Lactose

Lactose Function in the Human Body

Lactose is a source of energy; it can provide 3.9 Calories per gram, which is about the same as sucrose ^[2,3].

Lactose Sources

• Foods naturally high in lactose include milk, yogurt, custard, curd and ice cream ^[5].
• Lactose or milk can be added as a sweetener or coating agent to various commercial foods, including infant formulas.
• Lactose is used as a binder or diluent in certain pills and vitamin/mineral supplements.
• Lactose can be found in certain toothpastes.

Picture 1. Examples of foods high in lactose

Chart 1. Foods High in Lactose
FOOD	LACTOSE (g)
Beer: milk stout, cream stout, sweet stout (12 oz, 355 mL) [7]	6-46
Eggnog (1 cup, 254 g)	20
Whey powder, sweet (1 oz, 28 g)	18+
Human breast milk (1 cup, 237 mL)	18
Whey powder, acid (1 oz, 28 g)	17
Buffalo milk (1 cup, 237 mL) [11]	14
Cow’s milk (1 cup, 237 mL)	11-13
Goat milk (1 cup, 237 mL)	12
Acidophilus milk (1 cup, 237 mL) [6]	11
Buttermilk (1 cup, 237 mL) [6]	10
Yogurt, plain, regular (6 fl. oz, 180 mL)	10
Custard pudding (5.3 oz, 150 g)	8
Probiotic yogurt, active culture (4 oz, 113 g) [producer]	8
Greek yogurt (6 fl. oz, 180 mL)	7
Pudding (5.3 oz, 150 g)	7
Pancake, plain (180 g)	6
Ice cream (1/2 cup, 120 mL)	~5-6
Kefir, low-fat (6 fl. oz, 180 mL) [12]	<6
Vanilla milkshake (1 cup, 237 mL) [13]	~5
Soured buttermilk (1 cup, 237 mL)	5
Curd (1/2 cup, 135 g)	4
Blue cheese sauce (4 fl. oz, 120 mL)	4
Cottage cheese, creamed and uncreamed (4 oz, 112 g)	3
Milk chocolate (1 oz, 28 g)	3
Infant formula (1 fl. oz, 30 mL)	3
Milk, canned, evaporated ( 1 fl. oz, 30 mL)	~3
Scrambled eggs (1 cup, 220 g)	2
Cheese, neufchatel (2 oz, 57 g)	2
Mashed potatoes (1 cup, 210 g)	2
Cream for food preparation (1 fl. oz, 30 mL)	2
Soft ice cream (1 cornet, 135 g)	2
Cream cheese (2 tbsp, 28 g)	2
Feta cheese (2 oz, 57 g)	2
Sherbet, orange (1/2 cup, 120 mL)	~2
Gouda cheese (2 oz, 57 g)	1.3
Processed cheese, American cheese (2 slices, 30 g)	~1
Commercial foods that may contain lactose: candies, cakes, pastries, pies, quiche, snacks, bread, biscuits, crackers, waffles, breakfast cereals, potato chips, margarine, peanut butter, sport drinks, sauces, bologna, cream soups, salad dressings, canned food, frozen commercial foods, non-dairy whipped cream toppings, minced meat, sliced ham, bacon, hot dogs, mayonnaise, cream liqueurs, dry mixes, milk stout or sweet stout (beer) [5,14,15].	Probably < 2g/serving
DAIRY PRODUCTS LOW IN LACTOSE (1 g or less)
Cheeses (2 oz, 57 g): blue; brie; brick; camembert; cheddar; colby; edam; emmental; fondue; fontina; goat cheese; gruyere; limburger; mascarpone; monterey; mozzarella; muenster; oven cheese, parmesan; ricotta; romano; Swiss; tilsit	<1
Yogurt, low-lactose (6 fl. oz, 180 mL)	<1
Sour cream (1 tbsp, 12 g)	0.4
Cream liqueur (1 jigger, 1.5 oz, 45 mL)	0.3
Whey drink (1 cup, 237 mL)	0.3
Whipped cream, pressurized (1 tbsp, 3 g)	<0.3
Cream, half and half (1 fl. oz 30 mL)	0.05
Butter (1 tbsp, 14 g)	0.01
Ghee – Indian clarified butter (1 tbsp, 14 g)	traces

Chart 1 sources: ^[5,8,9]

Lactose Digestion

The enzyme lactase in the small intestinal lining breaks down lactose to glucose and galactose, which are then absorbed ^[15]. Most healthy people can absorb at least 50 grams of lactose (aka 1 liter of milk) in one sitting ^[16].

Lactose Intolerance

Some people have a decreased amount of the enzyme lactase in the intestinal lining, so they have a reduced ability to digest lactose; the condition is called lactase deficiency or–when lactose ingestion triggers gastrointestinal symptoms–lactose intolerance.

Symptoms

The undigested lactose drags water from the intestinal wall (an osmotic effect) and triggers diarrhea. When lactose passes to the large intestine, it is broken down (fermented) by the normal colonic bacteria into gases gases (hydrogen, carbon dioxide), which can cause abdominal pain, bloating and flatulence; other symptoms may include nausea, vomiting, constipation and, in women, depression one to two hours after eating lactose-rich foods ^[17,18,19].

Types of Lactose Intolerance

Primary lactose intolerance or lactase-nonpersistence

In many, genetically-predisposed individuals, after weaning the amount of lactase (the enzyme that digests lactose) in the intestinal lining drops for more than 10 times ^[19]. In blacks and Asians, this usually occurs after age of 2 or 3, but in whites usually in adolescence or early adulthood ^[20]. Once established, primary lactose intolerance probably does not worsen further with age. The prevalence of lactose intolerance is the United States is currently not known, but is probably low in whites and high in native, black, Hispanic and Asian Americans ^[21]. The prevalence of lactose intolerance may range from as low as 4% in Sweden to up to 100% in certain Asian regions ^[24,25].

Secondary lactose intolerance

Lactose intolerance can temporarily occur in gastrointestinal illnesses, such as Crohn’s disease ^[27], celiac disease, gastrointestinal infections, small intestinal bacterial overgrowth (SIBO) ^[28,29], diverticulitis in the colon ^[28], cystic fibrosis, in milk allergy, during chemotherapy or radiation, in iron deficiency, after gastric or intestinal resection ^[19] or gastric bypass (bariatric surgery) ^[30] or in Grave’s disease. Symptoms usually disappear completely when the underlying condition heals. Lactose intolerance in infants is usually caused by other disorders of the small intestine, such as rotavirus infection ^[19].

Congenital lactose intolerance, also known as alactasia or hereditary disaccharide intolerance type II is very rare–most cases have been reported from Finland. Severe diarrhea, which is present from birth, may lead to death if the cause is not recognized.

Developmental lactose intolerance may be present at birth in prematurely born infants. It improves with age.

Diagnosis

In lactose tolerance test a person usually consumes 50 grams of lactose in a water solution, which is about the amount of lactose in 1 liter of milk. Normally, lactose is completely digested to glucose and galactose, which are then absorbed and their blood concentrations rise. Individuals with lactose intolerance do not digest lactose completely, so they have lower blood glucose concentrations after lactose meal than healthy individuals. Both false positive and negative results are possible ^[19].

Hydrogen breath test. After drinking a solution with 50 grams of lactose, any unabsorbed lactose passes to the colon where the normal colonic bacteria break them to gases, including hydrogen, which is absorbed and can be measured in the exhaled air. In up to 20% individuals with lactose intolerance their colonic bacteria do not excrete hydrogen, so they have a false negative hydrogen breath test ^[19]. False positive tests are also possible ^[19].

Biopsy of the small intestinal lining and activity of lactase in the obtained sample can be measured, but this is rarely required ^[19,22].

Stool acidity test is usually performed in small children; increased acidity of the stool speaks for lactose intolerance.

Test-confirmed reduced lactose absorption without symptoms is officially called lactose malabsorption; when symptoms are present, the condition is called lactose intolerance.

Symptoms disappearance within few days of starting lactose-free diet can be observed in lactose intolerance but also in milk protein allergy ^[22]. Symptoms that speak for milk allergy include hives and itchy skin within few minutes of milk or other dairy products consumption ^[32].

Low-Lactose or Lactose-Free Diet

Symptoms of lactose intolerance can be prevented by avoiding foods high in lactose (see the chart above) and food products with the following ingredients: cream, milk, nougat or whey. The following ingredients do NOT contain lactose: casein, lactalbumin, lactate, lactic acid, lactone ^[9,33].

Facts about low-lactose diet in lactose intolerance:

Most individuals with lactose intolerance can tolerate at least 10-15 grams of lactose (aka 1 cup or 237 mL of milk) in one sitting, or 24 grams of lactose (two cups of milk) in divided doses per day, especially when consumed with other foods ^[19]. 50 grams of lactose per day (four cups or one liter of milk) triggers symptoms in most individuals with lactose intolerance ^[19].

Lactose-free milk, which contains “hydrolyzed lactose” (glucose and galactose) does not necessary trigger less symptoms in individuals with lactose intolerance than regular milk ^[34]. In one study, some lactose intolerant participants reported symptoms after ingestion of 200 mL lactose-free milk and no difference in symptoms after consuming milk with 0.5, 1.5 or 7 grams lactose, which means it was not lactose that triggered symptoms, but probably a psychological reaction to milk ^[35].

Skim milk by does not contain less lactose and is not better tolerated than whole milk.

Goat milk triggers less symptoms than cow’s milk in many lactose intolerant individuals ^[36].

• Pasteurized or boiled milk is better tolerated than raw milk by most lactose intolerant individuals; pasteurization and boiling do not significantly reduce the amount of lactose in milk, though ^[18,60].
• Raw milk does not contain bacteria that would help to digest lactose.
• Consuming milk with solid foods can reduce the risk of symptoms ^[37]. Lactose from solid foods tends to cause less symptoms that the same amount of lactose from liquids ^[19].
• Yogurts with live cultures, which excrete the enzyme lactase which can help digest lactose, or even pasteurized yogurts tend to trigger less symptoms than milk.
• Kefir is also often well tolerated ^[38].
• Lactose from drugs usually does not trigger symptoms in lactose intolerant individuals ^[39].
• There is no strong evidence about association between low-dairy diet and reduced gastrointestinal or bone health ^[21]. In one 2012 study, there were no differences in bone density between lactose-tolerant and lactose intolerant children ^[40].

Treatment of Lactose Intolerance

• Currently, primary and congenital lactose intolerance cannot be cured.
• Symptoms can be prevented by limiting lactose intake.
• Lactase supplements are available over-the-counter. In one study, lactase supplements effectively prevented symptoms after consuming 400 mL milk containing 20 grams lactose but not after consuming a water solution with 50 grams lactose ^[41].
• There is insufficient evidence about the effectiveness of treatment with antibiotic rifaximin, consuming yogurt, probiotics or increasing amounts of lactose in long-term reduction of symptoms ^[34].

Irritable Bowel Syndrome (IBS) and Lactose Intolerance

• The frequency of lactose intolerance in individuals with irritable bowel syndrome (IBS) is about the same as in general population ^{[23,42,43,44,45]}.
• Many people who self-report having symptoms after consuming lactose do not have positive hydrogen breath test for lactose so they actually do not have lactose intolerance, but they get irritated by milk ^[19,42].
• In individuals with IBS, lactose-free often does not help to relieve symptoms ^[43,45].

Lactose, Dairy Products and Dental Caries

• Cow’s milk does not seem to be a significant risk factor for dental caries ^[46].
• Breastfeeding at night and breastfeeding for longer than one year that may increase the risk of caries ^[46]. Small children fed by sweet drinks (formula, soda pop, fruit juices) by a bottle for more than a year may develop a “nursing bottle syndrome” or “nursing bottle tooth decay,” which mainly affects the upper front teeth; the condition is now called “early childhood caries” (ECC) ^[46].
• Consuming hard cheese, which are usually low in lactose, and unsweetened plain yogurt after meals, may even have a protective effect against dental caries due to its calcium, phosphate and casein content, among other ^{[48,49,50,51,52]}.

Effect of Lactose on Blood Glucose Levels and Diabetes

• Lactose has a relatively low glycemic index (GI = 48), which means it does not significantly increase blood glucose levels ^[53].
• A long-term lactose intake does not seem to increase the risk of developing diabetes 2 ^[54].
• Ingesting pure lactose increases blood insulin levels more than milk ^[55].

Lactose and Cooking

• Lactose is commercially available in the form of a powder and tablets.
• Lactose powder is a white crystalline substance without odor, about 20% as sweet as sucrose ^[56].
• Lactose solubility in water at 77 °F (25 °C)  is 21.6 g/100 mL ^[57]. Lactose solubility in 10% ethanol at 77 °F (25 °C) is 13 g/100 mL ^[58].
• Lactose is not a fermentable sugar: it is not fermented by baker’s yeast, so it can be used as a preservative and browning agent in baked goods ^[6]; it is also not fermented by brewer’s yeast, so it can be used as a thickener and sweetener in beer ^[7].
• Lactose melting point is 396-417 °F (202-214 °C) ^[56,59].
• Lactose decomposes to glucose and galactose at 220 °C (428 °F) ^[60], so it does not seem likely that any significant amount of lactose would be decomposed during usual milk boiling at 212 °F (100 °C).
• Caramelization of lactose starts at 302 °F (150 °C) and browning at 347 °F (175 °C).
• Lactose is a reducing sugar ^[61] so it takes part in the Maillard browning reaction with amino acids.

Lactose Production

Lactose as a foodstuff or food additive is usually extracted from whey, a byproduct of cheese production ^[4].

Food-grade lactose powder is commercially available either as anhydrous lactose or lactose monohydrate.

Anhydrous Lactose

Anhydrous lactose is dry lactose, without water; it is used as a filler and diluent in tablets ^[62-p.360].

• Anhydrous lactose is highly hygroscopic – it readily attracts moisture ^{[62-p.360; 63-p.86]}.
• Solubility of anhydrous lactose in water at 77 ° F (25 °C) is 40 g/100 mL ^[62-p.360].
• Melting point of commercial anhydrous lactose is 450 °F (232 °C) ^[62-p.360].

Lactose Monohydrate

Lactose monohydrate, a compound of lactose and water, is used as a filler in tablets and in sugar coatings in confectionery ^[62-p.365].

• Solubility of lactose monohydrate in water at 20° C is about 20 g/100 mL ^[62-p.366].
• Lactose monohydrate is very hygroscopic – it readily attracts moisture at 60% relative humidity ^[62-p.86].

Who should avoid lactose?

Individuals with the following conditions should avoid lactose:

• Lactose intolerance
• Glucose-galactose malabsorption ^[64]
• GLUT-1 deficiency syndrome
• Glycogen-storage disease type 1 ^[10]
• Galactokinase deficiency ^[47]