This 2015 New York rodent study found:

“Early stage [diet-induced] obesity, before the onset of diabetes or metabolic syndrome, produced deficits on cognitive tasks that require the prefrontal cortex.

These results strongly suggest that obesity must be considered as a contributing factor to brain dysfunction.”

The difference in the diets of the adult male subjects was that the control group ate 10% fat (20% protein, 70% carbohydrates) whereas the obese group ate 45% fat (20% protein, 35% carbohydrates). Significant changes in body weight were present after the first two weeks on the diets, but testing didn’t begin until after eight weeks.

I thought the study design prematurely terminated the experiments. The study didn’t justify the ultimate purpose of conducting rodent experiments, which is to find possible human applicability.

One study design possibility would have been to continue through old age to find how the conditions progressed. Another possibility would have been to reverse the high-fat diet to find whether the conditions reversed.

http://www.pnas.org/content/112/51/15731.full “Obesity diminishes synaptic markers, alters microglial morphology, and impairs cognitive function”