB light, or they use sunscreen irregularly, so sunscreen’s effects on vitamin D might not be that important. An Australian study that’s often cited showed no difference in vitamin D between adults randomly assigned to use sunscreen one summer and those assigned a placebo cream.
Skin color. Melanin is the substance in skin that makes it dark. It “competes” for UVB with the substance in the skin that kick-starts the body’s vitamin D production. As a result, dark-skinned people tend to require more UVB exposure than light-skinned people to generate the same amount of vitamin D.
Weight. Body fat sops up vitamin D, so it’s been proposed that it might provide a vitamin D rainy-day fund: a source of the vitamin when intake is low or production is reduced. But studies have also shown that being obese is correlated with low vitamin D levels and that being overweight may affect the bioavailability of vitamin D.
Age. Compared with younger people, older people have lower levels of the substance in the skin that UVB light converts into the vitamin D precursor. There’s also experimental evidence that older people are less efficient vitamin D producers than younger people.
For more information on the benefits of vitamin D as well as advice on making sure you get the proper amount of vitamins and minerals in your diet, buy Vitamins and Minerals, a Special Health Report from Harvard Medical School. |
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News and Views from the Harvard Health Blog
Despite improved survival, type 1 diabetes still shaves an average of 12 years off life expectancy. Better blood sugar control may be one key to longer survival.
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Does your diet deliver?
It’s easy enough to look up how much vitamin C or calcium you should get each day. It is also easy to read the back of a vitamin bottle to see just how much of a given vitamin or mineral the product provides. But how can you tell if what you eat — which should be your primary source of important nutrients — is giving you what you need?
One way is to focus on the big picture: eat a balanced diet that contains a variety of colorful fruits and vegetables, whole grains, beans, nuts, dairy products, seafood, lean meats, and poultry. Focus on nutrient-dense foods like these, which are packed with vitamins and minerals relative to the number of calories they deliver, and you should be fine.
To really optimize your diet, keep these two additional tips in mind.
Limit liquid sugars. Soft drinks, sports drinks, energy drinks, and other sugar-sweetened beverages can deliver up to 12 teaspoons of sugar in a single serving, with no other useful nutrients. These beverages offer no health or nutritional benefits. Worse, regular consumption of these drinks can increase your chances of becoming obese or developing diabetes — both of which raise your risk for heart disease and other chronic conditions. Unsweetened coffee or tea or sparkling water are better choices.
Cut back on refined carbohydrates. White bread, many breakfast cereals, packaged snack foods, and potato chips and French fries deliver mainly pure starch — which the body quickly converts to sugar — with few other nutrients or fiber. Better choices are whole grains, breads made with whole grains, high-fiber breakfast cereals, brown rice, steel-cut oats, fruits and vegetables, and beans. A good general rule is to choose foods that have at least one gram of fiber for every 10 grams of carbohydrate. Even better, aim for a gram of fiber for every 5 grams of carbohydrate.
For more information and advice on making sure you get the proper amount of vitamins and minerals in your diet, buy Vitamins and Minerals, a Special Health Report from Harvard Medical School. |
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Vitamins and Minerals
Featured content:
| • | Vitamins and minerals: The basics |
| • | Federal guidelines |
| • | Making sense of scientific studies |
| • | Cast of characters: From vitamin A to zinc |
| • | SPECIAL BONUS SECTION: Does your diet deliver the daily recommended dose? |
| • | ... and more! |
Click here to read more » |
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