Science writer Gary Taubes has a provocative article in the forthcoming New York Times magazine on whether sugar is toxic. Taubes has penned two well received books on metabolism and obesity recently – Good calories bad calories and Why we get fat?. In the context of the article, sugar is defined to be either sucrose, which is composed of 50% glucose and 50% fructose or high fructose corn syrup (HFCS), which is composed of 55% fructose and 45% glucose. The fact that sucrose, which is what you put in your coffee and HFCS, which until recently had replaced sucrose in many products like soft drinks, are so similar in composition has always been sufficient evidence for me that if one of them is bad for you then the other must be as well.
In order to understand why sugar could be unhealthful requires some background in human metabolism. The energetic portions of food consists of carbohydrates, fat or protein, which are used by the cells of our body for fuel. However, the brain only utilizes glucose (or ketone bodies when glucose is not available) and very little glucose is stored in the body (half a kilogram in the form of glycogen). Hence, glucose is tightly regulated in the blood. When we eat glucose, it enters the blood stream fairly rapidly. The body responds by secreting insulin, which activates transporters in non-brain cells to take up glucose. The cells will either burn the glucose or use it to replace depleted glycogen stores. Glucose can also be utilized by muscle cells during intense exercise. Any excess glucose will be taken up by the liver and be converted into fat in the form of triglycerides.
The reason fructose is considered bad is that it is only metabolized in the liver where it is converted into glucose or fat. The fat is either stored, which is thought to be bad, or gets secreted into the blood inside VLDL particles that are precursors of LDL particles, which is considered to be the bad cholesterol. This line of reasoning is highly plausible but I think it is incomplete. The argument is that since fructose leads to fatty liver, I must avoid fructose. However, what you really want to avoid is excess fat in the liver and high LDL and this can be caused by things other than fructose. So in addition to avoiding fructose, you must also make sure you don’t replace that fructose with something that is equally or almost as bad. For instance, if you decided to replace that fructose with lots of glucose, it could still end up getting converted into fat in the liver.
To me, the real problem is not the fructose per se but the imbalance between glycogen use and carbohydrate input because glucose or fructose will first replace depleted glycogen before getting converted to fat. Hence the amount of exercise one engages in cannot be ignored. Basically, any amount of carbohydrates consumed above your glucose/glycogen utilization level will necessarily be converted into fat. Thus, if you believe in this hypothesis, what you really should believe is that the increase in obesity, insulin resistance and Type II diabetes is not just due to any specific dietary element like fructose but a general imbalance between carbs burned and carbs eaten.