Taste of fructose revs up metabolism

Pancreas can pump more insulin in response to the sugar

Scientists have a greater appreciation of fructose’s full flavor. The sugar, which is found predominantly in fruit, honey and more recently high-fructose corn syrup, tickles taste cells found on the pancreas (that’s right, the pancreas) (SN: 3/27/10, p. 22). The interaction can crank up the body’s secretion of insulin, which may be a concern for people prone to diabetes, researchers report online February 6 in the Proceedings of the National Academy of Sciences.

Insulin-producing pancreatic cells (red) have proteins (green) that can taste fructose, which boosts insulin production in some instances. Tyrberg lab, Sanford-Burnham Medical Research Institute

Experiments with mouse and human cells and living mice reveal that fructose activates the same proteins in pancreatic cells that the tongue uses to taste sweets. When these cells are exposed to glucose — the sugar that is body’s main source of energy — and then get a hit of fructose, the cells pump out more insulin than with glucose alone, the researchers found.

“This is really beautiful mechanistic work,” says nutrition and metabolism expert Kathleen Melanson of the University of Rhode Island in Kingston. The research adds to a growing body of evidence demonstrating that taste cells are not just the province of the tongue, says Melanson, who was not involved with the new study.

Insulin is a master regulator, keeping the right amount of glucose in the blood. So it makes sense that fructose alone doesn’t trigger insulin secretion, says cell biologist and physiologist Björn Tyrberg of Sanford-Burnham Medical Research Institute in Orlando, Fla. If fructose triggered insulin release on its own, glucose levels in the blood could get dangerously low. “The system seems to be elegantly made to keep a balance,” says Tyrberg, who led the new work.

Fructose has recently taken some heat for whacking metabolism out of balance. An issue is where the sweet stuff enters the metabolic assembly line: Most sugars join the process at a point where a supervisory enzyme can control the flow of goods. But fructose comes in farther down, where it can lead to an overproduction of fat. And because fructose on its own doesn’t stimulate the same insulin response that glucose does, the hormone isn’t doing the other regulatory things it usually does, like moderating appetite. The sugar content of high-fructose corn syrup is typically 55 percent fructose; the rest is glucose. Molecules of sucrose, or table sugar, consist of a fructose linked to a glucose.

In general, people should keep an eye on their intake of all sugars, Melanson says.

“It’s amazing how much people consume — there’s a lot hidden, in things like stuffing and salad dressings,” she says. The quantities of fructose found in a spoonful of honey or an apple aren’t of concern, “but our metabolic pathways aren’t designed to handle Big Gulps.”

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