In 1981, Eli Hazum and his colleagues at Wellcome Research Laboratories in Research Triangle Park, N.C., reported a remarkable discovery. Analyzing samples of cow’s milk, they found traces of a chemical that looked very much like morphine. They put it to one chemical test after another. And, finally, they arrived at the conclusion that, in fact, it is morphine. There is not a lot of it, and not every sample had detectable levels. But there is indeed some morphine in both cow’s milk and human milk.
Morphine, of course, is an opiate and is highly addictive. So how did it get into milk? At first, the researchers theorized that it must have come from the cows’ diets. After all, morphine used in hospitals comes from poppies and is also produced naturally by a few other plants that the cows might have been eating. But it turns out that cows actually produce it within their bodies, just as poppies do. Traces of morphine, along with codeine and other opiates, are apparently produced in cows’ livers and can end up in their milk.
But that was only the beginning, as other researchers soon found. Cow’s milk—or the milk of any other species, for that matter—contains a protein called casein that breaks apart during digestion to release a whole host of opiates called casomorphins. A cup of cow’s milk contains about six grams of casein. Skim milk contains a bit more, and casein is concentrated in the production of cheese.
If you examined a casein molecule under a powerful microscope, it would look like a long chain of beads (the “beads” are amino acids—simple building blocks that combine to make up all the proteins in your body). When you drink a glass of milk or eat a slice of cheese, stomach acid and intestinal bacteria snip the casein molecular chains into casomorphins of various lengths. One of them, a short string made up of just five amino acids, has about one-tenth the pain-killing potency of morphine.
What are these opiates doing there, hidden in milk proteins? It appears that the opiates from mother’s milk produce a calming effect on the infant and, in fact, may be responsible for a good measure of the mother-infant bond. No, it’s not all lullabies and cooing. Psychological bonds always have a physical underpinning. Like it or not, mother’s milk has a drug-like effect on the baby’s brain that ensures that the baby will bond with Mom and continue to nurse and get the nutrients all babies need. Like heroin or codeine, casomorphins slow intestinal movements and have a decided antidiarrheal effect. The opiate effect may be why adults often find that cheese can be constipating, just as opiate painkillers are.
It is an open question to what extent dairy opiates enter the adult circulation. Until the 1990s, researchers thought that these protein fragments were too large to pass through the intestinal wall into the blood, except in infants, whose immature digestive tracts are not very selective about what passes through. They theorized that milk opiates mainly acted within the digestive tract and that they signaled comfort or relief to the brain indirectly, through the hormones traveling from the intestinal tract to the brain.
But French researchers fed skim milk and yogurt to volunteers and found that, sure enough, at least some casein fragments do pass into the bloodstream. They reach their peak about 40 minutes after eating. Cheese contains far more casein than other dairy products do. As milk is turned into cheese, most of its water, whey proteins, and lactose sugar are removed, leaving behind concentrated casein and fat.
Cheese holds other drug-like compounds as well. It contains an amphetamine-like chemical called phenylethylamine, or PEA, which is also found in chocolate and sausage. And there are many hormones and other compounds in cheese and other dairy products whose functions are not yet understood. In naloxone tests, the opiate-blocking drugeliminates some of cheese’s appeal, just as it does for chocolate.
