Dr Atkin's low carbohydrate diet seemed to work, but no-one knew why, and it was thought to be a grossly unhealthy thing to do. By traditional reckoning, it just shouldn't work at all. In fact it's the opposite of the traditional weight-loss advice.
Diabetes research had an answer.
Diabetes is a disease of carbohydrate metabolism. Fat and protein take a long time to metabolize.
Carbohydrates, however, can be digested very quickly. When you eat carbohydrates, your blood sugar levels rise much faster than when you eat fat or protein.
This is because carbohydrate is the active form of energy. Fat is for long term energy storage. Protein is for building actual body structures, and its energy value is a by-product.
But carbohydrate is how living beings store energy that they may need to use in a hurry.
There is little carbohydrate in nature. Animals such as ourselves contain it only as blood sugar. Most plants contain it only in the form of cellulose, a structural building block that plants use but animals don't.
Humans and our fellow omnivores can't even digest cellulose. Herbivores can, which is why they can make a living eating grasses, but they need complex digestive systems to break it slowly down.
Our ancestors would not have eaten much carbohydrate.
But there was some carbohydrate in our ancestor's world. Honey and fruit sugars are the obvious examples.
We love both, especially as children. Often the only way to get a child to eat a new thing is to cover it in honey or sugar.
If you're tired, and your blood sugar is low, a little honey or sugar can bring you back to life almost instantly. Athletes like rowers or cyclists know this. Another thing they know is that eating these things just before a race doesn't help. You end up being strangely tired. They only help for recovery. Weird huh?
But honey is very hard to get hold of. And fruits are not densely sugary. And what sugar there is in fruit needs to be extracted slowly from the fibrous matrix of the fruit.
Another source of carbohydrate is in the seeds of plants. But that would have been very fiddly to get at for our ancestors. Not least because modern forms of domestic grasses are, like the dog and the cat, very recent human inventions.
The ancestral grasses would have been very thin stuff. With tiny seeds. They might have been worth something in a pinch, but you'd have had a hard time living off them.
So our ability to metabolize large amounts of carbohydrate is a recently evolved thing, and it doesn't work very well.
Say you eat a large quantity of carbs, say a bowl of sugar or a few slices of bread or some rice or some of that delicious breakfast cereal with the pictures of the thin women jumping up and down on it. (they're not too different in terms of energy content or compostion. If you keep any of them in your mouth for a few minutes you'll taste it starting to turn into sugar).
It doesn't stay in your stomach very long. It's already so close to being blood sugar that a couple of simple chemical reactions can turn it into the right form, and at that point it goes straight into your bloodstream.
At that point, if you're diabetic, you're in big trouble. Your blood sugar levels go far too high and the stuff starts causing havoc.
If you're not diabetic, you have a coping mechanism. People whose ancestors were farmers do it better than people whose ancestors weren't.
You release insulin. This is like an alarm signal for 'blood sugar too high'. All your cells start sucking sugar in as fast as they can and frantically turning it into fat. Very soon you're safe. You actually overcompensate a bit. Your blood sugar levels are now too low because you've cleared all the glucose from your blood.
You know that feeling you get after a heavy meal, say a nice pizza or baked potato. Where you feel lethargic and don't want to do anything for a bit? People call it 'digesting'. Actually all the digesting was done a while ago. You're now hypoglycaemic, which is a greek word meaning that you're blood sugar's too low for you to be able to do anything.
Not to worry though. Your body soon notices, cancels its over-sugar panic response, and blood sugar starts being released from the places where it's stored, and goes back into your bloodstream.
You're now in a state where your blood sugar is a bit low, and your blood sugar reserves are a bit depleted, because you've just used some of them to recharge your blood.
The name for this new state is 'hunger'.
Rereading, there's a lot of stuff here that I've just picked up without noticing, and some of it looks suspicious now I come to write it down. I may have made some of it up! Does anyone reading this know one way or the other?
I've said that people whose ancestors were farmers are better at carbohydrate metabolism. The reason I think this is because I read a book where it was claimed that the introduction of the Western diet to Pacific Islands caused large scale diabetes and obesity. I can't even remember the name of the book.
And I don't know what the high blood sugar havoc actually is in diabetics. I think it's quite bad though.
I know several people who "don't get fat whatever they eat". All women. That may be selection bias. Men don't talk about weight much.
I wonder if these lucky girls are at risk of diabetes? Or perhaps they have some super-controlled insulin response that doesn't overcompensate and leave them hungry?
I wish I had a continuously sampling blood sugar monitor I could wear on my arm to test all this out!