Epidemiologic evidence of a protective role for fruit and vegetables in cancer prevention is substantial. A report commissioned by the World Cancer Research Fund and the American Institute for Cancer Research, estimates that diets high in fruit and vegetables (more than 400g per day) could prevent at least 20% of all cancer incidence (Table 2).
Table 2: Dietary factors associated with the different cancers
|Breast||· Energy intake
|Fruit and vegetables|
|Prostate||· Animal fat
· Red meat
Vitamin E supplementation
|Colorectal||· Red and processed meat
· High fat intake
· Overweight in men
· Physical inactivity
|Fruit and vegetables|
· Very hot drinks
· Charcoal broiled, fried and smoked foods
· Deficiencies of niacin, riboflavin, zinc and selenium
|Fruit and vegetables|
|Stomach||· Smoked, pickled and fried foods
· High salt intake
|Fruit and vegetables
|Lung||· High fat intake in men||Fruit and vegetables|
Fruit and vegetables are most effective against those cancers that involve epithelial cells such as cancer of the lung, cervix, oesophagus, stomach, colon and pancreas. A recent study from Greece, involving
2 400 women, noted that vegetable and fruit intakes were independently associated with significant reductions in the incidence of breast cancer. Woman who consumed at least 4-5 servings of vegetables per day had a 46% lower risk of breast cancer than women who had an intake of less than two servings per day. Women with the highest intake of fruit (6 servings per day) had a 35% lower risk than women who had the lowest intake of fruit (less than 2 servings).
The exact mechanism that explains the protective role of fruit and vegetables in cancer is yet to be determined, but is likely to be multifactorial. Antioxidant nutrients (e.g. vitamin C, vitamin E, beta-carotene and selenium) have been in the news for quite some time now regarding their role in cancer prevention. Theoretically, these nutrients should protect the body against the development and progression of cancer. There is, however, no conclusive scientific evidence regarding antioxidants, specifically, being the only active substances in fruit and vegetables which provide the protection against cancer. The results of many studies continue to suggest a much more complex role for specific micronutrients and “non-nutritive” substances, an area which is being actively and intensively investigated. It should also be borne in mind that fruit and vegetable consumption may also be an index, which reflects differences between individuals who eat or do not eat correctly rather than having any protective role against cancer.
Research has identified a host of active substances in foods that protect against cancer. These include allyl sulfates in garlic and onions; phytates in grains and legumes; glucarades in citrus, grain and solaceous vegetables; lignans in flax and soya beans; isoflavones in soya beans; saponins in legumes; indoles, isothiocyanates and dithiolthione in cruciferous vegetables; ellagic acid in grapes, strawberries, raspberries and nuts and a whole range of flavonoids, carotenoids and terpenoids in various plants (Table 2).
|Table 3. Classes of Vegetables|
|Cruciferous vegetables: cabbage, cauliflower, broccoli, brussels sprouts, kale, kohirabi, mustard, rutabaga and turnips.
Solanaceous vegetables: tomatoes peppers and eggplant.
Umbelliferous vegetables: carrots, celery, cilantro, parsley and parsnips.
Allium vegetables: garlic, onions, shallots, chives and leeks.
Cucurbutaceous vegetables: pumpkin, squash, cucumber, muskmelon and watermelon.
Some of the phytochemicals are widespread, whereas others are characteristic of particular classes of vegetables and fruit. For example (Table 3), cruciferous vegetables, such as broccoli, cauliflower, brussels sprouts and cabbage are unique in their high content of dithiolthiones and isothiocyanates. These are organosulfur compounds that have been shown to increase the activity of enzymes involved in the inactivation of carcinogens and other foreign compounds. Cruciferous vegetables also contain indole-e-carbinol, which has been shown to affect oestrogen metabolism. The allium vegetable family includes onions, garlic, scallions, leeks and chives and is notable for its content of compounds such as diallyl sulfide and allyl methyl trisulfide. Allium compounds have been shown to induce enzymatic detoxification systems. Furthermore, the antibacterial activity of these compounds may serve to inhibit the bacterial conversion of nitrate to nitrite in the stomach, thereby reducing the amount of nitrite available for reaction with secondary amines to form nitrosamines, which may be carcinogenic, particularly in the stomach.
Citrus fruit is known for its high content of vitamin C which, as an antioxidant, may protect cell membranes and DNA from oxidative damage. Vitamin C may further help prevent cancer via its ability to scavenge and reduce nitrite, thereby reducing substrate for the formation of nitrosamines. Vitamin C also plays a role in the synthesis of connective tissue protein, such as collagen. A deficiency of vitamin C may, therefore, affect the integrity of intercellular matrixes and have a permissive effect on tumor growth or hinder tumor encapsulation. Citrus fruit also contains coumarins (also found in some vegetables) and D-limonene (found specifically in citrus fruit oils), which have been shown to increase the activity of glutathione transferase, a detoxification enzyme.
Green leafy vegetables contain lutein, a carotenoid, and xanthophyll pigment that has no vitamin A activity, but that, as an antioxidant, may protect against cancer via its ability to block damage by free radicals. Green leafy vegetables are also a rich source of folic acid, a deficiency of which may lean to chromosomal damage at sites thought to be relevant to specific cancers.
Orange vegetables, such as carrots, sweet potatoes, winter squash and pumpkin are rich sources of beta carotene, as are some fruits, including papaya, mango and cantaloupe. Beta-carotene, like other carotenoids, is an antioxidant and through this function it may protect against free radical damage. Beta-carotene can also be metabolized to vitamin A (retinol), which plays a role in differentiation of normal epithelial cells. Because lack of differentiation is a feature of cancer cells, adequate intake of vitamin A (from either carotenoids or retinol) may help avoid the development of cancer. Tomatoes are particularly rich in a red pigment, lycopene, another antioxidant carotenoid.
Other potentially anticarcinogenic substances are not limited to one type of vegetable or fruit, but are more widespread. For example, selenium is found in produce in amounts proportional to the selenium content of the soil in which it is grown. Selenium functions as a cofactor for glutathione peroxidase, an enzyme that protects against oxidative tissue damage. Selenium may further alter the metabolism of carcinogens via its role in the mixed function oxidase system in the liver. Vegetables also contribute vitamin E to the diet (although the major sources are vegetable oils and margarine); vitamin E is an antioxidant that protects polyunsaturated fatty acids in cell membranes from oxidation. Vitamin E further keeps selenium in the reduced state, thus facilitating the antioxidant capacity of selenium. Additionally, vitamin E has bee shown to decrease the formation of nitrosamines in the stomach.
Flavonoids, such as quercetin and kaempferol are polyphenolic antioxidants that occur in vegetables and fruit (tea and wine are also important sources). In addition to being antioxidants, flavonoids may defend cells against carcinogens.
Fruit, vegetables and legumes are major sources of dietary fibre, which has been widely hypothesized to be protective against colon cancer. Dietary fibre many increase fecal bulk and decrease transit time; thereby, via dilution and a shorter period of contact, fibre may reduce the interaction between carcinogens and the intestinal epithelium. Fibre may also bind carcinogens and bile acids. Furthermore, certain types of dietary fibre are fermented by microflora in the colon, which leads to the production of short-chain fatty acids, one of which – butyrate- has been shown to be antineoplastic. The presence of short-chain fatty acids also lowers colonic pH and, subsequently, inhibits the conversion of primary to secondary bile acids, which stimulate colonic cell proliferation and are thought to promote carcinogenesis.
The fruit, vegetables and herbs with the highest anticancer activity include garlic, soybeans, cabbage, ginger and the umbelliferous vegetables. Onions, flax, citrus, turmeric, cruciferous vegetables and solanaceous vegetables have a modest protection against cancer.
Epidemiologic evidence is growing for a protective role of fruit and vegetables in coronary heart disease. Vitamin C, vitamin E, beta-carotene, selenium, flavonoids and magnesium are all dietary compounds that are known to decrease LDL oxidation, slowing the atherogenic process and protecting the vessel walls. Carotenoids found in yellow-orange fruit and vegetables are powerful antioxidants that act to quench free radicals and provide protection against oxidative damage. The many flavonoids in fruit and vegetables have antioxidant properties and protect LDL cholesterol from oxidation and also inhibit platelet aggregation. Quercetin is a major flavonol in red and yellow onions, kale, broccoli, red grapes, cherries, French beans and apples and inhibits LDL oxidation. The Zuptphen study of elderly men in the Netherlands found that flavonoid intake was inversely associated with heart disease and incidence of heart attack over a 5-year period. Those who had the highest consumption of flavonoids had 60% less mortality from heart disease than low flavonoid consumers.
Phenolic antioxidant compounds found in grapes and unfermented grape juice and phenolic flavonoids found in red wine and grape juice also protect against LDL oxidation and inhibit platelet aggregation.
Folic acid found widely in fruits and vegetables such as dried beans, green leafy vegetables, melons and oranges and Vitamin B6 and Vitamin B12 help to lower blood homocysteine, a known risk factor for heart disease.
Cholesterol: Several dietary fibre sources lower blood cholesterol levels, specifically that fraction transported by low density lipoproteins (LDL). Fibres that lower blood cholesterol levels include foods such as apples, barley, beans and other legumes, fruits and vegetables, oatmeal oat bran and rice hulls; and purified sources such as beet fibre, quar gum, karaya gum, konjac mannan, locust bean gum, pectin, psyllium seed husk, soy polysaccharide and xanthan gum. Several studies over the past 20 years have shown that persons who daily consume about 30-60g of soy for at least 4 weeks can decrease total cholesterol and LDL cholesterol with as much as 10% to 20% when their initial blood cholesterol levels are elevated. The blood lipid response tends to be more pronounced in younger individuals and those individuals who have a greater initial cholesterol level. A recent meta-analysis of 38 controlled clinical trials found that and average intake of 47g soy protein per day produced an average 13% decrease in LDL cholesterol and a 10% decrease in triglyceride levels.