Cow’s milk is roughly 87% water and 13% solids, but that 13% packs a surprisingly complex mix of proteins, fats, sugars, vitamins, minerals, and trace bioactive compounds. A single cup of whole milk delivers about 149 calories, 7.5 grams of protein, 12 grams of sugar, and a meaningful share of several nutrients most people don’t get enough of.
Water and the Basics
The vast majority of what you’re drinking is water. Of the remaining 13% solids, about 3.7% is fat and 9% is everything else: protein, lactose, minerals, and vitamins. The fat content is what separates whole milk (3.5% fat), reduced-fat (2%), low-fat (1%), and skim (virtually zero fat). Removing fat lowers calories but slightly concentrates the protein and calcium. A cup of 1% milk, for instance, has 102 calories and 8 grams of protein compared to whole milk’s 149 calories and 7.5 grams.
Protein: Casein and Whey
Milk protein splits into two families in roughly an 80/20 ratio. About 80% is casein, which forms soft curds in your stomach and digests slowly, providing a steady release of amino acids. The other 20% is whey, which stays liquid and digests quickly. This is why the supplement industry sells both casein and whey protein powders for different purposes.
Within the casein family, there’s a genetic split worth knowing about. Beta-casein comes in two main variants, called A1 and A2, that differ by a single amino acid at one position in the protein chain. When your body digests A1 beta-casein, it releases a small peptide called BCM-7 at levels about four times higher than A2 milk produces. Some people who experience digestive discomfort from regular milk report fewer symptoms with A2-only milk, which is now widely sold in grocery stores. Most conventional milk contains a mix of both variants.
Fat: More Complex Than You’d Think
Milk fat contains over 400 different fatty acids, making it one of the most complex natural fats. The three dominant ones are palmitic acid (about 31% of total fat), oleic acid (23%), and stearic acid (12%). Palmitic acid is saturated; oleic acid is the same monounsaturated fat found in olive oil. A cup of whole milk contains about 4.5 grams of saturated fat, 2 grams of monounsaturated fat, and half a gram of polyunsaturated fat.
Fat also carries the fat-soluble vitamins (A, D, E, K) in milk. This is one reason skim milk is typically fortified with vitamins A and D, since removing the fat strips those nutrients away.
Lactose: The Only Sugar
All of the sugar in plain milk is lactose, a natural sugar made of two simpler sugars (glucose and galactose) bonded together. A 150 ml serving of whole cow’s milk contains about 7 grams of lactose. Goat’s milk has slightly less at 6.3 grams per serving, and sheep’s milk falls in between at 6.6 grams. Higher-fat milk actually contains a bit less lactose per serving because the fat displaces some of it.
Your body needs the enzyme lactase to break lactose apart for absorption. People with lactose intolerance produce too little lactase, so the undigested sugar ferments in the gut and causes bloating, gas, and cramping. Lactose-free milk is regular milk with lactase added during processing, pre-splitting the sugar so your body doesn’t have to.
Vitamins and Minerals
Milk is best known for calcium, and the reputation is earned. A liter of milk contains about 1,100 mg of calcium. Drinking just two cups a day covers roughly 40 to 50% of the daily reference intake. But calcium isn’t even the nutrient milk delivers most efficiently.
Vitamin B12 stands out: half a liter of whole milk supplies about 90% of the daily reference intake. Riboflavin (vitamin B2) is another strong point, with the same amount covering 60 to 80% of daily needs. Potassium comes in at 322 mg per cup for whole milk and 366 mg for low-fat, making milk one of the more accessible potassium sources in a typical diet.
Vitamin D doesn’t occur naturally in milk at meaningful levels. In the United States, it’s added during processing at a standard level of 42 IU per 100 grams, which works out to roughly 100 IU per cup. Vitamin A is also commonly added back into reduced-fat and skim varieties to replace what’s lost with the fat.
Hormones and Growth Factors
Milk naturally contains small amounts of hormones and growth factors because it’s a biological fluid designed to support a growing calf. The one that gets the most attention is IGF-1 (insulin-like growth factor 1), a protein involved in cell growth. Levels in regular milk average about 4 ng/mL, which is far lower than the 103 ng/mL found in colostrum (the first milk produced after birth). As a cow’s lactation progresses, IGF-1 drops further, reaching about 1.6 ng/mL by day 210.
Milk also contains trace amounts of estrogens, progesterone, and other naturally occurring hormones. These are present in very small quantities, and the levels vary depending on whether the cow was pregnant at the time of milking.
Enzymes in Raw vs. Pasteurized Milk
Raw milk contains naturally occurring enzymes, including lipase (which breaks down fat) and plasmin (which breaks down protein). There’s a common claim that pasteurization destroys beneficial enzymes, but both of these enzymes are relatively heat-stable, meaning pasteurized milk retains much of their activity. More importantly, no physiological role in human digestion has been demonstrated for any of milk’s native enzymes. Your own digestive enzymes handle the work.
One enzyme that pasteurization does destroy on purpose is alkaline phosphatase. Dairy processors test for its absence as proof that milk has been properly pasteurized, since it breaks down at roughly the same temperature needed to kill harmful bacteria.
What’s Added During Processing
Beyond vitamin D and vitamin A fortification, commercial milk may be homogenized, a mechanical process that breaks fat globules into smaller, uniform particles so the cream doesn’t separate and float to the top. Homogenization doesn’t change the nutritional content; it changes the texture and appearance. Ultra-pasteurized (UHT) milk is heated to a higher temperature for a shorter time, extending shelf life to months when stored in sealed containers, but the nutrient profile stays largely the same.
Flavored milks add sugar, typically 10 to 15 grams per cup on top of the naturally occurring lactose. Organic milk follows the same compositional profile as conventional milk, with the difference being in how the cows are raised and fed rather than in what ends up in the glass.