This is a common question I receive. Vitamin D is commonly used in combination with calcium supplements because the body requires vitamin D in order to use calcium.
To understand why this is important, we need to talk about how amazing the bones that make up the skeletal system are. When we think of bones, we often associate them with long dead animals, like dinosaur bones in a museum. So, we tend to think that they don't change once we become adults.
In truth, however, our bones are actually living organs, just as vital to our survival as the heart or liver. Our bodies are constantly taking up old bone and laying down new bone in it's place.
Every cell in the human body requires calcium to function. From the beating heart to our muscles and nerves, if there isn't enough calcium, they cease to function. If the calcium levels in our blood get to low, the body takes calcium out of the bones to keep those levels high enough. When there is plenty of calcium in the blood, that is one of the factors that allows the body to lay down new bone using calcium.
Unfortunately, the average American diet has too little calcium. Vitamin D is also required for the body to absorb calcium from the gut (ie, from pills). If the body has too little vitamin D (also known as vitamin D deficiency or osteomalacia) the body can't absorb adequate calcium and--as a result--cannot lay down new bone.
Even if you took an entire bottle of calcium pills every day--which could kill you--if you don't have enough vitamin D in your body, you won't be able to use the calcium.
So how much vitamin D is needed? We'll talk about that on my next post, because there is a lot of controversy about that.