Even though the names sound very similar, lactic acid and lactose aren't a bit alike. If you're lactose intolerant, you can consume foods that contain lactic acid -- and the closely related compound lactate -- without any difficulty. In fact, your body produces lactic acid whenever you metabolize sugars under the right conditions.
Lactose Intolerance
Lactose intolerance is the result of insufficient production of lactase, which is the enzyme you use to digest milk sugar. Milk sugar, or lactose, occurs in milk and most other dairy products. You can't absorb it directly; instead, you must first break it into its smaller constituent compounds -- glucose and galactose -- which get absorbed by the bloodstream. If you're lactose intolerant, you can't consume lactose without digestive difficulty.
Lactic Acid
The lactic acid molecule is quite different from the lactose molecule, despite the similar sounding names. While lactose is a sugar, lactic acid is a small carbon-based acid that humans, other animals, and some bacterial species make when they break down sugars for energy. For instance, one common food source of lactic acid is yogurt. Bacteria break down the sugars in milk to make lactic acid. This causes the milk to thicken and become sour, producing the characteristic taste and texture of yogurt.
Foods To Avoid
If you're lactose intolerant, you need to avoid any foods that contain lactose. Milk and most dairy products will cause the typical digestive symptoms, but yogurt is unlikely to do so. This is because bacteria in the yogurt have already broken down much of the lactose, essentially "pre-digesting" it. Because lactose and lactic acid are very different molecules, however, you can freely eat foods that contain lactic acid -- whether it's naturally occurring or has been added as a preservative.
Metabolism
One interesting feature of lactic acid is that, in addition to occurring in some foods, it's also the molecule you make when your muscles burn sugar in the absence of oxygen. This is called anaerobic metabolism, or fermentation. It occurs when you're engaging in a very hard sprint or power effort, and it causes the characteristic muscle burn you feel during and after such an effort.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
