Dieting makes you fat. It shouldn't be a secret, since there is a huge amount of scientific evidence that people who diet today gain weight tomorrow.
Yet thousands of people across the country are currently dieting to recover a figure for their beach holiday in July or August.
They should google 'dieting doesn't work' and read the plethora of scientific articles on the calamity of failure in this near-universal human endeavour.
The chances are they will get roped into a cyclical pattern of dieting, bouncing back and struggling to diet again.
The one thing you can be sure of about someone who is dieting today is that they will, next year, or the year after, be even fatter than they would be if they hadn't bothered.
In between times, they may have had a moment of marvellous and elegant trimness. Most do not enjoy it for long.
One theory is that dieting triggers a metabolic alarm that says: this person has been through a calamity and needs protection against it happening again, more fat reserves to draw on than last time.
You need to move very slowly in your weight-loss programme to prevent that alarm going off. Mind you, none of the old hunger strikers seem to be fat.
Another theory is that the problem is lack of willpower - something you couldn't accuse the hunger strikers of, whatever else you might hold against them.
The high failure rate of diets may have much to do with the limited resolve of those who embark on them.
A lot of people lose weight for a holiday, focusing eagerly on the day they can drink sugary cocktails again. For them, it is temporary.
Others need not the latest fad, but a determination similar to what is required to give up smoking. And for those who need to lose weight for the good of their health, there can be few things more cruel than selling them a fad diet with a near-guaranteed bounce built into it.
So the next time you see a book, or programme, that offers you the chance to transform yourself by drinking gloop or avoiding carbohydrates, ask yourself: do I want to be fatter than I am now? If the answer is no, then don't touch it.
The best research on this was done by the University of California in Los Angeles, which monitored thousands of dieters over a period of years and found nearly all of them fatter than they were before they started. That's what makes diet promotion good business. People fail again and again; they live their lives in the desperate hope of losing weight and their very efforts fatten them and send them back to find another miracle cure for their problem.
How would diet promoters make money out of their products if their formulae worked? Each client would just come once and go away slimmer and contented.
The industry itself relies on serial dieting; people crashing out and trying many times again.
That's where the money is.
I shouldn't be talking like this, because I have benefited by substantial weight loss and resumed a cycling hobby that I had, until recently, thought I was permanently unfitted to.
And I don't need to read scientific articles to see how the bounce works. We all know people who have lost weight and piled it back on again. It's too early to be smug.
One of the problems, I suspect, is that people lose weight too quickly and eagerly. The sharper their trajectory towards slimness, the fiercer the recoil.
When I was diagnosed with type 2 diabetes two years ago, the medical advice I was given was to lose about two pounds a week.
I actually brought my weight down slower than that, losing two stone in about six months. What works is not a temporary readjustment of your eating habits, but an actual, permanent change of lifestyle, eating less and exercising more.
But there are no profits in good sense like that.
Dieting is marketed to people who expect quick results. The Atkins diet website offers to help you lose 15lbs in two weeks, more than a stone.
Participants in an Australian experiment into obesity were put on a drinks-only diet of 500 calories a day. No wonder their bodies thought they were in trouble.
I'm hoping to cheat the bounce by doing nothing so drastic. People ask me what diet I used and I tell them it was the smaller portions diet.
Instead of eating to feel pleasantly stuffed, I stop at one spud, instead of three or four on the plate. I'm hoping that this gentler approach won't recoil on me; that I won't wake up as desperate for a large fry as is a hungover alcoholic for a drink in the morning.
I take heart from the fact that I have held my weight level for 18 months, by maintaining the resolve I started off with two years ago and by riding my bike.
I am motivated by a desire to live long. Medical advice tells me that weight reduction has better outcomes than heart bypass surgery. With health at stake, I am not going to take my guidance from fad magazines and profiteers with magic potions, no more than I would take snake oil, or any other quack cure.