Vitamin D helps build strong bones by aiding in calcium absorption and blocking parathyroid hormones, according to Cleveland Clinic. Parathyroid hormones make bones brittle by reabsorbing bone tissue. Vitamin D helps muscle, nerve and immune system functioning, states the National Institutes of Health.
A lack of vitamin D can cause soft, brittle or thin bones, notes the National Institutes of Health. A combination of vitamin D and calcium can reduce the risk of osteoporosis in older adults.
Low levels of vitamin D may lead to a variety of health problems, including heart disease, weight gain, depression and certain cancers, according to WebMD. However, studies do not prove that low vitamin D causes the conditions or that additional vitamin D could prevent them.