Higher Dementia Risk in Very Thin

People who are thin in middle age have the highest likelihood of dementia in later life, according to a large scale study done in Great Britain. Thin was defined as a body mass index (BMI) of less than 20. Someone who is 5”4” and 117 pounds would have a BMI of 20.

The study looked back at health records of nearly 2 million people and found that the risk of later dementia consistently fell as the weights increased—and very obese people had the lowest risk. During the span of the study 45,000 people were diagnosed with dementia (people with previous dementia were excluded). The study was published April 10, 2015, in The Lancet Diabetes & Endocrinology. 

“If increased weight in mid-life is protective against dementia, the reasons for this inverse association are unclear at present,” wrote the authors led by Nawab Qizilbash, MSc, DPhil, an epidemiologist at the London School of Hygiene and Tropical Medicine. “Many different issues related to diet, exercise, frailty, genetic factors, and weight change could play a part.”

These results contradict some previous, smaller studies which showed that obesity is associated with higher risk of dementia later in life. The apparent protection conferred by obesity is sometimes called “the obesity paradox.” Studies have shown that people with diabetes of normal weight are twice as likely to die as those that are overweight. Obese people with other chronic diseases, such as heart failure and renal disease, also often live longer than normal weight patients and scientists are at a loss to explain why.

Qizilbash and colleagues found that at 80 years of age, the incidence of dementia was:
9.9% for underweight people
6.5% for those of normal weight
5.2% for overweight people
4.9% for obese people