I recently had the pleasure of seeing a new mother with her newborn baby. What made the visit even more special is that I used to be the mother’s pediatrician! And, the mom’s own mother and father also made the trip to the office — three generations before me, and two generations of patients! That really made my day! The baby’s grandmother asked me a very good question about the formula her granddaughter was taking: Are there any nutritional differences between “ready-to-feed” formulas and powdered formulas? I said I didn’t think so but would research it.
I found one article — by Lucas, et al published in 1992 in the Archives of Disease in Children — that showed that babies who took ready-to-feed formulas grew similarly to babies who were breastfed (the standard) in terms of fat deposition. Powdered formula drinkers were more likely to become overweight than the ready-to-feed group:
Despite similar nutrient composition of the two formulas those fed the powdered formula had significantly increased body weight and skinfold thickness gains, and became significantly heavier than a further group of 20 breast fed infants by 3 and 6 months
The authors felt that the cause was due to incorrect reconstitution of the powdered formula with water: If less water than directed is used to mix the powder, then the resulting formula will be more calorically dense per ounce, and the baby will gain more (undesirable) body fat and more weight as a result.
A review article by Renfrew, et al in the same journal in 2003 concluded that:
Only five studies were identified, none of adequate quality or size. All found errors in reconstitution, with a tendency to over-concentrate feeds; under-concentration also occurred.
In other words, they found a tendency to use less water when mixing the powder, leading to babies receiving more calories per ounce than they should. The authors of both articles called for more studies, but I cannot find a more recent examination of the subject.
So, the bottom line is that, nutritionally, there should be no difference between ready-to-feed formulas and powdered formulas as long as they are properly mixed with the correct amount of water.
The International Formula Council (IFC) has an informative website on infant feeding and nutrition. Bear in mind that the members who make up the IFC are formula companies. Still, I think their FAQ page here is credible and accurate.