Low body-mass index associated with increased risk of dementia

1. In this retrospective cohort study, being underweight during middle and old ages was associated with an increased risk of dementia; whereas very obese people had a lower risk of dementia.

2. After adjusting for age and sex, all-cause mortality was highest in underweight and very obese populations.

Evidence Rating Level: 2 (Good)

Study Rundown: Dementia is a decline in mental functions beyond expected age-related changes and affects an elderly individual’s ability to carry out activities of daily living (ADLs). The condition has variable clinical courses, but is typically irreversible and significantly impacts the quality of life for both the patients and their care-givers. Previous studies have linked obesity to an increased risk of dementia. In this retrospective cohort study, Qizilbash et al. explored whether the same held true for underweight populations. The study analyzed the link between body weight and diagnoses of dementia in 2 million people. The study observed a 34% increase in the risk of dementia in underweight people (BMI < 20 kg/m²) compared to people of healthy weights; whereas the risk was 29% lower for very obese people. The statistical analyses was strengthened by the large number of participants in the study. However, this retrospective cohort study was not designed to establish causal links. Public health implications of the results remain to be seen.

The study had no funding.

Click to read the study in the Lancet Diabetes & Endocrinology

Relevant Reading: The projected effect of risk factor reduction on Alzheimer’s disease prevalence

In-Depth [retrospective cohort]: This study assessed the link between low body weight during middle and old ages with the risks of subsequently developing dementia. The authors drew body height and weight information from the Clinical Practice Research Datalink (CPRD). A patient was classified as having dementia if s/he carried a diagnosis of dementia, Alzheimer, Lewy body disease, or Pick’s disease, or if dementia was recorded on a death certificate. The final analyses included 1958191 people (18786640 person-years of follow up). Being underweight was defined as body-weight index under 20 kg/m².

Compared with people of healthy weight, underweight people had a 34% (95% confidence interval, CI, 29-38) excess risk of dementia. Very obese people (BMI≥40 kg/m²) had a 29% (95% CI, 22-36) lower risk of dementia. All-cause mortality as related to BMI was J-shaped, with the highest mortality rates among the underweight people (rate ratio 1.53, 95% CI 1.51-1.55) and very obese people (BMI≥40 kg/m², rate ratio 1.57, 95% CI 1.53-1.62). Sensitivity analysis indicated that the increased mortality rate in obese people did not explain their low dementia risk.

Image: PD

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