Here is a link to the actual study. Sorry, only the abstract is free.
http://www.thelancet.com/journals/la...033-9/abstract
Here is a quote with numbers from the abstract:
"
Our cohort of 1 958 191 people from UK general practices had a median age at baseline of 55 years (IQR 45–66) and a median follow-up of 9·1 years (IQR 6·3–12·6). Dementia occurred in 45 507 people, at a rate of 2·4 cases per 1000 person-years. Compared with people of a healthy weight, underweight people (BMI <20 kg/m2) had a 34% higher (95% CI 29–38) risk of dementia. Furthermore, the incidence of dementia continued to fall for every increasing BMI category, with very obese people (BMI >40 kg/m2) having a 29% lower (95% CI 22–36) dementia risk than people of a healthy weight. These patterns persisted throughout two decades of follow-up, after adjustment for potential confounders and allowance for the J-shape association of BMI with mortality.
"
What they call underweight is a BMI of 20 or less gets a 34% risk increase.
At 6 feet tall, that would be 147 pounds.
At 5'6", that would be a weight of 124 pounds
Whoa, that's skinny for a guy. On my frame that's near zero body fat.
For the other end, a BMI of 40 gives the 29% risk reduction.
a person 6 feet tall would weigh 295 pounds
and 5'6" tall would weigh 248 pounds
Anyway, what is interesting is that beneficial effect (less dementia) increases steadily with BMI, so you don't have to have to get 40 BMI obese to benefit.
Notice that they are accounting for the, "J-shape association of BMI with mortality."
This means that the very fat don't get senile dementia as often because they are already dead.
For the numbers they gave, 2.4 cases in 1000 person years, being obese lowered that by 29%, so now we're talking about 1.7 per 1000 person years. I think I would shoot for being slightly overweight for a 10% dementia risk reduction instead of obese and a great increase in diabetes and worse.