When is a high fat diet not a high fat diet?

The observation that a high fat/low carbohydrate (CHO) diet has a beneficial effect on a mouse model of Alzheimer's disease (AD) published today is notable given previous results showing that high fat diets have a deleterious effect on AD. Van de Auwera, et al. [1] reported that mice fed a ketogenic diet (<1% carbohydrate, 80% fat) were found to have a 25% decrease in the protein Aβ42 compared to mice fed a standard high-carbohydrate, low-fat chow diet. Aβ42 is a particularly amyloidogenic mutant form of the amyloid precursor protein whose proteolytic product β-amyloid peptide is contained in the plaques and neurofibrillar tangles that are characteristic of AD.