Animals fed a Western-style diet demonstrate rapid impairments in hippocampal function and poorer appetitive control. Researchers examined if this also occurs in humans. One-hundred and ten healthy lean adults were randomized to either a one-week Western-style diet intervention or a habitual-diet control group. Measures of hippocampal-dependent learning and memory (HDLM) and of appetitive control were obtained pre- and post-intervention. HDLM was retested at three-week follow-up. Relative to controls, HDLM performance declined in the Western-style diet group (d = 0.43), but was not different at follow up. Appetitive control also declined in the Western-style diet group (d = 0.47) and this was strongly correlated with HDLM decline (d = 1.01). These findings demonstrate that a Western-style diet can rapidly impair appetitive control in humans an effect that could promote overeating in consumers of a Western-style diet.
Western-style diet impair Hippocampal-dependent appetitive control
In animals, an extensive literature suggests that even brief exposure
to a Western-style diet, rich in saturated fat and added sugar, results in selective impairment on tests of hippocampal-dependent learning and memory. Emerging evidence in humans suggests a similar conclusion. While it is well established that much hippocampal processing relates to memory, its functions extend into many other domains including the regulation of appetite. Animal data suggest that hippocampal lesions impair the use of internal state to resolve appetitive-related decisions. Presumably, the same should occur in humans, with a Western-style diet impairing hippocampal function, and thus relatedly, hippocampal-dependent appetitive control.
That a Western-style diet affects human hippocampal function in young lean healthy people, paralleling animal studies who typically use young lean healthy rats or mice, is a fairly recent finding. Early studies identified an association between consumption of a Western-style diet, assessed by a validated food frequency measure, and performance on neuropsychological tests that are known to be dependent upon hippocampal function. These studies included tests of verbal paired-associate learning, visual memory and episodic memory—with associations to a Western-style diet sustained even after controlling for other likely sources of variability (e.g. exercise, mental and physical health etc). These data suggest that the more a person conforms to a Western-style diet, the more likely they are to perform poorly on hippocampal-dependent neuropsychological tasks. This correlational approach has been further augmented by the finding that hippocampal volumes on neuroimaging are also similarly correlated with a Western-style diet, with this diet linked to smaller hippocampi in older samples. A more crucial test, previously only examined in animals, is whether experimental exposure to a Western-style diet actually causes hippocampal impairment. So far, only one study has reported this. Here, young healthy lean participants were randomly assigned to undertake 4 days of either healthy control breakfasts or a week of Western-style breakfasts, high in saturated fat and added sugar—all consumed in a laboratory setting. Before and after
the dietary exposure phase, all participants undertook control and hippocampal-dependent neuropsychological testing. The key finding of this study was that relative to controls, those in the Western-style breakfast group performed more poorly on one test of HDLM (Hopkins verbal learning
task; but not on another, logical memory), while showing no change on control tests (forward and backward digit span). These data provide the first and as yet only demonstration in humans, paralleling animal studies, that brief exposure to a Western-style diet causes impaired performance on a test of HDLM.
Appetitive control: role of hippocampal-dependent processes
A recent study attempted to more directly test the role of hippocampal-dependent processes in the control of appetite by interoceptive satiety cues. Using a cross-sectional design, hungry participants who habitually ate either a healthy or a Western-style diet, viewed familiar palatable snacks and reported how much they wanted to eat them. As the snacks were only viewed, these judgements rely upon memory. Next, after tasting each snack, participants rated liking for their flavor and whether they wanted more—with both of these ratings presumably more reliant on the actual sensory
properties of the food. After a filling meal, participants were asked to repeat this wanting and liking test. Three findings emerged. First, snack wanting declined more across the meal than snack liking. This represents a form of appetitive control, with the snack judged as significantly less desirable
(i.e. wanting) than it actually tastes (i.e. liking), thus presumably reducing the likelihood of further eating. Second, consumers of a Western-style diet reported smaller changes in wanting relative to liking across the meal, than consumers of a healthy diet. This finding reflects a loosening of appetitive control. Third, the smaller the changes in wanting relative to liking (i.e. our index of appetitive control), the poorer the performance on a neuropsychological measure of HDLM, suggesting mediation of this
effect by the hippocampus.
Karimi, H.
Master of clinical Nutrition & Dietetics
Reference
-Stevenson RJ, et al. Hippocampal-dependent appetitive control is impaired
by experimental exposure to a Western-style diet. Royal Society Open Science.2020.
