Avoid taking an iron supplement at the same time you take a calcium supplement. Iron and calcium will compete for absorption in the small intestine, and can therefore interfere with your body's ability to absorb either one. However, there are exceptions to this rule. Taking vitamin C or when using calcium citrate decreases this interaction and can enhance absorption of calcium and iron. Although vitamin D is needed for calcium absorption, it does not necessarily need to be present in your calcium supplement.
Sunlight can provide more than enough vitamin D to aid in the absorption of calcium.
People who are housebound or who live in far northern climates with little sun exposure especially need good sources of vitamin D from food. Test your supplement Some brands of calcium supplements do not quickly nor easily dissolve in the stomach. You may not absorb the calcium you are taking if this is the case. Try putting one of your calcium pills in a glass of vinegar at room temperature. Wait about 45 minutes, stirring occasionally.