There are a whole range of biologically active substances in foods, especially plant foods and herbal remedies, apart from those accepted as nutrients. Some of these are harmful or affect the availability of nutrients in the diet, but others may have beneficial effects on health. Many of the active substances have been isolated and are now available as dietary supplements; however, their presence, previously unrecognized, is probably the best advertisement there is for eating a varied diet, including plenty of vegetables, pulses, and fruits, s it is quite possible that there are still other substances that remain to be discovered. Much of the evidence for the benefit of such substances ahs come from epidemiological studies, where the prevalence of certain types of disease is related to the consumption of specific foods or food groups within the community. Further scientific study is then needed to identify the particular active component and demonstrate an effect in the body. The following outline the importance of some of the compounds that have been studied more extensively.
Oxidation is an essential process whereby the nutrients we obtain from foods are oxidized in a controlled manner involving the consumption of oxygen. Carried out at a cellular level, oxidation releases energy for metabolism and transformation of nutrients into body tissue and generation of heat. The oxygen is ultimately converted into water and excreted. However, during this process so-called free radicals or reactive oxygen species are formed that, unless mopped up by the body’s antioxidant defences, can damage the tissues, increasing the rate at which they age and potentially contributing to a range of degenerative diseases such as arthritis, immune disorders, cancer, stroke, coronary heart disease, and many others. Antioxidants are substances produced by the body, or consumed in foods, that significantly delay or prevent the oxidation of a particular substrate.
Some vitamins and trace elements in the diet contribute to the body’s antioxidant arsenal. Vitamins A (as beta-carotene), C, and E are known as the antioxidant vitamins, and selenium, copper, manganese, and zinc are components of antioxidant enzymes. In fact the carotenoids, the red-orange pigments in plants, comprise about 600 different substances, of which about 60 are precursors of vitamin A. Many of the non-provitamin carotenoids, including substances such as lycopene, zeaxanthin, and lutein act as antioxidants. Lycopene is the most interesting of these. It is present in tomatoes and, therefore, in food products such as ketchup and sauces. Cooking releases the lycopene and makes it more available, especially in the presence of a small amount of oil or fat. Recent epidemiological studies have suggested that consumption of tomatoes and products containing them is associated with a lower incidence of prostate cancer. Consumption of 10 or more servings per week of foods containing tomatoes, including soup, pizza, and pasta sauces afforded the greatest protection. In addition, non-nutrients such as phytoestrogens, flavonoids, phenolic acids, and polyphenols such as tannins are present in foods and drinks, and may help to prevent oxidation in the plant as well as in human tissues.
Flavonoids are phenolic compounds that are water soluble and occur widely in nature. There are hundreds of different flavonoids found in fruits, vegetables, and beverages such as tea and wine. The particular flavonoids in tea and wine have strong antioxidant effects. Epidemiological studies have suggested that the risk of coronary heart disease is substantially lower in people within populations with the highest flavonoid intake, possibly due to the prevention of oxidation of low-density lipoproteins and reducing blood clotting. The most widely distributed flavonoid in foods is quercitin, followed by kaempferol, but others include myrecitin, catechin, apeginin, and luteolin. In a Dutch study investigating flavonoid intakes, black tea was found to supply more than half the intake, followed by onions and apples.
Phytoestrogens are steroid substances derived from plants, that, it has been suggested, have several potentially beneficial actions in the body. Epidemiological studies suggest that in populations where there is a high intake of phytoestrogens the incidence of certain cancers, especially hormone-sensitive types such as some forms of breast cancer and ovarian cancer in women and prostate cancer in men, is lower. One group, known as lignans, are derived from the bacterial digestion of polyphenols, and many oilseeds such as soya bean, rapeseed, and flax are rich sources of the lignans or their precursors. Women in countries with high consumption of soya beans and soya products have been shown to have a lower incidence of breast cancer. This may be related to the phytoestrogen content of the foods as well as to the presence of flavonoids and other phenolic compounds. Soya is also a rich source of another class of phytoestrogens – the isoflavonoids – especially diadzein and genistein.
Phytoestrogens appear to increase the binding of sex hormones to the protein on which they are carried in the blood, thus resulting in lower levels of biologically active free hormone, but they also have other potentially beneficial effects. Some have antioxidant effects that are cancer-preventing, while others appear to reduce the proliferation of cells that respond to oestrogens (such as in the breast and uterus) either by inhibiting enzymes involved in cell proliferation or by competing with oestrogens for binding sites. Food manufacturers are taking the opportunity to make products in which the above potentially beneficial components of foods are concentrated naturally, or are adding tem to other foods. For example, soya, flax, and linseed may be added to breads to increase the phytoestrogen content, with the breads then being advertised as functional foods.
Phytoestrogens are also regarded as active principles in herbal remedies.
These are foods that appear to have health benefits beyond the provision of nutrients and energy. A symposium on the topic gave the following definition “a food can be said to be functional if it contains a compound, which may or may not be a nutrient, that affects one or a limited number of functions in the body in a targeted way so as to have positive effects on health”. The health benefits may be physiological or may take the form of a positive psychological effect.
Functional foods may be foods that contain the beneficial substance naturally, e.g. fruits and vegetables contain a variety of antioxidant substances that are not strictly nutrients but have beneficial effect: wholegrain cereals contain dietary fiber that may have beneficial effects on gut function and help prevent heart disease: soy beans contain phytoestrogens that may have beneficial effects as described above. However, increasingly food manufacturers are producing foodstuffs with ‘functional” added ingredients that may be of benefit to health. For example, spreads with plant sterols or plant stanols added may help lower cholesterol levels; addition of specific bacteria, called probiotics, to yoghurts and yoghurt drinks, may have beneficial effects within the gut and beyond; and chewing gum containing phosphatidylcholine is claimed to improve memory.