Fruit and vegetables are the only foods which collectively have been consistently associated with risk reduction in several diseases such as cancer, heart disease, diabetes, hypertension and age-related macular degeneration. Additionally, some new data is emerging to support a protective role for fruit and vegetables in the prevention of cataract formation and chronic obstructive pulmonary disease.
In a landmark study, an international panel of experts, which reviewed more than 4500 research studies to determine the relationship between food, nutrition and cancer, estimated that cancer rates would decrease by as much as 20% of people would eat five or more servings of fruit and vegetables per day.
Consuming a diet rich in a variety of plant foods provides a mixture of phytochemicals, “non-nutritive” substances in plants that possess health protective benefits. Fruit and vegetables contain an abundance of phenolic compounds, terpenoids, pigments and other natural antioxidants that have been associated with protection from and treatment of chronic diseases such as heart disease, cancer, diabetes and hypertension. Yet despite their complex composition, some of which is known (more is unknown or is being studied), we have tended to simplify their value in the daily diet. Indeed we have often erroneously assumed that they can be “replaced” with pills of single or multiple known nutrients.
What is the dietary composition or the so-called “magic compounds” of fruit and veg in general?
Dietary Fibre: Dietary fibre consists of the structural and storage polysaccharides and linin in plants that are not digested in the stomach and small intestine. Servings of commonly consumed grains, fruit and vegetables contain 1 – 3 g of dietary fibre. For instance, a large apple with the skin contains 3.7 g and half a cup of cooked spinach contains 2.8 g of fibre. A wealth of information supports, on balance, the protective role of dietary fibre in the treatment and prevention of diseases such as cancer, heart disease, diabetes mellitus, constipation and diverticulosis. Recommended intakes of 20-38 g/day for healthy adults and age plus 5 g/day for children, are not being met, because consumption of good sources of dietary fibre, such as fruits, vegetables, whole and high-fibre grain products, and legumes is inadequate. A diet adequate in fibre-containing foods is also usually rich in micronutrients and “non-nutritive” ingredients that have additional health benefits. A fibre-rich meal is processed more slowly, which promotes earlier satiety, and is frequently less energy dense, lower in fat and added sugars. All of these characteristics are important elements of a dietary pattern to treat and prevent obesity.
Phytochemicals: Plant foods contain large amounts of “non-nutrient” compounds call phytochemicals, which are biologically active, natural occurring chemical compounds (Table 1). Phytochemicals act as natural defense systems for their host plants protecting them against infection and microbial invasions. The phytochemicals also provide the colour, taste and aroma to fruits and vegetables. More than 2000 plant pigments are considered phytochemicals and include flavonoids, carotenoids and anthocyanins. Research has identified a host of active substances in fruit and vegetables, which are being investigated and some of them are thought to protect against disease.
Table 1: Some phytochemicals and their sources
|Anthocyanins||Strawberries, cherries, cranberries, raspberries, blueberries grapes and black currents|
|Butyrate||Fruit, vegetables and legumes|
|Carotenoids||Dark yellow, dark orange and deep green vegetables and fruit|
|Diallyl sulfite||Onions, garlic, scallions, leeks and chives|
|Flavenoids and phenois||Parsley, carrots, citrus fruits, broccoli, cabbage, cucumbers, squash, tomatoes, eggplant, peppers, berries, potatoes, broad beans, pea pods, coloured onions, onions, apples|
|Indoles||Cabbage, cauliflower, broccoli, brussels sprouts, spinach|
|Isothiocyanates||Cabbage, cauliflower, broccoli, brussels sprouts, horse radish, radishes|
|Flavonoids||Fruit, vegetables, wine, onions, kale, beans|
|Lycopenes||Tomatoes, red grapefruit, guava, dried apricots|
|Organosulfuric compounds||Garlic, onions, chives, citrus fruit, broccoli, cabbage, cauliflower, brussels sprouts|
|Terpenes and monoterpenes||Citrus fruits, parsley, carrots, celery, broccoli, cabbage, cauliflower, cucumbers, squash, tomatoes, eggplant, peppers|
In relation to cancer, phytochemicals in fruit and vegetables help metabolise drugs, toxins, carcinogens and mutagens. These overlapping and complementary mechanisms include the neutralizing free radical, inhibiting enzymes that activate carcinogens and inducing enzymes that inactivate carcinogens. They, therefore, act as blocking or suppressing agents and reduce the risk of cancer.
The antioxidants, vitamin C, vitamin E, selenium and phytochemical content in fruit and vegetables is thought to be primarily responsible for disease prevention. This has lead to an explosion in the food supplement market of supplements containing one or more of these compounds for the protection and treatment of the so-called diseases of lifestyle. In the process, the collective contribution and complex interaction of these compounds has been overlooked and, at times, indeed ignored. The available scientific experience indicates that the so-called “pill-popping” is not only ineffective, but it could also be dangerous, depending on the dose in the pill and the duration of the “pill” consumption. For instance, a study in Finland of male smokers who received either atocopherol, B-carotene, both or placebo revealed a 16% higher incidence of lung cancer associated with high dose B-carotene supplementation. Effective and useful as supplements may be in certain situations, they nevertheless remain a poor substitute for the complex composition of fruit and vegetables.
Macronutrients and Micronutrients: Fruit and vegetables are low in fat or virtually fat free, high in carbohydrates moderate in protein and excellent sources of vitamins and minerals. For instance, a serving of vegetables (1 cup raw or half a cup cooked vegetables) provides on average 5 grams of carbohydrates, 2 grams of protein, 0 grams of fat and 120 kJ in energy. A serving of starchy vegetables (potato, sweet potato, corn, pumpkin, peas) provides on average 15 grams of carbohydrate, 3 grams of protein, 0 grams of fat and 300 kJ in energy. A serving of fruit provides on average 15 grams of carbohydrate, 0 grams of protein, 0 grams of fat and 250 kJ in enery. Diets high in fruit and vegetables are, therefore, higher in fibre and micronutrients, but lower in fat and energy, which protects against the so-called disease of lifestyle such as obesity, heart disease and cancer.
Vitamins and minerals: Fruit and vegetables are by far the best dietary sources of antioxidants, folate, vitamin C, manganese, beta carotene and potassium. They are also a good dietary source of vitamin E, pantothenic acid, coition, choline, vitamin K, calcium, magnesium, iron (non-haeme), chromium, molybdenum and selenium.