Summary: Vitamin D plays an essential role in regulating calcium and phosphate metabolism and has many other vital activities that are important for the overall health of older people. Due to the limited skin biosynthesis of this vitamin and sometimes worse absorption and metabolism, it is necessary to supplement it widely. Some patients require assessing 25 (OH) D levels and using therapeutic doses of vitamin D to achieve its optimal levels. The article aims to present the importance of vitamin D for the human body, the frequency and causes of its deficiency in old age, and the approach to preventing, diagnosing, and treating these conditions.