Buttermilk.

Churning cream strips the milk fat globule membrane into the buttermilk phase, which is why buttermilk is unusually rich in phospholipids including phosphatidylcholine and sphingomyelin relative to its fat content. A phosphatidylcholine supplement supplies the same class of molecule from soy or sunflower instead of dairy. Anyone counting phospholipid intake should count both.

Buttermilk retains most of the lactose of the milk it came from, though cultured versions have some of it fermented to lactic acid already. People with low intestinal lactase activity take supplemental lactase with dairy to hydrolyse lactose to glucose and galactose before colonic bacteria reach it. Cultured buttermilk tends to be tolerated a little better than fresh milk for this reason.

Cultured buttermilk is milk fermented with lactic acid bacteria, usually Lactococcus strains, so the product is a delivery matrix for live cultures as well as a food. Dairy matrices buffer stomach acid and improve organism survival to the intestine compared with water. Whether the resident cultures reach the counts used in probiotic research depends entirely on the product.

Calcium stays in the aqueous phase during churning, so buttermilk carries close to the calcium content of the milk it came from while carrying very little fat. Someone using a calcium supplement alongside regular buttermilk is adding to an intake that is already substantial. The two are the same nutrient from different sources, not a partnership.

Calcium and casein-derived peptides in dairy reduce the fraction of non-heme iron absorbed when both are in the same meal. An iron supplement taken with buttermilk is therefore absorbed less completely than one taken with water or with a source of vitamin C. Separating the two by a couple of hours removes the problem.

Active vitamin D drives the calcium-binding protein machinery in the duodenum that handles calcium uptake at moderate intakes. Dairy calcium is well absorbed, but the active transport route still depends on adequate vitamin D status. This is standard mineral handling, which is why fortified dairy pairs the two.

Buttermilk protein is the same casein and whey mixture as the milk it came from, at a modest concentration. Whey isolate delivers the fast fraction alone, concentrated and with the lactose and fat removed. They overlap in amino acid supply rather than enhancing each other.

Some lactic acid bacteria used in dairy fermentation produce menaquinone-9 and related long chain forms, which is why fermented dairy shows up in dietary K2 analyses. The amounts depend heavily on the strain and the ripening, and are far lower in a fresh cultured drink than in a long-ripened cheese. Read it as a plausible minor contribution rather than a reliable source.

Lactoferrin is a whey protein and stays in the aqueous phase through churning, so some of it carries into buttermilk. Heat treatment denatures it, and pasteurised commercial buttermilk retains far less activity than the raw starting material. Supplemental lactoferrin is isolated and processed specifically to avoid that loss.