Vitamin D (cholecalciferol) is needed for bone strength and good immune functionality.
The importance of Vitamin D was discussed in Episode 1. As the Grand Solar Minimum is intensifying, we will continue to see Seasonal Affective Disorder (SAD) happening in even traditionally less extreme locations, such as was the case in Moscow over the summer of 2017. It is good to be aware we may need to consciously supplement through vitamin-D rich (and/or enriched) foods, or even supplementation.
Vitamin D is essential for promoting calcium absorption in the gut and maintaining adequate serum calcium and phosphate concentrations to enable normal mineralization of bone and prevent hypocalcemic tetany. It is also needed for bone growth and bone remodeling by osteoblasts and osteoclasts. Without sufficient vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults. Together with calcium, vitamin D also helps protect the elderly from osteoporosis. Vitamin D has other roles in human health, including modulation of neuromuscular and immune function and reduction of inflammation.
You really can't beat this. Go plant some stuff. It'll do you good in more ways than one, among them, you'll generate Vitamin D from the sunlight.
In areas (or times, perhaps like solar minima) when access to sunlight is reduced, very real issues can manifest; these are generally called Seasonal Affective Disorder.
In geographies (f.e. Alaska), they use blackout curtains to keep rooms dark in the summer (for sleeping), and special lights to ensure they stimulate Vitamin D production in the dark winters.