Vitamin D3 is fat-soluble. This means that excess amounts of it are stored in the body tissues. Long-term high doses may be deposited in the soft tissues, irreversibly damage the kidneys and cardiovascular system. Like other fat-soluble vitamins, vitamin D3 can be toxic. Symptoms of too much vitamin D3 include nausea, weakness, constipation, irregular heartbeat, weight loss, seizures, and irritability.

Vitamin D3 is one of those vitamins for which a deficiency can cause severe effects. Children that do not get enough vitamin D3 in their diets are at increased risk of developing rickets, a disease that causes malformations of bones and teeth in children. Adults with low levels of vitamin D3 are more likely to develop osteomalacia (similar to rickets) and to suffer from osteoporosis, a bone-weakening disease.

Vitamin D3 also regulates the nervous system, aiding in the treatment of insomnia. A glass of warm milk before bed may indeed help you sleep soundly! Vitamin D3 deficiency has also been linked to the development of other illnesses, including type I diabetes, muscle and bone pain, and cancer.