Nitric oxide (NO) is a well-known “gasotransmitter,” a gaseous signaling molecule that has a remarkable ability to induce a relaxation of the artery wall and a resulting drop in blood pressure. Endothelial dysfunction linked to cardiovascular disease is associated with impaired production of NO from arginine by eNOS, and it causes high blood pressure.22 Researchers have recently become aware that the skin is somehow able to release nitric oxide in response to sunlight exposure. Exactly where the NO comes from is somewhat of a mystery because it has become clear that it is not a result of direct synthesis by eNOS.23
A clue comes from the fact that glutathione reacts with nitric oxide to produce S-nitrosoglutathione (GSNO), which I believe serves as a temporary storage form of NO. Almost miraculously, visible light (green, blue and purple) can catalyze the release of NO from glutathione.24 Not only does this cause a relaxation of the blood vessels, but it also frees up glutathione to react with hydrogen sulfide gas to produce sulfate. As illustrated in Figure 3, glutathione reacts with reduced sulfur to form glutathione persulfide (GSSH), and this can catalyze the oxidation of the extra sulfur atom to sulfur dioxide in the presence of superoxide. eNOS binds to flavins that respond to visible light by releasing electrons that convert oxygen to superoxide. The sulfur dioxide produced by eNOS is then oxidized to sulfate by sulfite oxidase. What this means is that the visible light in sunlight is crucial both for the release of NO from the skin and the synthesis of sulfate in the skin—and both of these results are crucial aspects of the beneficial effects of sunlight exposure.
eNOS is a “moonlighting” enzyme that is able to switch between two synthesized products: nitric oxide and sulfur dioxide, depending on electromagnetic signaling that it receives from the circulating red blood cells, as described at length in a paper I published with colleagues in 2015,25
These results might prompt medical professionals to advise people in higher latitudes to take a vitamin D supplement. However, as we by now can guess, a large study on vitamin D supplements and hip fractures gave disappointing results.26 The study involved women over seventy years old who had at least one self-reported risk factor for hip fracture (low body weight, previous fracture, maternal history of hip fracture, smoker, or poor health in general).27
The intervention involved daily oral supplementation with 1,000 mg of calcium and 800 IU of vitamin D3.
Figure3: Pathways catalyzed by visible light that lead to synthesis of sulfate and release the nitric oxide from the skin, lowering blood pressure.