Although the fact that infant breastmilk substitutes (formula) are known to be vastly inferior to breastmilk nutritionally, much of the focus of its deficiencies has been on the lack of proper fatty acids, most notably docosahexaenoic acid (DHA).

However, this is by no means the only deficiency of formula, especially considering the fact that at least 100 additional substances have been identified in breast milk. A new study has shown that it is also deficient in important substances known as 'free amino acids' (FAA).

Whereas most amino acids exist bound together in various proteins, "Free Amino Acids" are unbound. Some amino acids only exist "free" and are never part of proteins. One example of this is the amino acid taurine.

The FAAs in human milk were compared to those found in 12 different infant formulas (seven powdered and four liquid whey-predominant formulas and one soy formula).

• The human milk was collected at the end of each feeding, in order to obtain the "hindmilk".

• Samples were obtained over 24 hours, in a group of 40 healthy lactating mothers of one-month old infants.
  

• In human milk glutamic acid, glutamine and taurine were the most prevalent FAA, accounting for around 50% of the total amount of FAA.
  

• In the formulas the total FAA fraction was 10% or even less than in human milk, and was mostly represented by taurine.
  

• In the soy formula, methionine was found to be highest. This was due to methionine supplementation of the soy formula, since soy protein is deficient in the amino acid.
  

• The total quantity of glutamic acid and glutamine in all the formulas was much lower than in human milk.

The study authors note that breastfed infants are supplied with significantly different quantities of FAA and also the breakdown of the proportions of individual free amino acids is much different as well.

In comments to the Optimal Wellness Center, study author Carlo Agostoni, Senior Attending in Pediatric Nutrition, Dept. of Pediatrics, San Paolo Hospital, Milan, Italy, notes that there is a general consensus that taurine is a "conditionally essential" nutrient for infants. While adults can synthesize taurine from the amino acid methionine, infants can not.

Dr. Agostoni also confirmed that even when infants are fed taurine-fortified formula, their blood taurine levels are well below the levels seen in breast-fed infants. Therefore even fortifying infant formulas with the same nutrients found in breast milk may not be enough to insure that adequate levels are absorbed.

In addition, it is impossible to replicate the FAA content of breast milk, since it is constantly changing. Dr. Agostoni notes a recent study where he and his colleagues found that the levels of free glutamine and glutamic acid in breast milk increase progressively during the first 3 months.

"The different FAA intake might be the origin of some functional differences," researchers suggest "between breast- and formula-fed infants."

Journal of the American College of Nutrition August, 2000; 19 :434-8