Research shows a strong relationship between insulin resistance and/or high blood sugar levels and the development of Alzheimer's disease in the typical, elderly, adult population.
A similar relationship has been found between increased Body Mass Index (ie. being over weight) and the risk of developing Alzheimer's disease.
It may be safe to presume that the relationship holds true for those with Down Syndrome and it would therefore be wise to control blood sugar levels and take steps to avoid diabetes and significant weight gain. This is clearly easier said than done, but a low sugar, low fructose, gluten-dairy-soy free diet from early childhood should achieve this and will hopefully translate into better eating choices and discipline as an adult.
This type of diet also arguably has many benefits for the development and function of the brain and associated behaviour.