Carbohydrates are one of the three classes of macronutrient compounds, which are nutrients that you need in large quantities each day to provide for your cellular energy needs and to serve as building blocks for cellular products. Lactose, or milk sugar, is a sugar, which is a subcategory of carbohydrates.
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Carbohydrates include sugars, starch, and fiber, all of which consist of one or more smaller units called monosaccharides. "Monosaccharide" means "single sugar unit," and carbohydrate properties vary depending upon how many monosaccharides of which they consist. Those made of 1 or 2 monosaccharides are sugars, while larger carbohydrates include starch and fiber. Sugars taste sweet, because they bind to sweetness receptors on the tongue, while larger carbohydrates don't bind to the sweetness receptor.
Lactose is a disaccharide, which means it's made up of two monosaccharide units. Specifically, the monosaccharides in lactose are glucose and galactose. Glucose is very common in nature and in food; it's also found in table sugar, starch and fiber. The chemical formula of lactose is C12H22O11, which is identical to that of sucrose, or table sugar. Despite their identical chemical formulas, however, lactose and sucrose taste nothing alike; sucrose is much sweeter.
When you consume lactose -- which occurs any time you consume dairy -- you break it into its constituent monosaccharides and absorb them separately. Your cells can then take up glucose and galactose and use them as sources of cellular energy. You can either burn the monosaccharides immediately, or convert them into either glycogen or fat, both of which are energy storage molecules.
All carbohydrates that you consume for energy are digested by different enzymes. The enzyme responsible for breaking down lactose so you can absorb the monosaccharides is called lactase. Some individuals -- those with lactose intolerance -- don't produce enough lactase. As a result, they experience symptoms of gastric upset, including cramping and bloating, upon consumption of lactose-containing foods. You can take supplemental lactase when you eat dairy to help you avoid these symptoms.