Diet to Protect against Asthma
Diet to Protect against Asthma
There is growing evidence that several aspects of the modern Western diet make asthma more likely to develop. Parts of this evidence are very convincing, while other findings are less conclusive as yet. Some people might argue that, until all the facts about diet and asthma are firmly established, no dietary changes should be recommended. However, all the dietary changes that might protect against asthma are also very valuable for general health.
This diet is potentially useful for:
0Atopic families who wish to reduce the chance of their chil- dren developing asthma. Other preventive measures, such as allergen avoidance and exercise (see Chapter 8), are obviously important as well.
• Anyone who already suffers from asthma – with this diet, their symptoms may diminish.
The main elements of the anti-asthma diet are:
• A high intake of fresh fruit. Researchers in Britain and the Netherlands have shown that people who eat more fruit have better lung function, and are less likely to develop asthma or bronchitis. Apples have a particularly good effect on the airways, according to one recent study. Many other studies show a link between Vitamin C – the major vitamin in fruit – and asthma prevention. This makes sense because Vitamin C is an antioxidant which inactivates the pro-inflammatory substances (called oxidants) that are found in cigarette smoke and other polluted air. In addition to Vitamin C, many fruits contain beta-carotene (see below) – mangoes and apricots are the richest sources.
• Regular helpings of carrots, which contain the orange pigment beta-carotene. This is another antioxidant that can help prevent inflammation in the airways. It should be obtained from food, not supplements (see p. 207).
• A high intake of fresh green vegetables, especially broccoli, spring greens, dark green cabbage, peas, parsley and courgettes. One Australian study has shown that children who eat fewer vegetables are more likely to wheeze. The benefits of vegetables may be partly due to the fact that they contain beta-carotene and (if eaten raw or only lightly cooked) Vitamin C. Dark green vegetables are also a good source of magnesium, and researchers find that people with a higher magnesium intake have healthier airways. Magnesium is believed to protect against asthma by helping the muscles of the airways to relax.
• Plenty of tomatoes and tomato products, such as tomato juice, tomato sauce, ketchup and paste. The special protective effect of tomatoes is not entirely explained by their Vitamin C or beta-carotene content – another antioxidant, called lycopene, may be the crucial ingredient here. Good news for fast-food fans – the benefits of tomato paste are even seen among pizza eaters who are significantly less vulnerable to asthma.
• Daily intake of sunflower seeds, or sunflower oil and margarine. These are by far the best natural source of Vitamin E, another antioxidant (see left) which helps to reduce the risk of becoming asthmatic. Vitamin E taken in supplements seems to have much less beneficial effect than natural Vitamin E from food.
• A good intake of the minerals zinc, manganese and selenium, as well as magnesium (see p. 206). Shortage of any of these minerals may be linked with asthma. It is important not to eat too much wheat bran or unyeasted wholemeal bread, especially with main meals, as these block the absorption of several minerals.
Good sources of zinc include meat, shrimps, clams and oysters, with smaller amounts in cheese and egg yolks. Nuts, lentils and beans are fairly good sources of zinc, while soya protein blocks its absorption.
As well as being found in dark green vegetables, magnesium is plentiful in sardines, peanuts, hazelnuts, walnuts and lentils. Other fish, lean meat, milk, cheese and bananas contain smaller amounts.
Manganese is found in eggs and milk, and though the amounts are small, these are good sources because the mineral in them can be absorbed easily. While green leafy vegetables, whole grains and tea apparently contain more manganese – and are frequently recommended as a source of this mineral – in fact very little can be absorbed from those foods. Lentils are a moderately good source of manganese.
Selenium is most plentiful in fish and meat. It may be scarce in home-grown plant foods in areas of the world (notably Finland and parts of New Zealand) where selenium is lacking in the soil.
• A limited intake of meat, especially red meat, plus a com-
- plete avoidance of kidney, liver and other offal meats. An entirely vegetarian diet incurs a risk of mineral deficiencies however (see above). On balance, it is probably best to eat meat once a week or less.
• A low intake of salt. Researchers in Kenya found that children eating a high salt diet (which equals the average salt intake in Britain and other parts of the developed world) were at greater risk of becoming asthmatic. For existing asthmatics, increasing the amount of salt eaten can make asthma worse, while reducing salt can lessen symptoms. Male asthmatics seem to be more vulnerable than females. Salt probably affects the muscles of the airways, making them more likely to contract.
The role of supplements
You should try to get all the nutrients you need from food rather than supplements. However, there are times when a supplement can be useful. Any asthmatic who has to cope with the effects of high air pollution, especially ozone and sulphur dioxide (see pp. 130-31), may find a supplement of Vitamin C beneficial. However, you should avoid very high doses of Vitamin C (e.g. I g/day) as they can cause disturbed sleep. Use natural sources for Vitamin E (see p. 206) if you can, but taking a supplement is better than nothing.
Vegans should think about taking a multi-mineral supplement, given the difficulties of ensuring an adequate intake of zinc, manganese and selenium from vegetable food (see left). Vegetarians may also benefit from a mineral supplement.
Some supplements, in certain circumstances, can do more harm than good. Omega-3 oils (also called w-3 oils, concentrated fish oils, or EPA and DHA) may make asthma worse for some people (see box on p. 221). Beta-carotene (sold alone and as part of mixed antioxidant supplements) may, according to some studies, promote cancer at the high doses used in many supplements. It should only be obtained from food.
Foods and drinks that bring on asthma attacks
The anti-asthma diet tackles the inflammation of the airways and the underlying tendency of the airway muscles to go into spasm – in other words, it is concerned with the long-term treatment or prevention of asthma. In addition, you should obviously avoid any foods which aggravate asthma in the short term. Various foods and drinks can bring on an asthma attack:
• Foods and drinks containing sulphur-based preservatives tend to give off the irritant gas sulphur dioxide while being chewed or swallowed. Some asthmatics are more sensitive to sulphur dioxide than others. The foods that most commonly cause problems are dried apricots and other dried fruit (except those labelled ‘unsulphured’), shellfish, french fries, ready-made salads and fruit salads. Sulphur-based preservatives are used widely in the catering industry. On packaged food, look for ’sulphite’ and’metabisulphil or E numbers 220-227. Soft drinks, wine, beer and cider almost always contain sulphur-based preservatives.
• Foods that cause heartburn (GER – see p.38) can aggravate asthma for some people.
• Alcoholic drinks may make the airways contract for some asthmatics (see box on p. 160).
• Some asthmatics need to avoid foods containing histamine (see box on p. 67).
• A few asthmatics respond badly to the smell of food cooking. The most severely affected can suffer an asthma attack from anyfood aroma. Cromog lycate -type drugs (see p. 148) or anti -choli nerg ics (see p. 156) may block this reaction.
Needless to say, if you have a sensitivity reaction to any food listed for the anti-asthma diet you should not eat this food.