LACTOSE OR dairy intolerance is an inability to digest lactose, a type of sugar found in milk and other dairy products. It's believed to affect almost 10 million people in the UK and is caused by not having enough of the enzyme lactase. If lactose is not broken down by lactase, components of the lactose remain in the intestinal tract, where it feeds bacteria which can cause cramps, bloating, flatulence and weight loss.

Babies make this enzyme so they can digest milk, including breast milk, but before humans became dairy farmers, the majority of people didn't continue drinking milk, so their bodies didn't make any more lactase. Interestingly, people from cultures in which dairy farming started earlier (such as northern Europe) are less likely to suffer from lactose intolerance, than those from Asia and Africa.

Sometimes, a dairy intolerance is dose-related, so symptoms are more likely to occur if milk is consumed frequently or in large amounts. It is also related what actual products are consumed as some things are higher in lactose than others. Many people who are lactose-intolerant can digest small amounts of dairy.

There are four types of dairy intolerance: primary, secondary, congenital and galactosaemic. Primary intolerance is a complete lack of lactase production, rather than diminished production, and usually starts in teenage years. Secondary intolerance is usually short-term and occurs if the lining of the intestines has become damaged as a result of illness. Congenital intolerance occurs in children born with no lactase production. These children cannot tolerate any milk. Finally, galactosaemia is a rare hereditary disease which prevents the sufferer from metabolising galactose, a part of lactose. Symptoms of intolerance include nausea and vomiting, wind, cramps, diarrhoea and bloating.

Breath and stool tests will determine if you are lactose intolerant. It is very common in adults and removing milk products from your diet usually improves symptoms. However, this can result in a shortage of calcium, vitamin D, riboflavin and protein so a milk substitute such as Alpro's soya range (£1.49, available from supermarkets) is needed. Goat's milk is low in lactose, but you need to combine it with essential amino acids and vitamins. Most people with low lactase levels can handle two to four ounces of milk at one time (roughly equivalent to half a glass) but tolerance levels vary greatly.

If specific lactase enzymes are introduced in a form your body recognises, they can break down the milk sugar and proteins that cause the problem. Immunecare Lactase Plus (90 capsules for £9.99, 0870 774 7008) contains the enzymes in a convenient capsule. One taken before each meal containing lactose or dairy proteins will help redress the lactase level in your gut.

Once diagnosed, it's helpful to consult a nutritional therapist. They will carry out a thorough health and lifestyle history check and ask you to complete a symptoms diary. This will help you to decide which foods need to be reduced or eliminated. Never do this yourself without getting professional advice first.

You also need to become label-aware when shopping. The following milk derivatives should be avoided: casein; buttermilk; artificial butter flavour; cheese; custard; ghee; lactulose; rennet casein; whey and yoghurt. Also some margarine, cold meats, artificial flavourings and high-protein flours contain dairy.

This may seem like a daunting list, but the market for products that cater to dairy intolerant people is growing rapidly. Bute Island (www.buteisland.com) does a wonderful range of dairy-free cheese. And, luckily, milk is one of the most easily substituted ingredients in cooking and baking. You can modify existing recipes by substituting water or fruit juice measure for measure for milk, or use soy or rice milk.

To ensure you get enough protein in your diet, there are four things you can do. Eat moderate amounts of animal protein, such as turkey, which is also a good source of vitamin D, which helps metabolise calcium. Eat vegetables containing calcium and take a calcium supplement. Good, easily-digestible sources of protein are oatmeal, watercress, spinach, shrimp, anchovies, sardines, apricots, tahini and celery.

While you may feel you're being restricted, many experts believe there are benefits to a dairy-free diet, although this is a contentious issue. It's been claimed it can help lower your cholesterol and fat intake; reduce sinus problems; improve protein digestion; and improve nutrient absorption. Planning and understanding of your tolerance levels, mean it doesn't have to be a big problem.