Vitamin and mineral consumption
Vitamins
Vitamins are either fat soluble or water soluble. Vitamins A, D, E, and K are fat soluble. They are found in fatty foods and are absorbed, transported, and stored with fat. Because excretion is minimal and storage in fat is abundant, deficiencies of fat-soluble vitamins are rare, but toxic amounts can accumulate if intake is excessive. Vitamin C and the B-complex group are water soluble; they are absorbed in the intestine, bound to transport proteins, and excreted in the urine. Because storage is minimal, water-soluble vitamins should be ingested regularly, and except for large doses of B3 and B6, toxicity is rare [see Table 3].
Although there is great disparity between popular beliefs about vitamins and their known physiologic effects, new medical information may narrow the gap. First, it is becoming clear that many persons in the United States, particularly the elderly and the poor, do not consume adequate amounts of vitamin-rich foods. Second, laboratory and animal experiments demonstrate that antioxidant vitamins can retard atherogenesis and suggest that antioxidants may lower the risk of carcinogenesis. Indeed, many epidemiologic and observational studies have demonstrated an association between a low dietary intake or low plasma levels of antioxidants and an increased risk of atherosclerosis and certain cancers. Similarly, studies have linked low levels of folic acid, vitamin B6, and vitamin B12 with elevated blood homocysteine levels and increased cardiovascular risks.39 People who consume multivitamins appear to have a reduced risk of coronary artery disease40 and colon cancer41; protection may be attributed principally to folic acid. However, with the exception of the Cambridge Heart Antioxidant Study, which demonstrated the efficacy of vitamin E in reducing the risk of myocardial infarction in patients with coronary artery disease, randomized trials have not demonstrated benefit from vitamin supplements.42 Moreover, |-carotene supplements actually appear to increase the risk of lung cancer in smokers,43 and hypervitaminosis A is linked to an increased risk of frac-tures.44 It is clear that additional studies are required to clarify the impact of vitamins on health.
Women of childbearing age, the elderly, and people with suboptimal nutrition should take a single multivitamin tablet daily; others may benefit as well.45 Strict vegetarians should take vitamin B12 in the recommended daily amount (2-4 |g); because many people older than 60 years have atrophic gastritis and cannot absorb B12 bound to food protein, they may also benefit from supplementary B12. Multivitamin supplements may also be necessary to avert vitamin D deficiencies, particularly in the elderly.46 A supplement that combines antioxidants with zinc can slow the progression of age-related macular de-generation.47 Use of so-called megadose vitamins should be discouraged. Expensive brand-name and so-called all-natural preparations are no more effective than reputable generic preparations. In any case, vitamin supplements should never be used as a substitute for a balanced healthful diet that provides abundant amounts of vitamin-rich foods.
Minerals
Although minerals are chemically the simplest of nutrients, their roles in metabolism and health are complex. At least 16 minerals are essential for health [see Table 4]; 10 are classified as trace elements because only small amounts are required.
Table 4 Essential Minerals and Trace Elements
|
Minerals and Elements |
RDA/ESADDI for Healthy Individuals |
|
Macrominerals |
|
|
Calcium |
1,000 mg before age 50; 1,200 mg after age 50 |
|
Phosphorus |
800 mg |
|
Magnesium |
Men, 350 mg; women, 280 mg |
|
Sodium |
1,100-3,300 mg |
|
Potassium |
1,875-5,625 mg |
|
Chloride |
1,700-5,100 mg |
|
Trace elements Iron |
Men and postmenopausal women, 8 mg; premenopausal women, 18 mg; pregnant women, 27 mg |
|
Zinc |
Men, 15 mg; women, 12 mg |
|
Iodine |
 |
|
Copper |
1.5-3.0 mg |
|
Manganese |
2-5 mg |
|
Fluoride |
1.5-4.0 mg |
|
Chromium |
 |
|
Molybdenum |
 |
|
Selenium |
 |
|
Cobalt |
Required in small amount as a component of vitamin B12 |
ESADDI—estimated safe and adequate daily dietary intake
RDA—recommended dietary allowance
Other minerals, such as boron, nickel, vanadium, and silicon, have been shown to be essential in various animal studies but have not been found to be necessary for humans. Many persons in the United States consume too little of some minerals (e.g., calcium and iron) or too much of others (e.g., sodium).
Sodium The body can conserve sodium so effectively that only small amounts are required in the diet. The Food and Nutrition Board of the National Academy of Science estimates that an intake of no more than 500 mg of sodium a day is needed for health; the average American diet contains more than 4,000 mg a day.
Population studies have demonstrated conclusively that a high sodium intake increases blood pressure, especially in older people.48 The Dietary Approaches to Stop Hypertension (DASH) trial demonstrated that reduction of sodium intake from high amounts to moderate amounts will result in lower blood pressures and that further reductions in sodium intake will produce additional benefits.49 When combined with other elements of the DASH diet (increased consumption of fruits, vegetables, whole grains, and low-fat dairy products, along with decreased consumption of saturated fat and sugar), sodium restriction can lower systolic blood pressure by an average of 7.1 mm Hg in normotensive persons and 11.5 mm Hg in patients with hypertension. Hence, reductions in dietary sodium could substantially reduce the risk of stroke and coronary artery disease. A high sodium intake also increases urinary calcium excretion, which increases the risk of osteoporosis.
There is no RDA for sodium, and additional controlled clinical trials will be needed to provide conclusive evidence that sodium restriction is beneficial to normotensive persons. Pending such information, the AHA recommends that daily consumption of sodium not exceed 2,400 mg,19 and the National Academy of Science proposes a 2,000 mg maximum. Patients with illnesses such as hypertension, congestive heart failure, cirrhosis, and nephrotic syndrome may benefit from substantially lower sodium intakes.
About 80% of dietary sodium comes from processed foods. Physicians should review these hidden sources of salt with patients who would benefit from sodium restriction.
Calcium A high intake of calcium, either from dairy products or supplements,50 improves bone density. Dietary calcium intake is inversely related to blood pressure51 and to the risk of stroke51,52; however, calcium supplements produce only small reductions in systolic blood pressure.53 Calcium supplements appear to reduce the risk of colorectal adenomas54 but may increase the risk of prostate cancer.
At present, fewer than 50% of persons in the United States consume the RDA of calcium [see Table 4]. Persons who do not consume enough calcium from foods should consider a supplement such as calcium carbonate or calcium citrate. High-calcium diets do not increase the risk of nephrolithiasis,57 but prolonged overdoses of supplements may produce hypercalcemia (milk-alkali syndrome) or nephrolithiasis.
Iron Iron deficiency is the most common cause of anemia. In the United States, 9% to 11% of women of childbearing age are iron deficient and 2% to 5% have iron deficiency anemia, but only 1% of men are iron deficient. Routine administration of iron supplements is recommended only for infants and pregnant women58; dietary sources should provide adequate amounts of iron for other healthy people.
A high intake of iron is harmful for patients with hemochro-matosis and for others at risk for iron overload. A Finnish study linked high iron levels to cardiac risk.59 However, studies in the United States have not confirmed these observations, and one study indicated a possible inverse association between iron stores and mortality from cardiovascular disease and other causes.59
Potassium Dietary potassium is inversely related to blood pressure and to stroke mortality in hypertensive men.60 Although potassium supplements may assist in the treatment of hypertension,61 current data do not justify the routine use of potassium supplements. Physicians should encourage a high dietary potassium intake in most individuals,19,49 but low-potassium diets may be necessary for patients with renal disease or other conditions that cause hyperkalemia.
Selenium Selenium is a cofactor of the free radical scavenger enzyme glutathione peroxidase. A randomized clinical trial reported that selenium supplements of 200 ^g/day appear to reduce mortality from various cancers.62 Selenium levels have been inversely associated with mortality from prostate cancer63 and gastroesophageal malignancies.64 These data, however, do not yet support the routine use of selenium supplements, which can be toxic in high doses. Selenium is present in many foods, including tomatoes, poultry, shellfish, garlic, meat, egg yolks, and grains grown in selenium-rich soil.
Chromium Although chromium plays a role in glucose metabolism, there is no scientific basis for the claims that chromium supplements contribute to weight loss or increased energy. Chromium supplements may be beneficial for persons with low HDL cholesterol levels, but more study is needed. Dietary sources of chromium include brewer’s yeast, whole grains, legumes, peanuts, and meats.
Magnesium Magnesium deficiency is common in diabetics, alcoholics, patients who take diuretics, and hospitalized patients. Persons with hypomagnesemia may require magnesium supplements, but others can rely on foods such as green vegetables, whole grains, bananas, apricots, legumes, nuts, soybeans, and seafood to provide magnesium.
Water and food consumption
Water
On average, adults consume about 2 L/day of water, with two thirds coming from beverages and the remainder coming from food. Healthy people have no need to track their water intake. Patients with conditions such as nephrolithiasis and urinary tract infections may benefit from consciously increasing their fluid intake; patients who are at risk for hyponatremia should restrict their water consumption.
Foods
Fruits and vegetables Fruits and vegetables provide many desirable nutrients, including complex carbohydrates, fiber, vitamins, and minerals. Deep-green and yellow-orange vegetables may be particularly beneficial because of their carotenoids, and citrus fruits may be valuable because of their vitamin C, soluble fiber, and potassium. Cruciferous vegetables, such as cabbage, may reduce the risk of certain cancers. Vegetables and fruits are low in sodium and calories; none contain cholesterol, and only coconut, palm oil, and cocoa butter contain saturated fat.
The findings of case-control and cohort studies strongly suggest that the consumption of fruits and vegetables is inversely related to the risk of coronary artery disease,65 stroke,66 malignancies of the respiratory and digestive tracts,29 chronic obstructive lung disease,67 and all-cause mortality.68 A dietary-intervention trial demonstrated that a diet rich in vegetables, fruits, and low-fat dairy products can substantially reduce blood pressure.49 The United States Department of Agriculture’s Dietary Guidelines for Americans recommends eating two to four servings of fruit and three to five servings of vegetables a day; at present, only 35% of women and 19% of men meet these standards.69
Legumes Often neglected in the Western diet, legumes (beans, peas, and lentils) are rich in complex carbohydrates with low glycemic indices, iron, and B vitamins. Legumes are an excellent source of dietary fiber, including soluble fiber that can reduce blood cholesterol levels. Because of their high protein content, legumes are an excellent meat substitute. Soy protein can reduce blood cholesterol levels, and soy intake is inversely related to the risk of prostate and breast cancers.
Legumes can increase intestinal gas, causing bloating, flatulence, and cramps. Distress can be minimized by use of the nonprescription a-galactosidase preparation Beano.
Grains The seed-bearing fruits of grains, called kernels, consist of three layers: the inner germ, which contains vitamins and polyunsaturated fats; the middle endosperm, which contains complex carbohydrates; and the outer bran, which contains dietary fiber. Because milling removes the bran and endosperm, whole grains are nutritionally superior to refined grain; whole-grain consumption is inversely related to the risk of coronary artery disease70 and stroke.71 Whole-grain flour can be used to make cereals, baked goods, and even pasta. Whole grains such as brown rice, couscous, and yellow cornmeal (polenta) are easily prepared and healthful side dishes. Oats and barley contain soluble fiber that can lower blood cholesterol levels.
Meat and poultry Although meat is a source of protein, vitamins, and iron and other minerals, its high content of saturated fat, cholesterol, and calories makes it a potentially unhealthy food. Patients who eat meat should be encouraged to select lean cuts, trim away visible fat, and use cooking methods that remove, rather than add, fat. It is even more beneficial to reduce the amount of meat consumed by reducing portion size and frequency; a reasonable goal is to eat about 4 oz one to three times a week.
Poultry is a more healthful source of protein and other nutrients. Chicken and turkey are best, but the skin should be removed before cooking to reduce the fat content.
Dairy products and eggs To reduce intake of saturated fat and cholesterol, nonfat or low-fat dairy products can be substituted for whole-milk products. The use of nondairy creamers, imitation cheese, margarine, and other products that contain trans-fatty acids in partially hydrogenated vegetable oils should be limited. The consumption of up to one egg a day does not appear to increase the risk of cardiovascular disease in healthy, nondiabetic people,72 but additional egg-yolk consumption should be limited. One egg yolk contains about two thirds of the total amount of cholesterol that is recommended for an entire day. Egg whites and egg substitutes are good alternatives to egg yolks.
|
Table 5 Dietary Guidelines for Healthy People |
|
Eat more vegetable products than animal products |
|
Eat more fresh and homemade foods than processed foods Less than 30% of calories should come from fat |
|
Limit cholesterol to less than 300 mg a day |
|
Eat at least 25 g of fiber a day |
|
55%-65% of calories should come from complex carbohydrates |
|
10%-15% of calories should come from protein |
|
Limit sodium to less than 2,400 mg a day |
|
Obtain 1,200-1,500 mg of calcium a day from food or supplement |
|
Eat 6 or more servings of grain products a day |
|
Eat 3-5 servings of vegetables and legumes a day |
|
Eat 2-4 servings of fruit a day |
|
Eat two 4 oz servings of fish a week |
|
Eat no more than two 4 oz servings of red meat a week |
|
Chicken and turkey should be eaten in moderation with skin removed |
|
Eat no more than one egg yolk a day, including those used in cooking and baking |
|
Use vegetable oils, preferably olive and canola oils, in moderation |
|
Have no more than two alcoholic drinks a day |
|
Adjust caloric intake and exercise level to maintain a desirable body weight |
|
Avoid fad diets and extreme or unconventional nutrition schemes |
|
Avoid untested nutritional supplements, including megavitamins, herbs, food extracts, and amino acids |
Fish A 1997 study reported that participants who consumed 245 g or more of fish a week enjoyed a 38% reduction in fatal myocardial infarctions over a 30-year period.73 Although at least three other observational studies did not find that fish consumption was protective, a 1989 intervention trial that randomized 2,033 myocardial infarction survivors to usual care or usual care plus fish consumption found that eating two or three fish meals a week reduced 7-year mortality by 29%.74 Fish consumption has also been associated with a reduced risk of primary cardiac arrest,75 hypertension, stroke,9 and prostate cancer.7677 As little as 4 oz of fish twice a week may provide protection.19 Fish should be baked, broiled, grilled, steamed, or poached rather than fried, and high-fat sauces should be avoided. Because of their higher content of omega-3 fatty acids, oily, deep-water fish may be best. People who are reluctant to eat fish may benefit from fish oil supplements in the modest dose of about 1 g/day.78 More study is required to confirm the value of fish and fish oil and to define the optimal types and amounts of fish.
Cooking oils Canola oil contains an omega-3 fatty acid, a-linolenic acid. High serum levels of a-linolenic acid have been associated with a decreased risk of stroke, and consumption of canola oil is inversely related to the risk of myocardial infarc-tion.12 Canola oil and olive oil have a high content of oxidation-resistant monounsaturated fatty acids. Olive oil may be a car-dioprotective element in the Mediterranean diet and may also reduce the risk of breast cancer. Although more study is needed, canola and olive oils appear to be the most beneficial oils for food preparation.
Nuts Nuts are high in monounsaturated and polyunsaturat-ed fatty acids and fiber. Nut consumption appears to be inversely related to the risk of coronary artery disease79 and diabetes.80
Garlic Medical studies of garlic have shown mixed results. Some meta-analyses suggest that garlic extracts can improve blood cholesterol levels, but others do not.81 The putative benefits of garlic on blood pressure and coagulation are even less clear.
Flavonoid-rich foods Flavonoids are polyphenolic antioxi-dants that are found in a variety of vegetable foods, including apples, onions, tea, and red wine. Although not all studies agree, consumption of these foods has been inversely related to the risk of coronary artery disease and stroke.82
Alcohol Rarely regarded as a nutrient, alcohol should be considered when dietary recommendations are formulated. Containing 7 cal/g, alcohol is a calorie-dense food. Numerous studies demonstrate that low to moderate alcohol consumption substantially reduces the risk of coronary artery disease, peripheral vascular disease, and all-cause mortality.83 The major mechanism of protection is alcohol’s ability to increase HDL cholesterol levels; favorable effects on blood coagulation mechanisms may also contribute. Protective doses of alcohol can be obtained from one to two drinks a day; 5 oz of wine, 12 oz of beer, or 1.5 oz of spirits is counted as one drink. Despite its an-tioxidant content, red wine is no more protective than other alcoholic beverages.84
Caffeine Studies have failed to confirm putative links between caffeine and peptic ulcers, hypertension, coronary artery disease, breast disease, or cancer. Caffeine can trigger migraines in sensitive individuals, and caffeine withdrawal can precipitate headaches or depression in habitual consumers. Caffeine can cause anxiety, insomnia, and gastroesophageal reflux. Brewed coffee can increase blood cholesterol levels, but filtered coffee does not. The effects of caffeine on pregnancy are not fully understood, but it is reasonable to discourage consumption of large amounts.85 Caffeine restriction does not reduce palpitations in patients with idiopathic premature ventricular contractions.
