We all know sugary diets can sabotage a waistline. Now it turns out they might make brains flabby too.

Sweet drinks scrambled the memories and stunted learning in lab rats in a new study—leading to "high concern" over what sugary diets may do to people, according to neuroscientist Fernando Gomez-Pinilla. (Read more about memory from National Geographic magazine.)

For the study, Gomez-Pinilla's team first trained rats to successfully navigate a maze, giving them only water and standard rat chow for five days. During the following six weeks, the rats' water was replaced with syrups that were 15 percent fructose.

"Most sodas people consume are about 12 percent sugar, so imagine if you drank soda with sugar added instead of water," said Gomez-Pinilla, of the University of California, Los Angeles.m

During the six-week period, half the rodents were also given flaxseed oil and fish oil—both rich in omega-3 fatty acids. These antioxidants may protect against damage to chemical connections in the brain called synapses, past research suggests.

After six weeks of the fructose syrup, all the rats were slower at running the maze. However, those that had received omega-3s were slightly faster than their counterparts.

By studying the dissected brains of the study rats, the researchers determined that the high-fructose diets had sabotaged the ability of synapses to change, a key factor in learning. The sugary drinks had also disrupted the sugar-regulating protein insulin in a brain area called the hippocampus, which play a role in memory formation in both rats and humans.t

"I was very shocked to see how strong an effect these diets could have on the brain—I have high concern that the foods people eat can really affect mood and cognition," Gomez-Pinilla said.

Everything in Moderation

Cheaper than table sugar, high-fructose corn syrup is commonly added to soft drinks, condiments, and other processed foods.

The average person in the United States consumes more than 60 pounds (27 kilograms) of it annually, according to the U.S. Department of Agriculture. Consumption of cane and beet sugar, which also contain fructose, is only slightly lower.

"I'm not saying that fructose is bad—you don't want to single out one nutrient," Gomez-Pinilla said. "The big issue is excessive consumption.

"We also want people to think about their whole diets—also think about eating omega-3 fatty acids," which can be found in foods such as salmon, tuna, walnuts, and olive oil. (Read "Why Are We So Fat?" from National Geographic magazine.)

The next task for the team "is to determine whether the long-term effects of a poor diet are reversible," said physiologist Jill Barnes at the Mayo Clinic in Rochester, Minnesota, who did not take part in this research.

Future research should also investigate the effects diet might have on aging and development. "For example, what happens in younger animals or humans when exposed to chronic high-sugar, low omega-3 fatty acid levels during growth?" Barnes asked.

In addition, study leader Gomez-Pinilla said, "we can look at how diet might affect disorders such as post-traumatic stress disorder (PTSD)" by adding stress to an already stressed brain. "Perhaps for someone who doesn't eat properly, the effects of PTSD might be much worse."