 One in four infants are born with low amounts of vitamin D stored in their body. This study is designed to test whether improving vitamin D status quickly after birth helps infants to build muscle and to normalize growth. This is important since the investigators have noticed in previous work that infants with low vitamin D have higher body weight relative to body length later on and that those who develop very good stores quickly have a leaner body type. Therefore, in this study infants with low stores early after birth will be given either the regular amount of supplementation or a higher amount to more rapidly build up the vitamin stores in the body. Infants in both groups will be measured for muscle and fat mass at standardized ages during the first year of life and into the toddler years. The information will inform health care professionals and parents of the importance of establishing good vitamin D stores early in life. Vitamin D supplementation is a modifiable factor that is already recommended for all term born infants. Knowing how much is needed in infants born with low stores has not been tested in a controlled manner in Canada. 