You may be surprised to discover that lactose intolerance is natural. In fact, most adult humans around the world are lactose intolerant to some extent. Lactose intolerance basically is an inability to digest the milk sugar lactose. The condition is caused when individuals don’t produce enough lactase, the enzyme needed to digest lactose.

People tend to think of lactose intolerance as being something abnormal — a condition that needs special attention, like an illness or a disease. However, lactose intolerance is actually a natural state of being for most of the world’s adults. Understanding what lactose intolerance is and why it happens, though, may change your perspective and make you better able to manage your diet and your symptoms.

Examining milk: Nature’s first food

For every mammal on Earth — including humans — milk is the first food they eat. A mammal’s milk is tailor-made to be exactly what a baby mammal of that species needs. Squirrels, for example, produce milk that contains precisely what a baby squirrel needs to grow and develop normally. Dogs produce milk that’s customized for their puppies. And cows feed their calves milk that’s formulated to help a tiny calf grow into a hulking herbivore in little more than a few months.

Humans produce milk for their babies, too, and it’s the optimal food for normal human growth and development. It’s better than baby formula, and it’s better for human babies than milk from a squirrel or a dog. No other mammal drinks the milk of another mammal species. So the question is: Why do humans drink milk from a cow?

Because milk is for babies, mammal mothers only make it until their infants are nourished and have developed well enough to tolerate solid foods. Until then, a mother’s milk provides her offspring with special substances that boost immunity and provide the calories, protein, carbohydrates, fat, vitamins, and minerals they need to grow.

One ingredient in a mother’s breast milk is lactose, a form of sugar. A baby’s body produces an enzyme — lactase — that’s specially designed to help digest the lactose in milk. With the help of lactase, the body breaks down lactose into small forms of sugar — glucose and galactose — that are readily absorbed into the bloodstream and used to produce energy.

Over time, though, babies grow and develop to the point where they can eat solid foods. Gradually, they’re weaned from breast milk. At that point, if they’re squirrels, they start eating acorns. And dogs, like humans, begin eating a wide variety of foods. Cows start eating grass. When babies of all species no longer need their mothers’ milk to survive, they stop producing lactase — at least most of them do.

Lacking lactase results in lactose intolerance

By the time humans and other mammals reach adulthood, they generally don’t produce lactase. So, if they drink milk or eat foods made from milk, they can’t digest the lactose in the milk.

The waning of lactase production that begins in late childhood and extends into adolescence and adulthood is sometimes referred to as lactase nonpersistence.

This natural lack of lactase in turn causes lactose malabsorption. When lactose malabsorption results in symptoms, the condition is considered lactose intolerance. The symptoms of lactose intolerance can all be attributed to the undigested lactose moving through the gastrointestinal system.