Vitamin D3 supplements are often recommended for people who are at risk of vitamin D deficiency. Low levels of vitamin D cause depression, fatigue and muscle weakness. Over time, vitamin D deficiency can lead to weak bones, rickets in children and osteoporosis in adults. Vitamin D, called the sunshine vitamin, is produced by the body when ultraviolet B rays come into contact with specific receptors in the skin. But using sunscreen prevents this process, leading to mild to severe vitamin D deficiency in about one in four people.