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No one can eat 70% fructose
The rodent study
The rodents showed odd changes in the liver, including in lipogenesis. At a graduate school mixer, another professor remarked that high fructose corn syrup is obviously fattening. The researcher replied that people simply overeat it, and when pressed on whether something inherent to it was at work, he answered that it is just calories.
The proof of concept
His group ran a proof of concept in which they tried eating 70 percent of daily calories from fructose and could not do it. High fructose corn syrup is only 55 percent fructose, so someone eating nothing but the syrup would still fall short of the level the rodents were fed.
Rethinking sugar
A graduate student overhearing the exchange had believed since about 2005 that sugar was fattening and bad for health independent of any other variable, meaning independent of everything, including hunger and total calories. The conversation made him question that belief. He set aside epidemiology, since people who eat more sugar also tend to eat more calories, and looked for randomized trials that match calories while varying sugar. For fat loss or fat gain, those show no difference.
Why does the liver handle fructose differently?
Most fructose is cleared by the liver, which converts it to fuel. Unlike glucose, fructose bypasses a key regulatory step in glycolysis controlled by the enzyme phosphofructokinase. That lets it flow quickly into the building blocks of fat synthesis, which is why very high fructose intakes in rodents change liver lipid production. The dose in those experiments is far above what people eat.
How did high fructose corn syrup come about?
Corn syrup is mostly glucose. In the 1960s, researchers in Japan developed an enzyme process using glucose isomerase to convert part of that glucose into fructose. Manufacturers blended it to reach versions with 42 percent and 55 percent fructose, the latter matching the sweetness of table sugar. Sucrose itself is half glucose and half fructose.
How do researchers separate sugar from calories?
They run controlled trials in which one group swaps sugar for another carbohydrate at equal calories, or adds sugar to a diet on top of its usual calories. A 2012 meta-analysis by John Sievenpiper and colleagues in the Annals of Internal Medicine found that swapping fructose for other carbohydrates at equal calories did not change body weight. Adding fructose on top of a diet's normal calories did raise weight.
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