If you're lactose intolerant, you can't digest milk sugar because you lack the digestive enzyme lactase. As a result, drinking milk gives you uncomfortable gastrointestinal symptoms. Boiling milk won't remove the lactose, but you can do other things to prevent symptoms upon consumption of milk, cheese and other dairy products.
Lactose
Lactose is commonly called milk sugar. It's a disaccharide, meaning it's made up of two smaller sugar units; these are glucose and galactose. When you digest lactose, the enzyme lactase breaks it down into glucose and galactose, which you then absorb into your bloodstream. The cells take up the sugars from there, and can burn them for energy or store them for later use. If you're lactose intolerant, you can't digest lactose, and consequently can't absorb it, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry."
Boiling Milk
Heating a substance increases the rate at which chemical reactions occur. The reactants necessary to split lactose into glucose and galactose are water and a trace amount of acid or base; these reagents are present in milk. However, the rate at which the lactose-splitting reaction would occur -- even in hot or boiling milk -- is so slow as to result in nearly inconsequential lactose breakdown.
Why Boiling Sometimes Helps
You may know someone who can't consume milk, but can eat it cooked into foods, or can drink it if it's been boiled. This individual, however, isn't lactose intolerant -- he likely has an allergy to a milk protein. Heating milk and other foods significantly alters the proteins in the food, explain Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry," which in some cases reduces the likelihood that an allergic individual will react to them. If you're lactose intolerant, however, you don't react to milk proteins, so this approach isn't valid for you.
Other Solutions
Rather than trying to heat milk if you're lactose intolerant, you might try a lactose-free milk. These milks have been pre-treated with lactase enzyme, which breaks down the lactose so that it doesn't bother your digestive tract. You can also try taking a lactase supplement; these are available over the counter, and while they don't treat your lactose intolerance, they provide you with a temporary supply of the enzyme you need to digest lactose.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
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