The conclusion that vitamin D levels are an effect not a cause came from exploring the mechanisms. And yes, it may turn out that NO similarly is a marker. It's

The conclusion that vitamin D levels are an effect not a cause came from exploring the mechanisms. And yes, it may turn out that NO similarly is a marker. It's ironic that the author didn't consider that getting sunlight may similarly be the side effect of the true cause of the stated benefits (being active?).