Milk, Kefir and Gut Flora

Milk is a dramatic manipulator of gut flora.  It is a baby’s first food and provides necessary nutrients, but of equal importance, it crafts a community of gut microorganisms that develop the gut and immune system of babies.  Breastfed babies receive protein, fat and sugar, but they also coat their tiny stomachs and even their respiratory system with maternal lymphocytes and bacteria.  The major carbohydrate in breast milk is lactose, but there are other prebiotic oligosaccharides (HMO, human milk oligosaccharides) and polysaccharides (GAGs, glycosaminoglycans) related to heparin and chondroitin, which carefully limit which bacteria can grow in babies.  There are firm reasons why exclusively breastfed babies have diapers that smell like yogurt and look like curds and whey.

Milk Shows an Exaggerated Interaction between Food and Flora

Evolutionary selection is extreme to favor women who can successfully birth large babies and nurture them for years on breast milk.  Fewer than 5% of women in a general population need medical intervention for gestation, labor, delivery and breastfeeding.  Essentially all women and babies are genetically predisposed to healthy childbirth and milk-based child development.  Clearly, milk is powerful and an examination of the composition of milk should yield information on the interaction between food and gut flora.

Milk is the Prebiotic for Dairy Probiotics

Traditional preservation of cow's milk produces fermented kefir, butter, yogurt, cheeses, etc.  These are all controlled fermentations that begin by converting lactose into lactic acid.  Michael Pollan devoted a major section of his book, Cooked, to the cultural ramification and biology of fermentation.  It seems magical that leaving milk to sour will reproducibly yield a common dairy beverage.  When I taught microbiology, I had students spike raw cow’s milk with E. coli and then measure the decreasing survival of these common gut bacteria that are actively excluded from dairy fermentation.  One of the lessons here is that milk stops the growth of adult gut bacteria and supports the growth of lactic acid bacteria found in baby diapers and used to make fermented dairy products.

Milk is Toxic to Most Microorganisms, until Digested
Enzymes in the stomach convert milk proteins into antimicrobial peptides.  Later in the small intestines, pancreatic proteases digest and inactivate the peptides until they are converted into amino acids and are absorbed by the intestinal microvilli.  Milk is a natural antibiotic and is used ritually for cleansing wounds and pruning hooks.  Ritual fire walking ends by walking through a pool of cow’s milk.  The spread of plant disease in orchards from tree to tree is minimized by dipping pruning tools in milk.  The proteins, fatty acids and carbohydrates in milk kill or inhibit the growth of viruses, bacteria and fungi.  Early studies of the bacteria in breastfed babies showed an exclusive group of lactic acid bacteria and an absence of adult gut bacteria.  Breast milk was shown to contain a “bifidus factor” that selected for baby gut flora and this special ingredient was later shown to consist of a complex mixture of short chains of sugars, human milk oligosaccharides.  Thus, human milk is good for babies, but bad for adult gut flora because most of the protein, fat and carbs are digested and no soluble fiber remains for colon gut flora. 

Formula Kills Baby Gut Flora

Formula made from cow's milk or soy is toxic to baby gut flora and even a single bottle of formula can permanently damage it.  The disastrous impact of formula on gut flora is readily observed in the change to smelly diapers.  Mothers trying to give the best start to their babies can tell when the night nurse got lazy and just fed her baby a bottle of formula!  Use of formula in hospitals instead of mothers nursing or using donor milk greatly increases contamination of babies with deadly strains of hospital bacteria, e.g. C. dificile, and causes necrotising enterocolitis.  The only reason that babies can survive formula and the growth of adult gut flora in the first weeks of life, is that the disrupted gut flora is highly inflammatory and the inflamed gut provides some protection from infection.  Babies are tough, but there is no reason for hospitals to continue to use formula when research clearly shows that it is a risk to the health of babies.  Health concerns are forcing hospitals to encourage exclusive breastfeeding, but more work needs to be done so that donor breast milk is the alternative.

Raw Avoids Risks of Pasteurization and Ultra Homogenization
Milk straight from the udder contains natural dairy probiotics that are fit for a calf.  Dairy probiotics are different from baby gut flora and calves are different from babies, so cow milk is not appropriate for babies.  Processing cow's milk by heat (pasteurization) or extreme mixing to make ultra small fat droplets (homogenization) changes the structure of milk to increase storage shelf life, but the restructuring also produces some health risks for gut and gut flora.  Since leaves are rich in short chain omega-3 fatty acids and seeds are rich in omega-6s, grass fed cows produce healthier (higher 3/6 ratio) milk that may not store as well.

Kefir is a Yeast and Bacteria Biofilm

Commercial dairy products are uniform, because they are made from milk using defined mixtures of pure cultures of bacteria and fungi.  These dairy probiotics can substitute but not replace gut flora, because they can't grow in a healthy gut.  Kefir is a little different, because the kefir grains are biofilms of yeast and bacteria held together by a polysaccharide called kefiran made by a bacterial enzyme that rearranges the glucose and galactose sugar residues of lactose.  The point here is that if you grow your own kefir, you may end up with many species of bacteria and some may be able to contribute to your gut flora.  Many supermarket "kefirs" are just a blend of common dairy probiotics and maybe some inulin, and have no benefits over commercial yogurt.

Dairy products are nutritious, but will not benefit the health of your gut flora (fermented vegetables are a better choice), because they lack soluble fiber and do not contain gut flora, but your gut flora may adapt to the inherently disruptive nature of raw milk.