This sugar occurs naturally in milk and is also called milk sugar. It's the least sweet of all the natural sugars and is used commercially in foods such as baby formulas and candies. Lactose intolerance or sensitivity is the inability to digest lactose because the small intestine doesn't produce enough lactase, the enzyme needed for lactose digestion. The degree of the intolerance is individual, with some people able to ingest more milk-based products than others. Cheese contains less lactose than milk, with hard, aged cheeses containing only a trace amount.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.