People vary in their degree of sensitivity to food chemicals, and whether or not they get symptoms depends on the dose ingested. If you’re not too sensitive (with a high dose threshold) you may only react after a particularly rich meal or after bingeing on highly preserved/flavoured/coloured foods. Avoiding these may be all you need to do to stay well.
However, if you’re at the other end of the spectrum (with a low dose threshold) you may develop symptoms over several days or weeks from the cumulative effects of small amounts of natural chemicals. Because these are present in many otherwise ‘healthy’ foods in a normal diet, you’ll have to be much more careful with what you eat on a daily basis.
Food intolerances can be distressing, but they don’t cause permanent damage to the body. If you have persistent symptoms it’s a good idea to first make sure some serious disease hasn’t been overlooked.
If you’re having trouble working out which foods are upsetting you, professional help may be needed to investigate the problem more systematically. This first step is to follow a strict elimination diet for 3 or 4 weeks to see if symptoms disappear. You may get a withdrawal effect in the first week or so (with a temporary flare-up of symptoms for a few days) so don't give up too soon. Once your symptoms have settled and you're feeling better for at least 7 days in a row, your're ready to start doing challenges to find out which chemicals in your diet cause reactions.
Challenges can either be done double-blind (with purified food chemicals taken in capsules), or with carefully selected foods each containing only one problem substance. Once your problem substances have been identified, a dietitian with experience in the field can advise you how to manage your diet.
Don’t be discouraged — food intolerances needn’t be permanent. You may well be able to build up your tolerance level by gradually increasing the amount and variety of ‘low’ and ‘moderate’ foods over several weeks or months, and eventually return to a more normal diet. Even if this is not possible, you’ll learn ways of avoiding severe reactions by looking out for the foods that upset you most.