Amino Acid Profile of Mother's Milk

Breast milk composition changes over time to meet your baby's needs.

Breast milk has a unique composition that makes it the perfect food for human babies. The amino acids -- the building blocks of protein -- and ratio of proteins in breast milk differ from those found in cow's milk. Because of this difference, cow's milk and milk-based formulas are harder to digest than breast milk. The amino acid content in breast milk will change as your baby grows, to suit his needs at the time.

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Amino Acids in Breast Milk

The body can make some amino acids, which means they're not an essential part of a diet. Nine amino acids are considered essential, meaning they can only be obtained from dietary sources. All nine -- histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine are found in breast milk. Taurine and cysteine, which are necessary for infants but not for older children and adults, are also present. Breast milk also contains nonessential amino acids alanine, arginine, aspartic acid, glutamic acid, glutamine, glycine, hydroxyproline, proline, serine and tyrosine.

Predominant Amino Acids in Breast Milk

Amino acids in food can be bound to proteins or free. Free amino acids don't need to be digested; they're immediately available for use. Glutamic acid, glutamine and taurine make up approximately 50 percent of the free amino acids found in breast milk, a study published in the August 2000 issue of "Journal of American Nutrition" found. In a 2009 review published in "Acta Universitalis Sapientaie," a Hungarian medical journal, researchers reported that glutamic acid constituted 20 percent of the overall amino acid content in breast milk, while the amino acids found in the smallest amounts were histidine, cysteine and methionine.

Variations in Amino Acids in Breast Milk

Breast milk composition can vary from one woman to another or vary in the same woman over time. Protein content is highest in breast milk right after birth and decreases over time. A mother's nutrition can affect the amino acid content of her breast milk. In a study published in the 2013 "European Journal of Experimental Biology," researchers found that Nigerian mothers who ate a diet low in the amino acids lysine and cysteine, found mostly in animal proteins, produced breast milk that was also low in these amino acids.

Comparing Breast Milk to Cow's Milk and Formula

Cow's milk contains approximately three times more protein than breast milk. The composition of amino acids also varies. Human milk is the only mammalian milk with a methionine/cysteine ratio less than 1.0, a characteristic common in vegetable proteins, according to "Mother and Child Nutrition in the Tropics and Subtropics." Cow's milk and formula contain little or no taurine, the prominent free amino acid in breast milk, the textbook also reports. Formula contains less glutamic acid and glutamine than breast milk, while cow's milk contain less cysteine.