ApoE4 and ApoE3 are two variants of a gene that codes for a protein, apolipoprotein E, which binds fats and cholesterol to transport them to the body's lymphatic and circulatory systems and to the brain.
The ApoE4 variant is linked to increased inflammation, Alzheimer's and cardiovascular disease and appears in around 10 to 15 per cent of the population, researchers from University of Southern California (USC) in the US said.
ApoE3, which does not increase the risk for the disease, is a much more common variant appearing in an estimated 70 to 75 per cent of the population, they said.
They placed a group of mice with ApoE4 on a control diet that was 10 per cent fat and seven per cent sucrose, while another group of mice with ApoE4 ate a Western diet that was of 45 per cent fat and 17 per cent sucrose for 12 weeks. A similar test was run on mice with ApoE3.
The team found that on the unhealthy diet, both the mice with ApoE4 and those with ApoE3 gained weight and became pre- diabetic.
But most significantly, those with ApoE4 on the unhealthy diet quickly developed more Alzheimer's plaques - a marker for inflammation - in their brains, that obstruct cognition and memory but those with ApoE3 did not.
However, Alzheimer's symptoms did not worsen for the ApoE3 mice that ate a Western diet, researchers said.
"What happens to you in life is a combination of the genes that you have, the environment and behaviours, such as diet," said Christian Pike, professor at USC.
"Our thinking is that the risk of Alzheimer's associated with obesity is going to be regulated to some degree by the genes that we have," Pike said.
The study was published in the journal eNeuro.