Nutrition (18 page)
Catechins
Tea leaves are the richest dietary source of a group of antioxidants known as catechins. As tea is the second most common drink in the world after water, those who regularly drink green, white or black tea obtain sizeable intakes of these beneficial catechins. Other dietary sources include apples, pears, berries and dark chocolate.
Tea is prepared from the young leaves and leaf buds of an Asian shrub,
Camellia sinensis.
Green tea is made by steaming and drying the fresh tea leaves immediately after harvesting, while black tea is made by crushing and fermenting the freshly cut tea leaves so that they oxidize before drying. This allows natural enzymes in the tea leaves to produce the characteristic red-brown colour and reduced astringency. White tea is similar to green tea in that it is not fermented, but is only made from new tea buds, picked before they open. These have a white appearance due to the presence of fine silvery hairs.
Camellia sinensis.
Green tea is made by steaming and drying the fresh tea leaves immediately after harvesting, while black tea is made by crushing and fermenting the freshly cut tea leaves so that they oxidize before drying. This allows natural enzymes in the tea leaves to produce the characteristic red-brown colour and reduced astringency. White tea is similar to green tea in that it is not fermented, but is only made from new tea buds, picked before they open. These have a white appearance due to the presence of fine silvery hairs.
DID YOU KNOW?
White tea contains around 15 mg caffeine per cup, compared to 20 mg for green tea and 40 mg for black tea.
A third of the dry weight of tea leaves consists of antioxidants, with green tea containing more catechins and black tea containing more complex theaflavins and thearubigins which, although originally thought to be less beneficial, are now known to have similar antioxidant power. Black tea, as brewed in the UK, contains around 200 mg flavonoids per cup. Although it has been suggested that adding milk to a cup of tea might blunt its antioxidant capacity, a number of studies have now found that milk is unlikely to reduce the bioavailability of tea flavonoids.
Tea and heart disease
Consumption of tea flavonoids increases the antioxidant capacity of blood, and has been shown to improve blood-vessel reactivity (so that blood vessels are able to dilate more readily, as needed) and blood flow through the coronary arteries. This might be expected to provide some protection against heart attack. A review published in the
European Journal of Clinical Nutrition
found clear evidence for a protective effect of black tea against coronary heart disease at an intake of at least three cups per day. A meta-analysis of 13 studies published in the
American Journal of Clinical Nutrition
concluded that for every cup of green tea you consume each day, your risk of developing coronary heart disease is reduced by another 10 per cent. Perhaps surprisingly, they did not find a protective effect from drinking black tea, and more research is obviously needed.
European Journal of Clinical Nutrition
found clear evidence for a protective effect of black tea against coronary heart disease at an intake of at least three cups per day. A meta-analysis of 13 studies published in the
American Journal of Clinical Nutrition
concluded that for every cup of green tea you consume each day, your risk of developing coronary heart disease is reduced by another 10 per cent. Perhaps surprisingly, they did not find a protective effect from drinking black tea, and more research is obviously needed.
Tea and metabolism
Catechins can boost your basal metabolic rate by inhibiting a metabolic enzyme (catechol-0-methyl transferase) that breaks down a neurotransmitter called noradrenaline. A rise in noradrenaline levels stimulates the amount of energy burned in body cells (thermogenesis). Tea extracts may also block the activity of intestinal enzymes (gastric and pancreatic lipases) needed to digest dietary fat, so that less fat is absorbed. Research also suggests that drinking tea can have beneficial effects on metabolism to suppress the high blood glucose levels and insulin resistance associated with diabetes, as well as reducing weight gain. These effects are still under investigation in humans, but a study of 300 elderly people living in Cyprus found that drinking one to two cups per day for at least 30 years was associated with a 70 per cent lower odds of developing Type 2 diabetes, even after adjusting for other factors such as age, weight, smoking, activity level and dietary habits.
Although the jury may still be out, if you enjoy a cup of tea then carry on drinking it – in fact, aim for at least two or three cups per day!
Anthocyanins
Anthocyanins are the water-soluble pigments that give many fruit, vegetables and flowers their lovely red, blue or purple colour. They are also responsible for the red coloration of autumn leaves. The colour of anthocyanins depends on the level of acidity present, and these pigments are often used as pH indicators (red = acid, blue = alkali) such as the famous litmus test based on a blend of dyes extracted from lichens. Within plants, anthocyanins act as a ‘sunscreen’ by absorbing blue-green and UV light, and also have antimicrobial actions to protect plants against infection.
FANCY A PURPLE TOMATO?
Scientists recently developed a genetically modified purple tomato by inserting two snapdragon genes. These genes switch on a tomato gene that regulates the synthesis of tomato anthocyanins so that the purple colour is ‘natural’ to the tomato plant and is not owed to a flower pigment that is not normally present in tomatoes.
Different plants contain different types of anthocyanin (e. g. cyanidin, delphinidin, malvidin, pelargonidin, peonidin, petunidin, myrtilin) and over 630 have now been identified in nature. Many of these are in the food we eat, and intakes are estimated at 23 mg per day in Europe and 650 mg per day in North America (partly accounted for by a particular love of bilberries). A serving of 100 g berries provides up to 500 mg of anthocyanins.
Numerous studies show that the anthocyanins you eat can have a wide range of biological actions in the body. They are highly antioxidant and can have effects that vary from reducing blood pressure, abnormal platelet clotting, glucose intolerance and cholesterol levels to reducing inflammation and protecting against coronary heart disease. As well as having an antioxidant action, they may act as biological response modifiers by switching off genes involved in abnormal cell proliferation and the growth of abnormal blood vessels (angiogenesis), which together may reduce the risk of cancer. These beneficial effects are undoubtedly part of the reason why high intakes of fruit and vegetables are so beneficial to health – and possibly why a rosy red apple a day might help keep the doctor away!
Carotenoids
At the other end of the colour scale, carotenoids are antioxidant plant pigments found in yellow, orange and dark green fruits and vegetables, including sweetcorn, carrots, pumpkins, mangoes, oranges, peaches, guavas, watermelons, spinach and other dark green leafy vegetables. A few carotenoids are red, such as the lycopene found in tomatoes, and astaxanthin – the red carotenoid pigment found in certain algae which, when consumed by flamingos, turns their plumage such a glorious shade of pink.
Over 600 carotenoids have been identified, of which around 50 can be converted into vitamin A in the body and are said to have provitamin A activity. This is inefficient, however, and it takes 6 mcg of betacarotene (one of the most common carotenoids) to yield 1 mcg of retinol vitamin A. For other carotenoids with provitamin A activity, twice as much is needed (12 mcg) to yield 1 mcg retinol. A carrot weighing 100 g contains around 2,400 mcg of betacarotene, for example, which if you divide by six is equivalent to 400 mcg of retinol. In contrast, 100 g of calf’s liver provides around 25,200 mcg of retinol vitamin A.
Some carotenoids without provitamin A activity are also important for human health, most notably astaxanthin, lycopene, lutein and zeaxanthin.
TURNING ORANGE
Eating excessive amounts of carotenes – for example, from carrot juice – can lead to carotenodermia, in which the skin acquires an orange colour that resembles cheap fake tan. This is considered harmless, and quickly resolves once intakes are reduced. Excessive intakes of retinol vitamin A, however, are highly toxic, as discussed in
Chapter 4
.
Chapter 4
.
Astaxanthin
As mentioned above, astaxanthin is a carotenoid produced by some red yeasts and microalgae to provide protection against ultraviolet light. High concentrations are found in a tropical alga,
Haematococcus pluvialis,
which remains green until exposed to strong sunlight, when it produces astaxanthin and turns red. When Antarctic krill, prawns, salmon, rainbow trout and crabs eat red microalgae, the astaxanthin becomes concentrated in their flesh, roes or shells, which acquire a pink colour. If you regularly consume krill-oil supplements or crustacea, therefore, you will obtain significant amounts of astaxanthin in your diet. Research into the effects of astaxanthin on human health is in its infancy. However, a number of recent reviews, including one in the
American Journal of Cardiology,
discuss its potential for beneficial effects on your antioxidant levels, cholesterol balance, arterial blood flow and coronary heart disease, as well as for visual acuity.
Haematococcus pluvialis,
which remains green until exposed to strong sunlight, when it produces astaxanthin and turns red. When Antarctic krill, prawns, salmon, rainbow trout and crabs eat red microalgae, the astaxanthin becomes concentrated in their flesh, roes or shells, which acquire a pink colour. If you regularly consume krill-oil supplements or crustacea, therefore, you will obtain significant amounts of astaxanthin in your diet. Research into the effects of astaxanthin on human health is in its infancy. However, a number of recent reviews, including one in the
American Journal of Cardiology,
discuss its potential for beneficial effects on your antioxidant levels, cholesterol balance, arterial blood flow and coronary heart disease, as well as for visual acuity.
WHY FARMED FISH AND EGGS LOOK SO GOOD
Astaxanthin is added to some animal feeds to colour farm-raised salmon and to intensify the colour of hens’ egg yolks. And do you know why lobster shells turn from blue to red when cooked? It’s because heating releases astaxanthin from the proteins to which it is bound.
Lycopene
Lycopene is a red pigment found in tomatoes, pink guava, papaya, red/pink grapefruit and watermelon. When cooked, tomatoes release five times more lycopene than is available from raw tomatoes, so concentrated products such as tomato ketchup, tomato purée, passata – and pizzas – are among the richest dietary sources of lycopene. At last – a nutritional reason for fully enjoying a juicy Margharita pizza! Like other carotenoids, lycopene is fat-soluble, so drizzling healthy olive or rapeseed oils over your pizza slice, in true Mediterranean fashion, increases your dietary absorption of lycopene as much as threefold. Why is lycopene so good for you? Because evidence is accumulating that it protects against both coronary heart disease and cancer – the two biggest killers in the developed world.
Lycopene and coronary heart disease
People who regularly eat tomatoes and tomato products are 30 to 47 per cent less likely to develop heart disease than those who eat them infrequently. Lycopene is the most likely protective factor. A recent meta-analysis of 12 studies show that it has a marked cholesterol-lowering action – taking 25 mg supplements daily can reduce cholesterol levels by 10 per cent which, as the authors point out, is comparable to the effect of low-dose statin drugs. In addition, the studies showed that lycopene supplements noticeably reduced systolic blood pressure by, on average, 5.6 mmHg (an effect equivalent to some prescribed antihypertensive medications).
Lycopene and cancer
People with the highest dietary intakes of lycopene have the lowest incidence of certain cancers, especially those of the mouth, oesophagus, stomach, lung, colon, rectum, cervix and prostate gland. Although it is possible that lycopene may just act as a biomarker for tomato consumption, and that other phytochemicals present in tomatoes (such as phytoene, phytofluene, quercetin, kaempferol, naringenin) are also involved, cell studies in the laboratory confirm that lycopene has a number of powerful antitumour effects in addition to its antioxidant action. In particular, it inhibits growth factors and cell-membrane proteins involved in the spread and invasion of cancer cells. Follow-up studies involving more than 48,000 male health professionals found that men who ate more than ten servings of tomato products per week were 35 per cent less likely to develop prostate cancer than those eating less than 1.5 servings per week. And, after accounting for smoking, people with the lowest levels of lycopene are three times more likely to develop lung cancer than those with the highest intakes. Of course, it’s possible that people who eat lots of tomatoes and lycopene generally select a healthy diet and make other healthy lifestyle choices such as not smoking, exercising regularly and avoiding excess alcohol. But no similar associations have been found for other carotenoids associated with a healthy diet, such as betacarotene, alphacarotene, lutein or beta-cryptoxanthin. So while you may not want to take lycopene supplements, it’s certainly worth eating more cooked tomatoes and indulging in the odd splash of tomato ketchup and the occasional slice of pizza. Drizzled with olive oil, of course!
Lutein
Lutein is a bright yellow pigment found in yellow-orange fruits and vegetables. It’s also abundant (though disguised by the green pigment, chlorophyll) in dark green leaves such as kale or spinach, and contributes to the colour of egg yolks.
Lutein is important for vision. It is preferentially concentrated within the macula – the part of the retina responsible for fine sight such as recognizing faces and reading this print. Within the macula, lutein is partly converted into another carotenoid called zeaxanthin. Both lutein and zeaxanthin protect the macula by filtering out harmful, visible blue light, and through their antioxidant action, which neutralizes damaging free radicals generated during the chemical processes involved in light detection.
As lutein cannot be made in the body, good dietary intakes are vital for eye health. Lack of lutein causes depletion of macular pigment and increases the risk of age-related macular degeneration – one of the leading causes of registered blindness in people over the age of 50 years. Age-related macular degeneration is a particularly distressing and isolating condition as it produces visual distortion followed by ‘blanking’ out of faces and print so you can’t read. There are two types of AMD: ‘wet’ in which new, fragile blood vessels leak fluid into surrounding tissues, and ‘dry’ where new blood-vessel formation has not occurred. Wet AMD is treated with laser therapy to seal leaking blood vessels and, increasingly, with drugs injected into the eye. For dry AMD, however, the mainstay of treatment – and prevention – is to obtain a good dietary intake of lutein-rich foods. A number of studies have found an inverse relationship between dietary lutein/zeaxanthin intakes and advanced AMD that is statistically important. People with macular degeneration have, on average, 70 per cent less lutein and zeaxanthin in their eyes than those with healthy vision, while good intakes of lutein and zeaxanthin are associated with a reduced risk of age-related macular degeneration of up to 40 per cent.
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