London: Middle-aged people who are underweight (with a Body Mass Index [BMI]
less than 20 kg/m2 [1]) are a third more likely to develop dementia than
people of similar age with a healthy BMI, according to new research
published in The Lancet Diabetes & Endocrinology journal. The findings, which come from the largest ever study to examine the
statistical association between BMI and dementia risk, also show that
middle-aged obese people (BMI greater than 30 kg/m2) are nearly 30% less
likely to develop dementia than people of a healthy weight,
contradicting findings from some previous research, which suggested that
obesity leads to an increased risk of dementia.
The researchers analysed the medical records of nearly two million
(1,958,191) people with an average (median) age of 55 years at the start
of the study period, and an average (median) BMI of 26.5 kg/m2, just
within the range usually classed as overweight. During an average
(median) of nine years follow-up, nearly fifty thousand (45,507) people
were diagnosed with dementia.
People who were underweight in middle age were a third (34%) more
likely to be diagnosed with dementia than those of a healthy weight, and
this increased risk of dementia persisted even 15 years after the
underweight was recorded.
As participants’ BMI at middle age increased, the risk of dementia
reduced, with very obese people (BMI greater than 40 kg/m2) 29% less
likely to get dementia than people in the normal weight range. An
increase in BMI was associated with a substantial steadily decreasing
risk of dementia for BMI of up to 25 kg/m² (classed as a healthy
weight). Above a BMI of 25 kg/m² (classed as overweight or obese),
dementia risk decreased more gradually, and this trend continued up to a
BMI of 35 kg/m² or higher.
The association between BMI and dementia risk wasn’t affected by the
decade in which the participants were born, nor by their age at
diagnosis. Adjusting for confounding factors known to increase the risk
of dementia, such as alcohol use or smoking, made little difference to
the results.
According to study author Professor Stuart Pocock from the London
School of Hygiene & Tropical Medicine, “Our results suggest that
doctors, public health scientists, and policy makers need to re-think
how to best identify who is at high risk of dementia. We also need to
pay attention to the causes and public health consequences of the link
between underweight and increased dementia risk which our research has
established. However, our results also open up an intriguing new avenue
in the search for protective factors for dementia – if we can
understand why people with a high BMI have a reduced risk of dementia,
it’s possible that further down the line, researchers might be able to
use these insights to develop new treatments for dementia.” [2]
“The reasons why a high BMI might be associated with a reduced risk
of dementia aren’t clear, and further work is needed to understand why
this might be the case,” adds Dr Nawab Qizilbash from OXON Epidemiology
in London, UK and Madrid, Spain, the study’s lead author. “If increased
weight in mid-life is protective against dementia, the reasons for this
inverse association are unclear at present. Many different issues
related to diet, exercise, frailty, genetic factors, and weight change
could play a part.” [2]
Writing in a linked Comment, Professor Deborah Gustafson from SUNY
Downstate Medical Center in New York, USA, says, “The published
literature about BMI and dementia is equivocal. Some studies report a
positive association between high mid-life BMI and dementia, whereas
others do not… Many considerations are needed in the assessment of the
epidemiology of the association between BMI and late-onset dementia, as
is the case for many recorded associations involving late-life
disorders. To understand the association between BMI and late-onset
dementia should sober us as to the complexity of identifying risk and
protective factors for dementia. The report by Qizilbash and colleagues
is not the final word on this controversial topic.”
[1] Although a BMI less than 18.5 kg/m2 is usually classed as
underweight, a slightly higher threshold (20 kg/m2) was used in this
study to enable comparison with earlier studies, which had taken BMI
lower than 20 kg/m2 as the threshold.
[2] Quotes direct from authors and cannot be found in text of Article