In the old days, medical certainties stayed certain for a hundred years or more, but nowadays they can change overnight.
You open the paper and find that red wine is not a killer any more – it's good for you!
Yesterday sunshine was a killer and gave everyone skin cancer. In Australia, it is so powerful that it makes Australian cricketers into raving racists who attack Indian batsmen without warning, bite them in the leg and leave them with rabies.
Today they have discovered that sunshine is the only reliable source of Vitamin D, and that we will die without it, so we may have to put up with Australians on the globe for a while longer.
The same with salt.
Yesterday bad, today good.
So how can things can change so quickly?
I'll tell you.
It is all down to one man, Dr Olaf Knuttsen.
Olaf Knuttsen is the head of the Medical Rumours Syndicate, suppliers of medical rumours to the press and media.
When the media think it is time for another overnight shock, they get on the blower to Olaf.
"Hi, Olaf!" they say. "We haven't had a good stand-up medical scandal for weeks. Got anything good lined up for us?"
"Yes," says Olaf. "How would you like a rumour to the effect that breakfast cereals cause cancer?"
"Breakfast cereals cause what?" say the media.
"Just joking!" says Olaf.
Even Olaf Knuttsen and the Medical Rumours Syndicate would probably not have the nerve to go into pitched battle against Kelloggs. Still, nice idea, especially if you loathe corn flakes as much as I do ...
"How about this?" says Olaf. "The Olympic Games are going to cause a general deterioration in British health."
"Are they?" say the media. "How do you reckon that one, Olaf?"
"I don't reckon it at all," says Olaf. "But if you like the sound of it, I could get together enough evidence to make it stand up for a few days."
That's the way Olaf works. Sell a rumour, then find the evidence. Then, when people are used to the evidence, turn it on its tail, get some more evidence and prove the opposite.
"And this year we have got the big one coming up," says Olaf.
The big one, Olaf?
"Sure," says Olaf. He looks ever so slightly furtive. "We look as if we may be going to upset the biggest medical fact of all time. You know that everyone thinks that olive oil is far better than butter?"
"And that fish is better than meat, and that fresh soft fruit such as peaches and nectarines are great...?"
Yes. This is all part of the great Mediterranean diet theory, isn't it? Whereby Sicilian farmers and Greek fishermen, however wrinkled the look, are the healthiest people in the world?
"This year," says Olaf, "we are going to have a bash at turning that one upside down. We are going to announce that the Mediterranean diet doesn't work after all, and that what really works is..."
"The Scandinavian diet," whispers Olaf, scarcely believing what he is saying. "Everything from up north. Soused herring. All fish in brine. Lumpfish. Dried reindeer meat. Dried strips of fruit..."
You'll never get away with it. It sounds revolting.
"You may be right," says Olaf. "But what was so great about olive oil and sun-dried tomatoes in the first place? For 30 years we have let these Mediterranean poseurs get away with it. The time has come to stand up to them! It will be fun to have a bit of a challenge for a change, anyway. Bring on the moose steaks, I say! Deep down, would you rather have a platter of venison and blueberries or a bit of pasta al dente?"
Mmmm... Well, actually...
"Don't think about it now. Wait till we bring out the big one. We're looking at May or June, I think. And for God's sake, not a word to anyone before then, eh?"
On my word as a British journalist, Olaf. On my word.