Reduced cancer incidence among the blind.
Level 3 - non-randomized controlled study
Retrospective cohort study comparing incidence rates in visually impaired cohorts to national registry data.
What was done
A Swedish cohort of 1,567 totally blind and 13,292 severely visually impaired individuals was linked to the Swedish Cancer Registry to evaluate cancer incidence. Standardized incidence ratios (SIRs) and 95% confidence intervals were calculated using person-years and national rates matched for age, sex, and calendar year, testing the hypothesis that higher unsuppressed melatonin levels in blindness protect against cancer.
What was found
Totally blind individuals showed a significantly reduced incidence of all cancers combined (SIR = 0.69; 95% CI, 0.59–0.82). This reduction was present in both men and women and was similar for hormone-dependent tumors and other cancer types. In contrast, severely visually impaired individuals had an SIR of 0.95 (95% CI, 0.91–1.00).
Why it matters
These findings provide early population-level epidemiological support for the hypothesis that unsuppressed melatonin production or circadian disruption plays a role in oncogenesis, particularly highlighting differences by degree of visual impairment.
Limits
Melatonin levels and light exposure were not directly measured; they were inferred based on visual status. Potential confounding factors such as differences in lifestyle, diet, occupational exposures, or screening behaviors between blind and sighted populations were not accounted for in the abstract.
Cited by
- context Blind individuals produce more melatonin and have half the cancer incidence compared to sighted individuals.