Although vitamin and nutrient deficiencies that cause disease are uncommon in Americans, certain populations are at risk for true deficiency of certain vitamins and minerals.

"For instance, vegans may be deficient in vitamin B-12, older adults in northern areas may be deficient in vitamin D, and alcoholics may be deficient in folate, thiamine and others," says Dr. Kathleen Fairfield, of the Maine Medical Center and author of a review on vitamins and minerals in the Journal of the American Medical Association.

These nutrients are of concern to specific populations.

VITAMIN D

Why you need it: Vitamin D is found in food but can also be made in your body with exposure to sunlight. It maintains normal levels of calcium and phosphorus, promotes calcium absorption and bone mineralization and is associated with fewer fractures when taken with calcium.

How much you need: 200 international units for men and women ages 19-50; 400 IU for men and women over 50.

Tolerable upper level: 2,000 IU.

If you don't get enough: Inadequate vitamin D causes rickets in children or osteomalacia in adults, skeletal diseases that weaken bones.

Who needs it most: Older adults may benefit from supplemental vitamin D. Vitamin D-deficient diets are also associated with milk allergies, lactose intolerance and strict vegetarianism. People with limited sun exposure, such as homebound individuals, may also be deficient.

Getting enough: Vitamin D can be difficult to obtain from natural food sources, so your best option might be fortified foods and adequate sunlight exposure. Salmon (3 1/2 ounces): 360 IU; fortified milk (1 cup): 98 IU; whole egg: 20 IU.

IRON

Why you need it: Iron is necessary for hemoglobin, which carries oxygen in the blood, and myoglobin, which carries oxygen in muscle tissue. It helps regulate cell growth and differentiation.

Who needs it most: Infants, teenage girls, pregnant women and the elderly.

How much you need: Men: 8 mg; women: 18 mg.

Tolerable upper limit: 45 mg.

If you don't get enough: Iron deficiency limits oxygen delivery to the cells, resulting in fatigue, poor work performance, decreased immunity and, during childhood, difficulty in maintaining body temperature.

Getting enough: The best sources include organ meats (3 ounces liver: 5.2-9.9 mg,) clams (3 ounces: 23.8 mg), oysters (3 ounces: 10.2 mg), pumpkin seeds (1 ounce: 4.2 mg). In general, dietary iron is absorbed poorly, although animal sources (including poultry, red meat and fish) seem to be better absorbed. Plant sources include dried fruits, leafy green vegetables, nuts, legumes and whole grains.

VITAMIN B-12

Why you need it: Vitamin B-12 is necessary for processing carbohydrates, proteins and fats and maintaining healthy nerve and red blood cells. It's also required to make DNA, the genetic material in all cells.

Who needs it most: Older adults and vegetarians may benefit from vitamin B-12 supplements or foods fortified with vitamin B-12. Up to 30 percent of adults 50 years and older may have a condition limiting B-12 absorption from food. They are, however, able to absorb synthetic B-12 added to fortified foods and supplements.

How much: 2.4 micrograms for men and women.

Tolerable upper limit: There is a very low risk of toxicity.

If you don't get enough: Problems associated with B-12 deficiency include anemia, fatigue, constipation, sore tongue or mouth, loss of appetite and weight loss. Deficiency also can lead to neurological changes such as numbness and tingling in the hands and feet, as well as balance problems, depression, dementia and poor memory. If left untreated, damage may be permanent.

Getting enough: B-12 is found naturally in animal foods such as meat, poultry, fish, milk, dairy products and eggs (be careful of saturated fat and high calories). You can also get B-12 from fortified cereals. Mollusks (3 ounces): 84.1 micrograms; 1 slice braised beef liver: 47.9 micrograms; 3/4 cup fortified breakfast cereal: 6.0 micrograms.

FOLATE

Why you need it: The B vitamin folate (also called folic acid) is widely known for its role in reducing the risk of serious birth defects, such as spina bifida. Folate helps the body form DNA and blood cells and may help reduce the risk of cardiovascular disease. A recent study in the American Journal of Clinical Nutrition has also shown that folate may help prevent mental decline as we age.

Who needs it most: Any woman who can become pregnant needs an adequate supply of folate.

How much you need: 400 micrograms.

Tolerable upper limit: 1,000 micrograms.

Getting enough: Folate is found in legumes, oranges and green leafy vegetables. Synthetic folate, folic acid, is added to grain products, making them good sources as well. Breakfast cereal (1/2 to 1 1/2 cup): 100 to 400 micrograms; 3 ounces beef liver: 185 micrograms; 1/2 cup lentils: 180 micrograms; 1/2 cup chickpeas: 141 micrograms; 1/2 cup asparagus: 132 micrograms; 1/2 cup cooked spinach: 131 micrograms; 1/2 cup black beans: 128 micrograms.

Charles Stuart Platkin is a nutrition and public-health advocate. Write to info@thedietdetective.com.