A Naturopathic Framework for Personalised Therapeutic Sunbathing: A Critical Review of Photobiological and Epidemiological Evidences

Abstract

Deliberate exposure of the skin to solar radiation for therapeutic purposes has been practised for more than a century and remains embedded in naturopathic care as an expression of the doctrine that healing is assisted by ordered contact with natural agents. The practice now sits between two robust and partially opposed bodies of evidence. Solar ultraviolet radiation is an established human carcinogen with a demonstrable dose-response relationship to keratinocyte cancers, while observational cohorts, mechanistic human studies and recent expert appraisals indicate that habitually low sun exposure carries measurable health penalties that are not adequately explained by circulating 25-hydroxyvitamin D. Naturopathic sunbathing protocols customarily prescribe duration, time of day and progressive acclimatisation, yet they seldom quantify radiant exposure, and population guidance seldom descends to the level of the individual. This review evaluates whether current photobiological, clinical and epidemiological evidence can support a defensible framework for personalised therapeutic sunbathing, and where such a framework would exceed the available evidence. Literature was identified through openly accessible bibliographic indexes and institutional repositories, appraised for design adequacy and consistency, and synthesised thematically rather than catalogued. Four findings structure the argument. Mechanistic pathways beyond cutaneous vitamin D synthesis, particularly ultraviolet-A-dependent mobilisation of cutaneous nitric oxide stores and photic entrainment of circadian physiology, are biologically credible and partly demonstrated in humans, but               their translation into durable clinical benefit rests largely on short-term physiological endpoints and observational associations. Failure of vitamin D supplementation trials to reproduce the benefits associated with sun exposure undermines the vitamin-centred rationale that most naturopathic protocols implicitly assume. Interindividual variation in erythemal sensitivity, cutaneous synthetic capacity, photosensitising comorbidity and medication use is large enough that any uniform prescription is indefensible, and self-assigned skin phototype is an unreliable basis for individualisation. Instruments capable of supporting genuine personalisation, including wearable dosimetry and validated exposure models, exist but have not been tested as components of a therapeutic protocol. A cautious, explicitly provisional framework is therefore justified for defined low-risk phenotypes, whereas therapeutic claims for specific disease outcomes remain unproven.