'Exercise? Look at you, you don't need to'

Not long ago, I was looking for a new gym to join. Being instinctively inactive I was doing it on the web, when I stumbled across a fitness calculator. Being a sucker for such things, I entered weight, height, waist circumference, wrist circumference. At 5ft 3in and 7st 5lb I was informed I was underweight (according to my BMI - more of that later) but had a healthy metabolic rate (though how they knew that from my wrist measurement I have no idea). The gym, I was promised, would help me deal with these issues if I would just come in "and ask a member of staff for their help and advice in constructing a tailored weight loss programme for you".

Hmm. A little contradictory, that. Possibly even dangerous. But perfectly in tune with general assumptions of what the gym is for. Shaving off pounds, fitting into dresses. But if you haven't got many pounds to lose - what are you doing there? Thin, goes the accepted wisdom, is equivalent to fit. It is at least as high a virtue, probably higher. And if you have somehow hit the 21st-century western jackpot and inherited the genes for thin, it is certainly something you shut up about. These days thin is an achievement. It is a declaration of self-control, of class, of aspiration. In the tyranny of thin, being so naturally while eating normally (roast with all the trimmings for a weekday lunch, for example) is pure smuggery, and there is no shyness about letting you know.

As for noting, en passant, that I feel terribly unfit and really must get to the gym sometime this millennium - that is taken almost as a personal affront. I have lost count of the number of times the answer has been a dismissive, "Look at you. You don't need to." I will decide for myself what I need, thanks very much. The thing is, though, it is hard not to remain impervious. Maybe I am special. Maybe I really don't need to do anything.

But, as I am learning with advancing years, slimness is not equivalent to being fit or healthy. Nor, for that matter, is it equivalent to anorexia. People seem to feel it is somehow OK to imply that I don't eat properly; some have even suggested an eating disorder. If I protest, then that is simply proof of an unhealthy relationship to food, isn't it? That's what the celebrity bobble-headed poppets say, and look at them. You can't win.

Until a few months ago, I went to the gym, oh, once every two weeks or so if I was being good - but I have been quite shocked recently by the effect on my lungs and legs of not that many flights of stairs.

Researchers at the Cooper Institute of Aerobics Research in Dallas recently looked at the relationship between weight and health (activity levels, cardiovascular and metabolic fitness) of more than 70,000 people, and discovered that weight is not a reliable indication of overall health and mortality risk. Obese men who do regular exercise, found the study, are at lower risk of cardiovascular disease than unfit lean men. What is more, the death rate for people who are thin but unfit is twice as high as that for their obese peers who are fit. The underweight are at higher risk of osteoporosis and, especially if they are on punishing diets, age faster. They are more susceptible to illness. Extreme dieting can result in depression. Strip away the warped and bony ideals, and the fact, as Paul Campos, author of The Obesity Myth, has forcefully put it, is that, "There is no valid medical reason why two women of the same height cannot weigh 7 and 14st respectively, while both maintain optimum cardiovascular and metabolic fitness, and excellent overall health".

So, having dismissed the importance of actual weight, I'm curious to know how fit, in empirical terms, I am. Are my intimations of decrepitude an illusion? Can you really sit at a desk for 10 hours a day and get away scot-free? Is it possible to be thin and fit, and healthy, and do no exercise at all? And so, largely because I think the name mildly, amusingly appropriate, I pay a visit to Slim Jim's, a 24-hour club in the City. I feel at a disadvantage before I start: I have been denied my morning coffee because it might skew results, so a headache builds throughout the morning, until I start to find it difficult to listen to what Graham Smith, a personal trainer, is trying to say. He takes my resting pulse. It's 74, slightly worse than average; a fit person's would be closer to 60. The record is held by five-times winner of the Tour de France, Miguel Indurain, who has a resting rate of 28; clearly we're not breaking any records here.

He measures my blood pressure (normal), then peak flow: a short sharp breath into the gym equivalent of a breathalyser, to judge lung capacity - 390 litres per minute. Not great, but average. I should be aiming for about 400. Then it is time for another encounter with my BMI (Body Mass Index, a height-to-weight ratio), this time on a fancy machine: it's 18.1 - 20-25 is optimal. I need to put on about 15lb, apparently. Granted, I could maybe do with a little filling out, but more than a stone?

The trouble with the BMI, says Smith, is that it is an extremely blunt tool. It may guide government policy and be a main weapon in the current war against weight, but it takes little to no account of fitness or bone structure, or body composition. Or, in fact, overall health. By this measure, many professional athletes and fitness professionals are obese. Conversely, I have a slight bone structure and not much muscle. And the state of that muscle is not great: an endurance test involving a squat against the wall, then lifting one leg for as long as possible (no, it is not dignified), is not a notable success. Body composition testing, during which a mild electric current is passed through your body (fat is more resistant to it than muscle), tells a clearer tale - and begins to make me feel a little like something you might find shrink-wrapped on a shelf at Sainsbury's.

It turns out that 70.2% of me is lean, and 29.8% fat, "which is not expected," says Smith, diplomatically. The reference sheets are less coy: 29.8% places me squarely in a category endearingly labelled "over fat". ("Acceptable" for someone in their early 30s, is 23-27%; slim, the category I might have been expected to inhabit, is 14-22%.) Which somehow feels truer to how I feel going up the stairs.

Several tests later, it turns out that the only thing I am unequivocally good at is touching my toes: according to the handy tables Smith provides, I am even better at that than excellently bendy 17-year-olds. Hurray! I will try to remember that the next time I miss a bus.

"Am I terribly unfit?" I ask Smith. "No, in all honesty, I can't say that. But there's work to do. I would recommend you put on some lean muscle. It would involve doing a bit of resistance training. Things that involve more than one muscle group - lunges, squats. Body-weight exercises are always good because they are functional as well, not just lifting weights. A minimum of 20 minutes steady state cardio three times a week."

Theoretically, though, there is no cause for immediate alarm. I could just keep on doing nothing. But actually I crave the things that exercise brings: alertness, appetite, vigour. The longer I go without it, the less well I feel. There may not be anything particular wrong now, but I feel unsettled, underpowered, flat. And I am just as vain as anyone else - slack muscle is slack muscle, whether there is 30 kg of it or 300: it needs toning. So I shall rejoin the exercising masses, to live longer, feel healthier, look better, sleep better - but not to lose weight.

Thanks to guardian.co.uk who have provided this article. View the original here.