Last week I told you about how the body uses vitamin C and foods that contain vitamin C such as citrus fruit and tomatoes. Now I want to discuss vitamin D which is important to human nutrition because it helps the body to absorb calcium. Calcium, as you know, is essential for maintaining healthy bones and teeth. Vitamin D deficiency can lead to serious bone diseases. So in the United States it is added to milk, a staple food in most people's diet. Vitamin D is sometimes called the sunshine vitamin. That's because the body requires sunlight in order to make the use of it. Here's how it works. When ultraviolet rays from the sun strike the vitamin D in our skin, the sunlight converts vitamin D to its active form and a substance called calciferol is produced. Calciferol is what carries the calcium to our bones. If our bodies are not exposed to ultraviolet rays from the sun we cannot convert vitamin D to its active form, and thus cannot make use of any of the calcium that we have consumed in our food. Eventually, we would suffer from a calcium deficiency and have weak bones. Luckily, it takes only 10 to 20 minutes of exposure to sunlight a day to ensure sufficient vitamin D production. Except in extremely atmospheric conditions like very heavy smog or a sunless climate, most people are able to get enough exposure to the sun. Doctors recommend that people who don't drink very much milk take vitamin supplements and that everyone spend at least 10 minutes in the sun everyday.