Constant drinking of sugary liquid caused lab animals to quickly become resistant to an important appetite control hormone even faster than eating a high-fat diet, a situation that could be completely reversed by taking them off the liquid, research at Augusta University found.
The key now is figuring out why the liquid was more damaging than eating even solid sugar, said Dr. Ruth Harris, a professor of physiology at AU. Though the work does not apply to humans, people often overlook the calories they drink and how much sugar they are consuming, an Augusta dietitian said.
Harris has a $1.6 million grant from the National Institutes of Health to study this mechanism of inducing resistance to leptin, a hormone produced by fat that is an important risk factor for obesity and provides a signal to the brain of fullness that should control appetite, but often goes awry when the brain no longer responds to the signal. She and her lab were trying to create an animal model for leptin resistance by feeding rats a high-fat diet but the approach proved to be too slow, Harris said.
“It was taking months,” she said. Instead, they switched the rats to a diet often used by stress researchers that included giving them the option of drinking a 30 percent sucrose solution in addition to their food “and that worked so well and so quickly that we started using it,” Harris said. “It turned out that it really was the sucrose. And they really don’t have to be fat to be leptin resistant.”
That’s an important distinction because obesity itself can lead to leptin resistance. It also turns out the effect from the sucrose is quickly reversible just by taking the rats off the sucrose solution, Harris said.
“We’ve taken rats off sucrose and within a couple of days they are responding to leptin again,” she said. “That’s why we feel it is a change in the metabolic pathway (responding to the sucrose) rather than a change in something like body fat, which would not go down that quickly.”
Her lab is focusing in on something called the hexosamine biosynthetic pathway as the potential culprit for creating the leptin resistance.
“It is essential for the function of hundreds of proteins,” Harris said. “So you have to have the pathway doing this work but if you over-activate the pathway, then it has been shown to lead to the development of cardiovascular disease, diabetes, it has been implicated in cancer. It is another of those systems where you have to have it but if you drive it too hard, then you get into trouble. Is this just another of those things that go wrong if you activate this pathway too much?”
The bad effect only comes from drinking the sucrose, she said. When an equal amount in dry form is part of the diet, the effect does not occur. Also, the rats drink the sucrose constantly in small amounts around the clock as opposed to normally only feeding at night, Harris said. The question then becomes whether that constant elevation of blood glucose drives the over-activation of the critical pathway, she said.
The rats chose the sucrose over water and “they will drink so that more than 60 percent of their calories are coming from sucrose,” Harris said. That makes it difficult to make these findings applicable to humans, she said.
“I don’t think people are drinking quite that much” sugar,” Harris said, laughing, “ Our rats are pushing this beyond what people do.”
But many people are not aware of just how much sugar they are consuming, not just from sweetened beverages like soda or sweet tea but in things like juices, said Bethany Suddreth, a registered dietitian with University Hospital Outpatient Nutrition Services. The American Heart Association recommends no more than 36 grams of added sugar per day for men and 25 grams of added sugar for women, she said.
“If you look at a can of soda and how much sugar is in a 12-ounce can of soda, you’ve already well exceeded your limit with one can,” Suddreth said.
That also applies to seemingly healthy things like juice, she said.
“Even 100 percent juice, while that may be nutritious in many ways and have vitamins and minerals your body needs, it still has natural sugars,” Suddreth said. “Even too much of a good thing can then be a bad thing.”
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