The sugar fructose is in trouble again. This time it is accused of leading to weight gain when combined with a high-calorie, high-fat diet.

Fructose, also known as fruit sugar, is found naturally in small amounts in fruits and vegetables. It also constitutes 50% of sucrose, or table sugar. And high-fructose corn syrup 55, or HFCS-55, contains 55% fructose, just a little bit more than table sugar.

Fructose has a low glycemic index, and it doesn't raise blood glucose (BG) levels as sucrose does. For this reason, some people have considered it a healthy sweetener for people with diabetes.

But it has been known for a long time that a high-fructose diet can increase triglycerides in the blood. Furthermore, as I pointed out in the appendix of my book "The First Year: Type 2 Diabetes," fructose attaches to proteins a lot more than glucose. And it's these proteins with sugars attached, advanced glycation end products, or AGEs, that can cause complications.

The new indictment against fructose is that it induces leptin resistance. Leptin is a hormone implicated in uncontrolled weight gain in people whose bodies make none at all and a tendency to weight gain in those who are resistant to its actions.

Some years ago, researchers found mice with no leptin, and newspapers and magazines throughout the country showed photos of obese mice who were "cured" of their obesity by giving them leptin injections. Everyone thought the problem of obesity would be solved.

However, it was later found that very few humans actually lack leptin, and although these few people could, in fact, be cured of their obesity with leptin injections, most overweight people actually have more leptin than normal, suggesting that they're leptin resistant.

In experiments with mice (and mouse experiments don't always translate into human results), mice fed a diet including 60% fructose appeared normal, except for high triglycerides, until the researchers injected them with leptin. The control mice ate less, as expected. The mice that had eaten a lot of fructose did not, suggesting leptin resistance.

When they switched the mice to a high-fat, high-calorie diet, the leptin resistant mice ate more and gained more weight than the control animals.

Thus there are many reason to believe that eating a lot of fructose is bad for our health, even if we're not mice. And a lot of people blame HFCS. Because of the term "high fructose," it sounds as if HFCS had huge amounts of fructose. In fact, it has approximately the same amount of fructose as table sugar, or sucrose, which is 50% fructose.

The word "high" is in comparison with regular corn syrup, which is almost 100% glucose. The producers get corn syrup from corn and then treat it to convert about half of the glucose in the syrup to fructose, trying to mimic the taste of table sugar in a product that is cheaper to produce.

Does this mean that because HFCS-55 has about the same amount of fructose as table sugar it's OK to eat (or drink, as it's HFCS-55 that is most often found in sodas)?