It is essential to maintain healthy vitamin D levels throughout all stages of life, from fetal development to old age. Vitamin D helps protect a number of important functions in the body. The current recommended daily vitamin D intake of 400 IUs is based on the minimum allowance to protect against bone disease. However, an upward revision of the FDA's guideline is now under consideration in light of new research, especially pertaining to immune function. Cutting-edge research suggests serum vitamin D levels should be in the range of 32-80 ng/mL 25-hydroxyvitamin D (25(OH)D). Although direct sunlight exposure is a good source of vitamin D, many people do not get enough sun due to season change, latitude, or lifestyle. Therefore, vitamin D supplementation can be of great benefit in these cases.