What does sunlight do for your bones, apart from producing vitamin D?

The studies in the available source contain no evidence that sunlight does anything for your bones beyond triggering the production of vitamin D. Every effect on bone health that is described runs through that single mechanism: UV radiation converts a compound in your skin into vitamin D, and it is that vitamin D which then has consequences for your bones.

Through vitamin D, the role of sunlight is reasonably well supported. Too little sunlight is a recognised risk factor for vitamin D deficiency. That deficiency can lead to osteomalacia, a painful bone disease in which bone does not mineralise properly, and to osteoporosis and muscle weakness. Weaker muscles in turn increase the risk of falling, which in older adults is directly associated with a higher rate of bone fractures.

How quickly and how much vitamin D you produce through sunlight is difficult to state precisely. Eight small randomised trials did confirm that UVB radiation raises blood levels of vitamin D, but the studies differed considerably in design. A clear dose-response relationship, meaning how many minutes of sun leads to how much additional vitamin D, therefore remains unknown.

During pregnancy and breastfeeding, sunlight also plays a role in maintaining adequate vitamin D levels. In women with little sun exposure, this may contribute to bone mineralisation problems, although the body often appears to adapt when nutrition is reasonable. The evidence for this is limited and based on associations, not on causal studies.

There is an unresolved safety question attached to the sun-for-vitamin-D approach: the studies were unable to determine how much sunlight is sufficient for a good vitamin D level without simultaneously increasing the risk of skin cancer. That trade-off is therefore not resolved in the available literature.