Maltose and lactose are both types of sugar that occur in some foods. They're both carbohydrates, and even though maltose is sweeter than lactose, they both provide you with the same amount of energy. Just as you can use table sugar and starch to fuel your cells, you can also use maltose and lactose.
Maltose is a disaccharide, meaning it's a carbohydrate made up of two smaller sugar units called monosaccharides. Specifically, the monosaccharides in maltose are two glucose units. In fact, the constituents of maltose — glucose molecules — and the way in which they're bonded together is identical to the chemical makeup of starch. The only difference between starch and maltose is that only two glucose units make up maltose, while starch consists of long glucose chains.
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Like maltose, lactose is a disaccharide. Its constituent monosaccharides, however, are glucose and a different but related molecule called galactose. Like glucose, galactose consists of 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms; your cells can do the same things with galactose that they can do with glucose. You can burn either monosaccharide for immediate energy, store them for later use in the form of the carbohydrate molecule glycogen or convert them into fat, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry."
Digestion and Absorption
Your small intestine can't absorb either lactose or maltose. Instead, digestive enzymes in your small intestine must break both sugars down into their constituent monosaccharides, which you then absorb into the bloodstream. You use different enzymes to digest the two sugars; lactase breaks down lactose, while an enzyme called sucrase-isomaltase breaks down maltose. Because enzymes are so specific with regard to function, you can't break down maltose with lactase or vice versa.
To most individuals, there's little difference between maltose and lactose other than taste. If you're deficient in either of the enzymes that digest the sugars, however, the difference is quite important. People who are lactose intolerant can't digest lactose because they don't produce enough of the lactase enzyme. This condition is relatively common, particularly in older individuals, since you produce less lactase as you age. Sucrase-isomaltase deficiency is much rarer, notes the Genetics Home Reference; it occurs in about 0.02 percent of the general population.