A milk allergy is different (and more serious) than lactose intolerance. It's also not as widespread, affecting just 1–2 percent of the population.

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If you have a dairy allergy, chances are you’ve heard this question many times: So, you have a lactose intolerance, right? Those with dairy issues commonly encounter confusion over lactose intolerance and milk allergy. Over the years, I’ve received questions from relatives who can’t understand why tiny amounts of milk could be deadly for me, from new acquaintances who are curious about my dietary needs and from restaurant wait staff who are confused by my special-diet requests. Sound familiar?

To get to the bottom of this widespread confusion, we compiled questions and put them to some of the nation’s leading allergists who specialize in milk allergy. We hope that this information helps you understand dairy issues better and gives you the tools to educate those around you.

What’s the Difference Between Lactose Intolerance and Dairy Allergy?

“A milk allergy involves an attack on milk protein by the immune system, the part of the body that fights infections,” explains pediatric allergist Scott Sicherer, MD, author of The Complete Idiot’s Guide to Dairy-Free Eating. In most cases, a type of antibody known as IgE (immunoglobulin E) responds to milk protein as if it’s a dangerous invader, triggering a cascade of immune responses that causes potentially dangerous symptoms.

By contrast, says Sicherer, “a milk intolerance, also called lactose intolerance, is trouble with digesting the sugar in milk.” Lactose is the milk sugar and lactase is the enzyme that digests it. Unlike an allergy, lactose intolerance involves problems in the digestive system that stem from this missing enzyme.

How Common are Milk Allergies and Lactose Intolerance?

Data on milk allergy prevalence is relatively limited. Evidence suggests that in Westernized countries, about 1 to 2 percent of children have experienced a milk allergy, which most ultimately outgrow. Milk allergy is found more commonly in boys than girls. “For lactose intolerance, persons of Asian descent have the highest rate—over 90 percent,” says Sicherer. “Additionally, about 70 percent of Native Americans and African Americans have some kind of lactase deficiency. Thus, being lactose intolerant is basically the norm for adults worldwide. However, rates are lowest (5 to 20 percent) among Caucasian adults.”

When Do these Problems Generally Develop?

“Milk allergy most often has an early onset,” explains Robert Wood, MD, chief of pediatric allergy and immunology at Johns Hopkins Hospital in Baltimore. “It most often develops by one year of age. The majority of milk allergy will be outgrown by age 5 or 6; only about 10 to 20 percent will keep it for their entire lifetime.”

In the case of lactose intolerance, babies and young children usually have sufficient amounts of the lactase enzyme to digest milk sugars. However, the amount of lactase produced by the body decreases over time. Thus, dairy intolerance more commonly develops in later childhood and adulthood and once established, it’s less likely to resolve.

How Do Symptoms Differ Between an Allergy and an Intolerance?

An allergic reaction to milk generally involves multiple body systems, affecting the skin, lungs, digestive system and circulatory system. Symptoms can include:

• itchy rashes
• hives
• swelling (often of lips and face)
• wheezing
• throat tightness
• trouble breathing
• trouble swallowing
• abdominal pain
• vomiting
• diarrhea
• paleness/weakness
• confusion
• fainting
• low-blood pressure

Allergic symptoms usually begin minutes to an hour after ingestion. A very small amount of milk can potentially trigger a reaction. Thus, those with a milk allergy typically need to avoid milk completely.

By contrast, lactose intolerance generally involves only gastrointestinal symptoms, including:

• abdominal bloating
• abdominal cramping
• gas
• vomiting
• diarrhea

These symptoms can occur up to several hours after ingesting dairy. Those with lactose intolerance may be able to eat small amounts of milk without a problem.

How Can You Tell Whether You’re Experiencing an Allergy or Intolerance?

One of the main causes for confusion stems from the fact that both milk allergy and dairy intolerance can cause gastrointestinal symptoms.

“There are two main ways to differentiate in people who are having abdominal pain,” explains Wood. “First, it’s rare for someone with an allergy to have just abdominal pain. Usually, there are other symptoms, such as hives or other rashes, breathing difficulty or anaphylaxis. Second, it’s likely that an allergy would cause problems with much smaller doses of dairy than an intolerance.”

Another important indicator, says Jennifer Kim, MD, a pediatric allergist at Mount Sinai Medical School, is the timing of symptoms. “A dairy allergy is generally a more immediate reaction,” she says. “Symptoms occur right away or at least within an hour or two of ingestion. In lactose intolerance, they can develop over several hours after ingestion.”

SEE ALSO:

Food Allergy vs. Food Intolerance
Strategies for Managing Lactose Intolerance
What Are Dairy Intolerance Symptoms? Gas, Nausea, Diarrhea, and More

This article originally appeared in Gluten Free & More magazine in 2015. It has since been updated.  The author, Joshua Feblowitz, who has severe milk and nut allergies, is Lead Writer, Pediatric Views, at Boston Children’s Hospital.