A food diary is a way of discovering hidden causes of your health problems. After you have kept it for a while, you will be able to detect patterns that you were never able to see before. In fact, the whole process of finding out what is going on becomes, as Sherlock Holmes might say, "elementary, my dear Watson".

If you or your medical adviser suspect that your health problems are caused by an intolerance to food, one of the best ways to find out for sure is to keep a food diary. That's unless you are happy to go for total exclusion of all possible candidates followed by careful, individual re-introduction after at least two weeks' total abstinence.

Most of us aren't really up for the deprivation routine, so here's how a basic explanation of what's involved in keeping a food diary. Doing this should help you work out what you're reacting to (if anything) - bearing in mind that more than one food group may be involved.

The first thing to do is to get yourself a suitable notebook. Probably the best type would be an A5 one - around 20cm (8") tall by 15cm (6") wide, spiral-bound so that it lies flat when you open it. Get one bound on the side, not at the top.

Use one page a day. Divide the page into 2 columns (a simple fold will do). At the top of each page, write that day's date.

The left hand column is for the food you eat. You need to keep a note of everything you eat or drink every day. This sounds like a bit of a pain, and truthfully it can be, but it's better than cutting everything out on the off-chance.

Write down the time, the type of food, if it's a branded product, the brand, and how much you ate of it (by volume or weight). Also write down the mood you were in when you ate it.

Be specific. For example, don't just write down "potatoes". If you eat chips (french fries), write down that you ate chips (french fries). If you ate mashed potatoes, write down that you ate mash (and the brand if it came out of a packet).

Don't forget snacks, such as crisps (chips) or biscuits, or sauces, such as gravy, salad dressing or custard (and include the make of gravy or whatever). Remember to include drinks, from water (bottled or tap water? There may be differences in the mineral content), through tea and coffee, to soft drinks and alcohol.

On the right hand side, you write down symptoms you experience, and the time they occurred. This includes things like changes of mood, headaches, stomach problems, dizziness, insomnia, in fact anything at all that might be considered not part of a normal healthy life. Even if you have got so used to the symptom that "for you" it's normal, write it down. Also write down the time it occurred, how long it lasted and how severe it was.

As you need to include a lot of information, it's important, if you can possibly do it, that you take your food diary with you whenever you go out. Explain to anybody who asks that you are engaged in detective work - they are likely to be intrigued.

After two or three weeks, look through the entries in your diary to see if you can see anything obvious. You might notice that every time you eat some particular type of food, 4 or 5 hours later you get a particular symptom. The less the time between eating something and getting a result from it, the easier it is to spot. But some foods can trigger a reaction up to 24 hours later, so you might have eaten many other different foods in the meantime. This is why an elimination diet is often recommended for tracing food intolerance.

Your medical adviser may also have software that make analysis easier, though this is quite high-tech, so don't be surprised if he/she hasn't.

I hope this article has been helpful. Don't forget, many niggling symptoms that you've put up with for years may be caused by your body's inability to cope with certain foods. Cut them out of your diet and you will cut out the symptoms as well.

I stopped eating gluten some time ago, and I've never felt better. Problems I've put up with since childhood (which is a fair few years ago) just disappeared. And I started to lose weight, as well - even though I was eating more!

Keeping a food diary is not that difficult - treat it as a puzzle or mystery that you want to solve. After a few weeks, you will have all the clues, and you may be able to pin the blame for your health problems on one particular culprit!

Happy detecting!