MASON VITAMIN D3 400IU X100 CHEWABLES is a critical nutrient for regulating phosphorus and calcium levels in the body. This is why Vitamin D3 may be important for healthy bones, teeth and heart health. Vitamin D3 also plays a critical role in supporting a healthy immune system as well. Without it, important immune cells such as T-lymphocyte (T-cells) would not develop fully or function properly. All of these things make Vitamin D3 especially important for older aging adults to maintain strong bones and teeth, as well as, a healthy heart and immune system.