Here are two more reasons to avoid a high-fat diet: it could affect endurance levels and memory, according to a new study. Rats fed a high fat diet showed a reduction in physical endurance—the time span a person is able to exercise—and cognitive ability after just nine days, according to a study by Oxford University researchers published in the FASEB journal.

 

“We found that rats, when switched to a high-fat diet from their standard low-fat feed, showed a surprisingly quick reduction in their physical performance,” says Andrew Murray, who led the work at Oxford University and has now moved to the University of Cambridge. “After just nine days, they were only able to run 50 percent as far on a treadmill as those that remained on the low-fat feed.”

 

The findings not only affect people eating high-fat diets, but could also have implications for athletes looking for the optimal diet for training and patients with metabolic disorders, the researchers say.

 

Physical endurance depends on how much oxygen can be supplied to muscles and how efficiently the muscles can release the energy supplied by food. Using fat as a fuel is less efficient than using glucose from carbohydrates, the scientists note. However, the metabolic changes caused by different diets are complex and it has been controversial whether eating high levels of fat for a short period of time would increase or decrease physical performance, the researchers add.

 

In the study, 42 rats were initially fed a standard diet with a low-fat content of 7.5 percent. Their physical endurance was measured by how long they could run on a treadmill. Their short-term memory was measured in a maze task. Half of the rats were then switched to a high-fat diet where 55 percent of the calories came from fat. After four days of getting used to the new diet, the endurance and cognitive performance of the rats on the low- and high-fat diets was compared for another five days.

 

“The high-fat diet, in which 55 percent of the calories came from fat, sounds high but it's actually not extraordinarily high by human standards,” says Murray. “A junk food diet would come close to that.” Murray says that some high-fat, low-carb diets for weight loss can even have fat contents as high as 60 percent.  He cautions, however, that it’s unclear how many direct conclusions can be drawn from the study as the high-fat diet the researchers used were not particularly low in carbohydrates.

 

On the fifth day of the high-fat diet and the first day back on the treadmill, the rats were running 30 percent less far than those remaining on the low-fat diet. By the ninth day, the last of the experiment, they were running 50 percent less far, according to the researchers.

 

The reserachers say the rats on the high-fat diet were also making mistakes sooner in the maze task, suggesting that their cognitive abilities were being affected by their diet, as well. The number of correct decisions before making a mistake dropped to an average of 5 to 5.5 from more than six.

 

An increased level of a specific protein could partly explain the reduction in treadmill running, the researchers say. The rats on the high-fat diet showed an increase of a protein called the uncoupling protein in the muscle and heart cells. The protein uncouples the process of burning food for energy in the cells, reducing the efficiency of the heart and muscles.