This is especially important for premature infants who may not have the digestive systems necessary to break foods down into what they need for life. Davis and Reeds found that amino acids, the building blocks of protein, are the same in breast milk throughout the animal kingdom. However, the concentrations of these amino acids can vary almost tenfold among species. That means that while a human and a sea lion may have different concentrations of amino acids, the basic makeup of both human and sea lion milk will be similar. That commonality gives researchers more confidence in their animal-tested formulas.
Cystine is believed to play more of a role in body maintenance than in growth.
So Davis and Reeds reasoned that the high cystine levels reflected the fact that human and ape babies take longer to mature than their primate relatives. In fact, the closer any two animal species are on the evolutionary tree, the more likely their milks are to be similar. Davis and Reeds wanted to know if elephants, another slow-growing animal, also had high cystine levels. Researchers now speculate that high cystine levels might relate to brain and eye development. That's because cystine is used to make another compound called taurine, which is highly concentrated in both the brain and eyes.