Being underweight in middle age is associated with raised dementia risk, large study finds
BMJ 2015; 350 doi: http://dx.doi.org/10.1136/bmj.h1897 (Published 10 April 2015) Cite this as: BMJ 2015;350:h1897- Zosia Kmietowicz
People
who are underweight in middle age are at increased risk of developing
dementia, while increasing weight and obesity offer protection against
the condition, a large cohort study has found.
A number
of previous studies have looked at the association between weight and
dementia, and most have found that the risk of dementia increased among
adults who are overweight or obese, although some studies have found the
opposite. Most of these studies have been small, which led the present
researchers to conduct a much larger analysis of the link.
The
researchers, from the London School of Hygiene and Tropical Medicine
and from OXON Epidemiology, both based in London, examined the medical
records of nearly two million people (1 958 191) from the Clinical
Practice Research Datalink (CPRD), a database of patient information
recorded during routine general practice over nearly 20 years.
The
cohort included people aged 40 and over whose body mass index was
recorded from 1992 to 2007 and whose average age was 55 at the start of
the study. Their records were followed for an average of nine
years—until death, transfer out of the practice, or a diagnosis of
dementia. During this time 45 507 people had dementia diagnosed.
The findings, reported in the Lancet Diabetes and Endocrinology,1
showed that people who were underweight (BMI less than 20) were 34%
more likely to have dementia diagnosed (rate ratio 1.34 (95% confidence
interval 1.29 to 1.38)) than those who were a healthy weight (BMI 20 to
24.9).This higher risk of dementia persisted even 15 years after the
underweight was recorded.
The incidence of dementia
continued to fall as weight increased, as very obese people (BMI greater
than 40) were 29% less likely to get dementia than people in the normal
weight range (0.71 (0.64 to 0.78)). Adjusting for confounding factors
known to increase the risk of dementia, such as alcohol use or smoking,
made little difference to the results.
The authors were
unclear as to why increased weight seemed to protect against dementia,
although they said that diet, exercise, frailty, genetic factors, and
weight change could all play a part.
The study had a
number of limitations. Not all of the adults included in the analyses
were middle aged when they entered the study, although the researchers
found only 18 cases of dementia diagnosed in people under 45. Also, the
average baseline BMI used in the study (26.4) was higher than that seen
in previous cohorts. Another limitation is that dementia was probably
under-diagnosed in the cohort, but the authors said that, even if
substantial, this would not change the associations found.
Stuart
Pocock, one of the study authors, said, “Our results suggest that
doctors, public health scientists, and policy makers need to rethink 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.”
Notes
Cite this as: BMJ 2015;350:h1897
