Diet is a crucial factor in promoting human health and avoiding disease. Research shows that one third of cancer cases are related to what we eat (1). Prevention of cardiovascular disease (heart disease, artherosclerosis and cerebrovascular disease) is also strongly associated with a healthy diet, together with a healthy lifestyle.
Two thirds of all deaths in Britain are caused by cancer and cardiovascular disease. There is great potential for reducing premature death through dietary improvement and this is considered to be second only to the avoidance of smoking, in cancer prevention.
The food we eat affects our health in a variety of ways.
It follows that the healthiest foods are those that combine the maximum content of beneficial ingredients with minimal harmful or toxic constituents. Among the foods known to be particularly good for you are vegetables and fruit, and of these, tomatoes have been found to contain several potentially protective ingredients.
Foods that are regarded as particularly beneficial for health are sometimes referred to as functional foods. These are defined as: "Any modified food or food ingredient that may provide a health benefit beyond the traditional nutrients it contains." (US Institute of Medicine, 1994). Tomatoes, by this definition, are valuable functional foods.
Recent medical research has concluded that free radicals may play a part in many major diseases, including cancer, cardiovascular disease, arthritis and even the ageing process itself. These gene-damaging molecules arise naturally but also commonly as environmental pollutants in, for example, exhaust fumes and cigarette smoke.
In addition to the body's own defence mechanism to deal with free radicals, there is strong evidence that the risk of developing some major diseases may be reduced by increased consumption of fruits and vegetables (2, 3). This may be because of their antioxidant properties, their ability to neutralise or "quench" free radicals.
Antioxidants include vitamins A, C and E, flavonoids and the natural plant pigments known as carotenoids. In the body, some carotenoids are converted to Vitamin A, which may offer cancer protection because of its role in normal cell differentiation.
Vitamin A (retinol)
This vitamin is important for:
Vitamin C (ascorbic acid)
This vitamin is important for:
Vitamin E is relatively abundant in tomato products.
Carotenoids are a group of some 600 naturally occurring plant pigments of which about 40 are consumed by humans. One of the best known carotenoids is beta-carotene, found in highly coloured fruit and vegetables such as carrots, peppers and tomatoes. Beta-carotene converts to vitamin A in the body.
Lycopene, the red pigment in ripe tomatoes, is a carotenoid. In contrast to most others, which are found in a variety of fruit and vegetables, about 85% of lycopene in the typical western diet is derived solely from tomatoes. The long chain molecular structure of lycopene makes it a particularly effective antioxidant (4, 5). Dietary intake of tomatoes and tomato products containing lycopene have been shown to be associated with decreased risk of chronic diseases such as cancer, in several recent studies (6, 7).
In laboratory research, lycopene has been found to be a more potent inhibitor of human cell cancer proliferation than either alpha or beta-carotene. It has been shown to be more than twice as effective as beta-carotene in protecting human lymphocyte cells (white blood cells) from the damaging effects of nitrogen dioxide, for example (8). Nitrogen dioxide is a common air pollutant, which is also found in high concentrations in cigarette smoke.
Unlike other carotenoids, blood lycopene levels are not consistently lower among smokers than non-smokers, suggesting that lycopene may have disease preventive activity in ways other than as an antioxidant. It may instead inhibit cholesterol synthesis and enhance degradation of low density lipoprotein (LDL or "bad" cholesterol) (9).
In a measurement of antioxidant status in a small group of healthy men following increased consumption of carotenoid rich vegetables, only tomato juice was found to reduce low density lipoprotein (LDL) oxidation (10). Carrot juice and spinach powder had no effect. Reducing the accumulation of cholesterol may be the mechanism by which lycopene protects against cardiovascular disease. Evidence shows that inner vascular wall thickness and risk of heart attack are reduced in persons with higher stored fat concentrations of lycopene.
In a Europe-wide study on cardiovascular disease, comparing men who had suffered heart attacks with healthy men, it was found that those with high levels of lycopene were only half as likely to have an attack as those with low lycopene. Two other carotenoids studied showed little or no effect (11).
Several studies have indicated reduced levels of digestive tract cancers with high tomato consumption and/or lycopene blood serum levels (12). High consumption of lycopene from tomato-based foods is associated with reduced cancer risk at several sites, but especially the stomach, colon and rectum.
A recent review (13) looked at 72 separate studies from around the world. Fifty-seven reported inverse associations between tomato intake or blood lycopene level and the development of various cancers (i.e. the lower the tomato intake or blood lycopene level, the higher the risk of developing certain cancers). Of these, 35 were statistically significant, with evidence being strongest for cancers of the prostate, lung and stomach. Other researchers have proposed that lycopene may inhibit or even reverse the growth of tumours (14).
There were 21,770 prostate cancer cases reported in the UK in 1997. This accounted for 17% of male cancer cases, the second most common form of cancer in men. In 2000 in the USA, 40,000 men were expected to die from this type of cancer. A number of studies have indicated an association between high intake of tomato products and reduced prostate cancer risk in men (15, 16). Two types of studies have been conducted:
In a six year study of 14,000 men, the development of prostate cancer was assessed in relation to dietary and lifestyle habits (17). Only tomato, bean, lentil and pea consumption was found to be related to reduced prostate cancer risk. Consumption of beta-carotene rich foods, such as carrots, was not related. Results from another population-based case-control study also suggested that vegetables rich in beta-carotene are not protective against prostate cancer, although lycopene was associated with some reduction in risk (18).
In a study of 47,894 prostate cancer-free health professionals by the Harvard School of Medicine, dietary intake of various carotenoids was assessed (15). Only high lycopene consumption subsequently showed a statistically significant (21 per cent) risk reduction. Of the 46 food items containing carotenoids, three of the four showing a significant reduction of prostate cancer risk contained lycopene - tomatoes, tomato sauce and pizza.
Those subjects consuming ten or more servings of tomatoes or tomato-based products per week had a 35 percent reduced risk of prostate cancer than those consuming less than one and a half servings. For the more advanced or aggressive prostate cancers, least likely to respond to treatment, the apparent protective effects of lycopene were even higher, at 47 per cent.
It should be noted that other studies (19, 20, 21) have reported little or no effect of lycopene on prostate cancer risk, although these studies included many fewer subjects. Assessments of lycopene intake, based on tomato consumption, will also vary with the type of tomato products consumed. In some countries, such as Japan, pink tomatoes are commonly eaten and these contain less lycopene than red ones.
Other dietary factors, such as the consumption of certain oils, may enhance the absorption of lycopene by the body. These factors need more careful attention in dietary studies.
In blood and tissue measurement studies, prostate cancer patients were found to have a six per cent lower blood lycopene level in a US study (22). Lycopene has also been found to concentrate in prostate tissue (23). In a cell culture study, lycopene in combination with Vitamin E prevented the growth of prostate cancer cells (24).
Studies at the University of Toronto have found that prostate cancer patients have lower blood serum and prostate tissue lycopene levels than control subjects (25). It is not known, however, if the low lycopene levels in these patients were due to low consumption, poor absorption, or increased lycopene metabolism in fighting the disease.
Overall there is strong evidence to suggest that consumption of tomato products and ingestion of lycopene reduces the risk of prostate cancer but more studies are needed before more definitive conclusions can be made.
Until recently there has been relatively little focus on possible associations between lycopene consumption and breast cancer incidence and some studies may appear contradictory. Laboratory research has suggested no protective effect of lycopene supplements (26).
Other studies have shown that a high level of a compound called insulin-like growth factor I (IGF-I) in the blood constitutes a risk factor in breast and prostate cancer (27). Growth stimulation of mammary cancer cells by IGF-I was markedly reduced by tissue lycopene. This information suggests that inhibitory effects of lycopene on breast cancer cell growth may be due, not to direct toxicity of the carotenoid in these cells, but to interference with the activity of IGF-I in receptor signalling or "triggering" cancer cell growth.
Some recent research suggests that women consuming high levels of tomatoes and tomato products are less likely to suffer from breast cancer (28). A University of Toronto study reported that breast cancer patients have very low levels of blood lycopene and high levels of oxidative damage (29). Toronto researchers have suggested that a daily lycopene intake as high as 25-30 mg per day would be beneficial (30).
Studies on the exposure of the skin to ultraviolet (UV) radiation have shown a marked and rapid reduction in skin lycopene levels with exposure, but no reduction in beta-carotene (11, 31). This suggests that lycopene may be active in protecting the skin from UV damage from the sun's rays. Such damage is an increasing cause of skin cancers, including malignant melanoma, associated with increased exposure of the skin to sunlight.
A number of other diseases may also be affected by lycopene ingestion. Studies have shown low levels of serum lycopene in children infected with HIV, both at the non-AIDS and AIDS stages of the disease (32). The clinical implications of this result require further evaluation.
Age-Related Macular Degeneration (ARMD) is the most common form of blindness in the western world. Lycopene is the only micro-nutrient whose serum level is inversely related to the risk of ARMD i.e. the higher the level the lower the risk. Since there is no lycopene in the eye, this is a surprising result which is not yet understood (33).
Research on the potential benefits of lycopene in the diet against other conditions, including exercised-induced asthma and osteoporosis, is in progress.
Flavonoids are a group of chemicals found in some plant tissues in relatively high concentrations. Individual flavonoids tend to have a restricted distribution in the plant kingdom, many appearing in only one genus or even species.
Within this group of compounds, flavonols and flavones are of particular importance as they have been found to possess antioxidant and free radical scavenging activity in foods (34). Studies have indicated that their consumption is associated with a reduced risk of cancer and cardiovascular disease (35, 36).
Vegetables, fruits and beverages are the main dietary source of flavonoids (37). Tea, red wine and tomatoes are important, depending on the relative amounts consumed in individual diets.
Tea is one of the main sources of flavonoids for adults in the UK but its limited use by younger people is declining in favour of carbonated drinks and coffee, which are relatively low in flavonoids.
Red wines are a major source of flavonoids in countries where consumption is high, such as Italy and France. Coupled with a high consumption of tomatoes and relatively little saturated fat, this may be one of the factors which makes the "Mediterranean" diet a particularly healthy one.
It is known that the processes leading to coronary heart disease and cancers are initiated many years before the diseases manifest themselves. Since children consume relatively little tea or red wine, the consumption of fruit and vegetables high in flavonoids and other potentially protective constituents is critical to their future health.
Cherry tomatoes have a higher flavonoid content, pound for pound, than larger fruited types (37). They may also contain high lycopene levels when fully ripe (48). Since cherry tomatoes are more appealing in flavour and size to children than larger tomatoes, or possibly other salads and vegetables, they represent a potentially important dietary benefit for youngsters.
The promise of antioxidants as possible anti-cancer supplements has failed to materialise in a number of clinical trials. Daily beta-carotene supplements were administered to patients who included smokers, former smokers and workers exposed to asbestos (38). Not only did the supplements fail to reduce levels of disease, but also the incidence and mortality from cancer were increased with supplementation.
A 1997 study of smokers between the ages of 50 and 69 found significantly more deaths from fatal coronary heart disease in the beta-carotene supplemented group versus placebo controls (39).
The precise reasons for the harmful effects of supplementation in smokers are not known. One explanation may be that smoking reduces the levels of other antioxidants that normally stabilise beta-carotene. This leads to increased levels of oxidised and potentially harmful beta-carotene. A balance or range of different protective constituents working in combination in food is also thought to be important (40, 41, 42).
No clinical trials have been conducted to compare the effects on health of lycopene supplements with lycopene derived from tomato consumption. In one trial, lycopene levels in blood plasma were found to be similar whether originating from tomato juice or lycopene supplements (43), but it cannot be assumed that the health benefits are similar.
Lycopene, like other carotenoids, may only be beneficial as a result of interactions with other molecules present in tomatoes. Using lycopene by itself, as a single micro-nutrient supplement, may not be as beneficial (44).
Lycopene supplements are available as tablets and capsules, one brand containing 5mg lycopene per capsule with a recommended dose of one capsule per day. One supplier's pack states that this amount "is equal to two large ripe tomatoes." This considerably underestimates the potential lycopene content of fresh tomatoes, as can be seen from the figures later in this report.
Commercial lycopene supplements vary in their formulation and composition. They may or may not contain other tomato extracts, commonly referred to as "phytonutrients" in the trade literature, in addition to lycopene. Their biological activity may vary as a result.
Research at the Rowett Research Institute, Aberdeen (45), has identified a component in the yellow jelly around tomato seeds which, it is proposed, stops platelet cells in the blood from clumping together. The aggregation of platelets triggers the cascade of reactions leading to blood clot formation (thrombosis). Heart attacks, strokes and blood vessel problems resulting from thrombosis currently kill or disable more people in developed countries than any other disease.
In tests on volunteers, the compound (codenamed P3) from as few as four tomatoes reduced platelet activity by up to 72 per cent. Larger scale studies are necessary to confirm these results but, if successful, P3 could represent a benefit over existing anti-platelet therapy, such as aspirin, which may have side effects, including stomach upsets and bleeding (45).
Minerals are important for cell metabolism, good health and growth. Tomatoes contain a high level of potassium, which is important for healthy blood, together with calcium and other mineral salts and trace elements.
Dietary fibre is important to maintain a healthy digestive system and may also help to control high cholesterol levels in the blood. Tomatoes are a good source of fibre, especially when eaten with the skin and seeds.
There is evidence that cooking tomatoes can increase the absorption of lycopene into body tissues (46). Cooking tomatoes in certain oils, such as olive oil, is also believed to help this process since lycopene is lipid soluble. This is another reason why a Mediterranean diet may be a healthy one.
Processing tomatoes into more concentrated forms will also increase the lycopene concentration but many processed products, such as canned tomatoes and sauces, contain high levels of additives such as salt, which represents a less desirable feature. Imported, processed tomatoes will also not have been grown to the same standards as fresh British tomatoes, in terms of pest and disease control and other production and environmental factors.
Cooking tomatoes is likely to reduce the content of some beneficial ingredients, such as flavonoids (37), vitamin C and vitamin E. The fibre content may also be affected, especially if the skins are discarded in cooking.
Since tomato consumption in the UK is low compared with other European countries, increased consumption of both fresh and cooked tomatoes is strongly recommended. Selecting high lycopene fresh tomatoes for cooking could be particularly beneficial.
Recent publicity sponsored by manufacturers such as HJ Heinz, has promoted the benefits of processed tomato products, for their lycopene content, in comparison with the fresh or raw fruit. No information has been given in this publicity about the source or selection of the fresh product, in terms of variety, stage of ripeness or production system, all of which have a considerable impact on fruit lycopene content. Other studies, such as those on flavonoids,(37) can be criticised for using non commercial varieties in their research. These results again lack details of the maturity and condition of samples tested.
The lycopene content of fresh tomatoes varies with the variety, storage conditions and especially the stage of ripeness (47). A lycopene content of 30mg/kg (3mg/100g) is quoted by Heinz as being typical of raw tomatoes. However, recent research in the UK has demonstrated that the lycopene content of fresh, uncooked, tomatoes can be up to three times this figure (48). This is affected particularly by the variety and the stage of ripeness of fruit.
It is true that some fresh tomatoes, especially long shelf-life and non-red types, do have a lower lycopene content. This demonstrates the possibility of selecting high lycopene tomatoes and the need to take account of the lycopene content of different tomato products in dietary studies.
Tomatoes produced in Britain are harvested at a mature, ripe stage when lycopene levels are at their peak. They are fresh at the point of sale through frequent harvesting and a short delivery time to stores. Many speciality types are now grown here, such as cherry, plum and cocktail tomatoes. These have higher lycopene contents, in the same way that cherry tomatoes have higher flavonoids.
The lycopene content of a number of tomato varieties and types is shown in the table below. Further research on the effect of cooking some of these high-lycopene varieties on lycopene concentration is desirable.
Lycopene concentration (mg/kg) and content (mg per fruit) of UK grown varieties
and two imported from southern Europe (48).
|Type||Variety/Brand||Lycopene Content||Lycopene Content||Fruitweight (g)|
|Midi plum||Baby Sweetheart||63.6||2.48||39.0|
The stage of ripeness of fruit has a greater effect on its lycopene content than any other factor. This is not surprising since the development of the red colour occurs during ripening, by the conversion of chlorophyll to lycopene. The effects with one cherry tomato variety are shown in the following graph. The pattern is similar with other red types, such as the classic variety Espero, which was also tested in this research (48).
Effects of stage of ripeness on fruit lycopene concentration of cherry tomato variety Favorita
Notes: Stage of ripeness
1 = first signs of colour change, sometimes referred to as "breaker" stage.
5 = half-ripe or orange.
9 = red.
9 + 7 days = colour stage 9 plus 7 days storage at room temperature (18°C)
The effects of the location of production and growing management system appear to have little if any effect on fruit lycopene content, if the same variety is grown and tested at the same stage of ripeness in each case (48). However, research conducted in France has shown that tomatoes produced by conventional methods had a higher vitamin C content than those produced organically. The taste, texture and mineral content showed little difference according to the production method, however (49).
A number of food constituents or contaminants may be harmful to health, or harmful when eaten in excess.
While fats are not directly toxic (all living cells require a source of energy for life), too high an intake can contribute to obesity, itself a risk factor for heart disease and some cancers. Saturated fats are more harmful and can give rise to raised blood cholesterol levels. These can increase the risk of heart disease and high blood pressure (hypertension). Common sources of saturated fats are animal products, such as fatty meat and full cream dairy products.
Tomatoes contain virtually no fat and no cholesterol. They are very low in calories, typically only 10-15 kcals for a medium size fruit (65g). The energy is in the form of natural fruit sugars, glucose and fructose.
Excess salt (sodium chloride) consumption also represents an increased health risk. Reduced sodium intake is recommended as one of the ways to reduce the incidence of heart disease and stroke. There is a lot of "hidden" sodium in the diet in fast foods, prepared foods and those with preservatives. Fresh tomatoes have a low sodium content, but that in canned tomatoes, for instance, may be 30-40 times higher because of the addition of salt.
Cancer of the stomach has been linked to the consumption of salted and pickled foods yielding direct-action carcinogens. Their formation is inhibited by Vitamins C and E.
Cancers of the colon, breast, prostate and pancreas may be caused by a new group of carcinogens, heterocyclic amines. These are formed when grilling or frying creatine-containing foods, including meat and fish. Their formation and action can be inhibited by antioxidants.
Contaminants in or on food could include:
natural toxins such as mycotoxins, which may arise from certain moulds growing on incorrectly stored food products, such as cereals and nuts. They are occasionally found in other products such as wine, beer and coffee. Some of these chemicals are among the most toxic known to man and can be powerful carcinogens. There are no known cases of such toxins being associated with fresh British tomatoes.
disease organisms such as those causing food poisoning. These can arise from the use of contaminated animal manures or poor hygiene in handling uncooked food. No recent cases of food poisoning in the UK have been associated with fresh British tomatoes. Conventional commercial growing systems do not use animal manures. Hygiene standards in British tomato production and packing are also very stringent.
Pesticide legislation in the UK is extremely rigorous and includes statutory limits on residues of pest control products in food. British tomato growers have even higher standards however and, in partnership with major UK retailers, produce the crop to the requirements of the Assured Produce Scheme, which is unique to this country.
This scheme incorporates environmental protection and natural means of pest control, rather than the use of pesticides, into strict production protocols. It is monitored by independent inspectors. One object is to control pests in tomato crops by the introduction of natural enemies and the intention of British growers is to eliminate the use of pesticides entirely. Additionally the bumble bees used for crop pollination in all British tomato crops will only thrive in the absence of harmful pesticides.
The range of beneficial ingredients in tomatoes, especially antioxidants such as lycopene, coupled with a low calorie and negligible fat content, makes tomatoes one of the most valuable additions to the human diet.
There is an increasing body of evidence linking the consumption of tomatoes generally, and constituents such as lycopene specifically, to a reduction in the onset of several chronic diseases, including cardiovascular disease and several cancers.
More detailed epidemiological studies and fundamental chemical and biological research are needed to establish the direct health benefits and to clarify the complex mechanisms at work.
In real terms, fresh tomatoes are available at a lower price than ever before and all income groups should be able to benefit from this valuable food source.
There are advantages in consuming both fresh and cooked tomatoes and the British diet would benefit from a substantial increase in tomato consumption.
Gerry Hayman, BSc, FIHort
The Tomato Growers' Association
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Did You Know?
British Tomato growers use an army of over 40 million beneficial predatory bugs called Macrolophus in summer to keep pests out of their crops, reducing the need for chemicals.