“Fructose is a strange sugar.” That’s what I wrote here in July 2006, and I still think that it’s strange.

But I didn’t know the half of it when I wrote my earlier article about fructose. I didn’t know why it was so strange. And in this case the "why" is crucial.

Fructose is strange because it is the sweetest sugar and yet has the lowest glycemic index, so it has little immediate effect on our blood glucose levels.

When we assign a baseline value of 100 to sucrose (table sugar), then fructose has a sweetness factor of 173, according to Dr. Robert Lustig, professor of pediatric endocrinology at the University of California, San Francisco. Glucose, which we digest even faster, rates at just 74. Maltose, the highest glycemic sugar, is just 32, and high-fructose corn syrup, a mixture of fructose and glucose, has a sweetness score of 120.

Yet the glycemic index of fructose is only 19. That’s the average of six studies.

I often wondered how fructose could be so sweet and yet so low glycemic. I also wondered why so many people think that high-fructose corn syrup is bad for us.

Joe Anderson has studied the tight relationship between fructose and advanced glycation end products (AGEs). His disturbing report, “AGEs and Aging – Sweet Suicide,” helped form my thinking about AGEs, which I wrote about here in May 2006, September 2006, and this May. While accepting his concern about AGEs, I nevertheless questioned his concern about fructose, because I didn’t understand it.

Now, after reading two other experts this week, I get the point. The first was science writer Gary Taubes. His new book, Good Calories, Bad Calories, is really shaking up my thinking, even though I haven’t finished reading it yet, much less having taken the time to digest his extensive and profound critique of the received wisdom that we must minimize the amount of fat that we eat.

Taubes’s book is controversial and bound to stimulate debate for years to come. Gretchen Becker, my good friend and fellow writer here, reviewed the book on October 28 and October 30.

Gina Kolata reviewed it in October 7’s New York Times, to which Taubes responded on October 28.

All of these articles are well worth reading. In addition, Taubes has agreed to answer our questions at HealthCentral.com about his new book.

Fructose is just one small part of Taubes’s book. Yet it was an important eye-opener for me.

“Because fructose barely registers in the glycemic index, it appeared to be the ideal sweetener for diabetics,” Taubes writes near the beginning of his discussion of fructose on page 197. “By defining carbohydrate foods as good or bad on the basis of their glycemic index, diabetologists and public-health authorities effectively misdiagnosed the impact of fructose on human health” (page 199).

The trouble with fructose is its impact on the liver, which almost exclusively metabolizes it. That’s the key point.