Are most Americans lactose intolerant? Do all dairy products contain lactose? How critical is it to stick to a lactose-free diet?

Although most of us know someone who is lactose intolerant, and “lactose intolerance” frequently comes up in health-related conversations, there are many myths and misconceptions regarding this common digestive issue. I’d like to take a look at this malady and at the findings of a recent National Institutes of Health (NIH) consensus conference on lactose intolerance, where experts in many medical fields pored over the relevant medical literature, discussed findings and developed a state-of-the-science statement that includes a few surprises.

(This post is about the common form of lactose intolerance—primary lactose intolerance—and is not about lactose intolerance that arises from illness or intestinal injury, or the rare cases of babies born unable to digest lactose.)

Lactose and lactose intolerance

Chemistry moment (this won’t take long): Lactose is a two- molecule sugar (or disaccharide) made up of one molecule of glucose and one molecule of galactose. In order to absorb lactose our body needs to break the bond between the two simple sugars using the enzyme lactase.

Lactase is abundant in the human intestine in infancy, but its level declines with age in many people. Lactase production is a fascinating example of our body’s ability to turn genes on and off; while the genetic code for producing lactase is in the cell, the cell can turn off production partially or completely.

Those of us with lower levels of lactase don’t break up some or all of the dietary lactose, which will reach the large intestine undigested; there, bacteria will happily feast on the sugar and produce gasses: carbon dioxide, hydrogen and methane. Undigested sugars can also draw water from the intestinal walls, causing bloating and diarrhea.

But low lactase by itself doesn’t define lactose intolerance; most people who have low lactase don't experience signs and symptoms (the common ones being diarrhea, abdominal pain, gas, bloating or nausea). Only people with both low-lactase levels (measured by a health professional) and associated signs and symptoms have, by definition, lactose intolerance.

How common is it?

Surprisingly, the NIH panel concluded that we don’t really know.

Many people with low levels of lactase have no symptoms. Many people with gastro-intestinal symptoms attribute symptoms to lactose intolerance even if they haven’t demonstrated deficiency of lactase; their symptoms may very well be due to other reasons.

What we do know is that low lactase varies across ethnic groups, occurs less frequently in European Americans and more frequently in African Americans, Asian Americans and Native Americans.

Do people with lactose intolerance need to avoid lactose?

Let’s start with an explanation of the big difference between lactose intolerance and serious food allergies or gastrointestinal immune diseases such as celiac.

Food allergies can cause life-threatening systemic reactions after exposure to minute amounts of the offending food. Likewise, the intestine of a celiac patient suffers changes to its surface—changes that can lead to malabsorption, anemia and even cancer—even if exposed to very small amounts of gluten.

But not absorbing some of the sugars in our food is part of everyday life for all of us.

Beans, for example, contain sugar chains—called raffinose oligosaccharides—which no human can break down. The sugar polymers are digested by bacteria in the gut resulting in the well-know bean related flatulence—the butt of many jokes—and can also cause symptoms resembling those of lactose intolerance.

Some people experience no discomfort after eating beans while others limit their intake of beans because large amounts cause them symptoms. There are even individuals who experience such great discomfort that they choose not to eat beans at all. Our individual anatomy and physiology, our intestinal microbial flora, as well as the way we perceive pain and social awkwardness differs a lot. And by no means are beans unhealthy—they are in fact very good to eat!

Milk contains a considerable amount of lactose, much more than the undigested sugars in beans, but the bean analogy can perhaps explain some of the symptom variability we see with lactose intolerance.

So how much milk can people with lactose intolerance tolerate?

The NIH panel looked at the best studies, and concluded that most lactose-intolerant individuals can take in 12 grams of lactose (the equivalent of one cup of milk) in a single sitting with minimal or no symptoms and can tolerate larger amounts if the lactose if ingested with meals or spread over the day. A quart of milk (50 grams of lactose) ingested without food in one sitting will induce symptoms in most lactose-intolerant people. There’s also some evidence showing that the body gets used to lactose, and can tolerate more lactose if routinely exposed to it.

The most important long-term health consequence of lactose intolerance may be calcium deficiency

There are many ways to eat healthy, and dairy is by no means necessary for a balanced diet. Balanced Asian-type and vegan diets are perfectly healthy with little or no dairy. Calcium sources—both naturally occurring or in supplements—are plentiful. (Foods rich in calcium include collard greens, turnip greens, kale, bok choy, soybeans, okra, broccoli, some fish, cultured soy yogurt, tofu, almonds, and calcium-fortified orange juices, soy milks and cereals.)

But dairy products are by far the most prevalent—and least expensive—source of calcium in the Western diet. Since many people with real or perceived lactose intolerance avoid dairy products, they may consume inadequate amounts of calcium and vitamin D (which is added to milk), leading to weaker bones and osteoporosis.

Therefore the diagnosis of lactose intolerance should be made judiciously, and the treatment plan should include assuring that dairy avoidance, if necessary, won’t lead to nutritional deficiencies.

Lactose sources

Milk products vary greatly in their lactose content.

Milk itself has a lot of lactose, but aged cheeses have very little (most of the lactose is drawn away in the whey, and what’s left in the curd is fermented by bacteria and mold). Yogurt with live cultures contains bacteria that break down lactose, and therefore causes fewer symptoms. Butter and full-fat cream cheese contain almost no lactose.

Practical tips

If you think you have lactose intolerance:

Talk it over with your doctor; getting an accurate diagnosis for your symptoms can be important. Some of the causes of recurrent abdominal pain are treatable and it’s not a good idea to commit to an elimination diet for the wrong reason.

Find a reduced-dairy eating plan that controls your symptoms, while enabling both good nutrition and enjoyment of food.

Eat dairy products with less lactose (cheese, yogurt) and spread your dairy consumption throughout the day. Consume dairy as part of a meal.

Make sure you get enough of the nutrients usually found in dairy, especially calcium.

Consider using lactase products; these are dietary supplements that help digest lactose.

Above all, listen your body—treatment and diet plans can’t ever be one size fits all. Find what works for you and stay healthy!

Dr. Ayala