Sun exposure leads to an increase in bone mass and a decrease in falls. It is generally assumed that since sun exposure also leads to an increase in blood-vitamin D levels, those levels are responsible for those positive effects of sun exposure. However, in the July issue of Osteoporosis International, there are indications that sun exposure may have some bone-building and bone-protecting effects of its own. The researchers used microtopographical skin changes to quantify cumulative lifetime sun exposure. This method is known as the Beagley-Gibson (BG) method of measuring cumulative sun exposure. After this measurement, they compared lifetime sun exposure to bone-mineral density, risk of falls and risk of fractures in older adults, aged 53-83 years. Vitamin D levels were also assessed in all subjects.
In women, an increasing BG grade (increasing sun exposure) was quite protective; their risk for vertebral fractures was reduced by 66%, and all major fractures were reduced by 25%. For men, bone-mineral density was increased in younger males who showed the most sun exposure. These results were independent of current vitamin D levels. Risk of falls were not changed. We know that sun exposure is very closely associated with better bone strength and have assumed that most of that bone strength is due to higher vitamin D. In this investigation, vitamin D levels had no predictive value. However, there may have been be other factors at play.
What, besides vitamin D, could be responsible for the reduction in fractures? One possibility is this: Outdoor living is necessary for sun exposure, and people who enjoy the outdoors are generally more physically active. It is well-known that exercise builds bone mass and/or strength due to the stress on the bones. That probably happens with any reasonable level of vitamin D.
I opine, that a study conducted in Spain, is perhaps the transcendent research on hip fracture and sun exposure: it showed that women who were sun seekers had only about one-eleventh the risk of hip fracture compared to those who stayed indoors. So, it may be, that to protect our bones, lots of sun exposure, rather than just a little, may be best. Just be sure not to burn. Daily exercise is also imperative. And be sure to eat a healthful, anti-inflammatory diet, which guards against both osteoporosis and skin cancer. Happy sunning.
 Thompson MJW, Aitken DA, Otahal P, Cicolini J, Winzenberg TM, Jones G. The relationship between cumulative lifetime ultraviolet radiation exposure, bone mineral density, falls risk and fractures in older adults. Osteoporos Int. 2017 Jul;28(7):2061-2068.
 Larrosa M, Casado E, Gómez A, Moreno M, Berlanga E, Ramón J, Gratacós J. Vitamin D deficiency and related factors in patients with osteoporotic hip fracture. Med Clin (BARC) 2008;130:6-9.