Health Benefits of Vegetarian Diets
Author: Winston Craig, MPH, PhD, RD.
Recently, there has been a renewed interest in vegetarian diets. Today there are countless books, cookbooks, and magazine articles promoting vegetarian diets and providing guidance for those who wish to follow a meatless diet.
A Short Historical Perspective on Vegetarian Diets
In the past, many viewed vegetarianism as strange and faddish but appropriately planned vegetarian diets are now recognized by many, including the American Dietetic Association, as being nutritionally adequate, and providing healthful benefits in the prevention and treatment of chronic diseases (1).
Choosing a nonvegetarian lifestyle has a significant health and medical cost. The total direct medical costs in the United States attributable to meat consumption were estimated to be $30-60 billion a year, based upon the higher prevalence of hypertension, heart disease, cancer, diabetes, gallstones, obesity and food-borne illness among omnivores compared with vegetarians (2).
A large body of scientific literature suggests that the consumption of a diet of whole grains, legumes, vegetables, nuts, and fruits, with the avoidance of meat and high-fat animal products, along with a regular exercise program is consistently associated with lower blood cholesterol levels, lower blood pressure, less obesity and consequently less heart disease, stroke, diabetes, cancer, and mortality (1,3, 4). In African-Americans, the frequent consumption of nuts, fruits and green salads was associated with 35-44 percent lower risk of overall mortality (5).
A vegetarian diet is distinguished from an omnivorous diet by its content of dry beans and lentils. These take the place of meat and fish as the major source of protein. And there are so many different kinds of beans you can choose from - kidney, lima, pinto, cranberry, navy, Great Northern, garbanzo, soy beans, and black-eyed peas. These can be served with rice, added to soups, stews, and salads or a variety of casseroles, and made into different ethnic dishes.
Tofu, or soy bean curd, can be used in dips and spreads, or served with pasta or stir-fried vegetables. Soy protein contains isoflavones, such as genistein and daidzein, that act as phytoestrogens and inhibit tumor growth, lower blood cholesterol levels, decrease the risk of blood clots, and diminish bone loss. These benefits clearly translate into a lower risk of heart disease, stroke, cancer and osteoporosis (6).
A major report published by the World Cancer Research Fund in 1997 recommended we lower our risk of cancer by choosing predominantly plant-based diets rich in a variety of vegetables and fruits, legumes and minimally processed starchy staple foods, and to limit the intake of grilled, cured and smoked meats and fish. These methods of preparing meat produce polycyclic aromatic hydrocarbons and heterocyclic amines which are carcinogenic (11).
Over 200 studies have revealed that a regular consumption of fruits and vegetables provides significant protection against cancer at many sites. People who consume higher amounts of fruits and vegetables have about one-half the risk of cancer, especially the epithelial cancers (7). The risk of most cancers was 20-50% lower in those with a high versus a low consumption of whole grains (8).
About three dozen plant foods have been identified as possessing cancer-protective properties. These include cruciferous vegetables (broccoli, Brussels sprouts, cabbage, cauliflower), umbelliferous vegetables and herbs (carrots, celery, cilantro, caraway, dill, parsley), other fruits and vegetables (citrus, tomatoes, cucumber, grapes, cantaloupe, berries), beans (soybeans), whole grains (brown rice, oats, whole wheat), flaxseed, many nuts, and various seasoning herbs (garlic, scallions, onions, chives, ginger, turmeric, rosemary, thyme, oregano, sage, and basil)(9).
These foods and herbs contain of host of cancer-protective phytochemicals such as carotenoids, flavonoids, isothiocyanates, isoflavones, ellagic acid, glucarates, curcurmins, liminoids, lignans, phenolic acids, phthalides, saponins, phytosterols, sulfide compounds, terpenoids, and tocotrienols. These beneficial compounds alter metabolic pathways and hormonal actions that are associated with the development of cancer, stimulate the immune system, and have antioxidant activity (10).
Regular fruit and vegetable consumption reduces the risk of ischemic heart disease. A recent survey of 47,000 Italians found that persons in the highest tertile of vegetable consumption had a 21and 11% reduced risk of myocardial infarction and angina, respectively, compared with those in the lowest tertile of vegetable consumption (12).
A British study found that daily consumption of fresh fruit was associated with a 24 percent reduction in mortality from heart disease and a 32 percent reduction in death from cerebrovascular disease, compared with less frequent fruit consumption. Daily consumption of raw salad was associated with a 26 percent reduction in mortality from heart disease (13).
In another study, lifelong vegetarians had a 24 percent lower incidence and lifelong vegans (those who eat no eggs or dairy products) had a 57 percent lower incidence of coronary heart disease compared to meat eaters (14). Healthy volunteers who consumed a vegetarian diet (25% of calories as fat) that was rich in green, leafy vegetables and other low-calorie vegetables (tomatoes, cucumbers, carrots, bell peppers, celery, green beans, etc.), fruits, nuts, sweet corn and peas experienced after two weeks decreases of 25, 33, 20 and 21 percent in total cholesterol, LDL cholesterol, triglycerides, and total/HDL cholesterol ratio, respectively (15).
Various factors exist in fruits and vegetables that provide possible protection against cardiovascular disease. These factors include folic acid, dietary fiber, potassium, magnesium, carotenoids, phytosterols, flavonoids, and other polyphenolic antioxidants. Typically, vegetarian diets are also somewhat lower in saturated fat and cholesterol. Vegetarians typically have lower blood cholesterol levels. Plant diets rich in soluble fiber (such as found in dry beans, oats, carrots, squash, apples, and citrus) are useful for lowering serum cholesterol levels.
The many flavonoids in fruits, vegetables, nuts and whole grains, have extensive biological properties that reduce the risk of heart disease. Flavonoids are among the most potent antioxidants. They protect LDL cholesterol from oxidation; inhibit the formation of blood clots; and have hypolipidemic effects and anti-inflammatory action (16). European studies found that those who had the highest consumption of flavonoids had 60 percent less mortality from heart disease and 70 percent lower risk of stroke than the low flavonoid consumers (17,18).
The yellow-orange and red carotenoid pigments in fruits and vegetables are powerful antioxidants that can quench free radicals and protect against cholesterol oxidation. Persons with high levels of serum carotenoids have a reduced risk of heart disease. The recent EURAMIC study found that a high intake of lycopene (the red pigment in tomatoes, pink grapefruit, and watermelon) was associated in men with a 48 percent lower risk of a myocardial infarction compared with a low intake of lycopene (19). Cholesterol synthesis is suppressed and LDL receptor activity is augmented by the carotenoids beta-carotene and lycopene, similar to that seen with the drug fluvastatin (20).
Berries, Beans and Grains
Anthocyanin pigments, the reddish pigments found in fruits, such as strawberries, cherries, cranberries, raspberries, blueberries, grapes, and black currants, are very effective in scavenging free radicals, inhibiting LDL cholesterol oxidation and inhibiting platelet aggregation. Various terpenoids in fruits and vegetables, and tocotrienols in nuts and seeds facilitate lower blood cholesterol levels, by inhibiting HMG-CoA reductase (21). Garlic, onions and other members of the Allium family, contain a variety of ajoenes, vinyldithiins, and other sulfide compounds that have antithrombotic action and may lower blood cholesterol and triglyceride levels.
A number of studies have shown that legumes lower blood cholesterol levels, improve blood sugar control, and lower triglyceride levels. Since beans are good sources of soluble fiber, vegetable protein, saponins, phytosterols and polyunsaturated fat, consuming a diet rich in legumes will lower risk of heart disease.
In the Nurses' Health Study, the highest consumption of whole grains was associated with about a 35-40% reduction in risk of heart disease, stroke, and type 2 diabetes. In the Adventist Health Study a regular consumption of whole wheat bread was associated with a 40 to 50% reduced risk of fatal and non-fatal heart disease.
Epidemiological studies have consistently reported that frequent nut consumption is associated with a 30-60% reduction in the risk of coronary heart disease (22). A number of clinical trials have demonstrated the effectiveness of diets containing almonds, pecans, peanuts, hazelnuts, pistachios, macadamia nuts, or walnuts to significantly lower LDL cholesterol levels by 7 to 16 percent, without much change in HDL cholesterol and triglyceride levels (22).
While nuts are high in fat, they are naturally low in saturated fat and most are quite rich in monounsaturated fat. Nuts also contain a number of vitamins, minerals and other substances important for cardiovascular health, such as potassium, magnesium, vitamin E, folic acid, copper, and dietary fiber. In addition, most nuts contain phytosterols, tocotrienols, and protective polyphenolics such as ellagic acid and flavonoids (23).
Stroke and Diabetes
Data from two prospective studie supports a protective relationship between fruit and vegetable consumption and risk of ischemic stroke (24). Cruciferous and green leafy vegetables and citrus fruits were the most protective. Data from the NHANES study revealed that consuming fruit and vegetables three or more times a day compared with less than once a day was associated with a 27% lower incidence of stroke, a 42% lower stroke mortality, a 27% lower cardiovascular disease mortality, and a 15% lower all-cause mortality (25). In the Adventist Health Study, non-vegetarians had a risk of fatal stroke that was 20-30% higher than the vegetarians. Data from population studies and human trials provide evidence that vegetarian dietary patterns lower blood pressure (26). Lower systolic blood pressures in elderly vegetarians has been reported to be best accounted for by their lower body weight (27). Vegetarians living in northern Mexico, were found to have lower body weights, higher potassium and lower sodium intakes, and lower mean blood pressures than non-vegetarians (28).
Higher consumption of nuts (29) and whole grains (30) has been associated with lower rates of diabetes. In a large prospective study, fruit and vegetable intake was found to be inversely associated with the incidence of diabetes, particularly among women (31). Men and women who reported seldom or never eating fruit or green leafy vegetables had higher mean HbA1C levels than those who had more frequent consumption (32). An increased consumption of fruit and vegetables appears to contribute to the prevention of diabetes.
The consumption of a generous supply of whole grains, legumes, nuts, fruits and vegetables provides protection against chronic diseases such as cancer, cardiovascular disease and diabetes. A plant-based diet is rich in its content of health-promoting factors such as the many phytochemicals.
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In postmenopausal women that had type 2 diabetes, daily supplementation with 30 grams of soy protein containing 132 mg of isoflavones (phytoestrogens) favorably altered their insulin resistance, blood glucose control, and serum lipids, and hence improved their cardiovascular risk profile. After 3 months of using the isoflavone-rich soy protein, the mean values for fasting insulin had dropped 8%, insulin resistance was 6.5% lower, and LDL cholesterol dropped 7%. No significant change occurred in HDL cholesterol or triglycerides.