This is an excellent question, and one that I think applies to almost everyone. In answering it, I'm going to give you a brief description of the biochemical events that take place when a person consumes sugar.
In explaining this subject to many people over a period of several years, I found the most useful metaphor to be that of an automobile engine. Let's take a Toyota Corolla, one of the most popular cars on the street today. The Toyota Corolla has an engine that's designed to burn a certain type of fuel. That fuel, gasoline, has a certain flash point and a given output of power per gallon of gasoline. There are other fuels that power other types of vehicles -- for example, jets run on a high-octane petroleum product that has far more energy per gallon than automobile gasoline. Diesel fuel, on the other hand, burns more slowly.
So you have different types of fuels for different vehicles. Most people understand the idea that if you took your Toyota Corolla, drove it down to the airport, and filled it up on jet fuel, you wouldn't get a faster Toyota Corolla. Instead, you would get a critically damaged Toyota Corolla engine, because the fuel would burn too hot, too fast, and it would overpower the design of the engine. In fact, you probably wouldn't drive more than 10 miles before the engine burned up, and you would have to overhaul the entire engine in order to drive your car again.
This is sort of what goes on when it comes to the human digestive system and sugars. The human digestive system was designed to consume foods that are readily available in the natural surrounding environment. These foods can be compared to certain types of fuels because each food releases energy at a given rate during digestion (primarily based on fiber and fructose content).
As an example, let's take a look at an apple. It contains fiber, vitamins and minerals, and lots of carbohydrates. The carbohydrates are the fuel. But the carbohydrates are bound up in the fiber of the apple so that it takes your body a fair amount of time and effort to release those carbohydrates and convert them into fuel. So you could call the apple a medium-burning carbohydrate, or in medical terms, it has a lower glycemic index than straight sugar.
But if you take straight sugar, that is, refined white sugar, which is something that does not occur naturally in the environment, and you put that in your mouth, then your body converts that into blood sugar very rapidly. It's like pouring jet fuel down your throat. This is the same as filling up your Toyota gas tank with jet fuel and trying to drive away. When you eat sugar you consume soft drinks -- which are even worse because they are liquid sugar -- you are trying to run your metabolic engine on jet fuel, and the human metabolic engine was not designed to run on jet fuel.
So what happens? When you first consume any sort of refined sugars or refined carbohydrates (like white flour), the digestion process begins immediately -- in fact, it begins even before you swallow the foods. There are digestive enzymes in your saliva that go to work on these sugars and start converting them into blood sugar, even before they hit your stomach. Once they're in your stomach, they are mixed with acidic digestive juices and physically churned through stomach muscle contractions so that it creates a liquid paste. This liquid, sugary paste is then very easily absorbed through the intestinal walls, causing a rapid spike in blood sugar levels.
So your blood sugar, which might have been around 80 or 90 before you drank the soft drink or ate that candy bar, now suddenly starts spiking up to 150 or 200, or perhaps even higher. This creates an emergency situation in your body. High blood sugar is very dangerous for human beings. If it is allowed to continue, it will cause symptoms that are more classically known as diabetic neuropathy, which means the nerves that feed various limbs in your body (feet, mostly) start to die. Diabetics who maintain high blood sugar over a long period of time often have to have their feet amputated because the nerves in their feet are wasting away.
In part two of this Q&A article, we'll talk about how your body regulates blood sugar and attempts to protect you from the dangers caused by sugar consumption.