The Nutritional Facts section covers topics relating to milk and human nutrition and health. A brief overview of food digestion and absorption of nutritional components is provided below as an introduction to this section. Topics covered are:
- energy, water, carbohydrate, fat, protein, vitamins, minerals, minor biological proteins and enzymes
- cow (whole, 2% fat, 1% fat, skim), goat, sheep, and water buffalo
- arthritis, cancer, cardiovascular disease, children's health issues, cholesterol, conjugated linoleic acid, hypertension, lactose intolerance, oral health, osteoporosis, probiotics and diarrheal diseases
Food undergoes mechanical, chemical, and enzymatic digestion as it travels from the mouth to the stomach to the intestines. Many chemicals, enzymes, hormones, glands, and organs are involved in the absorption of nutrients from food.
The first step is mechanical digestion in the mouth – chewing food to break it into small pieces. Saliva, secreted by the salivary gland, contains water that helps dissolve food to make it easier to swallow. Saliva contains the enzyme amylase that begins to break down larger carbohydrate molecules, which makes them easier to digest in the small intestine.
Mechanical digestion continues in the stomach as it churns the food and mixes it with gastric juice. The stomach contains hydrochloric acid that dissolves additional food components that didn't dissolve in saliva. The highly acidic environment in the stomach causes proteins to unfold (denature) so that they can interact with the enzyme pepsin, which breaks down proteins into smaller molecules (peptides) for easier digestion in the small intestine. Gastric lipase begins to break down fats for further digestion in the small intestine.
The majority of food digestion occurs in the first section of the small intestine, the duodenum. As food leaves the stomach and enters the duodenum, hormones signal the pancreas to release sodium bicarbonate to neutralize the stomach acid so that digestive enzymes can act. The pancreas releases enzymes for the digestion of all major food components – proteases to break down proteins into small peptides, amylases to break down carbohydrates into smaller molecules, and lipases to break down fats. Hormones trigger the release of bile that is produced in the liver and stored in the gall bladder to aid in emulsification of fats for digestion and absorption. Further breakdown of peptides into amino acids and small carbohydrates into individual sugar molecules occurs by enzymes that are secreted by the intestinal cells.
The main carbohydrate in milk is lactose, which is a molecule that must by broken down (hydrolyzed) into its two individual sugars (glucose and galactose) before absorption. Lactose is broken down by the enzyme lactase that is secreted by the intestinal cells. Lactase activity often decreases as people age or may be deficient in some populations, which can result in lactose malabsorption or lactose intolerance.
Absorption of nutrients occurs in the second and third sections of the small intestine, the jejunum and ileum. Proteins, carbohydrates, fats, fat-soluble vitamins, water-soluble vitamins, and minerals are absorbed using different processes by specialized cells. Some nutrients are absorbed by diffusion across the cell membranes, others require transporters or binding proteins, and the absorption of fats involves a complex network of lipid carriers.
After absorption of nutrients in the small intestine, the remaining mass moves into the large intestine. The function of the large intestine is to absorb water and sodium and prepare the remaining mass for excretion by the body.