Feeling sluggish? Some of your favourite foods could be causing the problem

Bunged-up nose in the morning, headaches after a salad, or bloated after a pizza? Does too much milk cause nausea, a cup of coffee a hand tremor, a cheese sandwich a rash?

Bunged-up nose in the morning, headaches after a salad, or bloated after a pizza? Does too much milk cause nausea, a cup of coffee a hand tremor, a cheese sandwich a rash?

For millions of people, their body's intolerance to everyday foods has become a way of life, and it is a problem that is affecting increasing numbers of adults and children alike. Some researchers say that as many as one in 20 people now have a food-intolerance problem, while others suggest that almost everyone suffers at some time, often without knowing.

"We all get food intolerance but we forget about it the day after. You get up one morning feeling a bit rough from what you have eaten the day before, but by the following day you are bright and bushy-tailed again. The people who run into problems are those to whom it happens day in, day out. Some of the people who come to see me have been suffering for years before looking for help,'' says Dr Peter Fell, an Oxford GP, specialist in food sensitivity, and co-author of Food Intolerance.

For much of the time, food intolerance gets pigeonholed with allergies, but although the symptoms of the two can be similar, the causes are usually very different.

Allergies occur when the body's immune system responds abnormally to a protein found in a particular food. Food intolerance does not usually involve the immune system and is triggered by a physical reaction to a food, a drink or an additive, often because the body lacks the enzyme necessary to digest that food. Lactose intolerance, for instance, is the inability to digest significant amounts of lactose, the predominant sugar in milk. The inability is caused by a shortage of the enzyme lactase that normally breaks down the sugar so that it can be easily absorbed into the bloodstream. When there isn't enough of the enzyme around, some lactose doesn't get properly digested, resulting in nausea, bloating and diarrhoea up to two hours after eating food containing lactose.

In coeliac disease, gluten, a protein found in wheat and other cereals, adversely effects the mucous of the wall of the small intestine and causes weight loss, anaemia, diarrhoea, and mouth-ulcers.

A variety of other foods, including eggs, nuts, shellfish, wheat flour, chocolate, artificial colours, sulphur preservatives, monosodium glutamate, cheese and yeast, are responsible for similar intolerant reactions.

Some foods can cause particular pharmacological reactions. Large intakes of caffeine can cause tremor, migraine and palpitations, while compounds like tyramine, tryptamine and serotonin in foods as diverse as red wine and avocados can trigger urticaria, facial flushing and headaches. Food additives may provoke urticaria, rhinitis and asthma, while yeasts are responsible for a number of reactions, particularly skin disorders.

Food intolerance may also be linked to irritable bowel syndrome: "IBS is a very common presentation through intolerance, and IBS represents about half of gastroenterology consultations. We think IBS is about 90 per cent due to food intolerance,'' says Dr Fell.

"Food intolerance is a toxic reaction. All these detoxification mechanisms were put into the body to get rid of the toxic rubbish that is part of food. They do it very efficiently most of the time and it is only when they get overloaded or something goes wrong that you get a problem.

Just why food intolerance has been increasing is not clear. Some of it may be down to increased reporting due to heightened awareness, but changing food patterns are also thought to be implicated. Thirty years ago, pizzas, for example, were a rarity in the UK, now they are among the top 10 foods. Other foods that were once seasonal, like strawberries, are now eaten all year.

Many people with persistent symptoms of food intolerance do seek help, although often, the only treatment is a change of diet to avoid or cut down on the foods that cause the problem. But for most, occasional food intolerance is part of the ups and downs of the digestive system, and it disappears as quickly as it came.