Book: Nutrition: A Very Short Introduction (Very Short Introductions)

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Functional foods

Functional foods are defined as foods that contain one or more added ingredients to provide a positive health benefit, over and above the normal functions of food to provide nutrients and satisfy hunger. This definition excludes vitamins and minerals added to foods to replace losses in manufacture.

The concept of functional foods was developed in Japan in the 1980s, with a formal definition of ‘foods for specified health use’ (FOSHU), accompanied by a regulatory system to approve the statements made on labels and in advertising, based on scientific evaluation of the evidence of efficacy and safety. While there may be good evidence of potential benefits of some functional foods, and well conducted trials show improvement in indices of physiological function (biomarkers) and risk factors, as yet there is little evidence of improved health or increased lifespan in most cases.

When a vitamin or mineral deficiency is widespread in a population, a common approach is to enrich or fortify a staple food. The problem here is that if enrichment is voluntary, so that consumers have a choice of whether to buy the fortified or unfortified product, it is likely that the most vulnerable groups of the population will not be reached. However, if enrichment is mandatory then political problems of freedom of choice arise. It is noteworthy that despite the excellent evidence that fluoride reduces dental decay very significantly, fluoridation of water supplies is not universal in Britain, because of (unfounded) fears of ‘mass medication’.

Two examples of functional foods for which there is some evidence of efficacy are foods that modify the intestinal bacterial population, and foods that reduce the absorption of cholesterol.

Intestinal bacteria—prebiotics, probiotics, and synbiotics. It is a sobering thought that we host ten times more bacteria in the intestinal tract than there are cells in the human body. Some of the 100 or more species are pathogenic, some are harmless, and some (especially the lactic acid producing bacteria) are beneficial, producing a variety of compounds that prevent the growth of pathogenic organisms. They ferment dietary fibre to provide short-chain fatty acids that are the preferred fuel for intestinal mucosal cells and may have anti-cancer activity.

The main lactic acid producing bacteria of interest are Lactobacillus and Bifidobacterium spp., and there is some evidence of beneficial effects of lactic acid bacteria in controlling allergies, preventing or curing constipation, and generally maintaining gastro-intestinal health. These are the bacteria that are present in probiotic yoghurts. An alternative approach to modifying intestinal bacteria is the consumption of undigested carbohydrates (dietary fibre and starch that is resistant to digestion) that provide a substrate for fermentation by the probiotic bacteria, and are therefore known as prebiotics. The combination of the two, probiotic bacteria and prebiotic carbohydrates, is known as a synbiotic.

There is good evidence for the effects of some prebiotics in alleviating constipation. The evidence is less good for the prevention of colon cancer, intestinal infection, and recurrence of inflammatory bowel disease, but a number of trials have suggested that prebiotics can prevent colonization of the intestinal tract with pathogens.

Plant sterols and stanols to lower serum cholesterol. Average daily intakes of cholesterol from the diet are between 300 to 600 mg/day; in addition to this, some 2,000 mg of cholesterol is secreted each day in the bile, much of which is reabsorbed. This means that anything that will reduce cholesterol absorption from the small intestine will have a much larger effect on whole body cholesterol (and hence serum cholesterol) than would be expected from the dietary intake alone.

Analogues of cholesterol, such as the plant sterol β-sitosterol and the stanols, inhibit the enzymes that convert cholesterol to cholesterol esters in the intestinal mucosa, for absorption, so that less enters the circulation. Unesterified cholesterol is actively exported from the mucosal cells back into the gut. There is abundant evidence that consumption of plant sterols and stanols lowers blood cholesterol, and the effect is additive to that of statins, the drugs that inhibit cholesterol synthesis. Plant sterols and stanols are found in moderate amounts in fruits, vegetables, nuts, and vegetable oils. A variety of low fat spreads, yoghurts, drinks, and cream cheeses enriched with plant sterols and stanols have been marketed.

Superfoods

The concept of ‘superfoods’ was developed in the USA in 2003–4 and was introduced in Britain by an article in the Daily Mail in December 2005. Superfoods are ordinary foods that are especially rich in nutrients or antioxidants and other potentially protective compounds, including polyunsaturated fatty acids and dietary fibre.

A web search for ‘superfoods’ gives the following list:

almonds, apples, avocado, baked beans, bananas, beetroot, blueberries, Brazil nuts, broccoli, Brussels sprouts, cabbage, carrots, cocoa, cranberries, flax seeds, garlic, ginger, kiwi, mango, olive oil, onions, oranges, peppers, pineapple, pumpkin, red grapes, salmon, soy, spinach, strawberries, sunflower seeds, sweet potato, tea, tomatoes, watercress, whole grain seeded bread, whole grains, wine, yoghurt.

There are very few surprises in this list. Most of these are foods that we know are nutrient dense—with a high content of vitamins and minerals/1,000 kcal. The nuts, seeds, and olive oil are an exception, but they are all good sources of polyunsaturated fatty acids and vitamins E and K.

The labelling and marketing of the foods as superfoods seems disingenuous (or a clever marketing strategy), but if such marketing leads people to eat more fruit and vegetables and to reduce their saturated fat, salt, and sugar intake then it can only help to reinforce the message about the prudent diet discussed in .

Supplements

The term ‘nutritional supplements’ covers a very wide range of preparations whose common denominator is that they are regarded as foods rather than medicines, and in most countries are regulated under food legislation rather than laws relating to medicines. In the USA specified health claims are permitted by the Food and Drug Administration (FDA), but manufacturers are also permitted to make further claims, provided that these bear a note to the effect that the claims have not been evaluated by FDA, and that the product is not intended to diagnose, treat, cure, or prevent any disease. In Europe, the European Food Safety Authority (EFSA) has begun the lengthy process of evaluating the evidence for health claims submitted by manufacturers of foods and supplements, in order to draw up a list of permitted claims. Arguably, the least controversial supplements are multi-vitamin and mineral mixtures that provide about 100 per cent of the reference intake per day. This is already an unnecessary amount for most people, since they will have at least some intake from their foods. A number of surveys in developed countries show that average intakes of vitamins and minerals from foods are adequate to meet requirements. Of course, for people with a low food intake (and this will include many elderly people with low energy expenditure, and therefore low food intake) such supplements may well be advisable, or at least a prudent precaution. Similarly, for people whose diet is relatively poor, supplements are advisable. Unfortunately, most surveys show that supplements are purchased mainly by people whose diet is already adequate, not by those for whom they might be desirable.

Supplements that provide higher amounts of vitamins and minerals are a cause for concern. As discussed in , excessive intakes can be harmful. It is highly unlikely that a manufacturer would deliberately market a supplement containing a dangerously high amount of a nutrient. However, it would be possible to achieve an undesirably high, and even hazardous, intake by taking a number of different supplements, each of which provided a high, but safe, amount of a vitamin or mineral. This is especially a concern for vitamin A, where the margin between adequacy and potential toxicity is relatively small—especially for young children and pregnant women (see ).

There is a good basis of evidence to recommend some single nutrient supplements. As discussed in , folic acid supplements of 400 μg/day are recommended for women planning pregnancy and vitamin D supplements of 10 to 20 μg per day are advisable for pregnant women and the elderly, since it is unlikely that they will meet the reference intake from the few foods that are rich sources of the vitamin. Indeed, there is increasing evidence for a benefit of vitamin D supplements for everyone living in temperate regions with little sunlight exposure. Long-chain polyunsaturated fatty acids, as found in fish oils, provide protection against cardiovascular disease. For people who do not like oily fish, or eat it only rarely, supplements of fish oil will be beneficial. People whose diet is poor in fruit and vegetables will probably benefit from vitamin C supplements, and there is some (relatively weak) evidence that supplements of vitamin C may alleviate the symptoms of the common cold.

There is considerably less evidence, if any at all, for the benefits of other single nutrient supplements. As discussed in , relatively high-dose antioxidant supplements, especially carotene and vitamin E, are associated with a higher risk of death. It is certainly difficult to justify supplements of individual amino acids. Nevertheless, many single nutrient supplements, or preparations containing a small group of related nutrients, are widely available to be bought—both over the counter and by mail order/online.

One suggestion for regulating the market in nutritional supplements, which has not met with approval from any of the regulatory agencies, is to consider them in three groups.

Supplements containing nutritionally relevant amounts of nutrients (perhaps up to five to ten times the recommended daily amount (RDA) could be readily available over the counter, as at present, and be considered to be foods.

Supplements containing about ten to 50 times the RDA, where there is some risk of excessive intake, especially if you are taking more than one product, should be considered to be medicinal compounds rather than foods, and should be sold by qualified pharmacists, who can ask about other supplements and medicines you are taking.

Supplements containing more than about 50 times the RDA, where there might be a real risk of adverse effects of a high intake should be available only on prescription, since part of medical education includes learning how to balance the risks and benefits of treatment. There may well be some people who would benefit from a high dose of a given nutrient, even though there is a potential hazard to their health from the supplement.

As discussed in , there is little evidence for the efficacy of protein supplements marketed to athletes and sportspeople, and there may be undeclared ingredients such as steroids in some supplements that are banned in competitive sports. Creatine is often marketed as an ergogenic aid to improve sports performance. The rationale is that creatine (as its phosphate) provides a reserve to replenish ATP in muscle. Creatine is made in the body from amino acids and there is little evidence that supplements are effective in increasing performance, although a few studies do suggest benefits for such sports as arm wrestling, which require very short bursts of very high intensity muscle activity. Similarly, although carnitine is essential for uptake of fatty acids and their use as metabolic fuel in muscle, it can be synthesized in the body, and there is no evidence that supplements improve muscle function or athletic performance. There is little evidence that supplements of fish oils, containing ω-3 polyunsaturated fatty acids, have any beneficial effect on mental concentration or intelligence once requirements have been met from foods.

Many herbal preparations are marketed and regulated as nutritional supplements, although some are permitted as medical products with claims based on traditional use for treating various conditions. Some will indeed be effective—after all, many of the conventional medicines in use today have been derived from traditional herbal medicine, and many herbal preparations contain pharmacologically active compounds. Some, of course, may also be toxic, or may be contaminated with toxic metals because of where they were grown.

There is a need for caution in purchasing any nutritional supplement. If you buy products from a reputable manufacturer you can be reasonably sure that they have good quality control and laboratory facilities; that they analyse each batch of ingredients bought from suppliers; and can trace each batch of ingredients into each batch of their final products. By contrast, if you go online to buy supplements, while you may be lucky, the chances are that you will buy from a company that does not have its own laboratory facilities, and does not keep precise records of each batch of ingredients and products. They may be buying from their suppliers in good faith, but they cannot be sure that what they are selling meets appropriate standards of purity and safety.

If you follow the guidelines for a healthy diet discussed in , a varied, moderate diet will meet nutrient requirements for most people. It is important to avoid extreme or radical diets, and to be wary of bold claims that are sometimes made about foods. If you are healthy, then eat when you are hungry but stop before becoming unpleasantly full; and drink when you are thirsty, but there is no need for an excessive fluid intake.

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