Those in middle age are overweight, have a much greater risk of developing Alzheimer's disease decades later, scientists of the University of Kansas in the United States.
In contrast, retirees overweight provides some protection from this disease.
Researchers believe that only now they are starting to understand this so-called "obesity paradox".
In their view obesity in middle age leads to Alzheimer's disease is more likely due to different medical factors - or "heterogeneous pathophysiology", as expressed by the researchers.
One explanation of the paradox lies in the "preclinical phase" of the disease, which may include changes in the brain without external signs and accelerated weight loss.
Alzheimer's disease and neurodegenerative changes in the brain can affect the shape".
Possibly, Alzheimer's disease can lead to weight loss, forcing people to forget to eat, and by reducing the amount of physical activity, thus people can become thinner.
"It is unclear whether low body mass index (BMI) is actually part of Alzheimer's disease, or a side effect caused by the disease.
"Although this study shows the relationship between weight changes and changes in the brain that are common to Alzheimer's disease, there was no interaction between BMI and other symptoms such as memory loss. You must spend a great deal of work before we can say if these results can be used to develop better ways to diagnose early stages of the disease, say experts.