Foods containing very high levels of fructose, such as soft drinks and processed food, may damage our liver’s ability to properly burn fat, says a new study.
Fructose is a natural sugar found in many foods like honey and fruits. Fructose is too sweet and is mostly made into high fructose corn syrup, used in soft drinks and processed foods.
“The most important takeaway of this experiment is that high fructose for diet is bad,” said the lead author Carl Ronald Kahn, Professor of Medicine at Harvard Medical School.
“It is not bad because it has more calories, but because it has effects on liver metabolism to make it worse at burning fat. As a result, adding fructose to the diet makes the liver store more and fatter, and this is bad for the liver and bad for the metabolism of the complete body,” said Professor Kahn, who also serves as the CAO at Joslin Diabetes Center, Massachusetts.
Interestingly, the researchers found that relatively high levels of glucose in the diet actually improved the fat-burning function of the liver.
The findings, published in the journal Cell Metabolism, say that high dietary fructose has more negative health impacts than glucose does, even though they have almost the same caloric content.
In a series of animal studies, the researchers compared effects on the metabolism of six different diets – regular chow, chow with high fructose, chow with high glucose, a high-fat diet, a high-fat diet with a high level of fructose, and a high-fat diet with high glucose.
The researchers analyzed different known markers of fatty liver to find out the effects of each diet. For, they looked at levels of acylcarnitines in the liver’s cells.
Acylcarnitines are produced when the liver burns fats. High levels of these are a bad sign since it means there is a lot of fat being burned in the liver. Acylcarnitines were highest in the animals who are on the high-fat plus high fructose diet.
The levels were lower in the high-fat plus glucose diet than in the plain high-fat diet, which reflected previous experimental findings and indicated that glucose performed an assistive fat-burning action in these animals.
Researchers also monitored the activity of a critical enzyme for fat-burning known as CPT1a. In the case of CPT1a, the higher the levels the better – they indicate that mitochondria, which provides energy for cellular functions, are performing their fat-burning jobs correctly.
However, in the case of a high-fat plus fructose diet, the researchers found that levels of CPT1a are low and their activity was very low, meaning mitochondria might not function properly.
These findings, combined with other markers they monitored, proved that both high-fat and high-fat plus fructose diets damage mitochondria and makes it easier for the liver to synthesize and store fat rather than burn it.