Milk Is Milk . . . or Is It?
Former Texas Southern University student Pattie Ross at the human milk bank
maintained at the Children's Nutrition Research Center in Houston, Texas.
It's easy to milk an elephant--if you know how. Just ask veterinarian Eric
Miller, director of animal health and research at the St. Louis Zoo.
Four years ago, one of the zoo's pachyderms, named Pearl, had trouble
nursing her newborn calf, Raja. Miller and other members of the zoo staff
devised a way to feed Raja. They gave Pearl the lactation-stimulating drug
oxytocin. Then the animal staff restrained her and used a human breast pump to
get enough milk to bottle-feed Raja.
After Pearl was able to nurse, some milk was left over--milk that helped
scientists in Houston, Texas, who were studying human infant nutrition.
Nutritionists Teresa A. Davis and Peter J. Reeds have been studying the
milks of various mammal species to find how the nutrients they contain
contribute to development of their young. Understanding the ways milks differ
may lead the scientists to better formulas to nourish human babies who can't
breast-feed. This is especially important for premature infants who may not
have the digestive systems necessary to break foods down into what they need
Human feeding formulas are routinely tested on animals--but the results
could be misleading if human milk differs dramatically from that of the other
mammals. Until Reeds and Davis did this study, most comparisons had been
between human and cow's milk--which is what most infants get after weaning.
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.
Three amino acids--glutamate, leucine, and proline--make up 40 percent of
the total amino acids in all animals' milk. That commonality gives researchers
more confidence in their animal-tested formulas. But why are some amino acids
present in different ratios between species?
While all primates--humans, chimps, and apes--have very similar milk
components, human milk has the highest cystine content, with the great apes
coming in second. 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.
Enter Miller's elephant milk: Davis and Reeds wanted to know if elephants,
another slow-growing animal, also had high cystine levels. They didn't.
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.
In fact, cystine levels were higher, the further up the primates were on the
And there seems to be a connection between amino acid content and a species'
particular needs. For example, both tiger and house cat milks have a high
arginine content. It's an important amino acid for all felines, because kittens
and cubs can't synthesize it. In fact, cat parents must get all the arginine
they need from food, while other mammals synthesize it.
"The good news is there is more similarity than difference between
species," said Davis. "And we will keep studying how these amino acid
combinations work to promote optimum growth."--By
Jill Lee, Agricultural Research
Service Information Staff, 6303 Ivy Lane, Greenbelt, Maryland 20770, phone
Teresa A. Davis and
Peter J. Reeds are at the
USDA-ARS Children's Nutrition Research
Center, Baylor College of Medicine, 1100 Bates St., Houston, TX 77030-2600;
phone (713) 798-7147, fax (713) 798-7171.
"Milk Is Milk . . . or Is It?" was published in the May
1998 issue of Agricultural Research magazine. Click here to see this
issue's table of contents.