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Food · Animal Foods · Dairy
food source · 3
Milk is a dependable source of calcium: a 250 ml glass holds about 300 mg, and about a third of it is absorbed. By the mechanism, phosphopeptides cut from casein during digestion, together with lactose, help keep calcium in an absorbable state, but how much they matter in adults is not settled.
Mechanism · Why calcium has to stay dissolved
Calcium has to be a free ion dissolved in the gut fluid before the carrier proteins in the gut wall recognize it. The farther down the small intestine, the more alkaline it gets, and the more easily calcium comes out of solution and clumps. Milk brings two possible helpers: casein phosphopeptides, cut from casein during digestion, hold calcium ions like small clips and keep them soluble; lactose may make it easier for calcium to seep through the gaps between the cells of the gut wall. Both make sense by the mechanism, but studies disagree on how much extra calcium they bring in for adults. Spinach is the opposite case: its oxalate binds calcium on contact into nearly insoluble calcium oxalate. The calcium content of spinach is real, but little of it is absorbed.
Numbers · How much calcium a glass holds
Milk has about 120 mg of calcium per 100 g, or about 300 mg in a 250 ml glass, close to a third of the US adult Recommended Dietary Allowance (RDA). About 30–35% of it is absorbed, against about 20% from almonds, while the oxalate in spinach locks up nearly all of its calcium. Skim milk loses no calcium: calcium stays in the watery part with the casein, and skimming removes only the fat. Natural milk has little vitamin D; many regions fortify it, which puts calcium and the vitamin that helps it into the blood in the same glass.
Milk protein is complete, with enough of all nine essential amino acids. About 80% is casein, which curdles in the stomach and digests slowly; about 20% is whey, which digests fast and is rich in leucine. A 250 ml glass holds about 8 g.
Mechanism · One slow protein and one fast
Casein curdles when it meets stomach acid and is released slowly and steadily; whey does not curdle, is digested and absorbed quickly, and is rich in leucine, and it is the raw material of the whey powder people drink after a workout. A glass of milk carries both, so one share arrives quickly and the other is released slowly.
Numbers · Whole milk versus skim
Milk has about 3.2–3.4 g of protein per 100 g, or about 8 g in a 250 ml glass. Whole milk has about 60–65 kcal per 100 g and skim milk about 34–36 kcal; a glass comes to roughly 150 kcal for whole and 85 kcal for skim. The gap is real but not large, and both carry about the same protein. Carbohydrate is about 4.8 g per 100 g, mostly lactose.
B12 comes almost only from animal foods. A 250 ml glass of milk supplies about 45% of an adult's recommended daily amount, which makes milk an important source for lacto-ovo vegetarians, who eat no meat.
Mechanism · Three steps after you drink it
Only bacteria and archaea make B12; cows get it from the microbes in their rumen and store it in meat and milk. The B12 in milk is dissolved in the watery part, so skimming does not remove it. After you drink it, B12 still has three steps to pass before it reaches the blood: stomach acid frees it from protein, intrinsic factor (an escort protein made by the stomach) picks it up, and the last part of the small intestine takes the pair in.
Numbers · The B12 in one glass
Milk has about 0.45 μg of B12 per 100 g; a 250 ml glass supplies about 45% of the adult Recommended Dietary Allowance (RDA). Milk supplies B12, calcium and protein, but not iron: it has almost no fiber, very little vitamin C and little iron, so adults cannot rely on milk for iron.
contrast · 1
The milk in plant milk describes how it looks. A glass of oat or almond drink has a fraction of the protein in the same glass of cow milk, and only soy drink comes close on protein; calcium, vitamin D and B12 are there only when added. Swapping plant milk for cow milk means choosing a fortified one and reading the label, not assuming the nutrition is the same.
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