In animals, a Western style diet–high in saturated fat and added sugar–causes impairments in hippocampal-dependent learning and memory (HDLM) and perception of internal bodily state (interoception). In humans, while there is correlational support for a link between Western-style diet, HDLM, and interoception, there is as yet no causal data. Here, healthy individuals were randomly assigned to consume either a breakfast high in saturated fat and added sugar (Experimental condition) or a healthier breakfast (Control condition), over four consecutive days. Tests of HDLM, interoception and biological measures were administered before and after breakfast on the days one and four, and participants completed food diaries before and during the study. At the end of the study, the Experimental condition showed significant reductions in HDLM and reduced interoceptive sensitivity to hunger and fullness, relative to the Control condition. The Experimental condition also showed a markedly different blood glucose and triglyceride responses to their breakfast, relative to Controls, with larger changes in blood glucose across breakfast being associated with greater reductions in HDLM. The Experimental condition compensated for their energy-dense breakfast by reducing carbohydrate intake, while saturated fat intake remained consistently higher than Controls. This is the first experimental study in humans to demonstrate that a Western-style diet impacts HDLM following a relatively short exposure–just as in animals. The link between diet-induced HDLM changes and blood glucose suggests one pathway by which diet impacts HDLM in humans.

A high-fat high-sugar diet predicts poorer hippocampal-related memory and a reduced ability to suppress wanting under satiety Copyright and reuse:Sussex Research Online is a digital repository of the research output of the University.Copyright and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable, the material made available in SRO has been checked for eligibility before being made available.Copies of full text items generally can be reproduced, displayed or performed and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. Animal data indicate that greater intake of fats and sugars prevalent in a Western diet impairs hippocampal memory and tests of behavioral inhibition known to be related to hippocampal function (e.g., feature negative discrimination tasks). It has been argued that such high fat high sugar diets (HFS) impair the hippocampus, which then becomes less sensitive to modulation by physiological state. Thus retrieval of motivationally salient memories (e.g., when seeing or smelling food) occurs irrespective of state. Here we examine whether evidence of similar effects can be observed in humans using a correlational design. Healthy human participants (N = 94), who varied in their habitual consumption of a HFS diet, completed the verbal paired associate (VPA) test, a known hippocampal-dependent process, as well as liking and wanting ratings of palatable snack foods, assessed both hungry and sated. Greater intake of a HFS diet was significantly associated with a slower VPA learning rate, as predicted. Importantly, for those who regularly consumed a HFS diet, while reductions in liking and wanting occurred between hungry and sated states, the reduction in wanting was far smaller relative to liking. The latter effect was strongly related to VPA learning rate, suggestive of hippocampal mediation. In agreement with the animal literature, human subjects with a greater intake of a HFS diet show deficits in hippocampal-dependent learning and memory, and their desire to consume palatable food is less affected by physiological state -a process we suggest that is also hippocampal related.

Animals fed a Western-style diet (WS-diet) demonstrate rapid impairments in hippocampal function and poorer appetitive control. We examined if this also occurs in humans. One-hundred and ten healthy lean adults were randomized to either a one-week WS-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 WS-diet group ( d = 0.43), but was not different at follow-up. Appetitive control also declined in the WS-diet group ( d = 0.47) and this was strongly correlated with HDLM decline ( d = 1.01). These findings demonstrate that a WS-diet can rapidly impair appetitive control in humans—an effect that could promote overeating in consumers of a WS-diet. 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.

Human and animal olfactory perception is shaped both by functional demands and by various environmental constraints seemingly peculiar to chemical stimuli. These demands and constraints may have generated a sensory system that is cognitively distinct from the major senses. In this article we identify these various functional demands and constraints, and examine whether they can be used to account for olfaction's unique cognitive features on a case-by-case basis. We then use this as grounds to argue that specific conscious processes do have functional value, a finding that naturally emerges when a comparative approach to consciousness across the senses is adopted. More generally, we conclude that certain peculiar features of olfactory cognition may owe more to limited neocortical processing resources, than they do to the challenges faced by perceiving chemical stimuli.

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