Clinical studies that assess parameters such as growth, as well as safety and efficacy of an infant formula, may provide the most informative results when the formula is fed as the sole source of nutrition from birth. Th is is the period with the most rapid growth, the highest nutrient requirements per kg of body weight, and the highest sensitivity to an unbalanced supply. However, in Europe, it is difficult to enroll infants fed formula from birth, since most infants are, fortunately, breastfed aft er birth.