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The seminal discovery that sunlight was important in the prevention of nutritional rickets was made in 1890 by Theobald A. Palm, a medical missionary who contrasted the prevalence of rickets in northern European urban areas with similar areas in Japan and other tropical countries.

He surmised that exposure to sunlight prevented rickets. Over the next 40 years his observation led to an understanding of ultraviolet irradiation and its role in vitamin D synthesis. This opened a new era of appreciation for the curative powers of the sun and “the sunshine vitamin”. While Palm’s observations were in some ways
obscure, they had a potent effect on the development of photobiology.

Nutrients. 2012 Jan;4(1):42-51. doi: 10.3390/nu4010042.
Theobald palm and his remarkable observation: how the sunshine vitamin came to be recognized.
Chesney RW.