Sweet Taste Signals Good to Eat. (Not poisonous)
In history, sugar was scarce, but today many people consume a kilogram of sugar a week. In addition, with 60% of our diet being carbohydrates, even the ones called "whole grains," become glucose in our digestive system very rapidly. When sugar is added to drinks, it's possible to consume an enormous amount in a very short time, and hardly notice.
Glucose in the blood, is used for energy, but excess glucose in the blood is toxic. That doesn't normally cause a problem because the body is very well adapted to cope with a temporary glucose overload. The body responds by producing great quantities of insulin, intended to remove the excess sugar from the blood quickly. Insulin has many functions; it transports glucose into cells and enables glycogen storage, it turns off fat burning in favour of glucose oxidation, and it turns on fat storage.
Sugar like any substance we eat all the time becomes incorporated into our homeostatic state. The body adapts to cope with it, in this case our brain also produces dopamine, the pleasure hormone. We are satisfied. However, over time homeostasis can be altered, to create a new "normal setting" so you begin to want more and more sugar. This creates a rejection process in body chemistry called carbohydrate intolerance. Insulin becomes less and less effective, and type 2 diabetes starts to develop.
Sucrose, Glucose and Fructose
Table sugar is sucrose. It's a compound of glucose and fructose in equal parts.
High Fructose Corn Syrup is not very different, 45% glucose and 55% fructose.
Some fruits are very high in sugars, dates, grapes and bananas stand out. Dried fruits are sugar dense.
Much has been written about the danger of fructose in the diet recently. Prof. Richard D. Feinman, thinks that issue is over-blown. The research needed to resolve some of the questions raised has never been done.
When people think about sugars, they imagine table sugar, iced cakes, lollies or candy, or deserts. We tend not to think of apples and peaches, or wine, or oatmeal as being sweets. High Fructose Corn Syrup is new in the diet, since about 1974, and it's use grew rapidly for ten years and has continued to be increasing as part of our diet. High Fructose Corn Syrup is hidden in most of the sweet drinks we use, and in many shelf-safe products sold in supermarkets. (Most manufactured foods.) High Fructose Corn Syrup is cheap, and it's not called sugar.
The key difference between various sugars, and other carbohydrates and starches, is how long it takes for the food eaten to be converted into energy, or glycogen or body fat.
Fructose is processed differently to glucose. Fructose is directed to the liver, and there most of it is converted into glucose. It's argued that fructose is the direct cause of non-alcoholic fatty liver disease, and therefore the chief culprit in the development of type 2 diabetes. Richard D. Feinman, thinks that case is not proven.
Fuel Storage in the Body
A person burns about 2000 kcal a day. The body can store almost that amount of energy in cell tissues as glycogen, but some of that is bound and can't be exported for use. So if we were entirely depending on glucose we wouldn't be able to survive illness, and famine or starvation for very long at all.
Free fatty acid burning, stops excessive glucose use, and when glucose levels drop, insulin turns "off" and the Acetyl-CoA from fatty acids becomes the main fuel. In starvation, or in the artificial starvation of a low-carbohydrate high-fat diet, the body will produce ketones to transport Acetyl-CoA to the brain, the central nervous system, the heart and all the other body tissues. This severely limits the demand for glucose.
There is a lot of energy in body fat. At least 80,000 kcal, enough to keep you going for about a month, if food is really in short supply. This is why endurance athletes, who used to try and fuel long distance events by carbohydrate loading, and feeding on small sandwiches, and sugary drinks, are now switching in large numbers to ketogenic diets.
On a ketogenic diet, and athlete can begin a three hour run, with glycogen stores half full, and end the run with the stored glucose in his muscles hardly changed, the entire run was sustained by burning fat.