New research explores whether everyday insecticide use could be silently impairing memory and thinking ability in older adults, and why it might be more common than you think.
Study: Association between insecticide exposomics and cognitive function in older adults: an observational study based on NHANES 2011–2014. Image credit: Oleksii Synelnykov/Shutterstock.com
An observational study based on the 2011-2014 National Health and Nutrition Examination Survey (NHANES) data revealed a link between insecticide exposure and the risk of cognitive impairments in older adults. The study is published in Frontiers in Public Health.
Background
Cognitive impairment is an intermediate stage between normal aging and dementia, predominantly affecting the daily activities and social functioning of older adults aged 60 years or above.
An estimated 5 to 17% of cognitive impairment cases annually progress to dementia, primarily in the form of Alzheimer’s disease. An estimated 50 million people are living with Alzheimer’s disease worldwide. This number is predicted to reach 150 million by 2050, mainly due to a rapid increase in the global aging population.
Despite its significant public health impact, dementia treatment remains largely ineffective. Timely predicting cognitive function levels and identifying cognitive impairment risk factors through cognitive test performance in older adults are valuable strategies to prevent dementia development and progression.
Exposure to pesticides, including insecticides, herbicides, and fungicides, has increased the risk of several diseases, including Hodgkin disease, non-Hodgkin lymphoma, and Parkinson’s disease. Both animal and human studies have highlighted these substances' neuro-disruptive functions.
In the current study, researchers at the Guangzhou University of Traditional Chinese Medicine, China, aimed to investigate the association between pesticide exposure and cognitive functions in older adults aged 60 years or above.
The study
The researchers analyzed data from 1,544 older adults from NHANES, providing a detailed overview of the socioeconomic, health, behavioral, and nutritional aspects of U.S. adults and children.
Questionnaires collected self-reported data on pesticide usage (insecticides and herbicides). Cognitive functions were assessed using a range of validated tests, including the word learning test, the delayed recall test, the animal naming test, and the digital symbol substitution test.
The impact of pesticide exposure, particularly insecticides and herbicides, on cognitive function was evaluated using appropriate statistical analyses, controlling for sociodemographic, lifestyle, and medical confounders.
Key findings
Initially, the study found significant associations between insecticide exposure and cognitive impairment risk in older adults. However, after full adjustment for potential confounding factors such as education, income, and medical history, these associations did not remain statistically significant. For example, in the fully adjusted model, the association between insecticide exposure and word learning test scores (CERAD-WL) had a p-value of 0.2284, indicating a lack of strong evidence for a definitive link.
Although the sample for herbicide exposure analysis was limited and not the primary focus of the study, herbicide exposure also exhibited a notable association with animal naming ability in unadjusted models only.
The study measured specific urinary metabolites of the two most common insecticides and herbicides to explore further the associations between insecticide exposure and cognitive impairment risk. Among these metabolites, trans-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropane carboxylic acid (Trans-DCCA), a metabolite of pyrethroid insecticides, showed a significant association with word learning (memory function).
A subgroup analysis conducted in the study revealed that most of the potential confounding factors, including age, gender, income, alcohol consumption, body mass index (a measure of overweight and obesity), and hypertension (high blood pressure), did not significantly alter the observed associations, suggesting a limited interactive effect on cognitive performance.
Study significance
The study findings reveal that insecticide exposure may be associated with an increased risk of cognitive impairment, particularly memory and delayed recall. However, a causal relationship could not be confirmed because of the cross-sectional study design and lack of statistically significant results after full adjustment.
There remains a possibility of reverse causation, meaning that cognitive impairment itself may influence participants' ability to remember and report about pesticide exposure rather than exposure causing cognitive impairment. Therefore, future research with a longitudinal study design and biomarker-based exposure assessments is needed to understand the exposure-outcome relationship more conclusively.
Insecticide exposure is known to increase the level of neurotransmitter acetylcholine in the brain by inhibiting acetylcholinesterase activity, leading to disrupted synaptic transmission and cognitive impairment. These substances also induce oxidative stress and inflammation in the brain, leading to nerve cell death and cognitive impairment.
These observations regarding the neuro-disruptive effects of insecticides support the current study findings and highlight the need for future research to establish causality and inform preventive strategies.
Some potential confounders, such as occupational exposure, rurality, genetic predisposition, and environmental factors, could not be considered in the study analysis because information was unavailable in the survey database. These factors can independently influence cognitive functions, so not considering them in the analysis may lead to biased estimates of the observed associations.
The study relied on self-reported data on pesticide use in domestic, horticultural, and garden contexts, which may introduce recall bias and not accurately represent exposure levels.
The study excluded participants aged below 60 years. However, early-onset dementia can develop in individuals over the age of 40. Including a broader age range in future studies would be effective in exploring the impact of pesticide exposure on the onset of dementia, which would provide valuable insights for developing better prevention and intervention strategies.
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