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What is Milk Sugar?

By Tracey Parece
Updated May 16, 2024
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Milk sugar is a single molecule of glucose linked to a single molecule of galactose to form a carbohydrate. It is also commonly known as lactose, which is a special form of carbohydrate called a disaccharide. Lactose is the first and only form of carbohydrates consumed by humans and other newborn mammals in significant quantities. Although all mammals' milk naturally contains lactose, it is produced industrially from cows' milk.

Research has shown that nearly 70% of the world's population is intolerant to milk sugar, a condition called lactose intolerance. It may cause digestive problems ranging from gas and bloating to gastrointestinal discomfort and diarrhea. Lactose intolerance may be caused by a deficiency of the enzyme lactase, which is present on the surface of cells lining the small intestine. The function of lactase is to break down this complex sugar into simpler forms of sugar, glucose and galactose, that can be more easily absorbed by the bloodstream.

Some dairy products are marketed to people who have lactose intolerance. These food items include everything from milk to ice cream and cheese that have had all the lactose removed. Dietary supplements are also available that enable people who have an intolerance to the substance to digest it.

For people who don't have trouble digesting lactose, dietary supplements containing milk sugar are commonly used as part of a high-calorie diet or to gain weight. It is sometimes used as a sweetener in fermented alcoholic beverages like beer as well. Lactose may also be used as a stabilizing ingredient in the production of aromas and fragrances.

Lactose is also commonly used in the pharmaceutical industry as an excipient — an inert substance used as a vehicle for a drug's active ingredients. Lactose tends to be inert, meaning it does not commonly react with other ingredients, and it is also non-toxic and inexpensive. This makes it ideal for use in pharmaceuticals such as tablets, capsules, and dry powder inhalers. Pharmaceutical lactose is isolated from milk whey through the processes of concentration, crystallization, and purification.

Fabricio Bartoletti was a doctor of philosophy and medicine who discovered the presence of lactose in milk in 1619. A professor of surgery and anatomy, Fabricio Bartoletti was also the first person to isolate the milk sugar from the whey. Lactose wasn't identified as a sugar until 1780 by a pharmaceutical chemist named Carl Wilhelm Scheele.

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Discussion Comments
By anon351292 — On Oct 12, 2013

Most lactose intolerance is due to pasteurizing milk. My son and nephew both had stomach pain after drinking milk. Once we all switched to raw milk, the pain ceased. The more I talk to people using raw milk, the more I hear this story.

By serenesurface — On Nov 05, 2012

@MikeMason-- I'm no expert but I think that lactose intolerance develops at a later age. Infants don't have lactose intolerance. Some infants have trouble digesting cow's milk so they need to switch to goat's milk.

I think that genetically, we are all supposed to become lactose intolerant after childhood. But because some cultures consume a lot more milk and continue to have it into adulthood, some of our genes have adapted to tolerate milk after childhood.

By stoneMason — On Nov 05, 2012

How can 70% of all people be lactose intolerant when we all have breast milk as babies?

If so many people were lactose intolerant, they wouldn't be able to have milk from the beginning right? And I have never heard of lactose intolerant babies.

By bear78 — On Nov 04, 2012

I think people are confused about lactose. Lactose is a sugar molecule but don't think of this as something like table sugar that is sweet. When we're talking about sugar molecules, we are talking about different types of carbohydrates. All carbohydrates turn into glucose, a type of sugar in our body. This is what we use for energy.

So the fact that a food has a type of sugar in it like lactose, glucose or fructose doesn't necessarily mean that it is sweet. Vegetables also have carbohydrates in them which turn into glucose when we eat them.

This is why milk doesn't taste sweet, but it does have lactose, a type of sugar molecule. Does this make sense?

By OeKc05 — On Sep 12, 2012

I would never have imagined that beer would be sweetened with milk sugar! Picturing milk and beer together makes me a little nauseous. I know it's not the actual milk that is used in the beer, but it's still a weird thought.

I wonder if adding milk sugar to beer makes it more fattening. Since lactose is used to help people bulk up, you would think that it would have a high-calorie content.

By lighth0se33 — On Sep 12, 2012

@Perdido – I am lactose intolerant, and I drink soy milk that's sugar free. It's flavored with vanilla and coconut, so it's very tasty, even though it doesn't contain lactose.

I found out I was lactose intolerant when I started having painful abdominal cramps after drinking milk. The first time it happened, I didn't make the connection, but I started to notice that every time I consumed dairy products, my symptoms would worsen.

It got to the point where I was very nauseated and even had diarrhea after drinking a milkshake. A friend told me that I should try avoiding dairy for awhile and see if I felt better. I did, so I switched to soy milk, and I haven't had any issues with it.

By Perdido — On Sep 11, 2012

I have heard of some milk being sugar free. Is this made for people who are lactose intolerant?

What happens when someone who is lactose intolerant drinks regular milk? I have always heard people say that they have this issue, but I've never heard anyone describe what happens to them when they consume lactose. Are the reactions really severe, or do they just have mild discomfort?

By seag47 — On Sep 10, 2012

@turquoise – It does seem kind of strange to think of milk containing sugar. To me, it doesn't taste sweet at all.

I suppose that if milk had no sugar, it might not go as well with cereal. Most cereals have been at least slightly sweetened, and the milk helps bring that out a little more, even though it does this subtly.

I think that naturally occurring sugars are better for you than added sugars, though. Your diet probably won't be negatively affected by the consumption of milk sugar.

By turquoise — On Aug 30, 2012

I was wondering about this the other day because I bought some plain yogurt from the grocery store. I'm on a no-sugar diet so I've made it a habit to check sugar content on foods. When I looked at the sugar content of the plain yogurt, it said four grams. But then I checked the ingredients list, there was no sugar listed.

I wondered about this for a while and then realized that there is sugar in milk. So I guess they put the amount of lactose in the yogurt on the label. I was confused about this at first but now I get it. I guess there is sugar in almost everything naturally.

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