What does sunlight do for your bones, apart from vitamin D?
Sunlight helps your bones through vitamin D, but a separate bone benefit outside that pathway has not been demonstrated and has not been studied; for your bones, what matters most here is how much vitamin D you produce.
Sunlight does something for your bones, but this works almost exclusively through vitamin D. UVB radiation converts a substance in your skin into the building block of vitamin D. That vitamin D is then essential for the absorption of calcium and phosphate, the minerals from which bone is literally built. About 90% of the vitamin D your body uses is produced by the skin itself under the influence of sunlight.
A vitamin D-independent effect of sunlight on bones has not been described in the available studies, nor has it been investigated. There is one indication that UVA radiation, the wavelength range that penetrates deeper into the skin than UVB, releases nitric oxide and can thereby lower blood pressure. But that has no demonstrated connection to bone health; it is an explanation for epidemiological observations, not a direct effect on bone.
The downside is directly linked to the mechanism itself: in exactly the same UVB range that triggers vitamin D production, sunlight simultaneously causes DNA damage in the skin. There is no vitamin D production without that risk. Protection against skin cancer and exposure for vitamin D are therefore in direct tension with each other.
In short: if you want to know what sunlight does for your bones outside the vitamin D pathway, the honest answer is that the research for that is lacking. The bone benefits of sunlight are, as far as has been studied, simply the bone benefits of vitamin D.
This answer is based on several review articles on sunlight, vitamin D and health. None of these sources describe a vitamin D-independent effect of sunlight on bones; in all sources, the bone pathway runs through vitamin D synthesis. The evidence for the vitamin D pathway itself is strong and consistent. No research has been provided for a direct bone effect independent of vitamin D.
Does vitamin D prevent me from breaking bones?
Vitamin D alone does not demonstrably reduce the risk of bone fractures. Combined with calcium it does help, especially in older adults and people with a deficiency.
Is eating plant-based bad for your bones?
Eating strictly vegan is associated with lower bone density, but if you adequately supplement calcium and vitamin D, the elevated fracture risk largely disappears. As a vegan, also pay attention to vitamin B12, and discuss any supplement use with your GP or dietitian.
What happens to your bones when you lie in bed for a long time?
Prolonged bed rest demonstrably leads to substantial bone loss, particularly in the hip, spine and heel bone. Recovery takes much longer than the period of bed rest itself, so resuming movement as soon as possible is the best thing you can do.
Does boron as a supplement do anything for your bones?
Boron appears useful if you are not getting enough of it, but there is no good evidence that extra supplementation makes your bones stronger if you are already getting sufficient amounts. Make sure you are getting enough calcium and vitamin D first, and discuss boron supplementation with your doctor if you are dealing with osteoporosis.
Do tanning beds give you vitamin D?
A tanning bed can raise your vitamin D level, but only if the light also contains UV-B, and most commercial beds emit very little of it. Because tanning beds demonstrably increase the risk of skin cancer, a vitamin D supplement is the safer choice.
Does sunbathing boost your immune system beyond the vitamin D effect?
Sunlight has real effects beyond vitamin D through nitric oxide and hormonal pathways, but randomised evidence for improved immune defences is lacking. At the same time, the immunosuppressive effect of chronic exposure is well documented, so moderate sun exposure makes more sense than a great deal of it.