Fruits and vegetables their constituents and modes of action

A considerable amount of effort has been invested in identifying biologically active components within fruits and vegetables. Much of this work has related to the development of chemical analyses to quantify composition, and development of experimental models (animals and in vitro systems) to assess the functional consequences of supplementation with single compounds, or simple mixtures. The extent to which data produced from supplementation studies in animal or cell culture models can be extrapolated to humans consuming complex diets is not certain, but such studies have provided insight into putative mechanisms of health protection and promotion.

Thousands of biologically active phytochemicals have been identified in food plants. Of these food plant groups, fruits and vegetables are the most botanically

Table 2.2 Richest fruit and vegetable sources of specific compounds

Substance Richest source

Table 2.2 Richest fruit and vegetable sources of specific compounds

Substance Richest source

Vitamin C

Citrus (and other) fruits, green vegetables, potatoes

Vitamin E

Vegetable oils, avocado

Folates

Green leafy vegetables, potatoes, oranges

Vitamin K

Green leafy vegetables

Calcium, iron, magnesium

Green vegetables

Potassium

Bananas, vegetables and fruits generally

Fibre, NSP, pectin

Fruits and vegetables generally

Mono-unsaturated fatty acids

Olive oil

Alpha and beta-carotene

Carrots, green leafy vegetables, yellow/orange

fleshed fruits

Beta-cryptoxanthin

Oranges and related fruits

Lutein

Yellow/green vegetables

Lycopene

Tomatoes

Flavonoids

Onions, apples, green beans

Flavanoids

Peach, strawberry

Anthocyanins

Red/purple berries

Glucosinolates

Brassicas

Alkenyl cysteine sulphoxides

Alliums

Glycoalkaloids

Potato, aubergine

Furanocoumarins

Parsnip, celery

Cyanogenic glycosides

Cassava, Prunus species, butter beans

NSP = non-starch polysaccharides

NSP = non-starch polysaccharides diverse, represented in the Western diet by more than 40 botanical families. Table 2.2 lists the richest fruit and vegetable sources of specific compounds. However, apart from one or two exceptions, these compounds are also present (in varying amounts) in most other fruits and vegetables.

There are several biologically plausible reasons why the consumption of fruits and vegetables might slow, or prevent, the onset of chronic diseases. They are a rich source of a variety of vitamins, minerals, dietary fibre and many other classes of bioactive compounds collectively called phytochemicals. Experimental dietary studies in animals, cell models and humans demonstrate the capacity of some of these constituents of fruits and vegetables to modify antioxidant pathways, detoxification enzymes, the immune system, cholesterol and steroid hormone concentrations, and blood pressure, and their capacity to act as antioxidant, antiviral and antibacterial agents.

There has been extensive focus on antioxidant effects, as oxidative damage to biomolecules has been hypothesised to be responsible for CVD, cancer initiation, cataract formation, inflammatory disease and several neurological disorders. Our antioxidant defence system prevents the formation of damaging free radicals, removes radicals before damage can occur or repairs damage that has occurred. Several trace elements, such as manganese, copper, zinc, iron and selenium, are essential constituents of the antioxidant metalloenzymes: superoxide dismutase, glutathione peroxidases and catalase. Vitamins C and E and the carotenoids, which have received most attention with respect to their antioxidant capability, can interrupt free radical initiated chain reactions of oxidation, or scavenge free radicals before they damage cellular components. The antioxidant effects of several other groups of compounds, such as the flavonoids, have been studied mainly in vitro, but their metabolism is complex and effects in vivo may be different in type and extent from those observed in in vitro model systems. Some of the same factors that contribute to oxidative damage can also lead to the production of reactive, potentially carcinogenic, nitrogen species. Vitamins C and E, and polyphenols, have been shown to inhibit N-nitroso compound formation by destroying nitrosating agents.

Compounds in fruits and vegetables have been shown to attenuate the formation of carcinogens from non-toxic precarcinogens in vitro, by affecting their metabolism by the phase I enzymes (such as cytochrome P450 (CYP)-dependent monooxygenases) which catalyse oxidation, hydroxylation and reduction reactions, and/or by the induction of phase II biotransformation enzymes (such as UDP-glucuronosyltransferases, sulphotransferases and glutathione transferases) that accelerate the detoxification of the active carcinogenic metabolite. Studies in vivo are hampered by lack of knowledge of the normal range of expression or activity of these enzymes in human populations, the influence of other environmental factors and the influence of genetic polymorphism on phenotype.

It is also known that many of the constituents of fruits and vegetables have the ability to influence the immune system, which in turn is known to be intimately involved in both the prevention and promotion of chronic disease. Enhanced immune and inflammatory responses are central to our ability to deal with unwanted and potentially dangerous foreign particles such as bacteria and play a major role in tumour surveillance and cancer prevention. However, abnormal activation of the immune system has the potential to promote debilitating disorders such as gout and rheumatoid arthritis, and suppression of pro-atherogenic inflammatory responses have been suggested as one mechanism for the association between fish oil consumption (and specific n-3 fatty acids) and reduced CVD. Dietary strategies need to optimise rather than maximise immune reactivity and this will depend very much on individual susceptibility.

Several vitamins are associated with improved delayed-type hypersensitivity skin responses; some nutrients and phytochemicals modulate the activity of natural killer cells (NKC, a component of the antitumour host defences); vitamins C and E supplementation has been shown transiently to increase cytokine production (which assists in T cell and NKC activation); and beta-carotene enhances the expression of functionally associated molecules on human mono-cytes. The complexities of the immune system and its interaction with nutrients have been reviewed comprehensively.10,11

Garlic and garlic extracts appear to reduce risk factors for cardiovascular disease by decreasing platelet aggregation and reducing cholesterol and triacyl-glycerol concentrations in a variety of conditions. Specific dietary fibres from fruit and vegetable sources also show hypocholesterolaemic effects. Results from studies with other foods and beverages (for example, carrots and spinach, red wine and the polyphenols it contains) are less clear-cut in terms of influence on platelet function and cholesterol metabolism. However, it has been shown that replacing animal products with vegetable products in the diet can reduce blood pressure in both normotensive and hypertensive volunteers. Trials using components isolated from fruits and vegetables have reported inconsistent results.

The ability of fruits and vegetables and their constituents to stave off or relieve the symptoms of bacterial and viral infection tends to rely on anecdote rather than science. However, studies reported in the literature in the 1990s indicate that perhaps some credence should be given to the folklore. There is evidence from a double-blind, randomised, placebo controlled trial that cranberry juice positively influences the microflora of the urinary tract and that its use as a treatment for urinary tract infection may be well founded. Garlic too has a long history of use, as an antibiotic, antiviral and antifungal agent, which appears to be borne out by results obtained in a number of in vitro studies; however, verification of this activity in vivo is required. The health effects of vegetables and fruits and possible mechanisms of action in humans have been reviewed.12

There is a large literature on the effects of specific compounds in model animal and cell systems, relatively less in humans and a much smaller literature on the effects of fruit and vegetable interventions. Table 2.3 provides examples of some general and specific fruit and vegetable intervention studies and their outcome. A point to note is that the 'doses' used in most studies, particularly those using a single food item, are beyond what could be introduced reasonably into the day-to-day diet without distorting that diet in terms of the variety of fruits and vegetables, or other foods, consumed.

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  • DOROTHEA
    Which are constituents in vegetables?
    2 years ago
  • gigliola
    What are the consistuents of fruits?
    7 months ago
  • NIALL
    What is the constituent of vegetables and fruits?
    3 months ago

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