I am, as they say, conflicted about obesity. You can say it's not your fault, but it is entirely your responsibility to do something about it. It is not a medical condition. You just have to lose weight. On the other hand, you often need help. And, at the moment, there is a generation of kids that the government and the professionals have abandoned. They probably got that fat because their families are ignorant or feckless about their weight, but that is precisely why they need the help. And there is nothing much out there - except if you count people such as Thandi Rudin.

The kids that Rudin works with through her WELLactive programme have a body mass index (BMI) of as much as 50; some weigh 152kg (24st).

So how "fat" are these kids? BMI is the basic index of fatness, your weight in kilograms divided by your height in metres squared. Below 18.5, you're underweight; over 26, overweight; and 31 upwards, you're obese. Mine is 27 - and, drat, Christmas is coming.

Talking to a couple of parents of obese kids was a curious experience. They are in denial; their kids have just got fat. "I have other kids and they eat the same," says one, "but they didn't put on weight. But cheese, crisps, biscuits are just not good for some children." Her daughter weighs 127kg (20st). As Rudin says rather tactfully: "The majority of them are not in a supportive enough environment to make any change happen."

That mother and her daughter have worked with Rudin, and the girl is losing weight. But if the family isn't interested, there's nothing Rudin can do. And, boy, are some of them uninterested! One family watched TV all the way through the introductory session while the boy played on his computer.

Another child Rudin works with had never really drunk water. If she was thirsty, her mother gave her a fizzy drink - and she was drinking two litres a day of pop. "I gave her a challenge," says Rudin, "to drink one glass of water a day and only five glasses of pop. The girl then discovered that her thirst was quenched by water as it never had been by fizzy drinks." She is now working hard to lose the weight.

Shankar Kanumakala, the consultant paediatrician Rudin works with, says: "It's not a medical problem. It's a cultural, political, social and medical problem. Everyone is saying something should be done, but very few people are doing anything. When it escalates, it becomes a medical problem by default."

And that makes everyone think that government should deal with it. Well, they are doing something. The NHS is mopping up the costs of obesity through the treatment of coronary heart disease, strokes, diabetes and some cancers - not to mention osteoarthritis and other muscular conditions - to the tune of more than £1bn a year.

It's an epidemic. The government's own report, Tackling Obesities: Future Choices, puts it bluntly: the people of the UK are inexorably becoming heavier and "although personal responsibility plays a crucial part in weight gain, human biology is being overwhelmed by the effects of today's 'obesogenic' environment, with its abundance of energy-dense food, motorised transport and sedentary lifestyles".

In other words, our bodies are failing to catch up with the fact that we've become a bunch of sedentary layabouts who drive everywhere, take no exercise and eat too much of the wrong things. Living in Britain today makes you fat. The really fat are just the top end of an overeating, underexercising population that guzzles, slouches, and then expects people like Rudin to put it right.

But it's more than just a question of personal willpower. The harsh truth is that we are killing ourselves with food. We are failing to adapt to our environment. Humans have a tendency to put on weight, and the size of the problem, according to the government's chief scientific officer, Sir David King, is "daunting".

We have to change our lifestyle, but whatever the complexity of the reasons, that still comes down to a matter of personal choice. Without parental concern, kids will get fat. And that family will need a Rudin to help them. Unfortunately, there are very few of her about.

· Simon Fanshawe is a writer and broadcaster.