Young shift workers more at risk of developing multiple sclerosis

Researchers at the Karolinska Institute in Stockholm studied two different groups of multiple sclerosis (MS) patients to investigate the effect of shift work on the incidence of MS. They found a strong relationship between shift work before the age of 20 and the risk of developing MS. The researchers argue that the effect on the immune system from the disruption of the body’s daily rhythms and sleep disturbance due to shift work could be behind the increased risk in young workers.

Introduction

Factors that underpin the development of multiple sclerosis (MS) have been extensively researched and today we know that the susceptibility to MS depends on both genetic and environmental factors. It has also been found that young immigrants who move from a low risk country to a high risk country take on the risk of the host country, suggesting that exposure to environmental factors at a young age elevates the risk of developing MS.

While the association between a number of environmental factors and the occurrence of MS had been investigated, the effect of shift working had not. This prompted a research group led by Anna Karin Hedström at the Karolinska Institute in Stockholm to undertake a project between 2005 and 2010 to study the possible effects of shift work on the development of MS.

Study methodology

The research was based on two study groups of MS patients tested against two control groups (Hedström et al, 2011). The first study involved Epidemiological Investigation of Multiple Sclerosis (EIMS) and included incident cases of MS. The second study concerned Genes and Environment in Multiple Sclerosis (GEMS) and included prevalent cases of MS. ‘Incident’ and ‘prevalent’ are common terms used by epidemiologists; ‘incidence’ is the number of cases ‘measured within a set number of people and in a time period’ while ‘prevalence’ gives ‘a figure for a factor at a single point in time’ (Shields and Twycross, 2003).

Participants in the two groups were asked to fill in a questionnaire containing questions on demography, socioeconomic factors, lifestyle and health. Information on working conditions, including whether the participant had ever done shift work, was also collected. Any respondent that had been exposed to shift work was asked to give further details regarding the age at which they had worked shifts and about the duration (years) and intensity (nights/month) of the shift work.

In the study, shift work was defined as work between 9 pm and 7 am. The study’s authors acknowledge that this assumption might affect its findings.

Key findings

Both studies demonstrated a significant relationship between working shifts before the age of 20 and the occurrence of MS. In addition, the study found no significant relationship between working shifts after the age of 20 years and the occurrence of MS. The results were adjusted for a range of factors that could affect them such as the duration and the intensity of the shift work among those who started to work shifts before the age of 20.

The study could provide a strong association between shift working before the age of 20 and the occurrence of MS, thus contributing to the understanding of the factors that trigger its development. The study’s authors argue that exposure at a young age to lifestyle and environmental factors such as obesity and glandular fever seems to increase the risk of developing MS, thus suggesting some kind of vulnerability at a young age.

The authors put forward two possible explanations as to why shift working enhances the risk of developing MS at a young age.

One is the disruption by shift work of circadian rhythms (biological processes in the body that take about 24 hours) such as the light–dark cycle. There is growing evidence that links the circadian and immune systems. For instance, a disturbance to the secretion of melatonin (a hormone that helps to regulate sleep) due to circadian disruption may lead to a weakening of the immune system, thus increasing its vulnerability to pro-inflammatory cytokines. Cytokines can be briefly described as messengers between cells. Some cytokines help the immune system fight inflammation, whereas others (that is, pro-inflammatory cytokines) support the development of inflammation.

The second possible explanation is the sleep disruption and deprivation suffered as a consequence of shift work. Sleep disturbance has been shown to affect the immune system negatively in several ways. Important in the case of MS, sleep disturbance and deprivation have been associated with an enhanced level of pro-inflammatory responses, which have been shown to be significant in the induction of other autoimmune diseases.

Implications

The results from the two groups studied show a strong association between working shifts before the age of 20 and the risk of developing MS. The obvious implication is that young people should be careful when considering working shifts, especially those who suspect they may have a greater risk of developing MS. However, one of the study’s authors concludes in an interview (in Swedish) for the magazine Dagens Medicin that the results are no reason to ban shift working at a young age since the risk of developing MS is still very low. Nevertheless, he says, the results indicate the importance of a fairly regular lifestyle when young.

References

Hedström, A.K., Åkerstedt, T., Hillert, J., Olsson, T. and Alfredsson, L. (2011), ‛Shift work at young age is associated with increased risk for multiple sclerosis’, Annals of Neurology, Vol. 70, No. 5, 2011, pp. 733–741.

Shields, L. and Twycross, A. (2003), The difference between incidence and prevalence (58Kb PDF), Paediatric Nursing, Vol. 15, No. 7, 2003, p. 50.

Mats Kullander and Jon Halling, Oxford Research

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