When Julius Caesar cried, “Let me have men around me that are fat”, he never realised that one day his wishes would come true. According to Barry Popkin, an American nutritionist, there are now more overweight people in the world than hungry ones.

There are a billion fatties worldwide and 800m of the lean and hungry. It’s an extraordinary turnabout in the human condition. Once, the mass of men worked to stave off hunger and famine; now it’s coronary heart disease and type 2 diabetes that they have to worry about.

Mind you, we should be grateful: personally I’d rather die of any obesity-related disease than starve.

The technical definition of obesity is a body mass index count of more than 30, and you work that out by dividing your weight in pounds by your height in inches multiplied by 703. Or whatever that is in metric. I can’t be bothered myself, especially now that some scientists have questioned whether this measurement tells us as much as we thought. But I do know the cause of obesity: we’re eating too much and exercising too little. Especially in Britain, which is, apparently, the fattest nation in Europe.

So far so straightforward. But the moment you suggest that fat individuals bring their fatness upon themselves you enter the minefield that is obesity politics, which has a language all of its own.

The World Health Organisation calls obesity a “rising epidemic”, which is fine if you think of the term simply as meaning a common condition. But the word is normally used about disease. And fatness is quite unlike most diseases in that it doesn’t fall impartially on the just and unjust alike.

There are any number of diseases occasioned by fatness, but the condition itself is brought about by your own actions. Or inaction. It’s not a contagion like measles. You don’t catch it like the plague. It’s even unlike those diseases that you do bring on yourself by bad behaviour— syphilis, say — in that it’s not transmitted by bacteria. To talk about an epidemic of obesity is like talking about a plague of inactivity or a contagion of overeating.

Which brings me to Anne Diamond, the formerly skinny television presenter who has shared her weight troubles with interested television audiences for about a decade. Now, by dint of weight-loss surgery, she is down to a respectable size 14. Her essay this week in Hello! magazine about the way society stigmatises fat people is a model of the sloppy thinking that characterises much of what passes for debate on the subject.

“We [viz fat people] are normally considered to be lazy, slobbish and lacking in moral fibre,” she declares. “Yet nothing could be further from the truth . . . it can happen to anyone . . . We’ve somehow been caught up in an epidemic.”

Don’t you love that word “somehow”, which suggests that there could be some mystery about cause and effect? Normal people attribute extra fat to the fact that they’ve eaten their body weight in Mars bars or never go out on two feet when they can use four wheels instead. Celebrity obesity victims take a different view. “Fat isn’t a sin,” says Diamond. “and it doesn’t demand punishment.” Well, no, fat isn’t a sin, but gluttony and sloth are. As St Thomas Aquinas, no lightweight himself, put it, “gluttony denotes inordinate concupiscence in eating”.

So Diamond has set up a website called Fat Happens! to enable overweight people who want to become slimmer to share views and find supportive friends. The aims are admirable and the means exemplary; it’s the underlying premise I take issue with. Fat doesn’t “happen”; you bring it on yourself.

I say all this as a greedy pig myself. Once, when I thought of using homeopathic medication, I dropped the idea as soon as I read the instructions on the label. Apparently you shouldn’t eat for an hour before and an hour after taking the drops. It dawned on me then that there weren’t two solid hours in any normal day when I wasn’t eating or drinking.

As the old gag has it, I eat like a bird — a vulture. My father, who became tubby in later life (he drove when he should have walked), used to laugh at the old Special K advertisement: “Can you pinch an inch?” He said gloomily that no matter where he tried he found that he was pinching a lot more than one inch — more like three.

Of course there are people who are unfairly genetically burdened with the sort of build and metabolism which mean that they can eat like fairies and still run to fat. There is no justice in this world. But it would be self-deceiving to suggest, as lots of fat people do, that fatness is really a metabolic problem.

This week’s issue of the British Medical Journal is devoted to China, where the chief problem is that the rate of obesity has nearly doubled in 10 years. Yet you don’t get the Chinese talking about fat happening. Wang Longde, the vice-minister of health, instead says that the Chinese population lacks knowledge of what is reasonable nutrition and diet. Some people, he says, are eating too much meat, oil and fat, and parents and grandparents are feeding their children excessively to make up for being undernourished themselves.

The upshot is that the Chinese are to be bombarded with instructions about eating properly and exercise. In proper communist style, an earlier state propaganda campaign had 2,300 children doing callisthenics in Tiananmen Square in front of a television audience of millions.

Here our bad habits have any number of contributory causes. The inexorable growth of the food industry (which means that we are conditioned to buy ready-made food); our (related) inability to cook food for ourselves at home; the replacement of cookery classes in schools with food technology; and the evil manipulation of children by fast food advertisers — they are all described in Joanna Blythman’s excellent book Bad Food Britain and Eric Schlosser’s Fast Food Nation. And then there is the obvious — troubling — connection between obesity and poverty.

However, obesity can’t be addressed by blaming the state, the manufacturers or the fast food industry. Unless we accept that fat doesn’t happen, we make it happen, we will still be fat — only we’ll call it victimhood.