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Feature

A slice of pizza.

Super Size Me was no fiction

Eating too much fat really can throw your liver for a loop

By Deane Morrison

Published on May 5, 2005

A University of Minnesota study is the first to show that if you eat too much fat, it can go straight to your liver and damage it. Although the study was performed on people with liver disease, it should serve as a warning that this is what can happen to people who do what the star of "Super Size Me" did: eat too darn much fat and gain a lot of weight. In the study, which examined obese people with nonalcoholic fatty liver disease (NAFLD), fat from the diet ended up "stuck" in the liver, where it doesn't belong. It was known that the livers of NAFLD patients accumulated fat, but its origin was unknown. The new work implicates fat from the diet as one cause of NAFLD and shows that fat buildup in the liver results when the liver loses its ability to manage the various influxes of fat that occur during transitions between the fasted and fed states. Identifying the origins of accumulated fat in the livers of NAFLD patients will be important in preventing and reversing this condition, which can lead to more serious liver trouble. The work was published May 2 in the Journal of Clinical Investigation.

"For more than 20 years, the American Heart Association has recommended a moderate intake of fat-no more than 30 percent of total calories," says Parks. "[Yet] most low-carb diets are high in fat--up to 50 percent of calories."

"This is the first scientific proof of dietary fat stored in the liver in humans," says Elizabeth Parks, an associate professor of human nutrition, who led a research team from the department of food science and nutrition and the Medical School. "In health, it's the liver's job to store glycogen--a storage form of carbohydrates--not fat." The clear implication is that too much dietary fat leads the liver to fail in its mission as the body's central shipping and receiving center for fat. No longer does it take in dietary fat, repackage it and send it on its way back out into the blood. Rather, in obesity, fat builds up in the liver. The fat comes both straight from the diet and also from sugars that the liver turns into fat. As a result, the liver functions poorly. In healthy people, about half the fat from a meal is burned for energy, and the rest is shunted to adipose tissue, where it is stored until needed during fasting. Very little fat is normally stored in the liver. Working with obese subjects who had NAFLD, Parks and her colleagues fed the subjects food containing fats labeled with deuterium, a rare but stable form of hydrogen that can be used to trace fats as they move through the body. The subjects were already scheduled for liver biopsies, and Parks' team gave the patients labeled fat for five days before their biopsy. The researchers analyzed the waste liver tissue from the biopsies and found that these patients' livers had globules of fat--about 20 percent of it from the diet. Furthermore, the liver's fat synthesis from dietary carbohydrates was also elevated. Once thought benign, fatty liver is now considered a component of a condition called metabolic syndrome, which occurs most often in overweight people and whose features include insulin resistance and cholesterol abnormalities. Fatty liver is also a precursor to the more advanced liver disease nonalcoholic steatohepatitis, which may progress to cirrhosis of the liver in up to 25 percent of patients, says Parks. "For more than 20 years, the American Heart Association has recommended a moderate intake of fat--no more than 30 percent of total calories," says Parks. "[Yet] most low-carb diets are high in fat--up to 50 percent of calories. "The bottom line is, this study is a clear implication that if one eats too much fat, as in the film 'Super Size Me,' fat becomes deposited in the liver. This leads to a kind of liver toxicity that would be good to avoid." The work was supported by the National Institutes of Health.