The study – published in the Journal of Physiology – suggests that a diet consistently high in fructose ‘slows’ the brain and hinders memory and learning. However the study also finds that consumption of omega-3 fatty acids can counteract the effect.
"Our findings illustrate that what you eat affects how you think," said Professor Fernando Gomez-Pinilla, of the University of California, Los Angeles (UCLA). "Eating a high-fructose diet over the long term alters your brain's ability to learn and remember information. But adding omega-3 fatty acids to your meals can help minimize the damage."
The researchers revealed that rats fed both fructose and the omega-3 fatty acid docosahexaenoic acid (DHA) were able to remember the route of a maze faster than those fed fructose alone.
Gomez-Pinilla said rats fed just fructose showed a decline in synaptic activity, noting that their brain cells had trouble signalling each other, “disrupting the rats' ability to think clearly and recall the route they'd learned six weeks earlier."
"DHA is essential for synaptic function — brain cells' ability to transmit signals to one another," Gomez-Pinilla explained. "This is the mechanism that makes learning and memory possible. Our bodies can't produce enough DHA, so it must be supplemented through our diet."
He suspects that fructose is the culprit behind the brain dysfunction in rats that were not fed DHA – suggesting that consumption of high amounts of the sugar could block insulin's ability to regulate how cells use and store sugar for the energy required for processing thoughts and emotions.
"Insulin is important in the body for controlling blood sugar, but it may play a different role in the brain, where insulin appears to disturb memory and learning," said Gomez-Pinilla. "Our study shows that a high-fructose diet harms the brain as well as the body. This is something new."
Sources of fructose in the Western diet include cane sugar (sucrose) and high-fructose corn syrup, an inexpensive liquid sweetener. The syrup is widely added to processed foods, soft drinks, and condiments – especially in the US. Where, according to the US Department of Agriculture the average American consumes roughly 47 pounds (21.3kg) of cane sugar and 35 pounds (15.8) of high-fructose corn syrup per year.
"We're less concerned about naturally occurring fructose in fruits, which also contain important antioxidants," explained Gomez-Pinilla. "We're more concerned about the fructose in high-fructose corn syrup, which is added to manufactured food products as a sweetener and preservative."
However the US-based Corn Refiners Association were keen to point out the study did not investigate high-fuctose-corn-syrup but looked at intake of fructose, fed to the rats in water.
Dr John White, a sweetener expert and president of White Technical Research, also noted that studies in rats “often do not translate well to human physiology, anatomy or nutrition.”
“Since one of the most important differences between humans and rats is brain anatomical structure, the applicability of rat brain research to humans must be questioned,” he said.
White added that the study data shows that rats consumed 7 grams of fructose per day – which is comparable to an adult human consuming more than one kilogram per day. “A consumer would have to eat 66 apples or drink 51 cans of soda per day to reach that level," he said. "Clearly this is a highly exaggerated and distorted version of the typical human diet,” he said.
Gomez-Pinilla and his team studied two groups of rats that each consumed a fructose solution as drinking water for six weeks. The second group also received omega-3 fatty acids in the form of flaxseed oil and docosahexaenoic acid (DHA) – which is known to protect against damage to the chemical connections between brain cells that enable memory and learning.
The animals were fed standard rat chow and trained on a maze twice daily for five days before starting the experimental diet. The UCLA team then tested how well the rats were able to navigate the maze, which contained numerous holes but only one exit. The scientists placed visual landmarks in the maze to help the rats learn and remember the way.
Six weeks later, the researchers tested the rats' ability to recall the route and escape the maze – what they saw surprised them.
"The second group of rats navigated the maze much faster than the rats that did not receive omega-3 fatty acids," Gomez-Pinilla said.
"The DHA-deprived animals were slower, and their brains showed a decline in synaptic activity … Their brain cells had trouble signaling each other, disrupting the rats' ability to think clearly and recall the route they'd learned six weeks earlier."