Once upon a time there was a poor widow who lived in a little cottage with her only son, Jack. There had been a hard winter, and after it the poor woman saw there was no means of keeping Jack and herself from starvation but by selling her cow. One morning she said to her son, "Take the cow to market for me and sell her."

On the way to market, Jack met a butcher who offered to trade the cow for a handful of magic beans that would grow into a towering beanstalk, at the top of which Jack would find a goose that lays golden eggs. "Yeah, sure," said Jack. He told the butcher where he could plant his beans and continued on his way.

Soon, he met a farmer who also had a cow and who told him of all the money they could make by joining forces with all other dairy cow owners and promoting cow's milk as the greatest food since manna. They formed the Golden Egg Dairy Council, which contributed millions to political campaigns and hired lobbyists to elicit favorable legislation for their industry. They spent more millions to advertise that milk "builds strong bones" and that dairy foods are nutritionally indispensable for man, woman, child and infant alike.

Many years later, in the second half of the 20th century, it became known to medical science that as much as 70 percent of the world's adult population, including 80 percent to 90 percent of adult Africans and Asians, is incapable of digesting cow's milk without suffering unpleasant symptoms. Most adults lose the ability to produce the digestive enzyme lactase, which is necessary for breaking down milk sugar, or lactose, into the absorbable sugars glucose and galactose.

Some 80 percent to 95 percent of these so-called lactose malabsorbers (LMs) suffer symptoms such as abdominal cramps, flatulence and diarrhea from ingesting the lactose in dairy products, thereby qualifying for the unenviable categorization of lactose intolerants (LIs). All in all, an estimated 30 percent of adult Americans lack sufficient lactase to digest the lactose in dairy products without experiencing physical distress.

What does the dairy industry say?

In recent years, lactose intolerance has become something of a political, economic and sociological football -- sociological, because of perceived racial bias in the U.S. Department of Agriculture's Dietary Guidelines for Americans (the "food pyramid"), which advises two or three daily servings of dairy products in spite of the fact that many adult African- Asian- and Hispanic-Americans may suffer by following this advice.

The National Dairy Council (NDC), aggressively promotes the interests of the 92,000 dairy farms in the United States, with their 9.14 million cows producing 173 billion pounds of milk, 99.4 percent of which is consumed by humans (the rest by calves), for total annual cash receipts of $20.4 billion, according to USDA data for 2002.

In addition, the NDC and several agricultural organizations fund research at universities. Among the conclusions stated in some of these studies are that true LI is, in fact, quite uncommon; that people who claim to be severely lactose-intolerant are likely to be mistaken; that up to eight ounces of milk can be tolerated by everybody without incurring more than negligible symptoms; and that exclusion of dairy products from the diet is not only unnecessary but also potentially harmful because calcium deficiencies, not to mention osteoporosis, could result.

It may well be pointed out, however, that cows don't have a monopoly on calcium. It is available in green vegetables and other foods, not to mention the plethora of calcium supplements on drugstore shelves.

According to studies funded by the NDC and other agricultural organizations, many people who claim to experience distressing symptoms after ingesting milk or other dairy products may be blaming them on lactose without justification. In one study partially supported by the NDC, a group of subjects who professed being severely lactose intolerant were given psychological personality tests to assess their propensity for lying. They were found wanting in reliability.

And yet, legions of LI sufferers know from experience that they can drink lactase-reduced milk, such as Lactaid, without distress and that they can eat dairy products with impunity only if they take supplementary lactase tablets along with them. I find it hard to believe that so many consumers are incapable of discerning what makes them sick.

One of the authors of the NDC-supported study mentioned above has suggested to me that these strategies may appear to work because of a placebo effect fueled by the pharmaceutical industry's promotion of their supplement products. (Pot calls kettle black?)

What do the doctors say?

I am not a physician, much less a gastroenterologist, and am not qualified to review the copious medical literature in an attempt to establish a medical consensus on LI. But one measure of the medical community's best judgment can be found in the "home medical adviser" books written for the lay public. Here's what a couple of them have to say about LI.

The American College of Physicians Complete Home Medical Guide, 2003, says, "One person may experience symptoms after drinking several glasses of milk, but another may feel discomfort after consuming only a small amount of a dairy product. The symptoms are often initially mild, but can become more severe with each subsequent episode." Or: "The symptoms can be completely relieved by eliminating lactose from the diet. It is important to remember that you should avoid all dairy products such as milk, yogurt, cheese, cream and butter."

On the other hand, the Harvard Medical School Family Health Guide 1999 says, "If you are lactose intolerant, you can help eliminate symptoms and still consume dairy products by taking the following measures: Consume small qualities of dairy products. You may be able to tolerate small amounts if you cannot tolerate normal amounts. Eat yogurt or aged cheeses instead of drinking milk. . . . Eat lactose-containing foods with other foods. Use enzyme supplements."

Obviously, there is no consensus, except for the uncontested fact that there are degrees of LI, ranging from mild to severe, depending on the individual's intestinal supply of lactase, and that lactose-depleted milk can be effective in the avoidance of symptoms.

Regarding the avoidance of dairy foods: Guy says to doctor, "Whenever I go like this with my arm, it hurts." Doctor says, "So don't go like that." It's the same with lactose. If your experience is that eating certain dairy products gives you cramps and gas, don't eat what gives you cramps and gas. Listen to your body and don't let any highfalutin scientific research paper talk you out of it.

Unfortunately, every LI individual must determine his or her lactose tolerance level by trial and error. I have found several lists of the amounts of lactose in various foods, but they are inconsistent because of wide variability in the foods themselves. One list, together with many practical tips on managing LI, can be found at www.wegmans.com/kitchen/ diet/lactose/tips.asp.

Armed with this information, you can do your own (unfunded) research and find your threshold of (in)tolerance for lactose. Good luck.

(Sigh!) It's enough to make you wish Jack had taken the beans after all.

This is the third in a series on lactose intolerance.

Robert L. Wolke (www.professor

science.com) is professor emeritus of chemistry at the University of Pittsburgh and the author, most recently, of "What Einstein Told His Cook: Kitchen Science Explained" (W. W. Norton, hardcover, $25.95). He can be reached at wolke@pitt.edu.