There are many claims made in the media and promotional literature about the qualities and benefits of specific (or groups of) compounds found in fruits and vegetables. We are told that wrinkles, absentmindedness, cancer and clogged arteries (among many other disorders) can be prevented, or alleviated, by consuming these compounds in the form of isolates or concentrated extracts. In such claims the words 'tested', 'effective', 'safe', 'essential' and 'proven' are freely used. In the world of nutritional science, however, the picture is not so clear. The following two quotes provide an example of this apparent contradiction. The first relates to a study of antioxidant vitamins and risk factors for cardiovascular disease, 'These results back-up the findings of previous studies and point to a positive role for antioxidant supplementation among those suffering from coronary artery disease'.25 The second statement is again related to antioxidants and chronic
Table 2.3 Selected general and specific fruit and vegetable (F&V) interventions and their outcome
Study period Outcome
F&V providing 1 meal
325mg vitamin C
F&V 8 weeks mean number of servings, 8.5 and 3.6
F&V 2 weeks
Raw apple (350-400g) 1 month Prunes (100g) 4 weeks
Guava (500-1000g) 4 weeks
Raw carrots (200g) Carrots providing 15 g fibre Brussels sprouts (300 g)
Cranberry juice (300 ml)
3 weeks 3 weeks
Urinary N-nitrosoproline 13
Serum total cholesterol 14
reduced by 4%
Systolic and diastolic 15
pressures lowered with 8.5 servings, particularly in hypertensives
Oxidation resistance of 16
low-density lipoprotein increased
Plasma cholesterol reduced 17
Low density lipoprotein (LDL) 18 cholesterol decreased in mildly hypercholesterolaemic males
Serum total cholesterol, 19
triglycerides and systolic and diastolic pressures all decreased
Serum cholesterol reduced 20
No effect on serum 21
Detoxification enzyme 22
Detoxification enzyme 23
decreased disease, 'Current evidence is not strong enough to recommend antioxidant vitamin pills'.26 These quotes, apart from highlighting apparently contradictory views of scientists on a very similar point, also serve as an example of the fact that, whilst the public health significance of fruits and vegetables has arisen largely from observations of people eating traditional diets rich in these foods, research and information are dominated by the potential benefits of isolated compounds.
Recent surveys in Europe indicate that far more people are concerned about their food and their health than in the past. However, while consumers say they want to eat in a healthier manner, the reality is that they want to eat more easily, hence the claim of an enormous market potential for dietary supplements, nutrient enriched and functional foods, each of which contains perhaps one, or just a few, of the hundreds of components present in a diet containing a variety of fruits and vegetables.27 Compounds isolated from plant foods, or synthetic copies of compounds that can be found in these foods, are promoted and used for their putative medicinal or health promoting properties. The literature that accompanies their sale can be very convincing to those who want to stay healthy. For those with a diagnosed condition, these compounds can appear a more natural and safer alternative to drug therapy and certainly a much easier option than trying to change the dietary habits of a lifetime.
Epidemiological data reveals that diets rich in particular foods are associated with reduced risk of a chronic disorder. At this stage, however, the association between diet and health is merely an observation. This observation needs to lead to some reasonable hypotheses, possibly supported by earlier experimental evidence. These hypotheses then need to be tested in a wider range of experimental systems, often in vitro and/or animal and cell-line model systems, followed by smaller studies involving human volunteers, perhaps leading to very much larger trials. As part of this process a risk-benefit analysis of any dose is a vital consideration, as exemplified by b-carotene.
The predominant carotenoids in blood and tissues are b-carotene found in carrots, some orange coloured fruits and green vegetables, b-cryptoxanthin found in oranges; lycopene found in tomatoes and lutein found in yellow/green vegetables. These compounds have significant antioxidant activity, at least in vitro, and are therefore thought to be capable of protecting the cells and tissues of our body against the ravages of living in a world full of potentially toxic oxygen. Carotenoids also have a range of other biological activities. They modulate immune and inflammatory response and have long been known to influence cell-cell communication, which is a vital part of our ability to control the activity of individual cells within a tissue. In vitro and animal studies strongly support some carotenoids as natural anticancer agents and populations consuming higher amounts of carotenoid-rich foods have lower rates of CVD, cancer and other chronic diseases. There are convincing hypotheses about why this should be, but little is known about what dose provides optimum protection or how this may vary depending upon individual sensitivity.
Human trials were undertaken. Volunteers were given relatively high dose supplements of b-carotene for several years, which substantially raised plasma and, presumably, tissue concentrations. These studies showed one of two things, either supplementation with b-carotene was not effective with regard to CVD, cancer or all-cause mortality or, in susceptible individuals like smokers and asbestos workers, the mortality rate from lung cancer was significantly increased. On the other hand, plasma b-carotene concentration (reflecting the consumption of carotenoid-rich foods) before supplementation was inversely and significantly associated with lower cancer rate.
As with the carotenoids, epidemiology has implicated vitamin E as protective, particularly with respect to cardiovascular disease. Human intervention studies, involving high dose supplementation in 'at-risk' individuals, however, have not consistently demonstrated a role for vitamin E but perhaps too much is expected.
Research has concentrated on the potential for single food components to reverse existing disease, whilst the primary role of these components (in the balance and amounts found in diets rich in fruits and vegetables) is, arguably, in the prevention or slowing of initiating events.
Evidence to date, albeit largely observational, remains heavily in favour of the protective effects of specific foods or food groups consumed as part of a traditional diet and the role of any one component of fruits and vegetables in isolation from all others remains to be established. As part of this process, demonstration that the component of interest is released from the (often) complex food matrix and is effectively delivered to its putative site of action within the human body, is obviously essential.
Reviews debating these issues, with appropriate reference to the scientific literature, have been published.28,29
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