A new experiment that studied cancer cells found results that defy the main competing theories of sugar metabolism. The traditional view of sugar metabolism holds that all sugars are metabolized the same way. A more recent, competing theory says that fructose is metabolized slower than other sugars because most cells cannot metabolize fructose in significant quantities. The new findings do not appear to be consistent with either view.
In the study (“Fructose Induces Transketolase Flux to Promote Pancreatic Cancer Growth,” in Cancer Research) tissues of pancreatic cancer cells in test tubes were fed either glucose or fructose separately. The cancer cells metabolized fructose very differently from glucose, and they grew much faster on fructose, apparently because fructose metabolism spurs the kind of protein synthesis that is essential for cell growth.
The headlines are predictably sensationalist, suggesting that high fructose corn syrup causes cancer, or even that cancer can be cured by avoiding high fructose corn syrup. I should emphasize that the experiment looked at just one form of cancer, and found only that it grew faster when given fructose in isolation. Still, the link between fructose and the growth of cancer cells is an important finding, and it does beg the questions that some news stories are presenting as answers. In particular, to what extent can cancer be avoided or cured by avoiding soft drinks?
The deeper questions, though, have to do with sugar metabolism. The metabolism of fructose is of particular concern because it has gone from being a micronutrient to being a macronutrient within the last century. Since the popularization of high fructose corn syrup in U.S. soft drinks around 1980, millions of people are getting as much as 15 percent of their food energy from fructose, compared to no more than 1 percent in any normal diet of the past.
My own practice, recently, is to limit total sugar consumption at any meal (or at any one time) to no more than 60 grams, with the idea that the fructose load will not exceed 30 grams, and the metabolic effects of a sugar buzz can be avoided. This basically means drinking soft drinks and fruit juice from a 5-ounce juice glass rather than a 12-ounce water glass — a tough idea to get used to. If more can be discovered about fructose metabolism, it may be possible to replace that guideline with something more specific.