You may not know it, but most people in the world can't drink milk--not without suffering the consequences of diarrhea, discomfort or gas. They have lactose intolerance. It's hereditary. So at least one of their parents has the same problem. They may not recognize the problem, because over the years they've learned to shy away from milk products. They don't like ice cream or milk, because instead of feeling good when they lick their ice cream cones, they feel bloated and uncomfortable within a few hours.
The reason is that the enzyme, lactase, that breaks down milk sugar in infancy fades away in those with lactose intolerance. Almost everyone in the world is born with lactase enzyme on the tip of the intestinal surface so they can absorb the nutrients in breastmilk (and milk-based formula). But those whose families came from many parts of Africa, Asia, the Mediterranean, that enzyme isn't produced and active after 7 or 8 years of age. You can trace that back to cultures that were more hunters rather than farmers, but that isn't the point here.
That enzyme can also be temporarily damaged if someone gets an intestinal infection where the virus, or bacteria disrupts the intestinal surface. The same is true for those with celiac disease where gluten has injured the surface. But after the infection is over or the gluten is removed, the enzyme usually returns within 4 weeks or so and lactose can be digested again.
Milk Allergy is Different
Yes, you can have diarrhea or discomfort with milk allergy, but you can also have vomiting or a rash--you won't have those with lactose intolerance. The reason is that instead of not tolerating the sugar, the lactose, an allergy is a reaction to a protein in the milk. There are actually several proteins in milk (casein and those in the whey fraction) and a number of them can cause an allergy.
In simple terms, an allergy is where the a certain fraction of gamma globulins recognizes a foreign protein and mounts a vigorous response against it, essentially trying to fight it off. The result is the vomiting and discomfort or the rash that will typically return whenever the body comes in contact with even a small amount of that protein. And that speaks of another difference. You don't have to drink but a swallow or two of the milk, and your body will begin an allergic response (if you have the allergy), but if you're lactose intolerant, you can often put a small amount of milk in your coffee or eat a little yogurt without symptoms developing.
What's also different is the time when milk allergy is a problem. Instead of the slow start of lactose intolerance in late childhood, milk allergy mainly affects babies--and most outgrow it entirely by 3 years of age. A word of caution, however: an allergy-like condition called eosinophilic esophagitis which is common in older children and adults often, in part, responds to milk elimination.
But That's Not All Milk Can Do
Milk for some people also increases mucus production even if they don't have true allergies. And milk fat can trigger gallbladder symptoms (for those who should be on a low fat diet) and can slow stomach emptying, making some quite uncomfortable and nauseous. And of course that doesn't speak of raising cholesterol and adding to calories, which is why most adults are switching from whole milk (which has 4% fat) to skim or one of the lower fat milks. And that's why health organizations suggest even children as low as 18 months begin drinking 2% milk.
Why Drink Milk At All?
No one needs milk. And the problems for people who are lactose intolerant and allergic has caused many to consume less. Enough less, that the American Dairy Association has continued a very effective campaign to help people understand that milk and milk products is an important source for protein, calcium and phosphorous. Additionally, cow's milk is supplemented with Vitamin D--an nutrient that most of us don't get in the quantity we need. Yes, you can get all of these nutrients elsewhere (thank goodness for those who are allergic and can't have any form of cow's milk).
Yes, lactose-free and lactose-reduced foods are in most grocery stores and yes, there's even a pill those who are lactose intolerant can take when they eat pizza or ice cream that will help them digest and enjoy other dairy products. And there are substitutes that call themselves milks that are drinks with a milky consistency and that have some of the nutrients that cow's milk does (more about that in my other posts and in What to Feed Your Baby).
Organic milk is available as a healthy choice, because the cows providing that milk are not given hormones, chemicals or medicines that might cause problems for those who drink it. On the other hand, unpasteurized milk cannot be recommended because tuberculosis or other infections will not be killed and can pass from infected cows to affect dairy lovers.
People can have different problems with milk. Lactose intolerance can cause diarrhea, bloating, discomfort, and gas--but never vomiting. Milk allergy can cause that vomiting or a rash. And others have problems with milk for a variety of reasons. But for most everyone else (and even for those with lactose intolerance who use one of the predigested milk or an enzyme replacement), milk can provide an important source of protein, calcium, and vitamin D.