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Defences against oxidative stress
By including more vegetables and fruits in your everyday diet, you may ward off several diseases.
Plants make a variety of compounds in response to environmental stress. Many of these compounds function as antioxidants when consumed. Thus, the plants’ defences against oxidative stress can be used for human benefit, prolonging life.
By eating plenty of vegetables and fruits, man significantly reduces the risk of many age-related degenerative diseases. For several years the focus on eating more vegetables and fruits has been a central plank of dietary advice from dieticians and health authorities in most countries. Indeed, many nutritionists would now say that this is the single most important piece of advice that the public should follow.
During the first three to four decades of the 20th century, most of the dietary requirements were studied. The vitamins, minerals, amino acids and fatty acids necessary to avoid diseases were identified and characterized. And the amount of nutrients required on a daily basis to avoid deficiency were roughly estimated.
Partly thanks to the application of nutritional science and partly due to the massive social and economic changes, the nutrient deficiency diseases that were widespread even a hundred years ago have largely been eradicated, at least in the developed world.
The preoccupation of modern nutrition has now shifted to trying to understand, and if possible to prevent, diseases in which diet is but one risk factor. Such diseases include many cancers, atherosclerosis, some types of hypertension, diabetes and obesity. The aetiology of these diseases is complex and multifactorial.
Pakistan is a green country where huge quantities of vegetables and fruits are grown and are easily available. We need to optimize the benefits.
It is quite remarkable that eating vegetables and fruits has a strong, inverse correlation with the risk of developing both cancer and cardiovascular disease.
Of the 156 separate epidemiological studies investigating the effect of vegetable and fruit consumption on the incidence of different cancers, 128 showed strong and significant benefits of consuming vegetables and fruits.
Even after controlling potentially confounding factors, it was found that a quarter of the population with the lowest habitual intake had at least twice the risk of developing certain types of cancer when compared to the quarter of the population with high intakes.
Similarly, so-called meta-analysis of the published epidemiological studies on the relation between the risk of cardiovascular diseases and diet showed that consumption of vegetables and fruits had significant protective benefits. The risk of Alzheimer’s disease, osteoporosis and age-related macular degeneration (a condition leading to blindness with age), have all been inversely associated with vegetable and fruit consumption.
If we take an overview of the nutrients in vegetables and fruits and try to identify factors responsible for the health benefits noted above, we are presented with a puzzle. In strict nutritional terms, commonly consumed vegetables and fruits provide, in addition to a high proportion of water, significant amounts of fibre and a handful of vitamins and minerals. For example, a 100g portion of peas will provide about 35 per cent of an adult’s recommended daily allowance of vitamin C, 40 per cent of folate, 20 per cent of iron and 30 per cent of fibre. Furthermore, the beneficial effects of consuming these vitamins have only been found at high intake levels — many times higher than could be obtained by consuming reasonable amounts of vegetables or fruits.
An alternative explanation for the benefits of consuming vegetables and fruits is that they may substitute for other, potentially harmful, components in the diet. For example, eating vegetables and fruits may simultaneously reduce the intake, or dilute the effects of food with a high fat or potential mutagenic activity.
A third and more attractive explanation comes from the realization that vegetables and fruits contain thousands of molecules that are not classified as ‘nutrients’. Many of these so-called phyto-chemicals will be absorbed from the gastrointestinal tract and exert more or less profound effects on metabolism.
Within the myriad of molecules found in vegetables and fruits, scientific attention has become focused on the group known as antioxidants. This is a very large group of chemically diverse molecules that share the ability to reduce or prevent oxidative changes in living systems.
Compounds with antioxidant activity are not restricted to vegetables and fruits; they are also found in many foods of plant origin such as tea and vegetable oils. Indeed, humans and other animals are able to synthesize some of their own powerful antioxidants such as the tri-peptidethiol, (glutathione).
It seems quite clear, however, that for an optimum defence against oxidative damage we need a regular intake of exogenous antioxidants and plants are a rich source of the same.
Antioxidants in plants
Plant antioxidants can be divided into three classes:
1) antioxidants, such as caro- tenoids, that suppress AOS formation;
2) antioxidants, such as ascorbate, that scavenge AOS and prevent them from doing damage, and;
3) antioxidants, particularly thiols, that are involved in repair processes.
It is no accident that plants, and foods derived from plants, are generally very colourful as they are rich in these three classes of antioxidants, many of which have highly coloured pigments. The green colour of chlorophyll, which is the basis for capturing light, often masks the colours of other pigments that protect the plant — for instance, the yellows, oranges and reds of the carotenoids.
Much attention has been focused on the red carotenoid pigment, lycopene, which is the major antioxidant of tomato. High consumption of raw tomatoes, tomato sauces and pizzas may contribute to the prevention of diseases such as prostate cancer. However, tomatoes as well as many other fruits and vegetables are rich in other carotenoids, vitamins, flavenoids and phenolics, which are powerful antioxidants.
It is probably the consumption of this balanced array of antioxidants, rather than any single one of them, that is beneficial in preventing disease.
Plant polyphenolic compounds are widely present in the plant kingdom and can be highly effective antioxidants depending on the number and nature of the phenolic hydroxyl groups. Even under favourable environmental conditions, plant tissues can show red or purple colouration caused by protective pigments of this type, such as the anthocyanins. Intense colour and antioxidant protection, more often than not, go hand in hand.
Anthocyanins and flavonols are a sub-group of flavonoid pigments: these are receiving increasing attention as pharmacologically active antioxidant treatments of disease.
Many large families of secondary plant metabolites (including carotenoids, tocopherols, flavonoids, anthocyanins and cinnamates) have powerful antioxidant activity. The flavonols, carotenoids and anthocyanin pigments provide colour for fruits and flowers to attract insects, but they are also important in protecting the plant against the harmful effects of too much sunlight. Flavonols and anthocyanins act as sunscreens, filtering out ultraviolet light and decreasing the light intensity within the leaf.
Of all the components contributing to the nutritive value of green vegetables and fruits, vitamin C (ascorbate) is the best characterized. Since the elucidation of its key role in the prevention of scurvy, vitamin C has been shown to fulfil many other essential functions in humans.
Vitamin C is gaining an increasing profile in plant biology due to its important roles in various functions, like photosynthesis and defence against environmental stress. In most plant tissues vitamin C is present in very high concentrations (up to 100mm in leaves in the field). It seems rather incredible, therefore, that the pathway of vitamin C biosynthesis in plants has only recently been established.
Plants cannot escape or flee from adverse environmental conditions, herbivores or invasion by pathogens. Antioxidant defences protect plants against premature aging triggered by environmental factors. Exposure to mild stresses increases the content of antioxidants in plant tissues and renders them more resistant to subsequent severe stresses.
In addition, exposure to one type of stress makes the plant resistant to several stress conditions. Cell and tissue death or visible symptoms of aging are observed only when the antioxidant defences are completely overwhelmed.
By including more vegetables and fruits in our everyday diet we can obtain a balanced array of natural antioxidants and benefit from the long evolutionary history of plants that harvest energy from the Sun.
The writer
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