Benefits
Better digestion of the lactose in a dairy meal
Several small placebo-controlled trials found that lactase taken with lactose lowers breath hydrogen, a direct sign that less lactose reached the colon undigested, and most also found milder bloating, cramping, flatulence or diarrhea. Examples: 16 adults given enzyme-treated milk (Corazza 1992), 24 adults given 9,900 FCC units with 50 g lactose (Sanders 1992), and 60 patients in a three-arm trial (Ojetti 2010). The benefit has limits. About 6,000 units worked better than 3,000 with 20 g lactose, but neither dose made a significant difference against 50 g lactose in water (Lin 1993). In a 10-person comparison of three brands, one lowered hydrogen without easing symptoms while another eased symptoms (Ramirez 1994). In 96 patients given a standard dose without placebo, only about 22% had a normal breath test afterwards (Ibba 2014). The EU authorises the claim that lactase improves lactose digestion at 4,500 FCC units or more per meal. An independent review found most people with lactose intolerance tolerate 12 to 15 g lactose, about a cup of milk, without any aid (Shaukat 2010).
Easier to keep dairy foods in the diet
For people who get symptoms from lactose, taking lactase with a meal can make milk and other dairy foods easier to eat, and dairy supplies calcium, protein, riboflavin and vitamin B12. No trial has measured whether taking lactase raises nutrient intake or improves bone health, so this is a practical point rather than a proven effect. Hard cheeses and lactose-free milk contain very little lactose, and yoghurt with live cultures is often better tolerated than milk, so these are other ways to keep dairy in the diet.
Use in children with lactose intolerance
In a double-blind trial of 18 children (average age 11) with lactose intolerance, lactase tablets taken just before a lactose drink cut peak breath hydrogen from about 60 ppm to 7 ppm and reduced pain, bloating and diarrhea compared with placebo (Medow 1990). For infants and young children with diarrheal illness, most trial evidence concerns switching to lactose-free formula or food (a 2013 Cochrane review of 33 trials). One 2026 trial in 50 hospitalised infants in Pakistan found fewer ongoing diarrhea episodes with lactase drops added to milk than with lactose-free formula, but it had no placebo group. That is medical care for an illness, not a supplement use: a child with diarrhea, or an infant with feeding symptoms, needs a doctor's assessment.
Background: how lactose-free dairy is made (not an effect of taking lactase)
Dairy manufacturers add lactase to milk in processing to make lactose-free products. At high lactose concentrations the enzyme also forms small amounts of galacto-oligosaccharides (GOS), a type of fibre, so some lactose-free milks contain traces of GOS. This happens in the factory. Taking a lactase tablet is not a way to get GOS, and no health effect of this is claimed for lactase supplements.
Mechanism of action
Hydrolysis of β-1,4 glycosidic bond in lactose
Lactase is a β-galactosidase enzyme that catalyzes hydrolysis of the β-1,4 glycosidic bond connecting glucose and galactose in the lactose disaccharide. The released glucose and galactose are absorbed via SGLT1 and GLUT2 transporters in the small intestine, while undigested lactose ferments in the colon producing gas, water, and short-chain fatty acids responsible for intolerance symptoms.
Two main supplement sources: Aspergillus oryzae vs. Kluyveromyces lactis
Aspergillus oryzae lactase works best in mildly acidic conditions (around pH 4.5 to 5), close to the stomach's pH after a meal, and is the usual source in tablets and capsules. Kluyveromyces lactis lactase works best near neutral pH (about 6.5 to 7) and is mainly used to pre-treat milk to make lactose-free dairy. Trials of commercial tablets disagree on whether brands are interchangeable: one found three products worked about equally per unit of enzyme (Lin 1993), while another found clear differences between brands (Ramirez 1994).
Brush border location of native lactase
Native human lactase is anchored in the brush border of small intestinal enterocytes (specifically the villus tip cells of the proximal jejunum). Lactase non-persistence is genetic: the LCT gene's lactase-phlorizin hydrolase enzyme is downregulated after weaning in most mammals; persistence into adulthood is the evolutionary exception (developed in Northern European, some African, and Middle Eastern populations who domesticated dairy).
Clinical trials
Placebo-controlled comparison of Lactogest soft gels (about 3,000 or 6,000 units), Lactaid caplets and DairyEase chewable tablets (about 6,000 units each), fed with milk containing 20 g lactose or with 50 g lactose in water. Outcomes: breath hydrogen and GI symptoms. Lin MY et al., Dig Dis Sci 1993;38(11):2022-7 (University of Minnesota).
Adult lactose maldigesters; group size is not stated in the abstract.
With 20 g lactose in milk, all three products sharply reduced peak and total breath hydrogen, and symptoms were significantly milder. About 6,000 units lowered total hydrogen more than about 3,000 units, and the three product forms performed about equally per unit of enzyme. With 50 g lactose in water, hydrogen fell only slightly and not significantly, and symptoms did not differ from control: the large lactose load overwhelmed both doses.
Randomized, double-blind, placebo-controlled crossover trial. Three chewable lactase tablets (9,900 FCC units total) or placebo, taken after fasting with 8 oz of whole milk to which lactose powder was added (50 g lactose in total), at least one week apart. Outcomes: breath hydrogen, plasma glucose and symptoms over four days. Sanders SW et al., Clin Pharm 1992;11(6):533-8; lead author affiliated with TheraTech, Inc.
24 adults with lactose intolerance, confirmed by a breath hydrogen rise of 20 ppm or more after lactose.
Peak breath hydrogen was significantly lower after lactase than after placebo, and 21 of 24 subjects had a lower hydrogen curve. Cramping, belching, flatulence and diarrhea ratings were lower in the first 8 hours, and bloating in the next 8 hours. Plasma glucose did not differ. A small single-dose trial with an industry-affiliated lead author.