Too much fructose in the diet prevents the liver from breaking down fat stored within the organ, American researchers state.
The study by the Joslin Diabetes Center suggests this only relates to fructose, with the findings concluding that high levels of glucose lead to an improvement in the fat-burning function of the liver.
Researchers say this could explain why diets with high levels of fructose lead to more negative health outcomes compared to glucose, despite both types of sugar having the same calorie content.
Lead researcher Dr Ronald Kahn, who is Joslin’s Chief Academic Officer, said: “This is one of a series of studies that we’ve been doing concerning what role high fructose in the diet plays in terms of insulin resistance and metabolic syndrome. Fructose makes the liver accumulate fat.”
It is useful to note that sugar (sucrose) is half glucose and half fructose. This means that a diet high in sugar is a diet that will be high in fructose, and therefore likely to be troublesome for the liver and metabolism.
Dr Kahn, who is also an Iacocca Professor of Medicine at Harvard Medical School, added: “The most important takeaway of this study is that high fructose in the diet is bad. It’s not bad because it’s more calories, but because it has effects on liver metabolism to make it worse at burning fat.”
The researchers looked at a series of animal studies and examined the impact on metabolism from six different diets, covering combinations of high glucose, fat and fructose. They examined the level of fatty liver from each diet.
Dr Kahn added: “Surprisingly, when you switch the sugar in the diet from fructose to glucose, even though they’re both equally caloric, the glucose doesn’t have that effect. In fact, if anything, overall metabolism is somewhat better than if they just were on a plain high fat diet. In this paper we wanted to figure out at a mechanistic level how this could be possible.”
When fat is burned by the liver, acylcarnitines are created, with high levels considered unhealthy. According to the results, the level of acylcarnitines was highest in high fat plus high fructose diet. Also, they were lower in the high fat and glucose diet compared to the plain high fat diet.
The scientists also looked at the activity of an enzyme called CPT1a, which is critical to the fat-burning process, with the higher the level of the enzyme the better. This illustrates that the fat-burning mitochondria are performing their functions correctly. The results revealed that the levels of CPT1a were low and their activity extremely low in the high fat plus fructose diet, indicating that mitochondria was not working efficiently.
Dr. Kahn concluded: “When mitochondria are healthy, they have this nice ovoid shape and crosshatching. In the high fat plus fructose group, these mitochondria are fragmented and they’re not able to burn fat as well as the healthy mitochondria. But looking at the high fat diet plus glucose group, those mitochondria become more normal looking because they are burning fat normally.”
The research was published in the journal Cell Metabolism.