Talk of weight gain “triggering” Alzheimer’s goes beyond what the study can tell us about BMI and Alzheimer’s risk.
Research into lifestyle factors that may increase the risk for developing Alzheimer’s disease in later life is very important considering the disease’s nasty profile as well as its reach among the elderly. The first of the 74 million “baby boomers” (born between 1946 and 1964) have been reaching 65-year-old senior status at the rate of 8,000 a day since 2010. Public health experts have been waving a warning flag to expect a doubling in Alzheimer’s cases by 2050 as the U.S. and global population ages.
In a study published earlier this month in Molecular Psychiatry, National Institute on Aging (NIA) researchers reported on one such lifestyle factor that they foretell may hasten the early onset of Alzheimer’s disease — weight gain in middle age. The NIA researchers turned to a long-term study on aging — the Baltimore Longitudinal Study of Aging — that’s been running for 14 years. As part of the original study, researchers checked in with 1,394 cognitively healthy adult volunteers every two years. At each check-in, the study participants were given neuropsychological assessments. As the NIA evaluated the study data, they found that out of the 1,394 in this cohort, 142 people went on to develop Alzheimer’s disease. The researchers surmised from the study that each unit increase in body mass index (BMI), say from 25 to 26, may predict earlier onset of Alzheimer’s disease by about 6.7 months. As another part of the study, researchers autopsied the brains of 191 of the volunteers and said they observed in them a higher amount of amyloid plaque — a hallmark of Alzheimer’s.
While some media reports on the obesity-Alzheimer’s link included some cautions about the findings, most didn’t go far enough in pointing out the limitations of this observational study. The published study doesn’t support a conclusion of cause and effect — it merely showed an association between higher BMI and earlier onset of Alzheimer’s. Only an experiment where researchers manipulated the weight of study participants could show definitively that increased weight caused earlier development of the disease. But reporters and researchers glossed over that key detail when presenting the findings to the public.
Check out some of the headlines, statements and quotes, where use of active verbs and other language — “trigger,” “speed up,” “delay,” “protective effect,” etc. — implies a causal relationship between weight and Alzheimer’s:
AP — “Middle-Age Weight Gain May Trigger Alzheimer’s”
“Weight in middle age may influence who’s at risk for Alzheimer’s later on, and when it strikes.”
“Maintaining a healthy BMI at midlife is likely to have long-lasting protective effects,” said Dr. Madhav Thambisetty of NIH’s National Institute on Aging, who led the study reported in the journal Molecular Psychiatry.
CBS — “Weight in midlife may affect onset of Alzheimer’s”
“How much you weigh at age 50 could be a factor in the onset of Alzheimer’s disease, according to new research.”
The study “indicates that if we can maintain a healthy body mass index even as early as midlife, it might have long-lasting protective effects towards delaying the the onset of Alzheimer’s disease later on,” according to the lead study author.
AFP — “Overweight in midlife may speed up Alzheimer’s: study”
A statistical comparison showed that every extra unit in body mass index (BMI, a height-to-weight ratio) in middle age corresponded to earlier onset of Alzheimer’s by about 6.5 months — what the authors termed a “robust” correlation.
“A healthy BMI at midlife may delay the onset of AD,” the study paper said, referring to Alzheimer’s disease.
HealthDay — “Too Much Weight in Midlife Tied to Earlier Alzheimer’s”
The HealthDay story included an important caveat that most other stories didn’t:
The study, published online Sept. 1 in the journal Molecular Psychiatry, does not prove that obesity, itself, hastens Alzheimer’s.
However, obese adults often have health conditions that have been linked to an increased Alzheimer’s risk, such as high blood pressure, high cholesterol and diabetes.
In addition to this being an extremely small sample size — there are 5 million Americans and 50 million people worldwide with Alzheimer’s disease, according to the World Health Organization — the average age of onset of Alzheimer’s among those unfortunate 142 individuals that developed it, was age 83. That’s hardly “early onset.” In fact, that’s four years beyond the average life expectancy for Americans.
We are all for proactively adopting lifestyle changes like improved diets and exercise, since those changes yield many important health benefits in addition to their potential to impact Alzheimer’s onset. But perhaps this would be a good time to note that researchers have also this year found an association being underweight in middle age and developing Alzheimer’s — and that being overweight or obese was associated with reduced risk in the study. And this research reviewed the health records of some two million people — a sample size that dwarfs the ~1,400 included in the current study.
Despite its huge size, the earlier study was also observational in design, meaning that it couldn’t tell us definitively whether more or less weight could “protect” against dementia. And yet many of the stories about that earlier study also used inappropriate cause-and-effect language to describe the results.
BBC — Being overweight ‘reduces dementia risk’
Being overweight cuts the risk of dementia, according to the largest and most precise investigation into the relationship.
LiveScience — “Being Too Slim at Midlife May Boost Dementia Risk”
In a surprising finding that contradicts some previous studies, the researchers found that being overweight or obese in middle age actually appeared to protect brain health.
Medical News Today — “Being underweight increases dementia risk, while obesity reduces it, study finds”
“…a new study – deemed the largest ever to assess the link between body mass index and dementia risk – suggests obesity could actually be a protective factor against the condition, while people who are underweight may be at increased risk.”
Not one of the stories on this month’s obesity-Alzheimer’s association study mentioned these contradictory findings from just four months ago.
Note: For help on framing observational data the correct way, please refer to our primer on the topic: Observational Studies – Does The Language Fit The Evidence? – Association Versus Causation
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