Sunshine In a Pill: Can We Truly Bottle the Benefits of Light Exposure?
As a follow-up to our previous comment on the New England Journal of Medicine article and the New York Times editorial that questioned the benefits of vitamin D supplementation, here we dive deeper into some of the nuances and controversies of sunlight exposure and vitamin D, particularly the conundrum: can supplements truly replace light exposure?
A question we often get asked is which is better: sun exposure or vitamin D supplementation. Could it be that we have found a way to distil the benefits of getting outside in the sun into a daily pill? Significant amounts of our circulating vitamin D levels result from its synthesis in the skin but the total amount obtained via sunlight versus dietary intake will vary according to the individual’s diet, genetics, skin type, latitude and season.
It is well known that the prevalence of some major health conditions are associated with not only latitude and solar irradiance but also low or deficient levels of vitamin D. Multiple sclerosis prevalence, for example, has a strong latitudinal gradient and a correlation to vitamin D deficiency, maternal vitamin D deficiency whilst in utero and the month-of-birth effect. This might suggest vitamin D synthesis through sun exposure is the underlying mechanism at play in these diseases. Data is conflicting, however, as to the relevance of UV exposure being the determinant of this latitudinal variation or not. Similarly, we see a lower incidence of autism in countries closer to the equator and some suggestion that maternal prenatal vitamin D deficiency may be contributory.
Which is more effective in raising our blood levels of vitamin D though? There have been a number of trials comparing sunlight exposure to supplementation in terms of the ability to increase blood vitamin D3 levels in those who are deficient. A recent review summed up all available published trials to conclude that vitamin D supplementation was superior to sunlight at raising vitamin D levels in those who were deficient. The presumption, here however, is that the sum of the benefits of sun exposure can be obtained via supplementing with vitamin D to prevent deficiency.
Whilst these findings are encouraging for those who opt for supplementation or struggle to get sufficient sunlight exposure due to their working hours, latitude and other commitments, one must be careful not to fall into the assumption that correlation equals causation. That is to say that, just because a condition may be associated with low vitamin D levels, the low vitamin D levels may be a symptom of the cause rather than the cause itself. Thus, there may be something else we derive from sun exposure that protects us from these diseases that vitamin D alone does not correct. In multiple sclerosis, for example, vitamin D supplementation is yet to be supported by strong evidence. The science does not yet give us a definitive answer here.
For those who have the choice between supplements and sun exposure, at the other end of the spectrum, we often get asked about the risks of sun exposure to build up vitamin D levels and prevent deficiency-associated diseases. The risks of excessive sun exposure on rates of skin cancer and skin ageing are well known and can generate a fear of sunlight exposure, especially in the context of a family history of skin cancer. Use of sunscreen to prevent sun damage is widely recommended to reduce signs of skin ageing and while also protecting from various forms of cancer, sunscreens appear not to impact levels of vitamin D. Indeed, an international opinion on the current evidence found that sunscreens have little impact on vitamin D levels and, in fact, some studies found an association of increased vitamin D levels with sunscreen use, presumably due to greater intentional sunlight exposure while wearing sunscreen.
Getting sun exposure, even with sunscreen protection, may, however, have the additional advantage of providing non-vitamin D mediated benefits of light exposure. Separate to the vitamin D generating effects, UV exposure can benefit some skin conditions, such as psoriasis, and correlates with improved blood pressure, mood, testosterone and atherosclerosis, with the usual caveat of correlation versus causation. A combination of light exposure in the early morning and early evening is well known to also positively impact our body clocks and can be a useful tool to reset these when travelling or for shift work. For certain individuals, though, aside from the skin ageing and cancer risks, there may be downsides to significant sun exposure, such as reactivation of shingles, cataracts and macular degeneration. For this population, the risks of supplementing with vitamin D are low and whilst toxic levels do not arise from sun exposure, excessive supplementation can cause hypervitaminosis so caution not to overdose is required.
So what are the take-aways?
The individual risk-benefit assessment of sun exposure versus vitamin D supplementation will depend on the specific condition to be prevented or optimised by exposure or avoidance of sunlight, family history, genetics, skin type, season and latitude.
There is convincing evidence for the harms of excessive sunlight exposure but the evidence for harms of sun avoidance in vitamin D replete individuals is lacking.
There isn’t strong evidence that protected sun exposure mitigates the full benefits of sun exposure while reducing the risks of skin cancers and signs of ageing.
The duration of sun exposure to gain benefits are not excessive, 10-30 minutes per day, depending on latitude and season
It is possible to build up enough vitamin D through sun exposure in the summer months to maintain sufficient levels for the winter months.
For those opting to prevent vitamin D deficiency through supplementation, efficacy may be better than via sunlight.