Rodents fed a diet where fructose represented 60 per cent of calories ingested during the day were found to perform poorly in tests of memory, compared to rodents fed a control diet, say findings in the journal Neurobiology of Learning and Memory.
The researchers, led by Marise Parent, suggest their findings are relevant to humans, and nod towards the use of fructose-containing sweeteners used by the food industry. Table sugar (sucrose) contains 50 per cent fructose and 50 per cent glucose, while high-fructose corn syrup (HFCS 55) contains 55 per cent fructose and 42 per cent glucose.
“Fructose, in many forms, is added to countless foods including carbonated beverages, fruit products, baked goods, cereals, and dairy products,” wrote lead author Amy Ross. “Indeed, North Americans would be greatly challenged to purchase processed foods not containing some form of fructose.”
Commenting on the applicability of the results to humans, Ross and her co-workers noted: “Although deriving 60 per cent of calories from fructose produces pathology in rodents that is similar to that experienced by humans, the level consumed is outside the current range of the human diet.
“Notably, determining what concentration would be comparable between humans and rats is difficult, given that a rat is expected to metabolize fructose at a different rate than a human and because rats typically require higher doses of drugs than humans to observe an effect.”
Parent and her research team divided Sprague-Dawley rats into two groups, one fed a diet where fructose represented 60 per cent of calories ingested during the day, and the other was fed standard rat chow containing 60 per cent vegetable starch.
“Rats are an excellent animal model to study the effects of fructose intake because their metabolism of fructose closely resembles that of humans,” explained the researchers. “The present research focused on male rats, given that men are the greatest consumers of fructose.”
The rats were placed in a pool of water to test their ability to learn to find a submerged platform, which allowed them to get out of the water. Two days later, the animals were returned to the pool with no platform present to see if the rats could remember to swim to the platform's location.
"What we discovered is that the fructose diet doesn't affect their ability to learn," said Parent. "But they can't seem to remember as well where the platform was when you take it away. They swam more randomly than rats fed a control diet."
Commenting on a potential mechanism, the researchers note that fructose, unlike glucose, is processed almost exclusively by the liver, and produces an excessive amount of triglycerides, which may interfere with insulin signaling in the brain, and affect the brain’s ability to adapt based on new experiences.
Results were similar in adolescent rats, added Parent, but it is unclear whether the effects of high fructose consumption are permanent.
"The bottom line is that we were meant to have an apple a day as our source of fructose," said Parent said. "And now, we have fructose in almost everything."
Research is ongoing in Parent’s lab with current research looking at the intake of fish oil on triglycerides increases and memory deficits.
Source: Neurobiology of Learning and Memory
Published online ahead of print, doi: 10.1016/j.nlm.2009.05.007
“A high fructose diet impairs spatial memory in male rats”
Authors: A.P. Ross, T.J. Bartness, J.G. Mielke, M.B. Parent