This nutrient plays a vital role in bone formation and maintenance, largely by regulating calcium and phosphorus concentrations in the blood.
In children, a shortage of vitamin D can cause rickets, in which bones soften and become deformed. In adults, vitamin-D deficiency can lead to bone loss and fractures triggered by an overactive thyroid.
Natural sources of vitamin D in food are limited to a few items such as liver and fat from saltwater fish, seals, and polar bears, as well as eggs from hens that have been fed vitamin D. For this reason, cow's milk is artificially fortified with 400 International Units (IU) of vitamin D per quart. Some margarines, too, are fortified. So are some cereals.
Vitamin D is also manufactured by the human body in response to sunlight. This complicates the setting of dietary requirements. People who live in sunny latitudes or spend most of their time outdoors may need no dietary vitamin D, while people at high latitudes who remain cooped up for long periods may need a lot. To further confuse the issue, race and age may play a role: darker skin needs more sunlight to produce vitamin D, and so may the skin of people over 60.
It is also relatively easy to get too much vitamin D. For older people, toxic levels may be less than five times the Food and Nutrition Board's (FNB's) Adequate Intake (AI). (See "Upper Limit," below.) Overconsumption can lead to calcium deposits in soft tissues and irreversible kidney and cardiovascular damage.
Vitamin D and Osteoporosis
Vitamin D supplements, when combined with calcium supplements, have been shown to help prevent bone loss and fractures in the elderly. Some experts recommend daily supplements, especially for people who are past 60, have dark skin, live at high latitudes, or spend most of their time indoors.
Your Daily Allowance
Diet Power sets your Personal Daily Allowance (PDA) of vitamin D at the FNB's Adequate Intake (AI). The AI is designed only for people who don't get "adequate exposure to sunlight," however. If this doesn't apply to you, see "Revising Your Allowance," below.
The AI, as measured in the International Units (IU) used on food labels, is 200 for people 0* through 50 years old, 400 for people 51 through 70, and 600 for people over 70. During pregnancy and lactation it is 200 IU.
Patients on glucocorticoid therapy may need extra vitamin D.
* But remember that Diet Power is not designed for people under 15.
Revising Your Allowance
If your doctor recommends changing your PDA, use the Personal Daily Allowance Editor.
For teenagers and adults, the Tolerable Upper Intake Level (UL) of vitamin D is 2000 IU per day. Getting more than the UL may be harmful to your health.
If a label cites vitamin D in micrograms¼
¼you can convert them to IUs. See International Units of Vitamin D.
Vitamin-D levels are not cited for the 11,000 foods in Diet Power's original dictionary, but may be cited for the foods you add. Hence, the vitamin-D intake showing in your Nutrient History may be far lower than the amount you've actually consumed. To see whether a particular food has a vitamin-D reading, open the Food Dictionary and check the food's nutrient profile. If you find a question mark beside "Vitamin D," it means the amount is unknown. (To see all foods with unknown vitamin-D readings, click the dictionary's PowerFoods tab and sort the foods by vitamin-D power; then scroll to the bottom of the list.)
Color Coding of This Nutrient
The vitamin-D bar in your personal Nutrient History is:
· blue for "good" if you've logged 100 to 150 percent of your PDA
· red for "bad" if you've logged less than 100 percent of your PDA or more than the Tolerable Upper Intake Level (UL)
· yellow for "caution" if you've logged more than 150 percent of your PDA
· missing if you've logged no vitamin D.
In the nutrient profile of a food or recipe, the vitamin-D bar is:
· green for "good" if getting your entire PDA of calories from this item would give you more than 150 percent of your PDA of vitamin D
· magenta for "bad" if getting all your calories from the item would give you less than 50 percent of your PDA of vitamin D
· blue for "neutral" otherwise
· missing if the amount of vitamin D is either zero or (when the term Vitamin D is grayed out) unknown.
Vitamin D on Food Labels
Most food labels are not required to report vitamin-D content, but vitamin D-fortified milk and margarine do. They may cite the amount in International Units (IU) or as a percentage of the Daily Value.
The Daily Value for vitamin D is 400 IU. As suggested above, this amount may not be right for you¾it is a gross estimate meant to accommodate most of the U.S. population.
For more on label regulations, see Labels, Food.