Nutrition (17 page)

Authors: Sarah Brewer

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BOOK: Nutrition

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Toxicity

Excess sodium leads to fluid retention and high blood pressure in those who are sodium-sensitive. Toxicity leads to vomiting, muscle weakness, kidney damage and metabolic acidosis and can be lethal. Children are especially vulnerable.

Zinc

Zinc is a metallic element that acts as a cofactor for over 200 metabolic enzymes. It regulates the activity of genes in response to hormone triggers, playing an important role in growth, sexual maturity and wound healing. Zinc is also important for immune function.

Dietary sources of zinc

These include red meat; seafood, especially oysters; offal; wholegrains; pulses; eggs; cheese and yeast.

Deficiency

In some parts of the world, dietary zinc deficiency is common and results in growth retardation, physical and mental retardation in children, impaired nerve function, dermatitis, hair loss, diarrhoea, loss of appetite, taste and smell, anaemia, susceptibility to infections, delayed wound healing and macular degeneration. In males, zinc deficiency is associated with low testosterone levels, delayed male puberty, reduced male fertility and increased risk of prostatitis (inflammation of the prostate gland).

Toxicity

Excess can cause abdominal pain, nausea, vomiting, lethargy, anaemia and dizziness. Zinc affects iron and copper uptake when taken at doses greater than 50 mg per day. Zinc supplements often contain copper at a ratio of 10:1 (10 mg zinc to 1 mg copper).

Functional foods: phytochemicals and probiotics

As well as providing macronutrients, vitamins, minerals and fibre, fruit and vegetables provide other non-nutrient substances that have beneficial effects on human health. Although not deemed essential, these make an important contribution towards good health as they influence human metabolism in a positive way. So although you could survive without them, your risk of some long-term conditions such as atherosclerosis (hardening and furring-up of the arteries), high blood pressure, raised cholesterol levels, heart disease, stroke and even cancer could be increased. These plant-based substances are known as phytochemicals (after the Greek word
phyton
meaning plant). As far as the plants are concerned, some of these phytochemicals provide colour to attract the organisms that help with their fertilization and dissemination (for example, purple anthocyanin pigments in blueberries), protection against the sun (red lycopene in tomatoes) or have an antimicrobial action against plant viruses and other predators (flavonoids in apples). When these plants are eaten, phytochemicals contribute to flavour (the astringent tannins in tea and wine) and can also affect your body’s metabolic functions. As they are not classed as macronutrients, vitamins or minerals, they deserve special attention, together with probiotics – the so-called ‘friendly’ digestive bacteria that contribute to bowel health and immunity. One of the hottest debates in nutritional medicine currently centres around probiotics and the beneficial effects they have on human health.

Phytochemicals

More than 30,000 phytochemicals have been identified, which can be divided on the basis of their chemical structures into phenols, carotenoids, phytosterols and sulphur-containing compounds. The average diet is believed to supply more than 1 g of phytochemicals per day, which is good, but obviously people who eat the most plant-based foods (including tea drinkers) will obtain the greatest amounts and obtain the most benefits.

Phenols

Phenols are a group of plant chemicals that have two or more aromatic phenol rings in their structure. They contribute to the colour, taste and smell of many foods – capsaicinoids give chilli peppers their fiery hotness, for example, while limonenes produce both the orange-lemon scents of citrus fruit and the more pungent smell of fresh pine nuts. The amount present in fruit and vegetables depends on the cultivars (for example, whether you are eating a green Granny Smith or a rosy Pink Lady apple) and on their growing conditions, their ripeness and how they are processed and stored.

Phenols form a diverse group that includes tannins, lignans, stilbenes, phenolic acids and flavonoids, of which the latter are the best studied and understood.

Flavonoids (sometimes known as bioflavonoids) are antioxidants that protect plants from attack by microbes, insects and UV light.

According to researchers from Harvard University, there is a growing body of evidence that regular consumption of some flavonoids can have a marked effect on human health, for example by protecting against heart attacks. Among the most important flavonoids are the isoflavones, the catechins and the anthocyanins.

Isoflavones

Isoflavones have a similar structure to the human hormone oestrogen and are therefore classed as phytoestrogens (i.e. plant oestrogens). Other phytochemicals with an oestrogen-like action in the body include lignans (present in high concentrations in linseed/flax seed) and stilbenes (e.g. resveratrol found in the skin and pips of grapes and in wines, especially red wine, which tends to remain in contact with grape skins for longer during maceration). The role of these chemicals within the plants is not fully understood, but it has been suggested that their oestrogen-like properties may form part of the natural defences of some plants. In theory, they might lower male fertility in the herbivore species that eat them, and thereby reduce the number of animals who are actively seeking a snack! Whatever their role in plants, they have a physiological action in humans, and global consumption of phytoestrogens is increasing.

Of the thousand or more isoflavones that have been identified, five are present in significant amounts in the human diet: genistein, daidzein and glycitein are mainly derived from soy, while formononetin and biochanin A (which are metabolized to form daidzein and genistein) are obtained from chickpeas, lentils and mung beans. In cultures such as Japan, where soy is a dietary staple, intakes of isoflavones are between 50 mg and 100 mg per day, compared with typical Western intakes of just 2 mg to 5 mg isoflavones per day. As a result, blood levels of phytoestrogens in people following a traditional Japanese diet are as much as 110 times higher than those typically found in the West, and this is thought to account for the unusually low incidence of heart disease and breast cancer seen in those following a traditional Japanese diet.

Dietary isoflavones are mostly present in an inactive form (attached to sugars to form glycosides) such as genistin and daidzin. Once ingested, bacteria in your large intestines remove the sugar to release the active forms (isoflavone aglycones) such as genistein and daidzein. Some people possess good amounts of beneficial probiotic bacteria (such as lactobacilli, bifidobacteria) that further metabolize daidzein to a more powerful oestrogen called equol. Equol has a higher antioxidant activity than any other isoflavone, and those classed as ‘equol producers’ may gain greater health benefits from dietary isoflavones than ‘non-equol producers’.

Isoflavones interact with human oestrogen receptors. Although this interaction is between 100 and 1,000 times weaker than that of human oestrogen itself, they occupy the receptors and block the access of the stronger human hormones. This can damp down potentially harmful, high oestrogen states (which have been associated with health problems such as breast cancer and endometriosis, for example). They also provide useful oestrogen activity when oestrogen levels are low after the female menopause, which may help to reduce oestrogen-withdrawal symptoms such as hot flushes and night sweats. On the other hand, some phytoestrogens are now recognized as endocrine disruptors, which may potentially interfere with reproductive health just like certain synthetic pesticides (for example, DDT), industrial lubricants (PCBs) and plasticizers (Bisphenol A). There is growing concern too from some nutritionists about the widespread use of isoflavone-rich infant soy formulas. In some countries, such as the UK, Australia and New Zealand, soy-based formulas are now mainly reserved for infants who do not tolerate those based on cows’ milk proteins. Women who are pregnant, breastfeeding or planning for a baby should probably also use soy foods with caution, but for older people at risk of heart disease, a soy-rich diet may offer some benefits, as follows.

Isoflavones and heart disease

Japanese people who eat a traditional soy-rich diet have one of the lowest rates of coronary heart disease in the world. Their high isoflavone intake is suggested as one explanation due to a variety of mechanisms, including their antioxidant action and their ability to interact with oestrogen receptors to promote arterial dilation and reduce blood pressure, cholesterol levels and abnormal platelet clumping. A meta-analysis of 23 trials, for example, found beneficial reductions in total cholesterol, LDL-cholesterol and triglycerides, with increased beneficial HDL-cholesterol. The evidence of benefit was so strong that, in 1999, the US Food and Drug Administration (FDA) approved the use of a health claim that daily consumption of 25 g soy protein – in a diet that is low in saturated fat and cholesterol – may reduce the risk of heart disease. This advice still stands. The European Food Safety Authority took a much more rigorous view, however, and rejected this scientific claim, stating that a cause-and-effect relationship had not yet been established between the consumption of soy protein and the reduction of LDL-cholesterol concentrations. (The EFSA also rejected a claim that regular consumption of large amounts of water can reduce the risk of dehydration, which illustrates just how rigorous their approach can be!) But a recent medical review looking at the pros and cons of phytoestrogens suggested that ‘people at risk of heart disease may want to consider replacing at least a portion of their meat intake with soy’.

Isoflavones and cancer

Because they block the interaction of stronger human oestrogens with their tissue receptors, isoflavones may protect against oestrogen-sensitive cancers of the breast and, in males, of the prostate gland. This is supported by a number of observational trials in which those eating the most soy are considerably less likely to have a high risk of breast or prostate cancer than those eating the least. A study published in the
Journal of the National Cancer Institute
involving 21,852 Japanese women, for example, found that those with the highest consumption of isoflavones were 54 per cent less likely to develop breast cancer than those with the lowest intake. This remained true even after adjusting for reproductive history, family history, smoking and other dietary factors. Another study published in the
British Journal of Cancer,
involving 35,000 women, found an 18 per cent reduced risk of breast cancer relative to those consuming the least intake of isoflavones. In both studies, the protection was highest in postmenopausal women. However, observational studies can only show an association – they do not provide definite proof.

Not surprisingly, there has been concern over a possible detrimental effect of isoflavone supplements in women with pre-existing breast cancer due to their oestrogen-like action. A critical review of the literature concluded that, overall, there is no impressive data suggesting that adult consumption of soy/isoflavones affects the risk of developing breast cancer, or that soy consumption affects the survival of breast-cancer patients. This has now been substantiated by the American Institute for Cancer Research (AICR) who recently stated that breast cancer patients and survivors no longer need to worry about eating moderate amounts of soy foods. Different doctors have different opinions, however, so seek advice from your own doctor.

Isoflavones and menopause

Phytoestrogens have a weak oestrogen-like action that may reduce the oestrogen-withdrawal symptoms associated with the female menopause. This may explain why the prevalence of hot flushes among Asian women is generally lower (10 per cent to 20 per cent) than is observed in most Western populations (70 per cent to 80 per cent).

The use of hormone-replacement therapy (HRT) has fallen out of favour due to fears that its use may increase the risk of breast cancer, so women are turning more and more to natural treatments such as isoflavone supplements. Although results from different trials have varied, and there is a large placebo effect, a recent systematic review and meta-analysis of the results from 19 randomized controlled trials found that isoflavone extracts reduced hot flushes by 39 per cent compared with placebo. Even though the overall combined results showed a significant tendency in favour of soy, the authors still stated that it was difficult to establish conclusive results given the highly variable results from different studies. This may be explained in part by the suggestion that only women who are equol producers will derive meaningful benefit.

INCREASING YOUR NATURAL INTAKE OF PHYTOESTROGENS

Aim to consume more of the following foods:

Beans,
especially chickpeas, lentils, alfalfa and mung beans, soybeans and soy products (e. g. tofu marinated in low-salt soy sauce)

Vegetables,
dark green leafy vegetables (e. g. broccoli, spinach, cabbage) and exotic members of the cruciferous family (e. g. Chinese leaves, kohl rabi), celery, fennel

Nuts,
almonds, cashew nuts, hazelnuts, peanuts, walnuts and cold-pressed nut oils

Seeds,
especially flaxseed, pumpkin, sesame, sunflower and sprouted seeds

Wholegrains,
especially corn, buckwheat, millet, oats, rye and wheat

Fresh fruit,
including apples, avocados, bananas, mangoes, papayas and rhubarb

Dried fruit,
especially dates, figs, prunes and raisins

Herbs,
especially angelica, chervil, garlic, ginger, parsley, rosemary and sage