A study published in The Royal Society’s Open Science journal was the first of its kind to evaluate the effects of a Western-Style diet on human hippocampal function, after previous studies performed on animals found that “animals fed a Western-style diet demonstrate rapid impairments in hippocampal function and poorer appetite control.”
 
A WS-diet, as defined by researchers, is rich in saturated fat and added sugar.
 
The participants of this study consisted of 110 lean, healthy adults, who were randomized into a one-week WS-diet intervention group, or a habitual-diet control group. Hippocampal-dependent learning and memory (HDLM) was tested pre-intervention, re-tested one week later at post-intervention, and re-tested again at a three-week follow-up.
 
The WS-diet group saw rapid declines in HDLM and appetitive control after the one-week evaluation, however, by the three week follow-up, there was no evaluated difference.
 
“These findings demonstrate that a WS-diet can rapidly impair appetitive control in humans—an affect that could promote overeating in consumers of a WS-diet,” researchers wrote. “The study also suggests a functional role for the hippocampus in appetitive control and provides new evidence for the adverse neurocognitive effects of a WS-diet.” 
 
The research was motivated by extensive literature detailing impairment on tests of HDLM seen in animals exposed to a WS-diet, and worked on presumptions informed by additional studies suggesting that hippocampal lesions impair appetite-related decisions in animals. Early studies on animals, typically young, lean, and healthy rats or mice, used tests known to be dependent on hippocampal function.
 
The associations of a WS-diet sustained in humans even after controlling sources of variability including exercise, mental, and physical health, Royal Society Open Science reported.
 
For four days, young, lean, and healthy participants consumed either healthy control breakfasts, or a week of Western-style breakfasts in a lab setting. The WS-diet group performed more poorly on the Hopkins verbal learning task test, but not on a logical memory test, compared to the control group, after eating a toasted sandwich and milk shake for breakfast on days 1 and 8, two Belgian waffles for breakfast or dessert on days 2-7, and a main meal from a popular fast food chain on two days.
 
Additionally, a “wanting and liking test” was performed before and after breakfasts were consumed, which evaluated the changes in wanting and liking a set of palatable breakfast foods before and after consuming breakfast. Following the week-long intervention, the control group saw smaller changes in wanting and liking ratings before and after breakfast, which researchers considered to be evidence that a WS-diet could have a “dampening effect on brain reward circuits.”
 
Researchers suggested that because a reduced HDLM in the WS-Diet group was strongly associated with change in performance on the wanting and liking test, it may indicate that appetite control is impacted by hippocampal function.
 
“The magnitude of these changes in HDLM and appetitive control were strongly correlated, implying a probable common basis for these effects in the hippocampus, and thus a role for the hippocampus in appetitive control. More broadly, this experiment, alongside those from the other animal and human studies cited here, suggests that a WS-diet causes neurocognitive impairments following short-term exposure,” the researchers wrote.