Gary "Big Fat Lie" Taubes wrote a long feature for the NYT Magazine analyzing the claims made by UCSF childhood obesity expert Robert H. Lustig in his infamous lecture Sugar: The Bitter Truth , which has gotten about a million YouTube views (it's also had other exposure: I watched it last year on UC cable access while in LA).
Lustig claims that sugar is a "chronic toxin" -- a poison that will make you sick if you eat it for long enough -- and he blames it for everything from cancer to heart disease. Taubes traces the history of this theory about sugar through the past century, and concludes that while not conclusive, the evidence is worrying. I've tried to eliminate sugar from my diet with varying success since 2003, when I did a year of "strict Atkins" and lost 80 lbs, most of which I've kept off since by avoiding processed carbs where possible. I find that eating a little sugar (or high-carb food like bread) generally leads to cravings for a lot more, which means that slight slips tend to snowball.
Lustig's argument, however, is not about the consumption of empty calories -- and biochemists have made the same case previously, though not so publicly. It is that sugar has unique characteristics, specifically in the way the human body metabolizes the fructose in it, that may make it singularly harmful, at least if consumed in sufficient quantities.
The phrase Lustig uses when he describes this concept is "isocaloric but not isometabolic." This means we can eat 100 calories of glucose (from a potato or bread or other starch) or 100 calories of sugar (half glucose and half fructose), and they will be metabolized differently and have a different effect on the body. The calories are the same, but the metabolic consequences are quite different.
The fructose component of sugar and H.F.C.S. is metabolized primarily by the liver, while the glucose from sugar and starches is metabolized by every cell in the body. Consuming sugar (fructose and glucose) means more work for the liver than if you consumed the same number of calories of starch (glucose). And if you take that sugar in liquid form -- soda or fruit juices -- the fructose and glucose will hit the liver more quickly than if you consume them, say, in an apple (or several apples, to get what researchers would call the equivalent dose of sugar). The speed with which the liver has to do its work will also affect how it metabolizes the fructose and glucose.