High levels of fructose in food may damage the powerhouses, mitochondria, in liver cells, according to a study which suggests that a fructose rich diet may inhibit the liver's ability to properly burn fat. The study, published in the journal Cell Metabolism, noted that high levels of glucose in the diet improved the fat-burning function of the liver, whereas fructose rich food had negative health impacts -- even though the two sugars had nearly the same calorific content.
"Fructose made the liver accumulate fat, almost having the same effects as a high-fat diet," said C. Ronald Kahn of Harvard Medical School and lead author of the study.
However, he added that adding more glucose to the diet, had a contrasting effect, promoting the liver's ability to burn fat, and making for a healthier metabolism. "The most important takeaway of this study is that high fructose in the diet is bad," said Dr. Kahn.
According to Kahn, the harmful effects of fructose was not because it had more calories, but because it had effects on liver metabolism "to make it worse at burning fat." He mentioned that adding fructose to the diet made the liver store more fat, "and this is bad for the liver and bad for whole body metabolism."
The study noted, however, that if the diet was switched from fructose to glucose, the fat accumulating effect disappeared. "In fact, if anything, overall metabolism is somewhat better than if they just were on plain high-fat diet," Kahn said.
The researchers, including those from Harvard Medical School in the US, compared in model animals, the effects on metabolism of six different diets: regular chow, chow with high fructose, chow with high glucose, a high-fat diet, a high-fat diet with high fructose, and a high-fat diet with high glucose.
They analysed the known markers of fatty liver to determine the effects of each diet. The results of the study reflected earlier findings that glucose assisted fat-burning action in these animals.
The researchers also observed a critical enzyme for fat-burning known as CPT1a, whose high levels indicated that the cell's powerhouses -- mitochondria -- were performing their fat-burning jobs correctly.
They found that both the levels and activity of CPT1a in the liver were low in the group fed high-fat plus fructose diet, suggesting that the mitochondria was not functioning properly.
"When mitochondria are healthy, they have this nice ovoid shape and crosshatching," Kahn said.
He added that in the high-fat plus fructose group, the cell's powerhouses were fragmented and unable to burn fat as well as the healthy mitochondria. However, he added that in the high-fat diet plus glucose group, the mitochondria become more normal-looking because they were burning fat efficiently.
The study noted that both high-fat, and high-fat plus fructose diets damaged the mitochondria, making it easier for the liver to synthesise and store fat, rather than burn it.