why can I eat cheese but not milk?

Lactose intolerance is very common.  In fact, 75% of the people world-wide have some degree of it.  However, as you may have noticed, some ethnic groups (Asians, African Americans, Native Americans, Ashkenazi Jews, and Latinos) are especially prone to it. Other risk factors include increased aging, premature birth, certain diseases affecting the small intestine (including bacterial overgrowth, celiac disease and Crohn's disease), and some cancer treatments. Lactose intolerance may also be a result of genetics (you are born without the lactase enzyme or born with an ineffective gene that has trouble producing the enzyme in sufficient quantities) or infections that injure the small intestine.

Lactose intolerance prevalence around the world

If you don't have lactose intolerance, let me tell you a little about what you are missing.  Lactose intolerance is essentially a condition that is characterized by the inability to digest the lactose in milk products (lactose is the sugar component of milk).  The symptoms of this include abdominal discomfort and other bowel related issues. (You do not want to sit on a plane next to someone who is lactose intolerant and just drank a few cups of milk!) Lactose intolerance is rarely serious, but can cause much discomfort.

The cause of lactose intolerance is the reduction or loss of lactase (the enzyme that is used to breakdown the lactose sugar into its subunits of glucose and galactose).  The unhydrolyzed lactose sugar remains in the gut and draws water to it (via osmosis) causing liquidy fecal contents to go through your system. (EW!) Additionally, the colonic bacteria naturally in your intestines ferment some of the remaining lactose, which leads to short chain fatty acid, carbon dioxide and hydrogen gas production that builds up and eventually needs to be released.

So, why can some dairy products be consumed more easily (and pain free) than others? 

Chocolate milk is a good option!

It has to do with the lactose content of the food item.  Most people can consume 1-2 cups of milk throughout the course of the day without significant symptoms (although some people's symptoms may be worse than others). Interesting fact- people can generally tolerate chocolate milk better than plain milk!  I don't know why....

Aged cheese are lower in lactose

Aged cheeses (like hard Parmesan, Swiss, or cheddar cheese) have less lactose than younger cheeses.  This is because bacteria (specifically the species of Lactobacillus) that are used in cheese making process also consume lactose within the cheese.  An aged cheese literally has more lactose broken down compared to other cheeses because of this natural process.  (BTW, this is the same bacteria responsible for sourdough bread, yogurt, kimchee, and many other fermented foods!) Processed cheese, on the other hand, has as much lactose in it as regular milk does, and sometimes it has more!  Generally, the harder the cheese is, the lower the lactose, and the easier it is to be digested in people with lactose intolerance.

Yogurts that contain live bacterial cultures are also usually acceptable for the same reason that aged cheese are.  Much of the lactose is already broken down prior to consumption.

Processed cheeses are high in lactose

People who avoid milk/dairy products entirely because of fear of discomfort should try to re-introduce small amounts of dairy at a time.  If you can gradually introduce milk/dairy into your diet (small amounts throughout the day) you can increase your body's tolerance to lactose.  It is a good idea to include sources of fortified dairy in your diet because they are important sources of calcium and vitamin D, in addition to protein, riboflavin, vitamin B12, magnesium, and potassium!  If you cannot tolerate lactose at all, try to get these sources from other non-dairy items like soymilk or calcium-rich foods.

Calcium content in common foods

Tips:

• Consume dairy with other food items. This helps slow down your digestion, which makes it easier for your body to absorb lactose.
• Test different dairy products in small amounts to see your individual tolerance.  People tolerate food differently and at different stages in life.  So, keep trying!
• Consider taking enzyme tablets containing lactase before consuming dairy products.

Remember- lactose intolerance is an intolerance, not an allergy.  There is no immune response triggered- it is mostly symptoms of discomfort.  A milk allergy is separate condition in which you are allergic to proteins (not sugars) within the milk.  This is a potentially serious and can result in anaphylaxis and potentially death.

For more information about what you can and can't eat with lactose intolerance- the FDA has a nice summary here!

Sources:

• "National Digestive Diseases Information Clearinghouse (NDDIC)." Lactose Intolerance. Web. 24 Apr. 2012. <http://digestive.niddk.nih.gov/ddiseases/pubs/lactoseintolerance/>.
• "Lactose Intolerance." Mayo Clinic. Mayo Foundation for Medical Education and Research, 04 Apr. 2012. Web. 24 Apr. 2012. <http://www.mayoclinic.com/health/lactose-intolerance/DS00530/METHOD=print>.