Vitamin D helps your body absorb calcium and keep bones, muscles and the immune system healthy. Mild deficiency frequently has no symptoms. When symptoms do appear they can be vague: fatigue, low mood, aching bones, muscle weakness or cramps, and in children softening of the bones (rickets). Long-term deficiency raises the risk of osteoporosis and fractures.
People at higher risk include those with little sun exposure, darker skin, older adults, people who are housebound or cover most of their skin, and those with conditions that reduce absorption. In many regions, supplementing during darker autumn and winter months is commonly suggested.
The only reliable way to confirm deficiency is a blood test. Do not self-prescribe high doses, because too much vitamin D can raise blood calcium and cause harm. Ask your doctor whether testing and supplementation make sense for you, and what dose fits your situation.