Although phytochemicals are not classified as nutrients, defined as substances necessary for sustaining life, they have been identified as containing properties for helping in disease prevention. Eric Russell reports.
Phytochemicals are non-nutrient plant chemicals that contain protective, disease-preventing compounds. But the best way to use them to gain the maximum health benefits is the subject of much current research.
This includes assessing the way the human body handles phytochemicals. Depending on the body's metabolism, it may modify certain phytochemicals, while others can be found in the bloodstream in their natural form. And while plant extracts have been available from health shops, for example, for a long time, there is still a question mark over the exact benefits that accrue from consuming them.
More than 900 different phytochemicals have been identified as components of food, and many more continue to be discovered. It is estimated that there may be more than 100 different phytochemicals in just one serving of vegetables.
The research into phytochemicals shows an increasing recognition of the importance of diet in disease and the importance of nutrition in maintaining optimal health within the population.
As research progresses, the physiological effects of complete foods has been found in general to be greater than that of individual plant constituents. This raises the question of which phytochemical or combination of phytochemicals confer the best protection.
Research needed
There is still much research to be carried out into the basic molecular functions and interactions with other dietary components. The in vivo action of isolated phytochemicals may differ from that of the substance consumed as part of the whole fruit or vegetable matrix, and the chemical form of a compound that reaches target tissues often differs from that in the food.
The carotenoids beta-carotene, lycopene, lutein and zeaxanthin have antioxidant activity, which protects against free radical damage to DNA and is thought to contribute to prevention of cancer and other health benefits.
The xanthophyll carotenoids lutein and zeaxanthin could reduce the risk of eye diseases. This will become increasingly important for the world's growing population of elderly people.
Carotenoid lycopene offers beneficial effects in the prevention of degenerative diseases. Epidemiological studies suggest a potential link between a diet high in lycopene and reduced risk of developing coronary heart disease (CHD) and certain cancers, such as prostate, gastric, cervical and lung. Of all the carotenoids, lycopene has been found to be the most efficient quencher of reactive oxygen species. These findings have fuelled research into enhancing the natural levels of lycopene in tomatoes via genetic modification, and interest in adding lycopene to foods and supplements is growing.
Flavonoids are a group of phenolic compounds based on the flavone nucleus. They occur widely in fruit and vegetables, mainly present as the glycosides. A structurally-similar series of compounds is the isoflavones, which act as phytoestrogens.
A number of different classes of flavonoids have been shown to have potential beneficial properties, in particular the anthocyanins, flavones such as luteolin in artichokes, flavonols such as quercetin, and flavans such as catechins and proanthocyanidins.
Protection
Several epidemiological studies suggest that diets rich in flavonoids may help protect against coronary heart disease. Besides antioxidant activity, some flavonoids inhibit platelet aggregation and show anti-viral, anti-bacterial, anti-inflammatory, anti-mutagenic and immune-stimulating properties.
Estimates of dietary intakes of flavonoids vary widely and are influenced by different national diets. Preparation and processing of fresh fruits and vegetables may lead to losses in flavonoid content of up to 50 per cent as a result of leaching into water or removal of flavonoid-rich parts of the plant. The bioavailability of dietary flavonoids is poorly understood and research continues into the true potential of these compounds as dietary supplements.
Dr Ann Walker, of the Hugh Sinclair Unit of Human Nutrition at the UK's Reading University, has carried out many research projects into the best ways to use natural products such as phytochemicals in her work as a phytotherapist. Among her results, she has found that high doses of standardised artichoke leaf extract (ALE) may reduce symptoms of dyspepsia, the most common reasons for consultation in general practice as they affect about one in three persons in the UK.
Irritable bowel syndrome is a problem reported to affect 22 per cent of the general population. It is characterised by abdominal pain and altered bowel habit, but has so far defied elucidation of its pathogenesis and proved difficult to treat. There is a growing body of evidence that indicates therapeutic properties for ALE.
Help for arthritis
There is preliminary clinical evidence to support the contention that the anti-inflammatory and analgesic properties of bromelain help to reduce symptoms of osteo- and rheumatoid arthritis.
Bromelain is the crude aqueous extract obtained from the stem and fruit of the pineapple plant.
Knee pain is a very common complaint it can help. Apart from those diagnosed with osteo-, rheumatoid, and other forms of arthritis, large numbers of otherwise healthy individuals suffer pain and inflammation brought on by injury. The knee is vulnerable to twisting or shearing forces, and twisting suddenly whilst walking or even crouching can be sufficient to traumatise the knee. The usual treatment of non-steroidal anti-inflammatory drugs carries the risk of serious side effects, especially gastrointestinal damage.
Dr Walker says that, over the last decade, there has been a quiet revolution in thinking amongst nutritional scientists. This has culminated in research being focussed on optimal nutrition as a way to avoid deficiency disease. The realisation that avoidance of chronic disease and protection against environmental toxins, is as much part of the subject of human nutrition as is the prevention of diseases of undernutrition.
Extracts of tonic medicinal plants have potential for phytonutrient enrichment of functional foods. However, such enhancement poses a challenge to the food industry to produce palatable functional food products.
A functional food may be defined as a food with an enhanced physiological action beyond that expected of a normal food. The introduction of functional foods directly relates to the changing concepts of human nutrition over the past few years.
It has emerged that the health benefits of fruit and vegetables is largely due to their minor components, the antioxidant vitamins supported by the large numbers of phytochemicals, some with even greater antioxidant properties than the antioxidant vitamins.
The target of using phytochemicals is homeostasis, the maintenance of the internal environment of the body fluids within narrow limits, consistent with the optimal function of the body's cells.
Key homeostatic systems of importance in phytotherapy include the digestive system. Digestion is important in the provision of nutrients, but also plays a role in reducing allergen load through adequately functioning digestive glands and gastrointestinal tract co-ordination.
Poor digestive performance will lead to large molecules being presented to the lymphoid tissue in the ileum at the location of the Peyer's Patches.
The main responsibility for elimination of toxins in the body lies with the two organs, the liver and kidneys. The liver is responsible for biotransformation of toxins and bioactive compounds entering the body from the digestive tract, but it also deals with the disposal of the end products of metabolism such as insulin and oestrogen disposal.
The detoxication function of the liver reduces molecular mass (phase 1 reactions) and enhances water solubility by conjugation (phase II reactions). These processes facilitate the removal of metabolites and toxins from the body via the kidneys.
Waste removal
Larger molecules such as steroids, which are too big to be excreted via the kidneys, are passed into the gut via the bile and are eliminated in the faeces. However, some of these large molecules suffer at least a degree of recycling (the enterohepatic circulation), the extent depending on the composition of the diet.
For example, a diet high in dietary fibre will bind certain high molecular weight compounds such as oestrogen and carry them out of the body in the faeces, hence reducing recycling. Thus it is that diets high in dietary fibre reduce circulating oestrogen levels. Therefore, elimination via the gut is also important, although to a lesser degree than that via liver and kidneys.
Fruit and vegetables provide a wide spectrum of phytochemicals, but have been selected for palatability, crispness and lack of bitterness: attributes that tend to lower their phytochemical content.
Professor Catherine Rice-Evans of the Division of Biomolecular Sciences at King's College, London, says recent studies highlight the potential of phytochemicals in a neuroprotective role. This could help reverse the reduced performance of elderly people in cognitive, memory and learning tasks. These neurological functions can be influenced by supplementation of single dietary flavenoids or as part of a flavonoid-rich preparation.
The quantity of research into phytochemicals shows that while our understanding of human mechanisms is extensive, there is much more to be discovered. But at this stage it perhaps suggests a return to less sophisticated food structures that are closer to nature than to the factory.
