Researchers Discover Inverse Association Between Sustained Outdoor Work and Breast Cancer Among Nordic Women Over 50-Years of Age

A newly published article, in the British Medical Journal’s Occupational & Environmental Medicine, has documented that women over the age of 50, with a long-term history of working outdoors, have a lower risk of developing breast cancer.[i]

The paper in question reported the results of a Scandinavian study, involving 38,375 women under the age of 70, all of whom were recorded in the Danish Cancer Registry as having been diagnosed with primary breast cancer.

Full employment history was retrieved from pension fund records and a job exposure matrix was created, in order to estimate each woman’s personal occupational sunlight exposure.

Across the board, there appeared to be no correlation between exposure and risk of breast cancer.

However, for women with the longest durations of exposure (>20-years), the chances of being diagnosed with late-onset breast cancer (≥50-years-old) were 17% lower (odds ratio of 0.83), when compared against the control group (women born in the same year and randomly selected from the Danish Civil Registration System).

Equally, for women with the highest levels of cumulative exposure (>75%), the probability of being diagnosed with late-onset breast cancer were 11% lower (odds ratio of 0.89) than the general population.

Given that ultraviolet-B (UVB) radiation from the sun is the body’s ‘most important’ means of synthesising vitamin D, the researchers speculate that prolonged outdoor working careers were associated with a lower risk of breast cancer, because vitamin D production ‘mainly occurs’ between the hours of 10:00 and 15:00.

Even though this study was designed to be observational, rather than to establish causality, their theory (if correct) complements research printed by the European Society for Clinical Nutrition and Metabolism (ESPEN) in 2019, which indicated that elevated concentrations of vitamin D in the blood were linked with fewer cases of malignant disease.[ii] However, this is by no means a conclusive answer.

If vitamin D does have a protective effect against cancer, in the same way that it has a well-recognised role in maintaining bone and musculoskeletal health,[iii] then it could be that rising incidence of breast cancer since the 2nd half of the 20th century can be attributed, at least in part, to vitamin D deficiency, as populations in the developed world have reduced their amount of time spent outdoors and increased their use of computers indoors (for both work and leisure purposes).

Data that potentially supports this assertion can be found in Public Health England’s latest National Diet and Nutrition Survey (from 2008 to 2012), which exposed that 23% of adults aged 19 to 64; 21% of adults aged 65 and over; and 22% of children aged 11 to 18 had low vitamin D levels, where ‘low’ was defined as, ‘at greater risk of developing a deficiency’.[iv]

The study authors have called for further research on the inverse association between long-term occupational sunlight exposure and late-onset breast cancer that they detected, acknowledging that the present study was not without its drawbacks.

For sunlight to be treated as an accurate ‘surrogate marker’ of vitamin D levels, one would expect the sunlight exposure estimates used to have been moderately accurate. They were, however, by their own admission, ‘relatively crude’, and even leisure-related sunlight exposure was not accounted for.

Other factors that could possibly have delegitimised sunlight exposure estimates (and thus vitamin D levels) in the Danish cohort, but were not considered, included:

  • Dietary/supplement-based vitamin D intake;
  • Use of the contraceptive Pill;
  • Treatment with hormone replacement therapy;
  • Alcohol consumption;
  • Obesity; and
  • Leisure time physical activity.

That being said, the fact that a recent systematic review by Sowah et al (2017) exhibited that indoor workers have significantly lower levels of serum vitamin D versus their outdoor counterparts was seen as an overall validation of UVB exposure as proxy for vitamin D levels.[v]

In short, there is growing evidence that the relationship between outdoor work and breast cancer is genuine and this could be ‘conceivably explained’ by inadequate vitamin D production in the body.

 

[i] Pedersen JE et al., Occupational exposure to solar ultraviolet B radiation and risk of subtypes of breast cancer in Danish women. Occupational and Environmental Medicine. 2021;0:1–7. <https://oem.bmj.com/content/oemed/early/2021/01/14/oemed-2020-107125.full.pdf> accessed 5 February 2021.

‘Working outdoors linked to lower risk of breast cancer among older women’ (1 February 2021 BMJ) <https://www.bmj.com/company/newsroom/working-outdoors-linked-to-lower-risk-of-breast-cancer-among-older-women/> accessed 5 February 2021.

Joe Pinkstone, ‘Women who work outdoors in the sunshine are 17% LESS likely to get breast cancer 'due to high vitamin D levels', study claims’ (1 February 2021 The Daily Mail) <https://www.dailymail.co.uk/sciencetech/article-9210815/Women-work-outdoors-sunshine-17-likely-breast-cancer.html> accessed 5 February 2021.

[ii] Hossain S et al., Vitamin D and breast cancer: a systematic review and meta-analysis of observational studies. Clin Nutr ESPEN 2019;30:170–84. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6570818/pdf/nihms-1031158.pdf> accessed 5 February 2021.

[iii] Holick MF, Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease. Am J Clin Nutr. 2004;80:1678S–88 <https://academic.oup.com/ajcn/article-pdf/80/6/1678S/23714706/1678s.pdf> accessed 5 February 2021. 

[iv] Public Health England, ‘PHE publishes new advice on vitamin D’ (21 July 2016 GOV.UK) <https://www.gov.uk/government/news/phe-publishes-new-advice-on-vitamin-d> accessed 5 February 2021.

[v] Sowah D et al., Vitamin D levels and deficiency with different occupations: a systematic review. BMC Public Health 2017;17:519. <https://bmcpublichealth.biomedcentral.com/track/pdf/10.1186/s12889-017-4436-z.pdf> accessed 5 February 2021.