Why Does Milk Have Sugar in it?

A cold glass of milk is both nostalgic and satisfying for various reasons, one of which includes the fact that it’s made up of 87 percent water. That said, many people don't think of milk being sweet, but a single cup of cow’s milk has the equivalent of three teaspoons of sugar. Why does this occur, and is the sugar in milk natural or is it added? We explore these questions and also explain what can happen if your body can’t digest the sugars found in milk.

Milk sugar: What is lactose?

Lactose, also called milk sugar, is the main source of carbohydrate found in the milk of mammals. Americans most commonly associate lactose with cow’s milk. It’s also found in humans, goats and sheep. Humans have the highest concentration of lactose in milk at 7 percent. Cow’s milk is usually made up of 4.5 percent lactose.

When females, whether humans or animals, are pregnant, they lactate to produce milk for their offspring. As part of this process, glucose (the main sugar found in the blood) passes through the mammary glands and is synthesized into lactose. 

Lactose is a disaccharide, meaning it contains two linked simple sugars (monosaccharides). Fructose, galactose and glucose are the main monosaccharides in the foods you eat. When combined, these simple sugars produce lactose (glucose and galactose), maltose (two glucose molecules) and sucrose (glucose and fructose).

Lactose remains a complex sugar until it reaches the digestive system, where the enzyme lactase hydrolyzes it into glucose and galactose. Glucose moves into cells with assistance from insulin to provide energy. Galactose is sent to the liver to be converted into glucose.

Why does milk have sugar but some cheeses don’t?

Since cheese is made from milk, it would be fair to assume cheese also is high in lactose. That would be incorrect, though.

Hard cheese and other aged cheeses have low amounts of lactose for two reasons: the production process and aging. Cheese is made by heating milk and separating the curd from the whey, the liquid that remains after straining the curd. This liquid contains salt and lactose, meaning the final product is low in lactose. As for aging, the longer cheese sits, the more time bacteria has to break down lactose.

If you’re trying to cut down on your sugar intake, many low-lactose dairy options exist.

Some examples of cheese with some low amounts of lactose include cheddar, Swiss, mozzarella and gouda, which all have less than 1 gram per 1-ounce serving. Cottage cheese also has just 3 grams of lactose per serving, and Greek yogurt has 4 grams of lactose per serving.

How much sugar is in milk?

A cup of milk, regardless of how much milk fat it contains, has 12 grams of sugars.

• Whole milk: 12g
• Reduced fat (2 percent) milk: 12g
• Lowfat (1 percent) milk: 12g
• Fat-free (skim) milk: 12g

Twelve grams of sugar equals 3 teaspoons (4 grams in a teaspoon). This explains why milk can taste sweet if you’re not used to a diet that includes sugar. However, lactose is about five to six times less sweet than sucrose (cane sugar and table sugar), so it doesn’t have the same effect on your taste buds.

What about ultra-filtered milk?

There is one exception to the sugar content found in commercial milk. Ultra-filtered milk has significantly lower levels of sugar than regular milk thanks to an extra filtration process.

Ultra-filtered milk passes through a membrane that collects extra water and lactose while allowing milk protein to pass through. The finished product is lower in sugar (6g) and higher in protein (13g). In other words, it’s a concentrated version of regular milk.

Is lactose a natural or added sugar?

The key here is lactose found in milk is naturally occurring as opposed to an added sugar. That said, it’s also used commercially as an additive. When processed, crystallized lactose can act as a binder, filler or browning agent to baked and packaged goods.

What’s the difference between how your body digests natural sugars and added sugars? Natural sugars, like those found in milk, contain other nutrients that help prevent massive blood sugar spikes. 

In other words, yes, milk has 12 grams of sugar, but protein is harder to metabolize and thus takes longer to digest milk. Plus, naturally occurring lactose breaks down into glucose and glycogen, which your body uses for energy.

Added sugars, on the other hand, contain no nutritional value and only add empty calories to your diet.

What happens if your body can’t digest lactose?

Because lactose is a complex sugar, it needs a digestive enzyme to break down the molecules into glucose and galactose. This enzyme is called lactase, which is found in the small intestine. In addition, a liver enzyme called galactose-1-phosphate uridyltransferase helps break down galactose.

Two conditions that can impact people are lactose intolerance and galactosemia. Of the two, galactosemia is more severe and can even be life-threatening if not diagnosed and managed properly.

What does it mean to be lactose intolerant?

People who don’t produce enough lactase have trouble digesting lactose, a condition called lactose intolerance. Whenever you eat or drink foods with lactose, the undigested lactose passes through the gastrointestinal tract and ferments in the colon. As bacteria feed on the sugar molecules, they produce gas that can cause bloating, stomach pain, nausea and diarrhea. 

People who are lactose intolerant can either avoid dairy products altogether or drink milk that has been supplemented with lactase. The added lactase helps break down the lactose before you consume it.

What is galactosemia?

As part of normal digestion, lactose breaks down into glucose and galactose molecules. Galactose is then further broken down into glucose to use as an energy source.

In rare cases, babies can be born with galactosemia, an inherited genetic metabolic disorder that inhibits the body’s ability to break down galactose. The galactose builds up in the blood and can cause damage to cells, organs and tissues. For example, if not addressed, galactosemia can cause liver disease, kidney damage and neurological complications such as speech disorders, intellectual disabilities and developmental delays in babies.

Babies with galactosemia must eliminate dairy and other foods that contain lactose, including breastmilk. In its place, soy-based formula helps these babies grow and develop during their infant years.

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