I WILL never forget where I was when I heard the news that Jim Fixx, the running guru, had died. I had just got home from a run. This summer it will be 20 years since Fixx, a 60-miles-a-week man and the author of the bestselling The Complete Book of Running, which set me on the road to fitness, popped out from a hotel for his daily jog and keeled over dead from a heart attack, aged 52.

Since then I have lost count of the number of smug, sedentary smokers who have evoked Fixx’s name as evidence that exercise is bad for you and that they will have the last, albeit wheezy, laugh. And now, if you are a devotee of the Atkins diet, you can be sure that the same crowd of dull, statistically illiterate bores will be shaking their jowls with mirth at you.

The leaked news that Robert Atkins was, at 18st 6lb, technically obese, is based on the blunt instrument known as the Body Mass Index, or BMI. To see whether you, too, are walking on the wide side with the deceased doctor, calculate your BMI by dividing your weight in kilograms by the square of your height in metres. (If you find this dull, many websites have a calculator: try http://nhlbisupport.com/bmi/bmicalc.htm).

Once you have your figure, you will find that you fall into one of four categories: Underweight, less than 18.5; Normal weight, 18.5-24.9; Overweight, 25-29.9; Obese, 30 or more.

Now, just like Dr Atkins, I stand six feet tall in my bare feet. Unlike him, I weigh 196lb, not 258lb, but this still gives me a BMI of 26.6 and, therefore, puts me securely in the overweight category. Instead of my 14st, I should weigh in at 13st 2lb, but, fat-wise at any rate, I’m not overweight. To reach my “ideal” weight I would have to lose 12lb of useless fat — my head, maybe?

Where the BMI falls down is that it fails to take account of extremes, overestimating the body fat in anyone with a muscular build and underestimating it in anyone, such as an older person, who has lost muscle mass.

And that’s where all such formulas fail: we are all wildly different. Even Fixx had his own bonkers ideas about weight: he thought that the ideal weight for a runner was 2lb for every inch of height. This would make me 10st 2lb — severely underweight on the BMI scale (and, quite probably, dead).

All of this, of course, obscures what all serious doctors and nutritionists have always known: that no faddy diet is a decent substitute for exercise. Knock out six-minute miles, like Fixx, for an hour, and as well as ending up ten miles from where you started out, you will have burnt well over 1,100 calories.

And if, like, Fixx, you fall foul of some hereditary heart problem, at least (unlike the good doctor) you’ll look good on the slab.