Fruit and vegetables

What do we get from vegetables and fruit?

Mostly we think of vitamin C and fibre, but thinking about these plant foods has changed enormously. Most fruit and vegetables (F&V's) contain dozens if not hundreds of different components, and more and more are being found to be of health benefit.

Vitamin C

Although some vegetables, especially potatoes, can be useful sources of vitamin C, it is fruit which, with the exception of the banana, provide most. The role of potatoes surfaces occasionally. They were credited with preventing scurvy in Britain during WWII, and have recently again been credited (as fries) with a similar role among the poor of the United States.

Vitamin C is easily damaged by heat and oxygen, but deep frying prevents oxygen entry and so preserves the vitamin. The main sources are shown in the Table below. This vitamin is a main protector against "free radicals" generated all the time but especially by smoking, toxic substances, and radiation and chemotherapy, which can oxidise critical cell components.

This B vitamin is especially found in green leafy vegetables and citrus fruit. It is much publicised as preventing certain spine malformations in the fetus, but it also has a very likely role in prevention of coronary heart disease (by lowering the blood level of homocysteine), may well help prevent some cancers, and it is critical for all cell multiplication in the body. Unfortunately it is easily destroyed by heat in its natural form (but not the synthetic kind used in food fortification), so such foods should best be eaten either raw or only lightly cooked, preferably by microwaving or steaming.

B-carotene is present in red, yellow and orange fruit and in dark green vegetables. It is important as a precursor of vitamin A in people whose intake of the preformed vitamin from animal foods is limited. It also has a role in its own right, see below.

Vegetables and fruits are generally rich in potassium and low in sodium. A proportion of people are susceptible to excessive intake of sodium which can cause high blood pressure.

Potassium (and calcium) has the opposite effect. So unless we add lots of salt in preparation, vegetables and fruit are excellent in helping prevent this common and serious disease (hypertension).

Other minerals which we get especially from vegetables and fruits are magnesium, which is present in legumes, green vegetables (as chlorophyll) and bananas. This is not a mineral we think much about but although deficiency is rare it is very important in many bodily functions.

F&Vs should be important sources of iodine, but in New Zealand our soils are poor in this nutrient. Manganese is also present in important amounts, and legumes and mushrooms are good sources of molybdenum.

F&Vs are poor sources of iron, zinc, copper and selenium.

Dietary fibre can best be defined as carbohydrate substances which are not digested by the secretions of the human intestinal tract.

All F&Vs contribute a variety of different fibres, some of which, such as pectin in apples or gums in legumes, are not at all "fibrous", and are soluble in water. The body needs a good intake of a wide variety of different kinds of fibres, soluble and insoluble, and they are critical for health of the colon and may play an important role in the prevention of colon cancer. Some kinds also reduce the absorption of cholesterol and help reduce high levels.

Others slow the absorption of food carbohydrates and help prevent, or at least improve control in patients with diabetes. A good intake of fibres is necessary for good bowel function and prevents or improves constipation and diverticular disease.

Starch in some root vegetables such as potatoes and kumaras is a major source of carbohydrate, which is the preferred source of energy (calories) in our diet. While these plants do require substantial cooking to make the starch digestible, this process also converts some of the starch to "resistant starch" which is not digested but acts as yet another kind of "dietary fibre".

So much for the conventional nutrients. But we now recognise that F&Vs contain literally dozens of substances which are beneficial to human health (and of course a few which are not). Some of these substances are antioxidants. We have already heard of b-carotene present in carrots. But other members of the "carotenoid" pigments are much better antioxidants.

These include lycopene, which is predominantly present in tomatoes, and which has been linked to helping prevent prostate cancer.

This is present in spinach, broccoli, tomatoes, oranges and greens, and has proved to be crucial to preventing damage to the eye by blue and ultraviolet light which results in age-related macular degeneration (AMD) and is a dominant cause of impaired vision in the elderly. The retina of the eye concentrates lutein, which is one of the few carotenoids which can enter the brain and related tissues.

This vitamin comes mainly from seed oils, but green leafy vegetables are also important sources. The family of substances under this name, and not just a-tocopherol, are also very important antioxidants in the body.

This is not a familiar nutrient. It has long been recognised as essential for blood clotting, but now is known to have an important function in bone health and probably in preventing osteoporosis. It is present in green leafy vegetables although more abundant in some other foods.

Various members of the tannin family such as ellagic acid have shown promise as anti-cancer agents, for example in the lung, at least in experimental studies. Polyphenols in red grapes (and wines) are probably beneficial as anti-cancer agents. Flavonoids also seem to have similar benefits, and the list increases almost daily. The cabbage family (crucifers) contains groups of substances called indoles and isothiocyanates which have been shown to have some anti-cancer properties also but are not antioxidants. We must not assume that evidence linking F&V consumption with health benefits are all due to antioxidant effects.

The whole matter can be summarised by stating that vegetables and fruits of all kinds offer a wide variety of substances which have actual or probable benefits for human health. So the key message is to consume an abundance and wide variety of vegetables and fruits daily, preferably uncooked or lightly cooked.

Green leafy veges, broccoli

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Peppers, tomatoes, strawberries, potatoes, berryfruit

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Legumes

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Citrus fruit

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Flavonoids

quercetin, kaempferol

shallots, beans, greens, apples

Ellagitannins

ellagic acid, esculetin

several fruits and veges

Indoles, isothiocyanates

various

cabbage, radish, mustard, garlic

Terpenes

+limonene

citrus, especially peel

Polyphenols

catechin

red grapes, tea

Beta-carotene Cryptoxanthin Lutein Zeaxanthin Lycopene

Apricot, carrot, peach, kumara, spinach

Peach, papaya, tangerine, orange

Spinach, broccoli

Maize, red pepper, cress, beet greens

Tomato, watermelon, guava, pink grapefruit