Vitamin D3 supplements are often recommended for people at risk of vitamin D deficiency. Low vitamin D levels cause depression, fatigue and muscle weakness. Over time, a lack of vitamin D 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.