Why do we need more vitamin D in fall and winter?
While autumn brings changing leaves, pumpkin spice everything, and the lead in to winter, it also brings the inevitable decline of natural vitamin D. For those of us who live in northern climates, the sun lays lower in the sky than the summer months limiting our time to absorb natural vitamin D. Also, our diets tend to change from fresh to frozen (another contributing factor to our relatively low access to vitamin D). Since we aren’t receiving all of crucial vitamin we need, it is important to enhance this loss with supplements such Ddrops, as well as making some time for winter sun despite the cold.
Less sun time and more indoor time
As the weather begins to get colder, we tend to retreat indoors. Children and their teachers return to school with a focus on studies rather than playing outside. This shift to spending time indoors means that we are not consistently making vitamin D in our skin as a result of direct exposure to sunlight. Because we absorb the most vitamin D from the sun, in the winter, we need to look for other ways to supplement.
Health authorities estimate that 10 to 20 per cent of reoccurring depression follow a seasonal pattern.  There are many reasons that people become depressed. The underlying causes of depression are complex and there is little conclusive evidence. Researchers are actively looking at links between sunlight, a lack of vitamin D, Seasonal Affective Disorder (SAD), and mood swings.  In a 2015 study from the Pacific Northwest, low vitamin D levels were associated with symptoms of depression. Still, health authorities are not ready to recommend vitamin D as a treatment or preventative measure for mood disorders. The evidence is still new and inconclusive. This is a promising area of study that we will continue to watch.
Victorian doctors routinely prescribed rest in a sanatorium soaking up the sun as a cure for tuberculosis, a treatment which fell out of favour with the discovery of antibiotics. A 2011 paper by Dr. Sylvia Arrow showed that vitamin D arms and triggers T cells, the foot soldiers of our immune system, which destroy viruses, bacteria and other threats. In a six-year study that tracked 19,000 patients, those with lower vitamin D levels reported more upper respiratory infections. The European Food Safety Authority considers vitamin D a contributor to the normal function of the immune system.  In North America, health authorities have not yet adopted this recommendation.
Chronic bone disease
Having sufficient vitamin D has been shown to prevent chronic illnesses such osteoporosis.Maintaining a healthy level of vitamin D, not too low and not too high, helps the absorption and transportation of calcium and phosphorous throughout your body. 
 EFSA Journal 2015;13(5):4096