Exploring Possible Causes of Decreasing Dementia Rates In the US

Better management of cardiovascular risk factors and higher levels of education may have contributed to this improvement.
Better management of cardiovascular risk factors and higher levels of education may have contributed to this improvement.

The prevalence of dementia in the United States appears to be declining, according to research published in JAMA Neurology.1 Higher levels of education and better management of cardiovascular risk factors may have contributed to this improvement, although the mechanisms behind these factors are still unknown.

Kenneth M. Langa, MD, PhD, of the University of Michigan Medical School and the Veterans Affairs Center for Clinical Management Research in Ann Arbor, Michigan and colleagues analyzed data from individuals in the Health and Retirement Study (HRS) in 2000 (n =10,546) and 2012 (n =10,511). The participants had an average age of 75 years (95% CI, 74.8-75.2 years) in 2000 and 74.8 years (95% CI, 74.5-75.1 years) in 2012 (P= .24). In 2000, 58.4% (95% CI, 57.3%-59.4%) of the cohort were women, and in 2012, 56.3% (95% CI, 55.5%-57.0%) of the cohort were women (P< .001).

The researchers found that the prevalence of dementia decreased from 11.6% in 2000 to 8.8% in 2012.

More years of education was associated with a lower risk of dementia, and the researchers found a significant increase in educational attainment between 2000 and 2012 (from 11.8 years [95% CI, 11.6-11.9 years] to 12.7 years [95% CI, 12.6-12.9 years]; P < .001). However, it is still unclear why higher levels of education are associated with reduced risk of dementia.

While Dr Langa and colleagues noted that their study provides further support to the theory of “cognitive reserve” — the theory that early-life and lifelong education, as well as cognitive stimulation, may give individuals the ability to better tolerate or compensate for cognitive abnormalities — they also noted that the associations between education, brain biology, and cognitive function are complex, and that there may be other explanations.

Ozioma Okonkwo, PhD, and Sanjay Asthana, MD, from the University of Wisconsin School of Medicine wrote in an accompanying editorial:

“Although the putatively protective role of education against dementia risk is presently well accepted, several aspects of this phenomenon continue to generate controversy. For example, is education simply a handy surrogate for a conglomerate of ‘advantage' including access to wholesome nutrition, better health care, safe neighborhoods, careers that are cognitively challenging, and an overall healthy lifestyle? Each of these elements has been shown to be conducive to cognitive health.”2

They did note, however, that Dr Langa and colleagues addressed this question somewhat by showing that increased education still decreased the risk of dementia, even after adjusting for net worth of individuals and race.

Drs Okonkwo and Asthana also noted that because diagnosing dementia relies on cognitive tests, the association between education and dementia may be a byproduct of the dependence of test performance on education.

Dr Langa and colleagues also found a significant increase in the prevalence of obesity and diabetes between 2000 and 2012. Cardiovascular risk factors did not explain much of the decrease in dementia after controlling for socioeconomic factors of education, wealth, and race/ethnicity, but diabetes was associated with a 39% higher risk of dementia after controlling for other factors.

However, “the trend toward a declining risk for dementia in the face of a large increase in the prevalence of diabetes suggests that improvements in treatments between 2000 and 2012 may have decreased dementia risk, along with the documented declines in the incidence of common diabetes-related complications, such as heart attack, stroke, and amputations,”3 they wrote.

Dr Langa and colleagues also found that underweight individuals (body-mass index (BMI) <18.5) were more likely to be diagnosed with dementia (adjusted odds ratio, 2.47; 95% CI, 1.88-3.24) compared with individuals with normal BMI, whereas those who were overweight or obese had an approximately 30% reduction in dementia risk.

This shows an “obesity paradox,” a term “used to describe the counterintuitive finding that lower BMI, usually deemed indicative of good health, might be associated with increased morbidity and mortality among elderly individuals,”4 wrote Dr Okonkwo and Dr Asthana.

This emphasizes the “importance of a life course approach to the investigation of exposures such as body composition,” Dr Okonkwo and Dr Asthana wrote. They also noted, however, that reverse causation cannot be ruled out. “There is evidence that weight loss may precede the onset of dementia by many years,”5 they wrote. This means that rather than increased BMI being protective against dementia, it is possible that because dementia dysregulates olfaction, gestation, and overall appetite, which leads to a reduced caloric intake, dementia may instead cause reduced BMI.

“The growing consistency of reports indicating a potential decrease in [dementia's] prevalence is encouraging,” Drs Okonkwo and Asthana concluded. “The focus now should be on better understanding the factors that underlie this trend, and translating that knowledge into interventions that can reduce the risk of dementia for both individuals and society as a whole.”

References

  1. Langa KM, Larson EB, Crimmins EM, et al. A comparison of the prevalence of dementia in the United States in 2000 and 2012. JAMA Neurol. 2016; doi:10.1001/jamainternmed.2016.6807 [Epub ahead of print]
  2. Okonkwo OC, Asthana S. Dementia trends in the United States: Read up and weigh in. JAMA Neurol. 2016; doi:10.1001/jamainternmed.2016.7073 [Epub ahead of print]
  3. Gregg EW, Li Y, Wang J, et al. Changes in diabetes-related complications in the United States, 1990-2010. N Engl J Med. 2014;370(16):1514-1523.
  4. Oreopoulos A, Kalantar-Zadeh K, Sharma AM, Fonarow GC. The obesity paradox in the elderly: potential mechanisms and clinical implications. Clin Geriatr Med. 2009;25(4):643-659, viii.
  5. Knopman DS, Edland SD, Cha RH, Petersen RC, Rocca WA. Incident dementia in women is preceded by weight loss by at least a decade. Neurology. 2007;69(8):739-746.
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