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Abstract
Diets that are high in fat and sugar are associated with cognitive deficits in humans. Rodent models using these diets have shown that they produce deficits on tasks that assess hippocampal-dependent spatial learning and memory. However, less is known about the effect of such diets on other hippocampal-dependent forms of cognition. To examine the nature and specificity of diet-induced impairments in hippocampal function, rats were fed standard chow supplemented with sucrose solution, high-fat chow, or both high-fat chow and sucrose solution. These rats were then assessed on their memory for the location and identity of objects, and their formation and use of hippocampal-dependent representations of context.
Chapter 2 showed that relatively short-term dietary intake of fat, sugar, or fat and sugar lasting two-months or less adversely affects performance on a number of hippocampal-dependent spatial memory tasks. This conclusion was based on a meta-analysis of the results from rodent studies using different diets (high in fat, high in sugar, or high in both fat and sugar) and different tasks to assess hippocampal-dependent spatial learning and memory (water maze, place recognition, radial arm maze, and spontaneous alternation). The analysis revealed that the largest effect was produced by exposure to a combined high-fat and high-sugar diet, with medium effects produced by high-fat diet or high-sugar diet. Of the different tasks used to assess spatial learning and memory, the largest effect was observed in the radial arm and radial water maze tasks, with medium effects in the place recognition, the spontaneous alternation, and the Morris water maze tasks.
Chapter 3 demonstrated that rats fed chow and a sucrose solution performed just as well as control rats fed chow on a perirhinal-dependent object-recognition task, but demonstrated impaired performance, compared with controls, on a hippocampal-dependent place-recognition task. Rats exposed to high-sugar diet also performed comparably to controls in a context fear conditioning protocol, although there was some evidence that high-sugar rats generalised the context fear memory to a similar context more than control rats. The generalisation effect was only observed when context fear was assessed using a within-subject design and when rats were tested in the similar context before the conditioning context.
Chapter 4 established that the selective impairment in place recognition memory observed in high-sugar rats extended to rats exposed to a diet high in fat, or high in both fat and sugar. There was no evidence that any one diet produced a more significant impairment than the other diets. There was also no evidence that the impairment worsened with longer exposure to the diet. Finally, rats exposed to any of these diets performed comparably to chow-fed rats in a context pre-exposure fear conditioning task that is critically dependent on hippocampal function.
The findings from this thesis are discussed in relation to the insights gained from observing impairments that appear to be selective to spatial learning and memory, future directions of research for examining the effect of high-fat and/or high-sugar diets on hippocampal-dependent configural processing, the role of the hippocampus in diet-induced deficits, and the translational implications for dietary effects on human cognition and eating behaviours.
