Fructose, or fruit sugar, has been getting a bad rep lately, primarily because of high-fructose corn syrup. Before HFC, fructose was a minor part of the American diet, according to the September ’11 Harvard Health: “A century ago, the average person took in about 15 grams a day (roughly half an ounce), mostly from eating fruits and vegetables. Today we get more than triple that amount, almost all of it from refined sugars and high-fructose corn syrup used to make breakfast cereals, pastries, soda, fruit drinks and other sweet foods.... That increase may be contributing to liver and heart disease.”
Fructose is metabolized in the liver—and fat is a by-product. If you get too much fructose, fat begins to accumulate there and can eventually cause liver damage. Too much fructose also results in elevation of blood triglycerides and LDL, the bad cholesterol. According to Harvard Health, “Two recent studies have linked higher intake of fructose with higher chances of developing or dying from heart disease.”
The problem is that the human body has evolved to handle fructose in moderate amounts, such as what occurs in a serving of fruit. For example, a cup of blueberries has 30 calories’ worth of fructose—but a can of Coke has three times that amount. What happens when you overload your system with fructose? Your body panics, and much of it is processed in the liver.
According to Gary Taubes in his book Why We Get Fat (New York: Knopf. 2010), “Our liver responds to a flood of fructose by turning much of it into fat and shipping it to our fat tissue. Meanwhile the glucose [in high-fructose corn syrup, which is about half glucose] raises blood sugar levels and stimulates insulin secretion and puts the fat cells in the mode to store whatever calories come their way—including the fat generated in the liver from the fructose.”
So a serving of fruit is fine, but overdosing on fructose, as in high-fructose corn syrup, makes you much fatter much faster. Your liver turns excess fructose into fat, and high insulin levels from the glucose escort it into the fat cells almost immediately.
As stated above, fruit is a very minor player in this, but you do need to avoid sugary drinks and any foods with refined sugar as much as possible.