Not defensive, just critical.
20%? Not sure where that number comes from. Here's a quote from the article I read after following your link:
Those with a body mass index (BMI) - which measures weight relative to height - greater than 30, who are classified as obese, were 288% more likely to develop dementia than those with a BMI between 20 and 25, according to the study.
The clinically overweight, who have a BMI between 25 and 30, were 71% more likely.
Those are the numbers that are throwing me off. Plus the fact that they are using BMI as basis for the their studies - BMI is vague at best, and not suitable to base ANYTHING on.
As for other illnesses it is a symptom of, the first one is Senility, also known as senile dementia. If your uncle is elderly, then that is most likely what he is suffering from. Other illnesses that can (but don't neccessarily have to) cause dementia are Alzheimer's disease, Huntington's disease, Niemann-Pick disease, Tay-Sachs disease, Wilson's disease, Gaucher's disease, Creutzfeld Jacob's disease and some others.
It can also be caused, in extreme cases, by encephalitis and even hyperthyroidism, and strokes can trigger dementia as well. And that's just a few from the top of my head, I'm sure even a quick check on wikipedia or something similar would give you a more comprehensive list.