Thin may not mean good health after all! Or, good mental health, any way. A retrospective study involving nearly 2 million people has shown that people with body mass index values below 20 in middle age were at significantly greater risk of dementia in their later years than their heavier peers. Worse, it seems that the threat of dementia reduces with every extra pound of padding!
Very obese people fared the best with the lowest risk of dementia according to the study published in April in The Lancet Diabetes & Endocrinology. However, here is an important disclaimer from Nawab Qizilbash, MSc, DPhil, a lecturer of epidemiology at the London School of Hygiene and Tropical Medicine who led the study: “If increased weight in mid-life is an antidote for dementia, the reasons for this are presently unclear.” “A number of issues ranging from diet to exercise, genetics and weight change could play a part.”
Deborah Gustafson, PhD, from SUNY Downstate Medical Center in New York, noted that the findings by Qizilbash and colleagues are not yet ‘weighty’ enough and need to be interpreted cautiously. At best the “published literature about BMI and dementia is equivocal,” she observed.
BMI records of people aged 40 and above (with the median age being 55) were collected from the United Kingdom Clinical Practice Research Datalink (CPRD) from 1987 to July 2013. Fifty-five percent were female. Anyone with a BMI of less than 20 was defined as underweight. Likewise the range for healthy weight was 20-24.9, overweight 25-29.9, and obese as 30-plus.
The findings militate against smaller studies conducted earlier, which showed that being overweight in mid-life increases the risk of dementia later in life. However, there are studies that have revealed the prevalence of something known as the “obesity paradox”– being overweight in late-life reduces dementia risk.
Qizilbash and colleagues found that at 80 years of age, the incidence of dementia was 9.9% for both sexes (95% CI 9.5%-10.3%) for underweight people, 6.5% (95% CI 6.4%-6.6%) for those with a normal weight, 5.2% (95% CI 5.0%-5.3%) for overweight people, and 4.9% (95% CI 4.7%-5.0%) for obese people for both sexes. Beyond age 80, the gradient for risk of dementia was steeper for women than it is for men. “When mortality of older people caused by all reasons was factored in, the above ratios became somewhat less pronounced. Still, more than 20% lower dementia risk persisted at all degrees of obesity,” the researchers observe.