Milk contains a variety of compounds that require different enzymes to be broken down into smaller end products, which are then absorbed and used by your body as energy sources or building blocks. Milk seemingly contains a little bit of almost everything, including sugars, fats, protein, vitamins and minerals. Separate enzymes are used by your body to metabolize complex sugars into glucose, fats into fatty acids and proteins into amino acids.
The digestion and metabolism of milk starts in your mouth. Although the primary sugar in milk is called lactose, other sugars are present in small quantities and begin being reduced in your mouth with alpha-amylase. Alpha-amylase is secreted from your salivary glands into your saliva and starts to reduce more complex sugars into simpler forms such as maltose and sucrose. Due to its presence in saliva, alpha-amylase is also called salivary amylase. Some gastric amylase in the stomach also works on the sugars before the milk enters your small intestine and comes into contact with intestinal and pancreatic amylase and maltase, which ultimately produce glucose. Glucose is also called blood sugar because it’s the primary energy source that travels in the bloodstream.
Milk also contains some saturated and unsaturated fats. Whole milk contains about 4 percent fat, whereas reduced and low-fat versions contain 2 percent and 1 percent fat, respectively. Some fat digestion starts in the mouth with the secretion of lipase enzyme into your saliva from glands underneath your tongue. However, the majority of fat metabolism and absorption occurs in your small intestine. Lipase secreted from your pancreas, and bile secreted from your gallbladder, reduce fats into fatty acids of different lengths. Some smaller chains are absorbed into the blood and used immediately for energy, whereas longer chains of fatty acids are processed by your liver and stored for later use.
Lactose is the main sugar in milk and related dairy products, which is why it’s often called milk sugar. Lactose starts being digested in the small intestine with the secretion of lactase enzyme, which breaks lactose down into simpler sugars called galactose and glucose. A lack of lactase production or secretion is called lactose intolerance and results in undigested lactose sugar entering the large intestine. Friendly bacteria in the large intestine feast on the lactose via fermentation and produce gas in the process, which leads to abdominal pain, flatulence and diarrhea.
Milk is also a fairly good source of protein. For your body to metabolize protein and absorb amino acids, enzymes called proteases are needed. The first protease milk encounters is in the stomach and called pepsin. Pepsin needs an acidic stomach to function effectively. Proteases are also secreted from your pancreas, which fully reduce protein into single amino acids that are absorbed further down the small intestine. Your body uses amino acids to build and repair certain tissues such as skin, fingernails and muscle fibers.
- Encyclopedia of Human Nutrition; Benjamin Caballero et al.
- Advanced Nutrition and Human Metabolism; James L. Groff et al.
Sirah Dubois is currently a PhD student in food science after having completed her master's degree in nutrition at the University of Alberta. She has worked in private practice as a dietitian in Edmonton, Canada and her nutrition-related articles have appeared in The Edmonton Journal newspaper.