"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.
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.
It’s different from glucose. That sugar goes directly into our bloodstream so that our tissues and organs can use it as energy, with only 30 to 40 percent passing through the liver.
"The more fructose in the diet, the higher the subsequent triglyceride levels in the blood," Taubes writes on page 200. While our health authorities have focused largely on the health risks of high LDL cholesterol levels, Taubes demonstrates that our triglyceride - fat - levels are even more important in terms of our risks for heart attacks.
And especially troublesome for people with diabetes is that high-fructose diets lead us to secrete more insulin, which in turn leads to more insulin resistance. That’s because fructose seems to block both the metabolism of glucose in the liver as well as the synthesis of glucose into glycogen, the way that the liver stores glucose.
It’s even worse, Taubes writes. Fructose is perhaps 10 times worse than glucose in the way our bodies form AGEs.
It happened that just as I was reading Taubes, my favorite Certified Diabetes Educator brought to my attention a thought-provoking interview with Dr. Lustig. This interview, broadcast originally on Australia’s ABC Ratio National, confirms the outlines of Taubes’s brief against fructose.
"The only organ in your body that can take up fructose is your liver," Dr. Lustig told interviewer Norman Swan. The first thing that eating fructose does is causing an increase in uric acid, Dr. Lustig said. Fructose inhibits nitric oxide, which would otherwise reduce our blood pressure. "So fructose is famous for causing hypertension (high blood pressure)."
"The second is that fructose initiates what’s known as de novo lipogenesis, excess fat production"¦.And then the last thing that fructose does in the liver is it initiates an enzyme"¦.What happens is that your insulin receptors in your liver stop working"¦.That means your insulin levels all over your body have to rise."
When I wrote Dr. Lustig today to ask him the name of the enzyme that fructose initiates in the liver, he told me that they call it "c-jun N-terminal kinase-1" or just JNK-1 or Junk-1. “It serine phosphorylates a protein in the liver called IRS-1 (insulin receptor substrate-1), thereby rendering it inactive. This induces hepatic insulin resistance.”
Dr. Lustig also sent me a PDF of a slides for a talk he recently gave that he called “The trouble with fructose.” I swear that this just happens to be the same title I had already decided to use for this article. I have uploaded his article to my site.
Damning stuff, this. In fact, "we’re being poisoned to death," Dr. Ludwig concludes.
The trigger for the recent trouble with fructose began in 1978, when high-fructose corn syrup entered the market. The most common form, HFCS-55, is 55 percent fructose and 45 percent glucose.
HFCS is now the most common sweetener in the U.S. It has replaced sucrose (table sugar) especially in soft drinks, but it is in many other foods too.
But HFCS is only the tip of the sugar crystal. Sucrose is half fructose and half glucose.
Sucrose goes by a lot of names on the nutrition labels of the products that we buy in supermarkets and natural food stores. Probably no ingredient is more ubiquitous or goes by so many names as sugar. Sucrose includes white sugar, brown sugar, granulated sugar, turbinado sugar, and most of the sugar in regular and blackstrap molasses and almost all of the sugar in maple syrup. One of the trickiest names is "organic dehydrated cane juice." I’m sure that it fools a lot of people into thinking they aren’t getting sugar. Sucanat is another name for dried sugarcane juice. So-called raw sugar includes demerara, muscovado, and turbinado. More than 100 different sucrose substances exist.
Even the natural sweeteners that I have been using occasionally have lots of fructose. From 90 to 97 percent of the sugar in agave nectar is fructose. Up to half of the sugar in some varieties of honey is fructose.
Even fruit, vegetables, and meat contain some fructose, both directly and as half of the sucrose in these foods. Among meats, corned beef and pastrami seem to have the most fructose, through their added sucrose content. But fruit, vegetables, and meat aren’t much of a fructose problem, because the total amount of fructose we get this way is minimal.
Between 1970 and 2003 our average consumption of fructose increased from less than half a pound per year to 56 pounds per year, according to the interview with Dr. Lustig. "We were never designed to take in so much fructose."
Now we know why we need to avoid this added fructose in our diet. But let’s not go to extremes. Our bodies were designed to eat the fructose in fruit and vegetables. That naturally-occurring fructose is not the problem. The trouble with fructose is all the fructose and sucrose that we have been adding to our food.
Update: September 16, 2009: A correspondent just brought this video of a talk by Dr. Lustig to my attention. It’s on YouTube at http://www.youtube.com/watch?v=dBnniua6-oM and well worth watching when you have the time.