Sugar should be avoided in the diet, especially added sugar. Sugar has 61 different names when listed as a food ingredient. For the complete list of names and a host of interesting facts about sugar visit Sugar Science: The Unsweetened Truth.
Alzheimer’s: Sugar increases insulin levels, which if chronically elevated leads to insulin resistance which is highly correlated with Alzheimer’s.
For a more in depth description of the effects on sugar on the brain see How Sugar and Insulin Damage the Brain.