Casein and lactose are two of the chemical components of milk, and are quite distinct from one another. Casein is a protein, while lactose is milk sugar, a carbohydrate. There is no casein in lactose, but because both compounds occur naturally in milk, many products containing lactose will also contain casein.
Casein
Casein is one of several proteins in milk. Protein is one of the three macronutrient compounds, along with carbohydrate and fat. A macronutrient is something your cells need in large amounts each day to provide energy and building blocks for the structural and functional components of cells. When you consume milk, you break down casein into smaller molecules called amino acids, which you absorb. You then use these to provide energy or to make your own proteins.
Lactose
Milk sugar, or lactose, is very similar in chemical structure to the more familiar table sugar, or sucrose. Lactose consists of two building-block molecules called monosaccharides that are chemically bonded together. When you consume milk and other lactose-containing dairy products, you break the lactose into its component monosaccharides -- glucose and galactose -- and absorb them separately. Breaking down lactose requires the enzyme lactase, which you produce in your small intestine.
Sources of Casein and Lactose
Because casein and lactose are both components of milk, it's reasonable to assume that almost anywhere you find casein, you'll find lactose -- and vice versa. There are a few exceptions to this. For instance, lactose-free milk still contains casein, but has been chemically treated to break down the lactose by adding lactase, the same enzyme your body manufactures. Lactase does not affect casein, however.
Intolerance and Allergies
If you have adverse reactions -- gastrointestinal upset, for instance -- when you consume dairy, you may have a casein allergy, a lactose intolerance or an allergy to one of the other proteins in milk. Casein allergies, like other food allergies, can be quite serious. Symptoms include hives, itching, discomfort and difficulty breathing. Lactose intolerance isn't the same as an allergy, and isn't severe, though it can cause uncomfortable bloating and painful gas. If you react negatively to dairy products, avoid them all until you see a doctor, who can help you determine which component of milk you're reacting to.
References
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
- MayoClinic.com: Milk Allergy: Symptoms
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