For more information on how to space your babies naturally, a good resource is Breastfeeding and Natural Child Spacing by Sheila Kippley. Most formulas are based on cow's milk, which is made for a calf. Because of its high protein content, cows milk must first be watered down. Then, sugar must be added to recoup the lost calories. Infant formula cannot duplicate all the subtle ways in which human milk supports infant growth and health.
Human milk is uniquely suitable for human infants because of the complex balance of ingredients.
Formula companies simply have not been able to duplicate human milk, therefore, they lack the different enzymes, amino acids and ingredients that we know are essential for the development of a human baby. Human milk is better digested and assimilated than formula is. That is why the formula fed infant feels fuller longer. It takes him a lot longer to digest all the components in the formula, which generally makes the baby more uncomfortable also. Breastfeeding helps give your baby the best start in life.