Lactose

Lactose is the sugar of milk. It is a sugar of the disaccharide type, made of one glucose joined to one galactose. An enzyme called lactase splits the two apart at the gut wall.

Lactose is the main sugar of human milk, which holds about 6.7 to 7.8 g per 100 mL when mature. In our USDA ranking, semi-skimmed cow’s milk has 5.01 g per 100 g. No daily need or limit is set for lactose on its own.

Most adults in the world make less lactase than they did as babies. A 2017 review puts lactase non-persistence (losing the enzyme after childhood) at 65% to 70% of the world’s adult population. Keeping the enzyme into adult life is the unusual trait.

It comes from small DNA changes next to the lactase gene. One, called C/T-13910, is found in Europeans, and three different ones in Tanzanians, Kenyans and Sudanese. That makes it a textbook case of the same adaptation arising more than once.

Here is what most readers have not been told: making less lactase does not mean milk makes you ill. An NIH (US National Institutes of Health) expert panel concluded in 2010 that most people with lactose malabsorption do not have clinical lactose intolerance. Malabsorption means not fully digesting lactose; intolerance means getting symptoms from it.

In a blinded trial, 30 people who said even a little milk upset them had only minimal symptoms on a cup of milk a day. That was no different from lactose-hydrolyzed milk, whose lactose is already split. A systematic review found that 12 to 15 g of lactose, about a cup of milk, is well tolerated by most adults.

In an analysis of 25 protected-origin Italian cheeses, most held too little lactose to measure. This page sets out what 23 studies and reports show, each at its real evidence level, including where the evidence is thin.

Health effects of lactose

• ★★★Lactase supplements aid lactose digestion (EU claim)The EU allows a claim that lactase supplements improve lactose digestion in people who have difficulty with it. In one trial of 47 adults with confirmed lactose intolerance, a single dose of lactase tablets cut breath hydrogen by 55% and eased symptoms.
• ★★Most tolerate 12 to 15 g of lactoseIn a review of 36 trials, most adults with lactose malabsorption tolerated 12 to 15 g of lactose, about a cup of milk. An NIH panel said most people do not need to cut out dairy completely.
• ★★Lactose-free foods may shorten children’s diarrheaIn 33 trials of children under 5 with acute diarrhea, it lasted about 18 hours less (17.77 hours) on lactose-free milk or foods. This applies to children who are not mainly breast-fed.
• ★Daily milk caused few symptoms in self-reported intoleranceIn two small week-long trials, adults who said they were lactose intolerant had no significant rise in symptoms on regular milk compared with lactase-treated milk. In one trial, people with malabsorption had 2.5 more episodes of gas a day.
• ★Yogurt lactose was digested better than milk lactoseYogurt with 18 g of lactose gave people with lactase deficiency about one third the breath hydrogen of milk with the same. Yogurt also brought fewer reports of diarrhea or flatulence.

Foods high in lactose

Lactose is the sugar of mammal milk, so the real sources are milk and foods made from it. In our USDA ranking per serving, semi-skimmed cow’s milk is the only food with a lactose value. It has 12.3 g in 1 cup (246 g), or 5.01 g per 100 g.

The ranking is short because our data holds a lactose value for very few foods, not because other dairy foods have none. So it cannot tell you about yogurt, cheese or ice cream. By our arithmetic, a cup of milk sits at the low end of a key range. Most adults with malabsorption tolerated 12 to 15 g in the 2010 review.

Fermented and aged dairy differ from milk. In a breath test, each test meal held 20 g of lactose. Plain yogurt gave 76 ppm·h of breath hydrogen and plain kefir 87, against 224 for milk.

In the Italian cheese analysis, most of 25 protected-origin cheeses held under 10 mg of lactose per kg. Fresh Mozzarella di Bufala, the positive control (a sample known to hold lactose), held 3,540 mg per kg (about 0.35%, our arithmetic). A Pecorino Toscano held 336.8 mg per kg at 20 days and 28 mg per kg at 60 days.

A 2017 review lists hard cheeses, yogurt and lactose-hydrolyzed milk among the lower-lactose foods that people with intolerance can use.

Lactose also hides outside the dairy aisle. It is the most common filler (excipient) in drug formulations, and the amount is not on the label.

On food labels, the 2025-2030 US Dietary Guidelines list lactose among the names under which added sugars can appear. They say the sugars naturally in plain milk are not added sugars. The WHO (World Health Organization) free sugars guideline likewise does not cover sugars naturally present in milk, because no adverse effects of them were reported.

How lactose works

The basic biochemistry is well established, and what it means for people is tested in the study cards above.

Four ideas cover most of it. Lactase splits lactose at the gut wall. Most adults make less lactase after weaning, under gene control.

Lactose that is not split is fermented in the colon. Yogurt brings its own lactase. Here is each one, and what has been seen in people.

Lactase splits it at the gut wall

Lactase, formally lactase-phlorizin hydrolase, sits in the cells lining the small intestine. It splits lactose into glucose and galactose. Lactose is the main source of calories in milk for infants.

Misselwitz and colleagues separate three terms that are often mixed up. Lactase deficiency means the enzyme is not made. Lactose malabsorption is any failure to digest or absorb lactose in the small intestine. Lactose intolerance is abdominal pain, bloating and diarrhea after lactose in someone with malabsorption.

Malabsorption can be primary, from genes, or secondary, from infection or other conditions that harm the lining of the small bowel.

Most adults make less lactase after weaning

Enattah and colleagues describe lactase non-persistence as a common autosomal recessive condition: lactase activity falls after weaning. The persistence changes sit in a stretch of DNA about 14 kb upstream of the lactase gene. In test-tube studies, that stretch acts as an enhancer (a switch that turns the gene up).

Ingram and colleagues explain that several separate changes within 100 bases of each other can each lead to persistence. The European T allele sits on a haplotype (a block of DNA inherited together) of 500 kb or more. That means it is recent and was strongly favored by selection.

Tishkoff and colleagues found three other changes in East African groups that raise activity of the lactase promoter. They sit on different haplotype backgrounds from the European one.

The simple story, that drinking milk caused the spread of persistence, is now under question. A 2009 simulation placed the first selection of the European allele about 7,500 years ago. That was among dairying farmers between the central Balkans and central Europe.

The 2022 pottery study found that selection did not track how much milk was used. It also found the persistence genotype only weakly linked with milk intake in modern Europeans.

A gene test can show non-persistence in northern Europeans, but not whether a person gets symptoms. The link between self-reported intolerance, test results and the response to a diet is variable.

Undigested lactose is fermented in the colon

Lactose that is not split reaches the colon, where bacteria ferment it. The NIAID (US allergy institute) food allergy guidelines describe the result as excess fluid in the gut, with abdominal pain and diarrhea. In stool samples from 28 Chinese adults, 5 consistently intolerant people fermented lactose to acetate, propionate, butyrate and lactate faster than 7 consistently tolerant ones. The rate of lactose breakdown and the bacterial mix did not differ.

The colon can also adapt. In 9 lactose maldigesters, fecal beta-galactosidase rose threefold after 16 days of lactose. In 20 maldigesters, 10 days of daily lactose cut the breath hydrogen rise after a lactose test from 385 to 9 ppm·h.

Yogurt brings its own lactase

In yogurt, live bacteria carry lactase with them. Kolars and colleagues found that 18 g of lactose in yogurt gave about one third of the breath hydrogen of the same lactose in milk. They explained it by lactase released from the yogurt organisms inside the gut.

15 adults with lactose maldigestion ate test meals with 20 g of lactose. Plain yogurt and plain kefir gave far less breath hydrogen than milk. Flavored kefir was in between, and yogurts and kefirs cut the perceived severity of flatulence by 54% to 71%.

What has been seen in people

The mechanisms match the cards above in one respect and surprise in another. Malabsorption is common, but symptoms depend on the dose. A cup of milk a day gave only minimal symptoms in people who said they were severely intolerant. And the 2010 review found 12 to 15 g well tolerated by most adults.

Feeling intolerant is not the same as malabsorption either. In 427 healthy people, only about half of those who reported intolerance were maldigesters, and irritable bowel syndrome and female sex also explained it.

Lactase itself works in trials. Tablets gave 55% less breath hydrogen in 47 adults, and lactase added to milk gave symptom scores of 0.36 and 0.96 against 3.7. Both are small, short crossover tests of single doses.

How much lactose do you need?

You need none: no daily amount of lactose is required. Most adults with lactose malabsorption tolerate 12 to 15 g at a time, about 1 cup of milk. No official body that we found sets a daily need or an upper limit for lactose on its own. The nearest official values cover all carbohydrate or all added and free sugars, and they are set out below.

Source	Group	Value	Notes
National Academies (IOM) 2005, as listed in the 2020-2025 Dietary Guidelines	People aged 2 and older	Carbohydrate RDA 130 g a day; AMDR 45 to 65% of energy	Total carbohydrate, not lactose. Neither value is set for any single sugar.
Dietary Guidelines for Americans, 2025-2030 (current edition)	General population	No amount of added sugars is recommended; one meal should contain no more than 10 g of added sugars. Dairy goal of 3 servings a day on a 2,000 kcal pattern, full-fat dairy with no added sugars.	Lactose is listed among the names added sugars can have on ingredient labels. The sugars naturally in plain milk are not added sugars.
Dietary Guidelines for Americans, 2020-2025 (previous edition)	People aged 2 and older	Added sugars under 10% of calories a day	About 200 kcal, or 50 g of sugar, on a 2,000 kcal diet (our arithmetic). The guidelines say people who are lactose intolerant can choose low-lactose and lactose-free dairy products.
WHO guideline, 2015	Adults and children	Free sugars under 10% of total energy; below 5%, roughly 25 g a day, for added benefit	The guideline does not refer to sugars naturally present in milk, because no adverse effects were reported.
Systematic review of 36 trials (Shaukat), 2010	Adults with lactose intolerance or malabsorption	12 to 15 g of lactose well tolerated by most	A review finding of moderate-quality evidence, not an official limit. About 1 cup of milk.
Any authority we found	Lactose on its own	No daily need and no upper limit	The site’s intake box also shows no FDA or EU reference value for lactose.

The number that matters for the gut is how much lactose arrives at once, and with what. In the trials, people took milk with breakfast or with food. Two cups a day, taken as two doses with food, gave no more symptoms than lactose-hydrolyzed milk in lactase non-persistent adults.

Above the 12 to 15 g range the evidence is thinner. The 2010 review found too little evidence that milk with 0 to 2 g of lactose beats milk with more than 12 g for symptoms.

The nutrient to watch is calcium, not lactose. The US Institute of Medicine sets the calcium RDA (Recommended Dietary Allowance) at 1,000 mg a day for adults aged 19 to 50. It is 1,200 mg for women aged 51 to 70 and for everyone over 70.

The NIH panel warned that people who avoid dairy, for real or perceived intolerance, may take in too little calcium and vitamin D. The 2025-2030 US guidelines set a goal of 3 dairy servings a day on a 2,000 kcal pattern.

A review found that lactose itself changes calcium absorption little in adults, so the calcium in lactose-free dairy should be absorbed about as well. See our pages on calcium and vitamin D.

Lactase deficiency: signs and who is at risk

Lactose in dairy products can bring abdominal pain and swelling, rumbling gut noises, gas and diarrhea in people who do not digest it. Primary lactase deficiency, from genes, is the usual pattern worldwide.

Secondary deficiency follows infection or other conditions that harm the lining of the small bowel. Young children with acute diarrhea may temporarily stop making lactase. A pilot study looked at 15 kidney transplant recipients. All 7 with chronic norovirus infection had positive lactose breath and tolerance tests, against 1 of 8 without it.

People with irritable bowel syndrome or visceral hypersensitivity are at higher risk of symptoms after lactose, whether or not they digest it well.

Tests include genetic, endoscopic and physiological ones. The lactose breath test is the usual one.

A North American consensus agreed on a 25 g dose. A rise in breath hydrogen of 20 ppm or more from baseline counts as positive for maldigestion. Bacterial overgrowth of the small intestine is excluded first, to avoid false positives.

A European guideline adds that a positive breath test shows malabsorption, and that intolerance needs validated symptom assessment alongside.

Misselwitz and colleagues define intolerance as symptoms after lactose that do not appear after a placebo, in a person with maldigestion. They note that such blinded testing is not part of current practice. Self-diagnosis is unreliable: in the cards above, about half of the people who reported intolerance were not maldigesters.

Forms of lactose: absorption and effects

Lactose reaches people in milk and dairy foods, and as lactose-free milk, where added lactase has split it. Some people take lactase drops or tablets with food.

Lactose is also in fermented dairy such as yogurt and kefir, in aged cheese, in infant formula, and in medicines as a filler. The forms differ in how much lactose arrives at once and whether lactase comes with it.

The honest bottom line: for people with malabsorption, the amount per sitting and eating it with food matter more than the form. A cup of regular milk is tolerated by most. For larger amounts, lactase and fermented dairy have the better evidence.

Some popular claims have no trial behind them. We found no trial showing that lactose-free milk is healthier than regular milk for people who digest lactose. We found none comparing one brand or form of lactase with another. And the 2010 review judged the evidence insufficient for probiotics sold to ease lactose intolerance (7 trials).

Form	What it is	What studies show	Source
Regular milk	Cow’s milk with its own lactose: 5.01 g per 100 g and 12.3 g in a cup of semi-skimmed in our USDA data	A cup a day gave minimal symptoms in people who said milk upset them; two cups a day in two doses with food gave no more symptoms than lactose-hydrolyzed milk. The 2010 review found 12 to 15 g well tolerated by most.	1, 2, 3, 4
Lactose-free (lactose-hydrolyzed) milk	Milk in which lactase has split the lactose into glucose and galactose	At a cup a day, symptoms did not differ from regular milk. The 2010 review found the evidence insufficient that milk with 0 to 2 g of lactose beats milk with more than 12 g. The 2020-2025 US guidelines name lactose-free products as a choice for people who are lactose intolerant. We found no trial showing it is healthier.	2, 4, 5, 6
Lactase drops, tablets and capsules	Lactase enzyme added to milk or taken with a lactose-containing meal	Added to 400 mL of milk, lactase cut peak breath hydrogen from 51 to 12 or 14 ppm and symptom scores from 3.7 to 0.36 or 0.96. Tablets cut cumulative breath hydrogen by 55% in 47 adults. The EU claim covers supplements with at least 4,500 FCC units taken with each lactose-containing meal.	7, 8, 9
Yogurt and kefir	Fermented milk with live cultures that carry lactase	Breath hydrogen was about a third of milk’s with 18 g of lactose in yogurt. With 20 g test meals, plain yogurt gave 76 ppm·h, plain kefir 87 and flavored kefir 156, against 224 for milk.	10, 11
Aged and hard cheese	Cheese in which most lactose is used up during making and aging	Most of 25 protected-origin Italian cheeses held under 10 mg per kg. Fresh Mozzarella di Bufala held 3,540 mg per kg, and a young Pecorino Toscano 336.8 mg per kg at 20 days and 28 mg per kg at 60 days. A review lists hard cheeses among lower-lactose foods. The analysis authors have ties to a lactose-free business.	12, 13
Breast milk and infant formula	Mature breast milk holds about 6.7 to 7.8 g of lactose per 100 mL; standard cow’s-milk formulas contain lactose, lactose-free ones use other carbohydrate, such as the glucose polymers in one trial	A lactose formula gave 10.3% more calcium absorption than a lactose-free one, with the same zinc absorption. A lactose-free formula supported normal growth over 112 days. In young children with acute diarrhea, lactose-free products shortened the illness by about 18 hours (low-quality evidence).	14, 15, 16, 17
Lactose in medicines	A filler (excipient) in tablets and capsules	Common in medicines for gut conditions, with the amount not on the label; lactose-free alternatives exist.	18

Here are the form-specific studies, strongest evidence first, 4 in all.

Is lactose safe? Limits and who should watch it

For people who digest lactose, food amounts are not a safety concern, and we found no upper limit for lactose set by any authority. The WHO guideline leaves out sugars naturally present in milk, because no adverse effects were reported.

For people who do not digest it, symptoms depend on the dose. The NIH panel found that most people with malabsorption do not have clinical intolerance. Several groups need more care.

• Lactose intolerance and irritable bowel syndrome. Gas, bloating, pain and diarrhea after dairy are common, and they depend on the dose. Irritable bowel syndrome can look like lactose intolerance, so ask a doctor before cutting out dairy for good.
• Classical galactosemia. This inborn error of galactose metabolism has an international treatment guideline. Lactose supplies galactose. Lactose-free milk is milk whose lactose has been split into glucose and galactose, so the galactose is still there. Anyone with galactosemia should follow their metabolic team’s advice. See our page on galactose.
• Milk allergy. An allergy is an immune response to milk protein. Lactose is not an allergen, because the response to it is not immune based. Lactase pills and lactose-free milk do not treat an allergy, so ask a doctor if you suspect one.
• Infants and young children. In young children with acute diarrhea who are not mainly breast-fed, lactose-free products may shorten the illness. But 22 of 33 trials excluded breast-fed children, and none were from low-income countries. Talk to a pediatrician before changing an infant’s formula.
• Medicines and supplements. Lactose is the most common filler in tablets and capsules, and the amount is not listed. People with marked symptoms can ask a pharmacist for lactose-free versions. Lactase supplements had no side effects in the 2005 trial. The EU claim requires a note that tolerance to lactose is variable and that people should seek advice. We found no report of an interaction between dietary lactose and a medicine.

How to handle lactose

• Keep to about a cup at a time, with food. Most adults with malabsorption tolerated 12 to 15 g. A cup a day with breakfast gave only minimal symptoms in people who said milk upset them. Spread larger amounts over the day.
• Pick fermented or aged dairy. Yogurt and kefir gave far less breath hydrogen than milk in test meals. Most of 25 protected-origin Italian cheeses held under 10 mg of lactose per kg, though each cheese differs and fresh cheese held far more.
• Use lactose-free milk or lactase, and keep the calcium. Lactase cut breath hydrogen and symptom scores in both trials, and the calcium in dairy is what dairy avoidance can lose. Adults need 1,000 mg a day at ages 19 to 50.

History of lactose

Reports in the early 1960s showed that adult lactase decline is common. A 1963 report in Gastroenterology described intestinal lactase deficiency and lactose intolerance in adults. A 1965 Lancet paper was titled “Lactase deficiency in the adult. A common occurrence.”

In 1970 Frederick Simoons proposed a culture-historical hypothesis linking adult lactose intolerance to the milking habit. Ingram and colleagues note that for some 40 years it has been known that lactase lasts into adult life in some people but not others.

The gene story came later. In 2002 the Finnish group of Enattah found C/T-13910. In 2007 Tishkoff’s group found three other changes in East Africa, so persistence arose several times. A 2009 simulation put the first selection of the European allele about 7,500 years ago among dairying farmers.

In 2022 a large team of archaeologists and geneticists studied about 7,000 pottery fat residues. They found that milk use did not explain the spread, and proposed that famine and disease exposure did.

The research argument is also young. A 2017 review notes that the dairy industry and Minnesota investigators have worked to minimize the influence of lactose intolerance on milk consumption.

Several tolerance studies on this page come from Minnesota investigators: Suarez, Savaiano and Levitt, and the 2010 review by Shaukat. A lactose-free business has links to the cheese analysis, so we state affiliations and funding on each card.

Frequently asked questions about lactose

What is lactose?

Lactose is the main sugar in milk: one glucose and one galactose joined together, which the enzyme lactase splits in the small intestine. Mature human milk holds about 6.7 to 7.8 g per 100 mL. In our USDA data, a cup (246 g) of semi-skimmed cow’s milk holds 12.3 g. See our pages on sugars, glucose and galactose.

Is lactose intolerance the same as a milk allergy?

No. A milk allergy is an immune response to milk protein, while lactose intolerance is trouble digesting the sugar, with pain, bloating, gas and diarrhea. The NIAID-sponsored guidelines say lactose is not an allergen because the response to it is not immune based. Lactase pills do not help an allergy, so ask a doctor if you suspect one.

How much lactose can someone with lactose intolerance have?

Most adults with lactose malabsorption tolerate 12 to 15 g at a time, about a cup of milk, says a 2010 review of 36 trials. In a blinded trial of 30 people who reported severe intolerance, a cup a day gave only minimal symptoms. Tolerance varies, so try it with food and find your own limit.

Which foods are lowest in lactose?

Aged hard cheeses are lowest: most of 25 protected-origin Italian cheeses held under 10 mg of lactose per kg. Fresh Mozzarella di Bufala held 3,540 mg per kg, and production rules vary, so check each cheese. Yogurt and kefir gave less breath hydrogen than milk, and lactose-free milk has had its lactose split.

Do lactase pills work?

In a crossover trial of 47 adults with lactose intolerance, lactase tablets cut breath hydrogen by 55% against placebo and eased symptoms. The EU authorizes a claim for supplements with at least 4,500 FCC units taken with each lactose-containing meal. The trials are small and short, and we found none comparing brands.

Is lactose-free milk healthier than regular milk?

Not shown. In a blinded trial, a cup of lactose-hydrolyzed milk gave the same symptoms as regular milk. We found no trial showing health benefits of lactose-free milk for people who digest lactose. It is a useful swap for people with symptoms, and the previous US guidelines (2020-2025) list lactose-free products for people who are lactose intolerant.

Why can some adults digest milk and others cannot?

Lactase activity falls after weaning in most people, and a 2017 review puts non-persistence at 65% to 70% of adults worldwide. Adults who keep it carry DNA changes near the lactase gene: C/T-13910 in Europeans and three different changes in Tanzanians, Kenyans and Sudanese. A 2022 study found that how much milk people used in prehistoric Europe did not explain how the European change spread.

Can I get enough calcium if I avoid dairy?

It takes planning: the NIH panel warned that people who avoid dairy may take in too little calcium and vitamin D. The calcium RDA is 1,000 mg a day for adults 19 to 50, and 1,200 mg for women 51 to 70 and everyone over 70. A review found that lactose itself changes calcium absorption little in adults, so the calcium in lactose-free dairy should be absorbed about as well. See calcium.