At this time of year when the suns rays are not strong enough shining through our atmosphere to produce vitamin D3 by exposure to our skin. And who wants to expose their skin in this cold? It is important to supplement with cholecalciferol/Vit-D3. Personally as an adult I like to take at least 2,000 IU a day for a few reasons. It is actually a hormone that helps mood swings like "seasonal affective disorder" due to a lack of this vitamin. It also is critical for calcium absorption and building bone. For those men in mid life it tends to lower the PSA (prostate specific antigen) at certain dosages. These are just some of benefits known from this important nutrient.