Ours are more stringent." (Pritikin also recently bought back the rights to its prepared food brand from Quaker Foods because, according to Lehr, "they didn't meet our health guidelines." )

The Pritikin guidelines include fat content (no more than 15 per cent of total calories); sodium (1,200-1,500mg per day, approximately three-quarters of a teaspoon from all sources); and sugar (less than 4 per cent of total calories, about one tablespoon per 1,000 calories).

"People like to hear good news about their bad habits," Lehr said. "If you read the South Beach Diet marketing - 'Lose all the weight you want, no exercise' - it's not true. The Atkins Diet is 'eat all the meat and cheese you want'. It's just not true."

What doesn't lie is the blood's chemistry. At the beginning and end of the week, each guest's blood is taken and sent to a diagnostic lab. It is then later deconstructed at length with the client by a team of doctors who counsel them twice a week. The numbers powerfully focus the mind both on

the need for change and, as they improve, on the need to stick with the programme.

What would my numbers show? I thought ruefully as I stepped on Pritikin's electronic scale last month. I was a 66-year-old man who still felt young, who wasn't looking for retirement, who ached for more and a better life, who still wasn't ready to give up anything but bubble gum and hard times. I weighed in at 200lb; my girth was 43 inches. In two weeks, I thought, this will be changed. How will my body respond?

In Pritikin's sedate residential setting of the Turnberry Isle Club - which in the early 1970s housed Miami's hottest disco - the denizens were now obsessed with getting it off instead of getting it on. But the scene was no less intense. After all, there's no drama like the drama of life and death.

TOMORROW: CALORIE CRACKHEADS