Mounting evidence shows that overweight and obesity in middle age is associated with poorer cognitive function in later life. Imaging studies have reported that increased BMI is associated with reduced hippocampal volume, a critical brain region in memory across species. Poor diets contribute to both obesity and cognitive decline, and Australians now consume over a third of their daily energy intake from so-called discretionary (snack) foods that are often high in sugar, fat and salt. While early research on dietary effects on cognition focused on the effects of saturated fat, followed by burgeoning interest in sugar, recent studies have found cognitive impairments in rodents fed high-salt diets (e.g. Faraco et al., 2018). Here we evaluate evidence for the effects of these three nutrients on cognitive function, drawing on our work using a varied, palatable ‘cafeteria’-style diet where rodents self-select foods and nutrient intake is monitored over time. Our experiments have shown consistent impairments in hippocampal-dependent memory following consumption of cafeteria relative to healthy diet. In our cafeteria diet model, salt intake increases proportionally with total energy intake. However, we have also found that selectively increasing the proportion of energy from sugar or saturated fat independently impaired cognition, even when total energy intake was matched (Beilharz et al., 2016). Thus, we argue that adverse effects of high salt are often only observed at supraphysiological levels of intake that are unlikely to be reached by humans (Kendig & Morris, 2019). While salt may promote overconsumption by increasing palatability, we believe that dissociating its effects from total energy intake is difficult, and that sugar and fat are more likely to drive the detrimental impacts of poor diet on cognition.
Faraco et al. (2018). Nature Neuroscience. doi:10.1038/s41593-017-0059-z
Beilharz et al. (2016). Behav Brain & Immunity. doi:10.1016/j.bbi.2016.07.151
Kendig & Morris. (2019). APJCN. doi:10.6133/apjcn.201903_28(1).0002