Vitamin D3 is a popular supplement because it plays an important role in many different biological processes linked to overall health. First, it acts as a shuttling nutrient; meaning that it helps to shuttle other nutrients like calcium and phosphorus through intestinal walls and into the bloodstream. Without vitamin D's help, neither mineral could find their way to help build new bones or repair existing ones, which is why it may be critical for healthy bone density and bone strength. Vitamin D3 isn't just an essential nutrient for healthy bones. It is also popular for helping to support healthy immunity as well. Without it, important immune cells such as T-lymphocyte (T-cells) might not develop fully or function properly. While our bodies do produce Vitamin D, many people choose to supplement with this important nutrient because it plays such a vital role in supporting optimal health and wellness.