Is anything more stigmatising nowadays than having a fat kid? Can't think of anything, unless it's being a fat kid. A horrible story about the possibility of a child being taken away from his parents and put into care because he was so fat attracted wall-to-wall media coverage. My nine-year-old tells me that the most commonly used playground taunt is "fatty".

Not only are parents made to feel awful if their child is fat, but the advice about how to deal with it is confused and confusing. And now the Early Bird study in Plymouth, which is investigating childhood obesity, is in jeopardy due to funding cuts.

Weight, like height, follows a bell-shaped curve with most people falling within a fairly narrow range and a small percentage at the lower and higher ends. There is no doubt that the weight curve has been shifting to the right so more of us are at the higher end of the range. But there is also no doubt that our life expectancy has been continually climbing and today's children are expected to live longer than ever before. So if being heavier is so bad, how come we live longer?

The Early Bird study is trying to unpick the complicated subject of childhood obesity. It's tracking the weight and activity of 300 local children. So far, it has found no link between weight and activity. It is already known that fat kids tend to become fat adults and that fat parents have fat children. It is probably partly genetic and partly to do with eating habits. It doesn't seem to have much to do with levels of activity though that doesn't mean that it is healthy to sit around all day. It just means that the words "fat" and "lazy" don't necessarily belong together.

The other surprising finding from the Plymouth study is that kids who seem to do a lot of structured exercise in or out of school, are not necessarily more active than kids who appear to do very little. I have often thought that picking up my daughter from school by car to drive her to a gym class is completely daft. On non-gym class days, we walk home and she bounces down the hill, climbs on walls and runs ahead of me to hide. I can see that she gets more exercise on non-gym days!

Many diseases are being linked to the concurrent rise in obesity: diabetes, heart disease, colon and breast cancer to name but a few. There is some evidence that people who live unusually long are particularly thin. It is likely though not certain that if you are genetically predisposed to any of the diseases I mentioned, having a healthy diet (opinions vary as to what that is) and doing exercise (how much and what type is not yet clear) may lessen your risk. However, it remains the case that some thin people get diabetes, and some obese people don't. In other words, lifestyle and weight are factors but they clearly are not the whole story.

But even if we agree that being thin lessens your risk of certain conditions and being fat increases that risk, I still don't get the moral tone that creeps into any debate on obesity. In my own experience, I'm most likely to eat well and exercise sensibly when I'm feeling good about myself. Duvet days when I feel too fat, ugly or sad to face the world, are the worst times to tell me to stop eating chocolate and get on my bike.

So being horrible to and about fat people is bound to be counterproductive. Accepting that we come in a range of shapes and sizes, have different food preferences and enjoy different levels of activity seems a better starting point to me. Kids need guidance and parents need sensible advice. Let them walk, cycle or scoot to and from school and run around in the park whenever possible. Turn off the TV and computer after an hour. Feed them a bit of proper fish, meat or chicken with some vegetables in the evening and only offer fruit in between. And don't let yourself buy into the notion that thin people are good, fat people bad.