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For generations millions of people offered the opinion that eating too much sugar causes diabetes. Although that's an over- simplification, new research finds that this old idea is partially right.

Type 2 diabetes is a disease caused by something called insulin resistance. Insulin resistance occurs when cells shut down receptor sites on their surfaces that ordinarily react with insulin, so they aren't flooded with sugar. In type 2 diabetes, however, there is something very strange about insulin resistance. Insulin doesn't just help cells absorb sugar. It also helps cells absorb fat. Why should a disease that "turns off" insulin receptors for insulin's use in moving sugar "turn on" insulin receptors for insulin's use in moving fat? Shouldn't diabetics at least have the pleasure of staying skinny?

It turns out that consuming sugar makes cells "selectively" insulin resistant. Diabetic fat cells and liver cells lose their ability to absorb sugar so blood sugar levels soar, but keep their ability to soak up fat. Eliminating consumption of the kind of sugar known as fructose can alleviate this problem, but before you can do that, it helps to understand the interrelationships between sucrose, glucose, and fructose.

Sucrose, Glucose, and Fructose

Sucrose, glucose, and fructose are all common sugars. Sucrose is the sugar molecule found in table sugar. Glucose is the form of sugar our bodies use for fuel. Fructose is the super-sweet sugar found in fruit and corn syrup.

Our bodies can use a very small amount of it directly as fuel, but most fructose gets converted into fat in the liver before it can be used to power the body.

Glucose is essential to life, and our bodies usually make it from carbohydrates. However, the human body can also make glucose out of the amino acids digested from protein. That doesn't mean you don't need to eat any carbohydrates at all, because the process of turning amino acids into glucose releases urea, which "acidifies" the bloodstream. Your blood doesn't actually become acidic, but your kidneys have to find calcium and, ironically, other amino acids to keep it from becoming acidic. There is a metabolic cost to getting your glucose from protein foods.

Every molecule of sucrose is made from one molecule of glucose and one molecule of fructose. The body can't use sucrose, so it breaks it down with an enzyme called sucrase in the small intestine. When you eat table sugar, you are eating glucose, which your body can use easily, and fructose, which it cannot. Fructose whether it's from corn syrup or fruit or table sugar once it has been digested is one of the culprits in insulin resistance.

The Complicated Way In Which Fructose Causes Insulin Resistance

Most of what scientists know about insulin resistance is derived from studies of animals, not humans, but the process through which fructose causes insulin resistance probably occurs something like this:

  • Your liver combines glucose with water to make glycogen, a storage form of energy for times you aren't eating. This glucose can come from carbohydrate foods other than sugar, protein foods, or sugar. Your liver can also turn glucose into fat.
  • Your liver can use a small amount of fructose directly for making energy. Up to 25 grams or 100 calories a day, whether it's from fruit, corn syrup, or table sugar, usually can be tolerated. If you consume more than that, however, your liver stores fructose as fat.
  • When your liver has to store fructose as fat, it in effect says "No more!" and shuts down insulin receptors so sugar stays in your bloodstream.
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