Bloating or diarrhea after drinking milk usually means the small intestine is short of the enzyme that splits lactose (lactase). The unsplit lactose travels on to the large intestine, where bacteria ferment it. Milk is not toxic; the chapter on lactose intolerance walks through the chain.
Milk itself is not just water with a little protein dissolved in it. It is a structured liquid: protein, fat, lactose, minerals, and vitamins are packaged in particular ways, and some components may help others get absorbed, calcium being one example.
Whole, semi-skimmed, skim, lactose-reduced, UHT, and pasteurized milk differ by more than their fat.
Numbers · What 100 g of milk holds
Per 100 g, whole cow's milk has roughly 3.2 g of protein, 3.5 g of fat, 4.8 g of carbohydrate (mainly lactose), and about 60-65 kcal. A 250 ml glass holds about 8 g of complete protein.
Skim milk has the fat removed while protein and calcium largely stay, but the fat-soluble vitamins (A, D, E, K) leave with the fat, which is why many skim milks are fortified.
Background · Skim, lactose-reduced and UHT compared
Whole, semi-skimmed, and skim milk differ in fat, and in the fat-soluble vitamins that travel with it. Lactose-reduced milk has had lactase added at the factory. Ultra-high-temperature (UHT) milk keeps at room temperature; pasteurization is gentler and keeps some heat-sensitive nutrients slightly better. Calcium and protein differ little across these forms.
Chapter 2
Protein, fat and carbs, all moderate
Milk is one of the few natural foods that supplies all three macronutrients, none of them in extreme amounts. Its protein is complete: about 80% is casein, which curdles in the stomach and digests slowly; about 20% is whey, which digests fast and is rich in leucine (see Protein & Amino Acids).
The carbohydrate is mainly lactose, which has to be split by lactase before it can be absorbed. Without enough of the enzyme, this double sugar reaches the colon intact.
Numbers · Protein and calories in one glass
Protein: about 3.2-3.4 g per 100 g. That is not as dense as chicken breast, but a 250 ml glass still delivers about 8 g of complete protein. Casein curdles in the stomach and releases slowly and steadily; whey is the raw material of the protein shake many people drink after a workout (see Protein & Amino Acids).
Fat: whole milk has about 3.5 g per 100 g, mostly saturated with some monounsaturated fat; skim milk has almost none. Carbohydrate is about 4.8 g per 100 g, mostly lactose, and digesting it takes lactase, which the chapter on lactose intolerance covers.
Calories: whole milk has about 60-65 kcal per 100 g, skim about 34-36 kcal. A 250 ml glass comes to roughly 150 kcal for whole and 85 kcal for skim. The gap is real, but not enormous.
Chapter 3
Whole or skim is not clear-cut
"Switch to skim to eat less saturated fat" treats all fat as one substance. But when milk reaches your mouth, its fat arrives packaged with calcium and protein.
One hypothesis holds that this packaging (the dairy matrix) changes what the fat does in the body. It is still a hypothesis, not proof that whole milk protects the heart: what supports it is argument in a review on one side and population cohorts, which can only show associations, on the other (see Fat Types).
Numbers · Which fatty acids are in milk fat
Roughly 60-70% of whole milk's fatty acids are saturated, 25-30% are monounsaturated, and a small share are polyunsaturated. The saturated group includes butyric acid and medium-chain fatty acids as well as long-chain palmitic acid, so their metabolism does not follow a single path (see Fat Types).
Translating "eat less saturated fat" straight into "you must switch to skim" skips the packaging.
Evidence · What backs the dairy-matrix idea
Astrup 2020 is a state-of-the-art review in the Journal of the American College of Cardiology (JACC). It argues that the health effect of saturated fat depends on the whole food it comes in, and that fermented dairy and cheese look neutral to slightly favorable for cardiovascular risk. It is an argument about how to read the evidence, not a license to drink unlimited whole milk.
The PURE study (Dehghan 2018) is a prospective cohort covering 21 countries: people who ate more dairy, whole-fat included, had lower death rates and fewer major cardiovascular events. That is an observed association, not a . People who drink more milk may already live differently, so it cannot be turned into "whole milk protects the heart".
The dairy-matrix hypothesis tries to explain this: when calcium, phosphorus, protein, and fat come packaged together, digestion and metabolism may play out differently from eating the same fat on its own. It explains why cohort results do not match models that look at fat alone; it does not turn milk into a heart-protecting food. For people who already eat a lot of fat, or who have familial hypercholesterolemia, low-fat milk remains a safe choice. Skim milk keeps all its calcium and protein but loses fat-soluble vitamins, so check the label for fortification (see Vitamin D).
Chapter 4
Rich in calcium, B2 and B12
Milk's standout micronutrient is calcium. Calcium has to stay a dissolved ion to cross the gut wall. By the mechanism, phosphopeptides cut from casein during digestion, together with lactose, help keep it in an absorbable state; how much they matter in adults is not settled.
Natural milk does not contain much vitamin D, and many regions fortify it, which puts calcium and the vitamin that helps it into the blood in the same glass (see Calcium, Vitamin D). Milk also brings B2, B12, and phosphorus.
Mechanism · Why calcium from milk gets through
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 (). About 30-35% of it is absorbed, more than from many plant sources (the oxalate in spinach locks up nearly all of its calcium; almonds come in at about 20%).
The absorption rate is the part of the calcium you drink that actually crosses the gut wall into the blood. The rest is not rejected by the body. It never became a form that could be carried, and it leaves in the stool as it came.
To understand the gap between those numbers, start with the precondition for absorbing calcium: it has to be free calcium ions dissolved in the intestinal fluid. Once calcium forms an insoluble salt with something else, it becomes a speck of precipitate in the gut. The proteins on the gut wall that do the carrying recognize only ions, not precipitate.
The trouble is that the farther down the small intestine you go, the harder it is for calcium to stay dissolved. Food leaving the stomach still carries acid, which is neutralized step by step on the way down, and calcium in a more alkaline setting tends to come out of solution and clump. So calcium is racing against clumping the whole way: whatever is still dissolved when it reaches the absorption site has a chance to be carried in.
Milk brings two possible helpers.
The first comes from protein. Casein curdles in the stomach, and digestive enzymes then clip it into short peptides. One class of those peptides carries phosphate groups; these are casein phosphopeptides (often shortened to CPP). They act like little clips that can hold calcium ions and keep them soluble even in the more alkaline lower small intestine.
The second comes from sugar. Lactose in milk is split slowly in the gut, and its presence may make it easier for calcium to seep through the gaps between the cells lining the intestine. This route of squeezing between cells does not need vitamin D to open the door; it is a supplementary channel alongside active transport.
Both helpers make sense by the mechanism, but studies disagree on how much extra calcium they actually bring in for adults.
Where spinach loses. Spinach is rich in oxalate. The moment oxalate and calcium meet in the gut, they form calcium oxalate, an almost insoluble salt. Calcium locked into that precipitate is no longer a free ion; the carrier proteins do not recognize it, and the whole lump leaves in the stool. So spinach's calcium content is real. What is misleading is what that number implies about how much you get: what is in the food and what enters your body are separated by a whole ledger of solubility.
Almonds lose in a different way. Some of the calcium in nuts is bound by phytate, and some is locked inside tough cell structures that have to be chewed apart and opened by digestive juices before it is released. It is not precipitated out; it is never let out. About 20% is absorbed.
Dissolving is only the first half. Once calcium has kept its ion form and reached the lining of the small intestine, it still has to cross that layer of cells to count as absorbed, and the capacity of that active channel is set by vitamin D. Natural milk does not contain much vitamin D, and many regions fortify it. So vitamin-D-fortified milk is not one more good thing piled on. It puts the material and the door-opener into the same glass (see Vitamin D).
Once the chain is clear, you can work out the next few yourself:
Comparing spinach's calcium content with milk's is basically meaningless: you are comparing what goes into the mouth, while the outcome is decided at the dissolving step.Why skim milk's calcium is not reduced: calcium sits in the watery part and travels with casein. Skimming removes fat; the calcium and the casein both stay.Why calcium from yogurt and cheese absorbs just as well: fermentation mainly changes the lactose. The casein is still there, so the little phosphate clips are still there too.
Numbers · B2, B12 and phosphorus
Riboflavin (vitamin B2): about 0.17-0.18 mg per 100 g, so one glass supplies roughly a third of the US adult Recommended Dietary Allowance (). It is the raw material for two coenzymes, FAD and FMN, which stand at the entrance to energy metabolism in the cell (see Riboflavin).
To make entrance concrete: the sugar and fat you eat are eventually broken down into electrons and handed to the respiratory chain in the mitochondria before they can be turned into energy. FAD and FMN are the two clips that take those electrons and pass them to the next runner, and their backbone is riboflavin. When riboflavin runs short, there are not enough hands to pass electrons along, and what shows up is often not one organ going wrong but a little less strength everywhere.
Vitamin B12: about 0.45 μg per 100 g; a 250 ml glass supplies about 45% of the adult RDA. This matters most for lacto-ovo vegetarians, who eat no meat: B12 in natural foods comes almost only from animal foods (plus fortified foods), and milk is an important source for them (see Vitamin B12).
Phosphorus: together with calcium it forms hydroxyapatite, the mineral that gives bone its hardness.
The phosphorus point connects to the chain on how calcium is absorbed. Calcium that has entered the blood is only halfway there; turning into bone means calcium and phosphorus crystallizing together on the collagen net laid down by bone-building cells (osteoblasts). Milk packs calcium and phosphorus into the same glass, handing over both building materials that step needs at once.
Chapter 5
What it lacks · what milk can't cover
Milk has no dietary fiber, very little vitamin C, and little iron. Adults cannot rely on milk for iron; its companions are whole grains, vegetables, legumes, and berries (see Iron).
Infants under 1 year should not be given ordinary cow's milk in place of breast milk or formula: milk is low in iron, and drinking a lot of it too early raises the risk of iron deficiency.
In practice · Filling milk's gaps at the table
Dietary fiber is zero. Milk does not feed the gut microbes; its companions are whole grains, vegetables, and legumes. Vitamin C is minimal, and pasteurization and UHT processing lose a little more. Fruit rich in vitamin C fills the gap, and vitamin C also helps you absorb the iron in plant foods (non-heme iron) (see Iron).
Iron is very low. Infants who drink a lot of ordinary cow's milk too early, in place of breast milk or formula, face a higher risk of iron deficiency.
A practical pairing is oats with milk for breakfast. The oats add fiber and beta-glucan; the milk adds protein, calcium, B2, and B12; a handful of berries adds vitamin C and polyphenols. The three complement each other, and the meal ends up dense in micronutrients.
Safety · Stacking calcium pills on dairy
Stacking high-dose calcium supplements on top of plenty of dairy is not always better. In one large randomized trial, postmenopausal women who took calcium plus vitamin D supplements for years had slightly more kidney stones. A link between very high calcium intake (including the calcium in milk) and prostate cancer comes only from observational studies, and those do not agree. For kidney stones, calcium from food has not shown the same problem (see Calcium).
Chapter 6
What lactose intolerance is
Lactose intolerance means not enough lactase; it does not mean milk is toxic. Lactose is a double sugar made of glucose and galactose. Without enough enzyme, it reaches the colon intact, where bacteria ferment it into hydrogen, carbon dioxide, and short-chain fatty acids, and the result is bloating, cramps, and diarrhea.
How much of the enzyme an adult keeps is set mainly by variants near the lactase gene (LCT), and it differs between populations. It is not an allergy. Many people have far fewer symptoms if they drink small amounts, drink milk with meals, or switch to yogurt and cheese.
A CLOSER LOOK
Unsplit lactose travels to the colon
This explains a general mechanism, not a self-diagnosis; tolerated amounts differ between people.
1The splitting enzyme is short
With insufficient lactase, lactose is not fully split into glucose and galactose.
2Fermentation in the colon
Bacterial fermentation produces gas and short-chain fatty acids, explaining symptoms such as bloating.
Illustration for understanding; not to scale. Saved figures include explanations and sources.
Mechanism · Where undigested lactose goes
In normal digestion, lactase on the surface of the cells lining the small intestine splits lactose into two simple sugars, which are then absorbed into the blood. When the enzyme is short, lactose travels into the large intestine intact. There gut bacteria ferment it, producing hydrogen, carbon dioxide, and short-chain fatty acids, which is where the bloating, cramps, and diarrhea come from.
This is not milk being toxic, and it is not an allergy to milk protein. An allergy is an immune reaction; intolerance is a question of how much enzyme you have.
Background · Why some adults still digest milk
In all mammals, lactase activity falls after weaning, and humans are no exception, but how far it falls differs greatly between populations. A review by Ingram 2009 explains that adult lactase activity is set by variants near the lactase gene (LCT), with large differences between populations. In Northern Europe and some traditional herding populations, most people carry a variant that keeps lactase working into adulthood. In East Asian, West African, and Indigenous American populations, most people's lactase activity drops sharply in adulthood. This is not a disease. It is a difference left by evolution and the history of farming and herding.
How much people can tolerate varies widely. Many have clearly fewer symptoms if they drink small amounts, drink milk with a meal rather than on an empty stomach, or switch to fermented forms such as yogurt and cheese. Lactose-reduced milk, which has lactase added at the factory, is another option.
This is general information only and does not replace a doctor's or clinical dietitian's assessment of your symptoms.
There is no universal answer to whether whole or skim is better. Plant milks are not cow's milk unless they are fortified, and on protein, soy milk comes closest.
Infants under 1 year should not drink ordinary cow's milk. Milk-protein allergy and lactose intolerance are different things: the first is an immune reaction and means avoiding milk; the second is a shortage of enzyme and can be managed by changing how you drink it.
In practice · Whole, skim or plant milk
Whole milk keeps the fat-soluble vitamins, along with the dairy matrix that may change how its fat is handled (still a hypothesis; the chapter on choosing whole or skim covers it). Skim milk has fewer calories with the same protein and calcium, a reasonable choice for people who need to limit fat. If you choose skim, check the label for added vitamins D and A.
Plant milks: soy milk comes closest to cow's milk on protein (about 3-4 g per 100 g when fortified); oat milk is higher in carbohydrate and low in protein; almond milk has very little protein. Plant milks usually need added calcium and vitamin D to match cow's milk, so drinking plant milk does not mean getting the same nutrition; read the label. Vegetarians who need B12 should choose a plant milk fortified with B12.
Safety · How to use it, and who should be careful
How to use it: milk with whole-grain oats is a classic breakfast; adding milk to soups or sauces raises their protein and calcium; making coffee or tea with milk instead of water is an unremarkable but effective way to add calcium.
Who should take care: infants under 1 year are not advised to drink ordinary cow's milk as a main drink (it is low in iron and puts a heavier load on the kidneys). People with cow's-milk-protein allergy should avoid it; that is an immune reaction, not the same thing as lactose intolerance.
This is health education and does not replace a doctor's or registered dietitian's judgment of your situation.
Chapter 8
Three common claims that fall short
Three lines are often said as if settled, and the evidence does not support that tone: "milk is inflammatory", "skim is always healthier", and "plant milk is nutritionally the same as cow's milk".
No high-quality has shown that milk causes body-wide inflammation in healthy adults. The dairy matrix is still a hypothesis and cannot settle whether whole or skim is better. Among plant milks, only fortified soy milk comes close to cow's milk in the main nutrients.
Myth · Does milk cause inflammation?
"Milk is an inflammatory food and adults should not drink it" is a widespread claim that rests on thin evidence. No high-quality has shown that ordinary healthy adults develop body-wide inflammation from drinking milk. Some people's lactose-intolerance symptoms get misread as an inflammatory reaction. True cow's-milk-protein allergy does exist, but it is an immune reaction and not common. Turning some people's symptoms into "milk causes inflammation" swaps one idea for another.
Myth · Skim is healthier; plant milk is the same
"Skim is always healthier than whole" is taken apart in the chapter on choosing whole or skim. For most people, moderate whole milk is not clearly worse for the heart and blood vessels than skim. The dairy matrix makes the question more complicated than "saturated fat is always harmful", but the matrix is still a hypothesis, not proof that whole milk protects the heart. If you already drink whole milk, there is no need to switch to skim out of health anxiety; choosing skim to control calories is just as reasonable.
"Plant milk is nutritionally the same as cow's milk" needs a lot of footnotes. Only soy milk fortified with calcium, vitamin D, and B12 comes close to cow's milk in the main nutrients; oat and almond milks fall well short on protein and minerals. Plant milks are good alternatives, but choose fortified versions and compare labels rather than assuming they are equivalent.
References · 10
U.S. Department of Agriculture, Agricultural Research Service. (2019). FoodData Central: Milk (whole 3.25%, reduced-fat, fat-free, with added vitamin D). ~3.2 g protein and ~120 mg calcium per 100 g; commonly fortified with vitamin D. fdc.nal.usda.gov
Astrup, A., Magkos, F., Bier, D. M., et al. (2020). Saturated fats and health: a reassessment and proposal for food-based recommendations: JACC state-of-the-art review. Journal of the American College of Cardiology, 76(7), 844-857. Argues saturated fat's health effect depends on the whole food matrix, with fermented dairy and cheese neutral-to-favorable for cardiovascular risk. 10.1016/j.jacc.2020.05.077
Dehghan, M., Mente, A., Rangarajan, S., et al. (2018). Association of dairy intake with cardiovascular disease and mortality in 21 countries from five continents (PURE): a prospective cohort study. The Lancet, 392(10161), 2288-2297. Higher dairy intake, including whole-fat, tracked lower mortality and major cardiovascular events. 10.1016/S0140-6736(18)31812-9
National Institutes of Health, Office of Dietary Supplements. (2024). Vitamin D — Fact Sheet for Health Professionals. Fact sheet (updated June 27, 2025; Wayback snapshot 20 September 2026): 25(OH)D below 30 nmol/L (12 ng/mL) is associated with deficiency, 30 to below 50 nmol/L is generally considered inadequate, 50 nmol/L (20 ng/mL) or more is sufficient for most people, and above 125 nmol/L (50 ng/mL) can be associated with adverse effects; RDA 600 IU (15 mcg) to age 70 and 800 IU (20 mcg) above 70; adult UL 4,000 IU (100 mcg); older people and people with darker skin (more melanin) make less vitamin D from sunlight (fact sheet). ods.od.nih.gov/factsheets/VitaminD-HealthProfessional
National Institutes of Health, Office of Dietary Supplements. (2024). Vitamin B12 — Fact Sheet for Health Professionals. Fact sheet (updated July 2, 2025; Wayback snapshot 20 September 2026): multivitamin/mineral supplements typically contain 5 to 25 mcg B12, B-complex products 50 to 500 mcg, B12-only supplements typically 500 to 1,000 mcg; absorption is only about 2% at 500 mcg and 1.3% at 1,000 mcg; a 2018 Cochrane review of 3 RCTs (153 participants) compared very high oral doses (1,000-2,000 mcg) with intramuscular B12; high oral doses (e.g. 1,000 mcg/day) might be equally effective in Crohn's disease and appear as effective as hydroxocobalamin injections after Roux-en-Y bypass. These are product contents and trial doses; the sheet gives no recommended daily supplement range (fact sheet). ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional
National Institutes of Health, Office of Dietary Supplements. (2024). Iron — Fact Sheet for Health Professionals. Fact sheet (updated September 4, 2025; Wayback snapshot 21 September 2026): RDAs 8 mg/day for men and for women 51+, 18 mg women 19-50, 27 mg pregnancy; UL 45 mg/day from age 14; bioavailability about 14%-18% from mixed diets with meat, seafood and vitamin C and 5%-12% from vegetarian diets; serum ferritin below 30 mcg/L suggests iron deficiency and below 10 mcg/L IDA, but inflammation can raise ferritin; supplemental iron of 45 mg/day or more may cause nausea and constipation; people with hereditary hemochromatosis are at risk of iron overload (fact sheet). Heme vs nonheme: heme iron (lean meat and seafood are the richest sources) has higher bioavailability than nonheme iron, and other dietary components affect it less; calcium might reduce the bioavailability of both forms; heme iron is about 10%-15% of total iron intake in western populations. The sheet gives no separate heme and nonheme absorption percentages (fact sheet, Wayback 2026 snapshot). ods.od.nih.gov/factsheets/Iron-HealthProfessional
Ingram, C. J. E., Mulcare, C. A., Itan, Y., Thomas, M. G., & Swallow, D. M. (2009). Lactose digestion and the evolutionary genetics of lactase persistence. Human Genetics, 124(6), 579-591. Adult lactase activity is governed by LCT-region variants, with large between-population differences. 10.1007/s00439-008-0593-6