The World Health Organisation (WHO) advises us to eat at least five 100g portions of fruit and vegetables a day to keep healthy and lower our chances of getting such illnesses as heart disease and certain types of cancer. What evidence is there that this is good advice?
Five years ago the WHO annual report listed 'a lack of fruit and vegetables in our diets' as the sixth risk factor contributing to death after high blood pressure, smoking cigarettes, high blood cholesterol, being underweight, and unsafe sex. And following fruit and vegetables in the long list, were alcohol, lack of exercise, contaminated drinking water, and air pollution. What makes fruit and vegetables so important to our well-being?
Minerals, vitamins and antioxidants
Fruit and vegetables are low in fats, high in fibre and packed with minerals and vitamins. Potassium, phosphorus, magnesium, manganese and selenium, and the vitamins A, C, E, K and B9 are found in various amounts in most fruit and vegetables. These minerals and vitamins are all essential components to our cells. Fruit and vegetables are also rich in 'antioxidants'. It is these chemicals that are thought to play a major role in protecting us from chronic disease - heart disease, stroke, Alzheimer's disease, AMD (age-related macular degeneration, the main cause of loss of sight in the over 50s), and certain cancers, including those of the pharynx, oesophagus, stomach, large bowel, bladder, lung and breast.
Antioxidants inhibit oxidation reactions. While some oxidation reactions are critical to life, others produce highly reactive free radicals. These very short-lived species can be any atom or molecule with one or more unpaired valence electron. Free radicals react with other molecules, resulting in chain reactions and the formation of new radicals.
Some free radicals, eg nitric oxide (NO•) and the superoxide radical (O2-•) play important roles in our cells. For example NO•, which is made in the walls of blood vessels, acts as a messenger molecule in controlling blood pressure, muscle relaxation, and memory. Nitric oxide, like the superoxide radical, also plays a part in defending our cells from invading bacteria. However, other free radicals - notably the hydroxyl (OH•) and peroxyl (O2•) radicals - can damage our cells.
Dangerous free radical reactions
Hydroxyl radicals are produced by many chemical reactions in the body. These highly reactive species attack proteins, lipids (fatty acids and vegetable oils, for example) and DNA (deoxyribonucleic acid) within our cells. Chain reactions are started which can go on for many years and damage cells. In the case of DNA, for example, OH• attacks all four bases of the nucleotide strands - adenine, guanine, thymine, and cytosine - resulting in chemical changes to DNA that can lead to cancer.1
Polyunsaturated fats (PUFAs) - fats containing two or more double bonds - are found in many foods and in cell membranes, where they are essential for membrane function. When an OH• attacks PUFAs, a PUFA radical is generated, which itself reacts with oxygen to generate another free radical - PUFA-O2• - the PUFA peroxyl radical.
The peroxyl radical leads to cell damage because this radical attacks another PUFA molecule and the chain continues:1
PUFA-O2• + PUFA → PUFA• + PUFA-O2H
Eventually all the PUFAs are oxidised to lipid peroxide, which will weaken and destroy the cell membrane.1
Heart disease is also associated with the formation of free radicals in the body. Cholesterol is an essential constituent of all cell membranes, and is a precursor in the production of vitamin D, steroid hormones and bile salts. We have 'LDL-cholesterol' or 'bad cholesterol' in the blood, and 'HDL-cholesterol', which is considered to be good. The LDL and HDL refer to the low density and high density lipoproteins that transport the cholesterol in the blood. An accumulation of LDL-cholesterol in the blood leads to the formation of plaques on artery walls, which narrows the arteries and restricts blood flow to the heart (leading to heart failure) and brain (leading to a stroke). Scientists believe that the mechanism of the formation of plaques involves the oxidation of LDL.
Scientists have also discovered that Alzheimer's disease (AD), a debilitating form of dementia, involves the overproduction and accumulation of a specific protein (beta-amyloid) in the brains of people suffering with AD. The presence of this protein is thought to lead to the production of damaging free radicals.
Antioxidants react with free radicals and control their levels in the body, and thus have a potential role in protecting us from such diseases. Even the levels of useful free radicals need to be controlled. Excess NO• and O2-• can lead to the formation of other toxic species, including the hydroxyl radical.
Antioxidants in fruit and veg
Fruit and vegetables contain a plethora of compounds that have antioxidant properties. Vitamins A, C, E, K and B9 are all antioxidants. Vitamin E, found in green leafy vegetables and berries, for example, reacts with the PUFA peroxyl radical to generate a less reactive free radical. This radical does not attack PUFAs at the same rate as PUFA-O2. does, and thus stops the damaging chain reaction. In addition, fruit and vegetables contain a host of carotenoids and flavanoids, which are also powerful antioxidants.
There are over 600 known carotenoids (1) - naturally occurring pigments with a polyene structure, ie alternating double and single carbon-carbon bonds. They are responsible for the orange colour of carrots, peaches and apricots (beta-carotene (2)), the yellow hue of corn (lutein (3)), and reds of peppers, tomatoes and sweet potatoes (lycopene (4)). Carotenoids are also present in varying amounts in spinach, cabbage, broccoli and beans, though their colour in these vegetables is masked by the presence of chlorophyll.
Since our cells cannot make carotenoids, they are obtained from our diet. As antioxidants carotenoids are thought to terminate free radical reactions by, for example electron transfer from the carotenoid to the hydroxyl radical to produce a less reactive carotenoid radical. Alternatively they can form adducts with free radicals, resulting again in a less reactive free radical.2
Flavanoids are polyphenolic compounds (they contain one or more phenol, C6H5OH, group). Over 4000 flavanoids have been identified, many in fruit, vegetables and drinks - specifically tea, coffee, beer, wine and fruit drinks. There are different types of flavanoids, classified according to their structure. Some of the most potent antioxidants include: quercetin (5), found in vegetables, fruit skins, especially apples, and onions; xanthohumol (6) found in hops and beer; and genistein found in soy.
So what other evidence do we have that points to antioxidants as protectors against disease, and supports the Government's advice that we should eat five portions of fruit and vegetables each day? (Incidentally, the Canadian Government recently recommended that adults should eat seven-10 servings of fruit and vegetables a day to keep healthy.)
Epidemiological and other research
Much of the evidence comes from epidemiological studies, which correlate incidence of disease with factors specific to a locality, in this case eating fruit and vegetables.
In the early 1990s, for example, a WHO European cross-cultural study revealed an inverse relationship between blood levels of vitamin E and death from heart disease in men from 16 different population groups. The number of men dying from heart disease sloped across Europe: those in Southern Europe having higher blood vitamin E levels and lower mortality than those from Northern Europe with lower vitamin E levels in the blood and higher mortalities. The study found that a low intake of antioxidant nutrients, especially in smokers, was likely to be a contributory factor to the higher incidence of coronary heart disease in Scotland. (Cigarette smokers tend to eat less fruit and vegetables.)3
According to Professor Sue Fairweather-Tait from the school of medicine, health policy and practice at the University of East Anglia, the evidence for the health benefits of fruit and vegetables for heart disease and certain cancers are stacking up. Among several studies she cites is a recent Nurses Health Study and Health Professionals Follow-up Study done at Harvard University, which found 30 per cent less heart attacks and strokes in people who ate eight, in contrast to 1.5, daily portions of fruit and vegetables. Other recent studies, she says, have revealed a protective effect of fruit and vegetables against certain cancers, including those of the stomach, colon, rectum and breast.
This year's American Chemical Society (ACS) meeting in Chicago revealed some of the latest research by chemists to identify the growing number of antioxidants in fruit and vegetables that are showing promise in the fight against cancer. Researchers at the Rutgers University, New Jersey, and the US Department of Agriculture identified pterostilbene in blueberries and found that in animal studies this antioxidant shows promise in preventing colon cancer.
In another study, German chemists reported that drinking two to three glasses of cloudy (unfiltered) apple juice (from cider apples) per day may also reduce our risk of colon cancer. In a 10-week study mice fed either cloudy apple juice or a potent extract of the juice showed a 38 per cent and 40 per cent (respectively) reduction in tumours of the small intestine in comparison to a control group that were fed water instead of the juice. According to Dr Clarissa Gerhauser of the Cancer Research Centre in Heidleberg, speaking at the ACS meeting, the anticancer effect is likely to be caused by the presence of myriad antioxidant chemicals found in apples.
So maybe the Government has got it right this time - eating five portions of fruit and vegetables a day may keep the doctors away.
My thanks to Professor Sue Fairweather-Tait for information on epidemiological studies on antioxidants in fruit and vegetables.
- Barry Halliwell, Educ. Chem., 1995,32 (5), 123.
- Richard M. Faulks, Educ. Chem., 2004, 41 (1), 21.
- K. F. Gey et al, Am. J. Clinical Nutrition, 1991,53, 326S.