First, I would like to state a personal observation. I was a late child (very late). My mother was born in 1928 and had 7 sisters and 4 brothers. They worked very hard when they were young and young adults. Systematically, almost all of them eventually ended up on a low fat diet for cholesterol or just because the doctor recommended it. Now, every single female that is living (including my mother) has adult onset diabetes. Also, if you look at the history of man, you can tell when he started consuming a lot of carbohydrates, their teeth started rotting.
This is what it says in this book I have. I am not endorsing it or anything, just quoting something
To begin with they state that there are three macronutrient, protein, fat and carbohydrates. The human body required fat and protein to survive but actually does not require any carbohydrates stating you body has all the biochemical machinery necessary to make all the blood sugar you need to nourish the tissues that require it - red blood cells, some parts of the eye, brain, and kidney.
"Bearing in mind that protein and fat are essential to health and carbohydrate isn't, what happens when we cut back our fats as the nutritional establishment recommends? Since we can't for the most part remove the fat from the food, we end up replacing foods that contain fat with those that don't. Since most sources of good-quality protein - meat, eggs, adn dairy products - contain a fair amount of fat, to cut back on fat we end up cutting back on protein as well and replacing them both with carbohydrate. Most vegetable sources of protein - beans and grains - are incomplete unless combined carefully and contain far more carbohydrate than protein. In the end, if we follow the low fat prescritpion we can end up deficient in protein (it's difficult to be deficient in fat because the only essential fat is linoleic acid, which is found in vegetable oils).
But probably the worse news of all is that eating more carbohydrates stimulates your body's fat storage. In attempting to reduce fat intake, you wind up actually getting fatter, becasue some macronutrients stimulate profound metabolic hormonal changes - chiefly insulin and glucagon - you wouldn't see much activity, because fat is essentially metabolically inert. Carbohydrate, however, would set off a Mad Hatter's tea part of metabolic activity. Eating a hand full of grapes while hooked to the same device would initiate a wild swinging of gauge needles indicating a rapid increase in insulin and a decrease in its opposing hormone glucagon, all perfectly normal metabolic responses kindled by the consumption of carbohydrate. It follows logically that the constant consumption of large quantities of carbohydrate would then produce large quantities of insulin, which indeed it does.
Even complex carbohydrates stimulate the response because all carbohydrates are basically sugar. Various sugar molecules - primarily glucose - hooked together chemically compose the entire family of carbohydrates. Your body has digestive enzymes that break these chemical bonds and release the sugar molecules into the blood, where they stimulate insulin and the other metabolic hormones. This means that if you follow a 2,200 calorie diet that is 60 percent carbohydrate - the very one most nutritionists recommend - your body will end up having to contend with almost 2 cups of pure sugar a day."
Whew