There's man benefits to the high carbohydrate, low fat diet, but it's never become really popular, due to a number of drawbacks. The first popularizer of the idea was Nathan Pritikin, who claimed it would lead to a long healthy life - then he died ironically in his 50's. The torch was taken up by Dean Ornish, who has recommended it mostly for people with heart disease; a very low fat diet will actually reverse some heart disease, removing plaque from arteries. Though a few top ultramarathoners have used it, it has not become the standard and here's why:
The first two books by Pritikin had recipes that were little more than gruel and pap; no one would voluntarily eat them for long. Over the decades, the recipes have improved, but they still are low fat versions of commonly eaten high fat foods, rather than what people who have always lived on these diets actually eat. Even if one creates flavorful low fat foods, one quickly tires of them; one simply wants to eat something else.
The Pima indians have gone from lean and healthy to having the highest rates of obesity, heart disease and diabetes in the world in only two generations. Their traditional diet was high carb and required a great deal of effort to obtain adequate sustenance; it is commonly believed that their bodies have adapted to continuous near-starvation by being very efficient at storing calories as fat. Attempts have been made to have them return to a more traditional diet, but with little success. Who would work 15 hours a day to gather a handful of vegetables, when they can sit back and order take-out? This is an exaggerated version of the problem of American society and diet.
2) Timing ,quantity and other logistics
We've all had the experience of eating a rich dessert, only to find ourselves even hungrier after eating it than we were before. The problem comes from there being different types of carbohydrates (unfortunately commonly referred to as "good" and "bad") and the way the body uses insulin. Some carbs, such as table sugar (sucrose), cause the level of glucose in one's blood to increase quickly, whereas others, such as the fructose found in fruits, cause a slower response. Insulin lowers blood sugar when it is too high, but it does not discriminate between a short burst of high glucose due to a small amount of a high-glycemic index ("bad") food and a large amount of a low-glycemic index ("good") food. A sugary snack causes one's body to react as if one's just eaten a large meal and the insulin lowers the blood sugar level; then, because one hasn't a lot of sugar in one's blood to begin with, the low blood sugar level causes one to become hungry.
The way around this problem is complex. One has to eat only low-GI foods in small quantities and this requires one to eat frequently, "grazing" throughout the day, rather than eating large meals - hard for most people to do when at work. There is also the matter of preparation; many of the foods eaten in this diet require cooking and are not as palatable when cold or reheated (this has been my downfall when attempting this diet - I don't want to spend the day cooking, so I make a large amount which I either eat all at once, or which I end up throwing away as I go in search of something else to eat later).
During a brief fad of low-fat diets, manufacturers created a variety of snack foods that would seem to overcome this obstacle, but in order to make them tasty, fat was simply replaced with sugar. This does not work in the low-GI scheme. People ate them and became hungry quickly, so they ate more. What was good for sales was bad for the consumers and people who tried this ended up spending a lot of money and gaining weight.
3) It's the fat, stupid.
A common thought among ultrarunners is that the exercise they do burns 99% fat, so fat is all-important and carbohydrates are meaningless. I'll explore this in detail in the next couple of installments of this overly long stream-of-conciousness ramble. There is one over-riding benefit to a high-carb diet and that is in how the body stores energy:
If you eat fat and your body doesn't need the calories from it immediately, it gets stored as fat. If you eat protein and your body doesn't immediately need it for replacing (or developing) its own proteins - muscle and enzymes, mostly - it gets converted to fat, except in the case that one's body has an extremely low amount of stored carbohydrates, when part of it is made into the needed sugars and the rest into fat. Carbohydrates, however, can be stored in two different ways; either as glycogen or by conversion to fat and the body replenishes the glycogen first. Thus, if one does exercise which lowers the level of glycogen, less of the calories one takes in is turned into fat. Rigorous exercise and a high-carb diet works best in decreasing the amount of fat one's body stores, but people tend to do only one or the other when trying to lose weight.
Skipping ahead a bit... the body craves a certain amount of fat, or else the high-carb diet would be popular. The way around this is by consuming carbs during endurance exercise, when they are converted to fuel. These calories during exercise will bring the overall diet into the high-carb range, while leaving regular meals in a more palatable range of fat content.
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