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Gut Microbes Boost Dairy Tolerance in Adults Lacking Lactase

    A young woman holds a cup of milk in her hands. A new study shows that lactose intolerance symptoms may be managed with daily consumption of lactose.

    Written by: Brittany T. Truong, Ph.D. | Issue # 120 | 2024

    • Without the lactase enzyme, adults with lactase non-persistence (LNP) cannot digest milk sugars. The lactose sugars are fermented by bacteria in the large intestine, leading to gas production and gastrointestinal (GI) symptoms such as bloating, flatulence, and cramping.
    • Bifidobacteria are beneficial microbes found in the gut that can digest lactose without producing gas.
    • In this clinical trial, repeated consumption of lactose by individuals with LNP led to an increase in bifidobacteria in the gut and a decrease in intestinal gas, as measured by a hydrogen breath test.

    Approximately 70% of adults across the world are lactase nonpersistent (LNP), meaning they cannot digest the milk sugar lactose [1]. They lack the lactase enzyme, which breaks down lactose into glucose and galactose in the small intestine. Without lactase, the milk sugar travels through the gut to the large intestine, where bacteria then ferment the sugar, producing gasses that can cause abdominal cramps, bloating, and flatulence.

    The threshold for experiencing these lactose intolerance symptoms varies among individuals with LNP. Many people with the LNP genotype can digest 12 g of lactose, equivalent to a full glass of milk, without symptoms [2]; however, many still choose to avoid dairy, reaching instead for alternatives, such as oat, almond, or soy milk or lactose-free products. Others choose to consume lactase capsules with their dairy to aid digestion. New studies suggest individuals with LNP can build a tolerance for lactose [3, 4]. 

    In a new clinical trial, published in The American Journal of Clinical Nutrition, researchers show that repeated daily consumption of lactose in LNP individuals alters the gut microbiome and improves their lactose tolerance. These individuals had an increase in beneficial gut microbes called bifidobacteria and produced less intestinal gas, as measured by a hydrogen breath test [5].

    “Bifidobacteria are able to break down lactose into glucose and galactose in such a way that no gas is formed,” says Lonneke Janssen Duijghuijsen, a lead researcher on the study and a Clinical Research Scientist at Wageningen Food & Biobased Research. The bacteria use an enzyme called β-galactosidase to digest lactose. “If you have lots of these beneficial bacteria, you may still be able to consume lactose without impactful intolerance symptoms.”

    In this single-blind study, the researchers gave LNP individuals, who carry two copies of the LNP gene, a daily dose of lactose. The daily dose gradually increased from 6 to 24 g over the course of 12 weeks. To determine the effect of increasing lactose on the gut microbiome, the researchers collected fecal samples from participants before and after the intervention. They performed shotgun metagenomic sequencing to determine the types of bacteria present as well as their abundance. The researchers also measured β-galactosidase activity in the fecal samples and intestinal gas using a hydrogen breath test. Finally, they tracked classic lactose intolerance symptoms, such as bloating, diarrhea, and flatulence, over time [5].

    The researchers found that repeated daily consumption of lactose led to an increase in beneficial bifidobacteria. β-galactosidase activity also increased, suggesting that the lactose was properly digested. This finding was confirmed with a hydrogen breath test, which showed that the LNP individuals produced significantly less intestinal gas while taking lactose. Interestingly, the lactose intolerance symptoms were mild, both at the start and end of the intervention, and variable. Therefore, the participants did not experience a significant improvement [5]. 

    “Symptoms are always subjective and will, therefore, always show high variability,” says Janssen Duijghuijsen. “Moving forward, it would be valuable to explore the factors contributing to this variability … and determine individual thresholds for lactose consumption.”

    She says that in hindsight, the researchers could have used a higher dose of lactose to induce symptoms at the start of the intervention. They could have also screened patients at the beginning of the study and only included individuals with clear lactose intolerance symptoms. 

    “It’s important to note that these results were backed by the relevant and significant finding that the production of gas after the intake of the high dose of lactose was substantially decreased,” says Janssen Duijghuijsen. “This is an objective indication for lactose intolerance.”

    The study adds to the growing body of evidence that links daily lactose consumption with beneficial gut microbes and improved tolerance [4, 5]. Together, these results suggest that individuals with LNP do not necessarily need to avoid dairy products. This is important regarding food security and maintaining good health, particularly in developing countries. Milk is affordable and nutrient-dense, consisting of high-quality proteins, vitamins, minerals, and antioxidants

    However, it is still unclear how long the lactose tolerance persists. “The intestinal microbiota is highly dynamic,” Janssen Duijghuijsen says. “The beneficial effects will not be long-lasting, and continuous exposure is a prerequisite in preventing lactose intolerance symptoms in the long run. It would be valuable to find out how long the observed effects would last.” 

    “We’ve learned a lot about the community of microorganism living in our intestines and their impact on our health,” Janssen Duijghuijsen says. “There is much to discover in this field, which may ultimately lead to more personalized approaches in nutrition and health research.” 

    Janssen Duijghuijsen adds, “One thing is certain — a ‘one-size fits all’ approach does not apply.” 


    1. Storhaug CL, Fosse SK, Fadnes LT. Country, regional, and global estimates for lactose malabsorption in adults: A systematic review and meta-analysis. Lancet Gastroenterol Hepatol. 2017;2(10):738–46.
    2. Savaiano DA, Levitt MD. Milk intolerance and microbe-containing dairy foods. J Dairy Sci. 1987;70(2):397–406.
    3. Szilagyi A. Adaptation to lactose in lactase non persistent people: Effects on intolerance and the relationship between dairy food consumption and evalution of diseases. Nutrients. 2015;7(8):6751–79.
    4. Kable ME, Chin EL, Huang L, Stephensen CB, Lemay DG. Association of estimated daily lactose consumption, lactase persistence genotype (rs4988235), and gut microbiota in healthy adults in the United States. J Nutr. 2023;153(8):2163–73.
    5. JanssenDuijghuijsen L, Looijesteijn E, van den Belt M, Gerhard B, Ziegler M, Ariens R, et al. Changes in gut microbiota and lactose intolerance symptoms before and after daily lactose supplementation in individuals with the lactase nonpersistent genotype. Am J Clin Nutr. 2024;119(3):702–10.