The article itself is behind a pay wall, but the abstract can be found here: http://www.thelancet.com/journals/landia/article/PIIS2213-8587%2815%2900033-9/abstract It was published in the Lancet, which gives it some credibility. It seems they monitored retrospectively studied close to two million people's BMI and occurrence of dementia for two decades. That seems a large enough population, over enough time, but I don't have sufficient understanding of endocrinology and/or dementia to see where actual issues may lie. At least they state further research is necessary, so that's a solid recommendation.