In the journal, Frontiers in Human Neuroscience, academics at Colorado State University have not long published the results of an investigation of 99 adults (70 women and 29 men), between the ages of 60 and 79, to see what impact demanding work has on brain health.[i]
To enable this exploration, the research team characterised the study participants’ most recent employment (full-time or part-time and lasting 2-years or longer) by measures of cognitive complexity (task variety, job complexity, information processing, problem solving, skill variety, and specialization), psychological stress (workload and interpersonal conflict) and physical stress (physical demands and work conditions).
Interestingly, what they found was that physical stress had the strongest relationship with negative neurological consequences. Specifically, magnetic resonance imaging (MRI) and tests of cognitive function showed physical stress to be associated with smaller hippocampal volume and poorer memory performance. A weak association was also detected between psychological stress and poorer memory performance.
Smaller Hippocampal Volume?
The hippocampus is a sea horse-shaped structure, located in the inner region of the temporal lobe of the brain, which is thought to be principally responsible for the storage and recall of long-term episodic memories, but also plays a role in the regulation of emotional responses, spatial processing and navigation.[ii]
Both healthy ageing and dementia can prompt the hippocampus to reduce in volume.
To-date, ‘the evidence for the effects of occupational stimulation and stress on the hippocampus is scarce’.
However, results from the present study endorsed Nyberg et al’s (2017)[iii] brain maintenance model of cognitive ageing, which states that the presence of a stressor can result in the depletion of brain health and memory, while the absence of that stressor can be protective.
Looking more deeply at the individual components of physical stress as ‘the stressor’, correlations indicated that it was physical demands (e.g. the need for muscular strength, endurance, and physical effort), as opposed to work conditions (e.g. environmental hazards), that were mostly driving the observed negative association with hippocampal volume.
What is more, since the majority of the test subjects did not, objectively speaking, have physically demanding occupations, it was significant that the workers themselves considered their own occupations (subjectively speaking) to be physically demanding, e.g. excessive reaching or lifting of boxes onto shelves.
Perhaps the most intriguing and unforeseen consequence of the team’s findings was that occupational physical demands had the inverse effect of leisure-based physical demands.
Indeed, aerobic fitness, aerobic exercise and resistance training have all been shown, in extant literature, to increase hippocampal volume and improve cognitive performance in older adults. In this instance, scores from a 10-item Physical Activity Scale for the Elderly (PASE) conveyed a positive association between heightened activity levels and size of the hippocampus.
It was therefore acknowledged that more research is needed to appreciate the connection between occupational physical stress and the structure and function of the hippocampus, a site where occupational stimulation and stress may coalesce [it was originally hypothesised that those who experienced more cognitive complexity at work would have larger hippocampi and better cognition, per Suo et al (2012),[iv] but this theory was not confirmed by the Colorado investigation].
Worse Memory Retention?
Over the past decade, multiple published works have demonstrated that those who report high levels of physical work strain suffer drop offs of memory function and general cognition – see Gow et al (2014),[v] Sindi et al (2017)[vi] and Dong et al (2018).[vii]
Similarly, older epidemiological papers have professed that physically demanding occupations (e.g. farmers, service employees and blue-collar workers) carry a higher risk of memory or cognitive deterioration than those in less physical occupations (e.g. professional and managerial roles) – see Dartigues et al (1992a),[viii] Dartigues et al (1992b)[ix] and Frisoni et al (1993).[x]
The fact that participants in this latest study, who endured greater levels of occupational physical stress, performed less successfully in episodic memory testing, converges with the collection of extensive of research that preceded it and does not appear to be coincidental.
Nonetheless, more research has been requested to better understand the weak positive correlation drawn between occupational psychological stress and memory performance. This is warranted by the fact that cognitive regression is still yet to be definitively affiliated with stress of this kind by neuroscientists.
If validated, Agbenyikey et al (2015)[xi] signals that poorer memory performance and intensified memory decline can be expected around 15 to 21-years after experiencing psychological stress.
Despite having conceded that this study is ‘just one piece in the puzzle’ in a field of ‘fragmented’ research, co-author and Assistant Professor in the University’s Department of Human Development and Family Studies, Aga Burzynska, heralded it as the ‘the first evidence that occupational stress can accelerate brain and cognitive aging’.
It is of course possible that neither psychological stress, nor physical stress, were responsible for the patterns documented. Other potential causes, which were omitted from the research, include quality of sleep, metabolic disorders (e.g. obesity and diabetes), chronic pain and inflammation, depression and earlier employment. Recall bias may also have played a part in skewing findings.
[i] Burzynska AZ et al., Occupational Physical Stress Is Negatively Associated With Hippocampal Volume and Memory in Older Adults. Front. Hum. Neurosci., 15 July 2020 <https://www.frontiersin.org/articles/10.3389/fnhum.2020.00266/full#B57> accessed 1 October 2020.
Jeff Dodge, ‘CSU study links physical stress on the job with brain and memory decline in older age’ (16 July 2020 Colorado State University) <https://chhs.source.colostate.edu/csu-study-links-physical-stress-on-the-job-with-brain-and-memory-decline-in-older-age/> accessed 25 November 2020.
[iii] Nyberg L, Neuroimaging in aging: brain maintenance. F1000Research 6:1215. (2017) <https://f1000research.com/articles/6-1215/v1/pdf?article_uuid=28bff923-5f4b-45d3-9148-f78c80839fa4> accessed 24 November 2020.
[iv] Suo C et al., Supervisory experience at work is linked to low rate of hippocampal atrophy in late life. Neuroimage, 12 Aug 2012, 63(3):1542-1551. <https://europepmc.org/article/med/22902920> accessed 25 November 2020.
[v] Gow AJ et al., Occupational Characteristics and Cognitive Aging in the Glostrup 1914 Cohort. The Journals of Gerontology: Series B, Volume 69, Issue 2, March 2014, Pages 228–236. <https://academic.oup.com/psychsocgerontology/article/69/2/228/543042> accessed 25 November 2020.
[vi] Sindi S et al., Midlife work-related stress is associated with late-life cognition. J Neurol. 2017; 264(9): 1996–2002. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5587634/pdf/415_2017_Article_8571.pdf> accessed 25 November 2020.
[vii] Dong L et al., Job Strain and Cognitive Change: the Baltimore Epidemiologic Catchment Area Follow-up Study. Occup Environ Med. 2018 Dec; 75(12): 856–862. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6476297/pdf/nihms-1525934.pdf> accessed 25 November 2020.
[viii] Dartigues JF et al., Principal Lifetime Occupation and Cognitive Impairment in a French Elderly Cohort (Paquid). American Journal of Epidemiology, Volume 135, Issue 9, 1 May 1992, Pages 981–988. <https://academic.oup.com/aje/article-abstract/135/9/981/137685?redirectedFrom=fulltext> accessed 25 November 2020.
[ix] Dartigues JF et al., Occupation during life and memory performance in nondemented French elderly community residents. Neurology. 1992 Sep;42(9):1697-701. <https://pubmed.ncbi.nlm.nih.gov/1513456/> accessed 25 November 2020.
[x] Frisoni GB et al., Principal Lifetime Occupation and MMSE Score in Elderly Persons. Journal of Gerontology, Volume 48, Issue 6, November 1993, Pages S310–S314. <https://academic.oup.com/geronj/article-abstract/48/6/S310/549966?redirectedFrom=fulltext> accessed 25 November 2020.
[xi] Agbenyikey W et al., Job Strain and Cognitive Decline: A Prospective Study of the Framingham Offspring Cohort. Int J Occup Environ Med. 2015 Apr; 6(2): 79–94. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5282587/pdf/ijoem-6-79.pdf> accessed 25 November 2020.