National Institutes of Health Consensus Development Conference: Lactose Intolerance and Health
Authors: Frederick J. Suchy
Journal: Annals of Internal Medicine (2010)

NIH Conferences15 June 2010National Institutes of Health Consensus Development Conference: Lactose Intolerance and HealthFREEFrederick J. Suchy, MD, Patsy M. Brannon, PhD, RD, Thomas O. Carpenter, MD, Jose R. Fernandez, PhD, Vicente Gilsanz, MD, PhD, Jeffrey B. Gould, MD, MPH, Karen Hall, MD, PhD, Siu L. Hui, PhD, Joanne Lupton, PhD, Julie Mennella, PhD, Natalie J. Miller, BS, Stavroula Kalis Osganian, MD, ScD, MPH, Deborah E. Sellmeyer, MD, and Marshall A. Wolf, MD*Frederick J. Suchy, MDFrom Mount Sinai School of Medicine of New York University, Mount Sinai Kravis Children's Hospital, New York, New York; Cornell University, Ithaca, New York; Yale Center for X-linked Hypophosphatemia, Yale School of Medicine, New Haven, Connecticut; The University of Alabama at Birmingham, Birmingham, Alabama; Children's Imaging Research Program, Children's Hospital Los Angeles, and Keck School of Medicine, University of Southern California, Los Angeles, California;Stanford University School of Medicine, Stanford, California; University of Michigan Medical School and Geriatric Research, Education, and Clinical Center, Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan; Center for Aging Research, Indiana University School of Medicine and Regenstrief Institute, Indianapolis, Indiana; Texas A&M University, College Station, Texas;Monell Chemical Senses Center and School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania; Harvard University, Children's Hospital Boston, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts; and Metabolic Bone Center, Johns Hopkins Bayview Medical Center, Baltimore, Maryland., Patsy M. Brannon, PhD, RDFrom Mount Sinai School of Medicine of New York University, Mount Sinai Kravis Children's Hospital, New York, New York; Cornell University, Ithaca, New York; Yale Center for X-linked Hypophosphatemia, Yale School of Medicine, New Haven, Connecticut; The University of Alabama at Birmingham, Birmingham, Alabama; Children's Imaging Research Program, Children's Hospital Los Angeles, and Keck School of Medicine, University of Southern California, Los Angeles, California;Stanford University School of Medicine, Stanford, California; University of Michigan Medical School and Geriatric Research, Education, and Clinical Center, Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan; Center for Aging Research, Indiana University School of Medicine and Regenstrief Institute, Indianapolis, Indiana; Texas A&M University, College Station, Texas;Monell Chemical Senses Center and School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania; Harvard University, Children's Hospital Boston, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts; and Metabolic Bone Center, Johns Hopkins Bayview Medical Center, Baltimore, Maryland., Thomas O. Carpenter, MDFrom Mount Sinai School of Medicine of New York University, Mount Sinai Kravis Children's Hospital, New York, New York; Cornell University, Ithaca, New York; Yale Center for X-linked Hypophosphatemia, Yale School of Medicine, New Haven, Connecticut; The University of Alabama at Birmingham, Birmingham, Alabama; Children's Imaging Research Program, Children's Hospital Los Angeles, and Keck School of Medicine, University of Southern California, Los Angeles, California;Stanford University School of Medicine, Stanford, California; University of Michigan Medical School and Geriatric Research, Education, and Clinical Center, Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan; Center for Aging Research, Indiana University School of Medicine and Regenstrief Institute, Indianapolis, Indiana; Texas A&M University, College Station, Texas;Monell Chemical Senses Center and School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania; Harvard University, Children's Hospital Boston, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts; and Metabolic Bone Center, Johns Hopkins Bayview Medical Center, Baltimore, Maryland., Jose R. Fernandez, PhDFrom Mount Sinai School of Medicine of New York University, Mount Sinai Kravis Children's Hospital, New York, New York; Cornell University, Ithaca, New York; Yale Center for X-linked Hypophosphatemia, Yale School of Medicine, New Haven, Connecticut; The University of Alabama at Birmingham, Birmingham, Alabama; Children's Imaging Research Program, Children's Hospital Los Angeles, and Keck School of Medicine, University of Southern California, Los Angeles, California;Stanford University School of Medicine, Stanford, California; University of Michigan Medical School and Geriatric Research, Education, and Clinical Center, Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan; Center for Aging Research, Indiana University School of Medicine and Regenstrief Institute, Indianapolis, Indiana; Texas A&M University, College Station, Texas;Monell Chemical Senses Center and School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania; Harvard University, Children's Hospital Boston, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts; and Metabolic Bone Center, Johns Hopkins Bayview Medical Center, Baltimore, Maryland., Vicente Gilsanz, MD, PhDFrom Mount Sinai School of Medicine of New York University, Mount Sinai Kravis Children's Hospital, New York, New York; Cornell University, Ithaca, New York; Yale Center for X-linked Hypophosphatemia, Yale School of Medicine, New Haven, Connecticut; The University of Alabama at Birmingham, Birmingham, Alabama; Children's Imaging Research Program, Children's Hospital Los Angeles, and Keck School of Medicine, University of Southern California, Los Angeles, California;Stanford University School of Medicine, Stanford, California; University of Michigan Medical School and Geriatric Research, Education, and Clinical Center, Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan; Center for Aging Research, Indiana University School of Medicine and Regenstrief Institute, Indianapolis, Indiana; Texas A&M University, College Station, Texas;Monell Chemical Senses Center and School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania; Harvard University, Children's Hospital Boston, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts; and Metabolic Bone Center, Johns Hopkins Bayview Medical Center, Baltimore, Maryland., Jeffrey B. Gould, MD, MPHFrom Mount Sinai School of Medicine of New York University, Mount Sinai Kravis Children's Hospital, New York, New York; Cornell University, Ithaca, New York; Yale Center for X-linked Hypophosphatemia, Yale School of Medicine, New Haven, Connecticut; The University of Alabama at Birmingham, Birmingham, Alabama; Children's Imaging Research Program, Children's Hospital Los Angeles, and Keck School of Medicine, University of Southern California, Los Angeles, California;Stanford University School of Medicine, Stanford, California; University of Michigan Medical School and Geriatric Research, Education, and Clinical Center, Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan; Center for Aging Research, Indiana University School of Medicine and Regenstrief Institute, Indianapolis, Indiana; Texas A&M University, College Station, Texas;Monell Chemical Senses Center and School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania; Harvard University, Children's Hospital Boston, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts; and Metabolic Bone Center, Johns Hopkins Bayview Medical Center, Baltimore, Maryland., Karen Hall, MD, PhDFrom Mount Sinai School of Medicine of New York University, Mount Sinai Kravis Children's Hospital, New York, New York; Cornell University, Ithaca, New York; Yale Center for X-linked Hypophosphatemia, Yale School of Medicine, New Haven, Connecticut; The University of Alabama at Birmingham, Birmingham, Alabama; Children's Imaging Research Program, Children's Hospital Los Angeles, and Keck School of Medicine, University of Southern California, Los Angeles, California;Stanford University School of Medicine, Stanford, California; University of Michigan Medical School and Geriatric Research, Education, and Clinical Center, Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan; Center for Aging Research, Indiana University School of Medicine and Regenstrief Institute, Indianapolis, Indiana; Texas A&M University, College Station, Texas;Monell Chemical Senses Center and School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania; Harvard University, Children's Hospital Boston, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts; and Metabolic Bone Center, Johns Hopkins Bayview Medical Center, Baltimore, Maryland., Siu L. Hui, PhDFrom Mount Sinai School of Medicine of New York University, Mount Sinai Kravis Children's Hospital, New York, New York; Cornell University, Ithaca, New York; Yale Center for X-linked Hypophosphatemia, Yale School of Medicine, New Haven, Connecticut; The University of Alabama at Birmingham, Birmingham, Alabama; Children's Imaging Research Program, Children's Hospital Los Angeles, and Keck School of Medicine, University of Southern California, Los Angeles, California;Stanford University School of Medicine, Stanford, California; University of Michigan Medical School and Geriatric Research, Education, and Clinical Center, Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan; Center for Aging Research, Indiana University School of Medicine and Regenstrief Institute, Indianapolis, Indiana; Texas A&M University, College Station, Texas;Monell Chemical Senses Center and School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania; Harvard University, Children's Hospital Boston, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts; and Metabolic Bone Center, Johns Hopkins Bayview Medical Center, Baltimore, Maryland., Joanne Lupton, PhDFrom Mount Sinai School of Medicine of New York University, Mount Sinai Kravis Children's Hospital, New York, New York; Cornell University, Ithaca, New York; Yale Center for X-linked Hypophosphatemia, Yale School of Medicine, New Haven, Connecticut; The University of Alabama at Birmingham, Birmingham, Alabama; Children's Imaging Research Program, Children's Hospital Los Angeles, and Keck School of Medicine, University of Southern California, Los Angeles, California;Stanford University School of Medicine, Stanford, California; University of Michigan Medical School and Geriatric Research, Education, and Clinical Center, Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan; Center for Aging Research, Indiana University School of Medicine and Regenstrief Institute, Indianapolis, Indiana; Texas A&M University, College Station, Texas;Monell Chemical Senses Center and School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania; Harvard University, Children's Hospital Boston, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts; and Metabolic Bone Center, Johns Hopkins Bayview Medical Center, Baltimore, Maryland., Julie Mennella, PhDFrom Mount Sinai School of Medicine of New York University, Mount Sinai Kravis Children's Hospital, New York, New York; Cornell University, Ithaca, New York; Yale Center for X-linked Hypophosphatemia, Yale School of Medicine, New Haven, Connecticut; The University of Alabama at Birmingham, Birmingham, Alabama; Children's Imaging Research Program, Children's Hospital Los Angeles, and Keck School of Medicine, University of Southern California, Los Angeles, California;Stanford University School of Medicine, Stanford, California; University of Michigan Medical School and Geriatric Research, Education, and Clinical Center, Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan; Center for Aging Research, Indiana University School of Medicine and Regenstrief Institute, Indianapolis, Indiana; Texas A&M University, College Station, Texas;Monell Chemical Senses Center and School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania; Harvard University, Children's Hospital Boston, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts; and Metabolic Bone Center, Johns Hopkins Bayview Medical Center, Baltimore, Maryland., Natalie J. Miller, BSFrom Mount Sinai School of Medicine of New York University, Mount Sinai Kravis Children's Hospital, New York, New York; Cornell University, Ithaca, New York; Yale Center for X-linked Hypophosphatemia, Yale School of Medicine, New Haven, Connecticut; The University of Alabama at Birmingham, Birmingham, Alabama; Children's Imaging Research Program, Children's Hospital Los Angeles, and Keck School of Medicine, University of Southern California, Los Angeles, California;Stanford University School of Medicine, Stanford, California; University of Michigan Medical School and Geriatric Research, Education, and Clinical Center, Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan; Center for Aging Research, Indiana University School of Medicine and Regenstrief Institute, Indianapolis, Indiana; Texas A&M University, College Station, Texas;Monell Chemical Senses Center and School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania; Harvard University, Children's Hospital Boston, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts; and Metabolic Bone Center, Johns Hopkins Bayview Medical Center, Baltimore, Maryland., Stavroula Kalis Osganian, MD, ScD, MPHFrom Mount Sinai School of Medicine of New York University, Mount Sinai Kravis Children's Hospital, New York, New York; Cornell University, Ithaca, New York; Yale Center for X-linked Hypophosphatemia, Yale School of Medicine, New Haven, Connecticut; The University of Alabama at Birmingham, Birmingham, Alabama; Children's Imaging Research Program, Children's Hospital Los Angeles, and Keck School of Medicine, University of Southern California, Los Angeles, California;Stanford University School of Medicine, Stanford, California; University of Michigan Medical School and Geriatric Research, Education, and Clinical Center, Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan; Center for Aging Research, Indiana University School of Medicine and Regenstrief Institute, Indianapolis, Indiana; Texas A&M University, College Station, Texas;Monell Chemical Senses Center and School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania; Harvard University, Children's Hospital Boston, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts; and Metabolic Bone Center, Johns Hopkins Bayview Medical Center, Baltimore, Maryland., Deborah E. Sellmeyer, MDFrom Mount Sinai School of Medicine of New York University, Mount Sinai Kravis Children's Hospital, New York, New York; Cornell University, Ithaca, New York; Yale Center for X-linked Hypophosphatemia, Yale School of Medicine, New Haven, Connecticut; The University of Alabama at Birmingham, Birmingham, Alabama; Children's Imaging Research Program, Children's Hospital Los Angeles, and Keck School of Medicine, University of Southern California, Los Angeles, California;Stanford University School of Medicine, Stanford, California; University of Michigan Medical School and Geriatric Research, Education, and Clinical Center, Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan; Center for Aging Research, Indiana University School of Medicine and Regenstrief Institute, Indianapolis, Indiana; Texas A&M University, College Station, Texas;Monell Chemical Senses Center and School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania; Harvard University, Children's Hospital Boston, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts; and Metabolic Bone Center, Johns Hopkins Bayview Medical Center, Baltimore, Maryland., and Marshall A. Wolf, MD*From Mount Sinai School of Medicine of New York University, Mount Sinai Kravis Children's Hospital, New York, New York; Cornell University, Ithaca, New York; Yale Center for X-linked Hypophosphatemia, Yale School of Medicine, New Haven, Connecticut; The University of Alabama at Birmingham, Birmingham, Alabama; Children's Imaging Research Program, Children's Hospital Los Angeles, and Keck School of Medicine, University of Southern California, Los Angeles, California;Stanford University School of Medicine, Stanford, California; University of Michigan Medical School and Geriatric Research, Education, and Clinical Center, Veterans Affairs Ann Arbor Healthcare System, Ann Arbor, Michigan; Center for Aging Research, Indiana University School of Medicine and Regenstrief Institute, Indianapolis, Indiana; Texas A&M University, College Station, Texas;Monell Chemical Senses Center and School of Veterinary Medicine of the University of Pennsylvania, Philadelphia, Pennsylvania; Harvard University, Children's Hospital Boston, Harvard Medical School, and Brigham and Women's Hospital, Boston, Massachusetts; and Metabolic Bone Center, Johns Hopkins Bayview Medical Center, Baltimore, Maryland.Author, Article, and Disclosure Informationhttps://doi.org/10.7326/0003-4819-152-12-201006150-00248 SectionsAboutVisual AbstractPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinkedInRedditEmail National Institutes of Health (NIH) consensus and state-of-the-science statements are prepared by independent panels of health professionals and public representatives on the basis of 1) the results of a systematic literature review prepared under contract with the Agency for Healthcare Research and Quality; 2) presentations by investigators working in areas relevant to the conference questions during a 2-day public session; 3) questions and statements from conference attendees during open discussion periods that are part of the public session; and 4) closed deliberations by the panel during the remainder of the second day and morning of the third. This statement is an independent report of the panel and is not a policy statement of the National Institutes of Health or the U.S. government. The following statement is an abridged version of the panel's report, which is available in full atconsensus.nih.gov/2010/lactosestatement.htm.Lactose intolerance is the syndrome of diarrhea, abdominal pain, flatulence, or bloating occurring after lactose ingestion. These symptoms, which are produced by malabsorption of lactose, a sugar found in milk and other dairy products, often cause afflicted individuals to avoid dairy products in their diets. Lactose malabsorption is caused by a decreased ability to digest lactose that is due to a deficiency in the levels of the enzyme lactase. Lactase breaks lactose down into 2 simpler sugars, glucose and galactose, which are readily absorbed into the bloodstream. This enzyme is produced by expression of the lactase–phlorizin hydrolase gene in the cells lining the small intestine.All infants produce lactase and successfully digest lactose provided by human milk or by infant formulas. However, sometime after weaning, a genetically programmed decrease in lactase (lactase nonpersistence) occurs in most children worldwide.The symptoms of lactose intolerance result from bacterial fermentation of undigested lactose in the colon. Lactose malabsorption can be diagnosed by having individuals ingest a standard dose of lactose after fasting and measuring breath hydrogen; elevated breath hydrogen levels are caused by bacterial fermentation of undigested lactose in the colon. Other diagnostic tools include measuring lactase activity in an intestinal biopsy sample or genetic testing for the common polymorphism that is linked to lactase nonpersistence. The demonstration of lactose malabsorption does not necessarily indicate that an individual will have symptoms. Many variables determine whether a person who malabsorbs lactose develops symptoms, including the dose of lactose ingested, the residual intestinal lactase activity, the ingestion of food along with lactose, the ability of the colonic flora to ferment lactose, and individual sensitivity to the products of lactose fermentation.Current management often relies on reducing lactose exposure by avoiding milk and milk-containing products or by drinking milk in which the lactose has been prehydrolyzed with lactase. Alternatively, persons with lactase nonpersistence may tolerate moderate amounts of dairy products ingested with other foods. Many individuals, however, mistakenly ascribe symptoms of diverse intestinal disorders to lactose intolerance without undergoing testing. This misconception becomes intergenerational, when parents with self-diagnosed lactose intolerance place their children on lactose-restricted diets (even in the absence of symptoms) in the mistaken belief that the children will develop symptoms if given lactose.The public health burden from deficiencies attributable to lactose intolerance has not been established. Many adults and children who avoid dairy products—which constitute a readily accessible source of calcium, vitamin D, and other nutrients—are not ingesting adequate amounts of these essential nutrients. For example, most African-American adolescents consume inadequate amounts of calcium and vitamin D because they avoid dairy products. Deficient intakes of calcium and vitamin D are risk factors for decreased bone mineral density. This may increase the risk for fracture throughout the life cycle, especially in postmenopausal women. Very low intake of vitamin D can lead to the development of rickets, especially in children of African descent and other highly pigmented persons. Although reduced-lactose dairy and nondairy alternative products are typically fortified with calcium, vitamin D, and other nutrients, they may be more expensive and less widely available than conventional dairy products. The bioequivalence of these and other calcium supplements is uncertain.To examine this important topic more closely, the Eunice Kennedy Shriver National Institute of Child Health and Human Development and the Office of Medical Applications of Research of the National Institutes of Health convened a Consensus Development Conference. This conference, which addressed several questions, was informed by a systematic review conducted by the Minnesota Evidence-based Practice Center.Question 1What is the prevalence of lactose intolerance, and how does this prevalence differ by race, ethnicity, and age?The prevalence of lactose intolerance in the United States cannot be estimated from available data. The potentially relevant studies identified in the systematic review used the definition of lactose malabsorption rather than an accurate and appropriate definition of lactose intolerance and did not evaluate a representative sample of the U.S. population. Studies that assessed self-reported lactose intolerance provided limited insight because the self-diagnoses were not confirmed by testing for lactose malabsorption, and the symptoms seen in true lactose intolerance may result from several other conditions, including the irritable bowel syndrome. Some studies evaluated only the genetic predisposition to lower-than-expected lactase levels in adults (lactase nonpersistence) without assessing lactose malabsorption or intolerance directly.Despite the limitations of available studies, several noteworthy observations emerged. First, lactose intolerance determined by self-report or nonblinded lactose challenge is less frequent across all ethnic groups than is lactose malabsorption determined by breath hydrogen tests or lactase nonpersistence determined by biopsy or genetic testing. Second, lactose intolerance, lactose malabsorption, and lactase nonpersistence vary across racial and ethnic groups, with the lowest reported occurrence in European Americans and higher (although variable) occurrence in African Americans, Hispanic Americans, Asian Americans, and Native Americans. Finally, lactose intolerance with nonblinded lactose challenge and lactose malabsorption was low in young children but increased with age. In children younger than 6 years, lactose malabsorption was low in all the studies and peaked between 10 and 16 years of age. Little evidence suggests that lactose intolerance increases in older persons. These trends need to be verified by representative population studies by using the case definition of lactose intolerance.Question 2What are the health outcomes of dairy-exclusion diets?The health outcomes of dairy-exclusion diets depend on whether other sources of nutrients, such as calcium and vitamin D, occur in the diet in sufficient quantities to replace dairy products as a source of these nutrients, and to what extent other components of milk are beneficial.Calcium is necessary for normal growth and bone development, as well as subsequent maintenance of bone density. The strongest argument for promotion of dairy ingestion is the beneficial effect of calcium (and fortified vitamin D in milk) on growth and development of the skeleton. Calcium is necessary for adequate bone accretion and optimal peak bone mass, which is a major determinant of risk for osteoporosis and fragility fractures later in adult life. Evidence suggests that certain age groups, such as children and teenagers, may be at increased risk for deficient bone acquisition if their diets are deficient in calcium or vitamin D. Weak evidence indicates that children with calcium-deficient diets have increased fracture rates. The maximal accumulation of bone mineral, and therefore the maximal calcium requirement, occurs during puberty. Although studies indicate that young children who drink milk are likely to meet or exceed the adequate intake for calcium, teenagers as a group tend not to take in enough calcium to meet recommended needs. This problem is exacerbated by dairy avoidance in individuals who consider themselves lactose intolerant, regardless of whether they have undergone objective testing for lactose intolerance.Studies show that the presence of lactose does not necessarily affect the efficiency of calcium absorption across the intestine and that persons with lactase nonpersistence do not have substantial impairment in calcium absorption. Thus, the limiting factor in achieving optimal peak bone mass in young individuals is the intake of calcium. Similarly, in older individuals, low calcium intake rather than deficient absorption is probably a major factor contributing to loss of bone mass. Replacement of calcium using supplements or dairy products slows the rate of bone loss in older people, possibly as a result of an overall decrease in bone turnover. Across the age spectrum, the factor that limits adequate calcium accrual in many individuals probably is dairy avoidance.Dairy-exclusion diets may decrease gastrointestinal symptoms (bloating, cramps, flatus, and diarrhea) in symptomatic persons who have lactose malabsorption or intolerance. The degree of relief is probably related to the degree of expression of lactase and the quantity of lactose ingested. People who remain symptomatic on a dairy-exclusion diet may have other causes for their gastrointestinal symptoms, such as the irritable bowel syndrome, celiac disease, inflammatory bowel disease, or small-bowel bacterial overgrowth.Question 3What amount of daily lactose intake is tolerable in persons with diagnosed lactose intolerance?Among persons with appropriately diagnosed lactose intolerance, differences in several factors—including lactase activity, gastric emptying rates, fecal bacterial metabolites, colonic mucosal absorptive capacity, and intestinal transit time—can greatly influence their susceptibility to development of intolerance symptoms after ingestion of foods and beverages containing lactose. Individuals differ in the intensity of symptoms of lactose intolerance because of differences in abdominal pain perception and the psychological effect of pain and social discomfort. Determining the amount of lactose that can be tolerated is necessary to develop evidence-based dietary recommendations that meet the needs of the individual.High-quality evidence that addresses the above question is limited. Pertinent studies used different definitions of lactose intolerance, sample selection criteria, lactose administration procedures, and assessment and follow-up methods. Most studies used a single dose of lactose administered without food and evaluated short-term responses. Efforts often were not made to mask the taste difference between lactose-free milk and milk containing lactose. Only a handful of studies tested the participants in a double-blinded manner with increasing amounts of lactose administered throughout the day to determine the daily tolerable lactose dose. Most studies examined small numbers of participants, and few or no studies focused exclusively on children, pregnant women, or lactating women.In most studies, participants were classified as malabsorbers or absorbers on the basis of breath hydrogen measurement or a blood glucose test, and symptoms of lactose intolerance were not always required for study entry. A blinded control was rarely used to define lactose intolerance at study entry; thus, it is probable that some individuals would have reported symptoms after ingestion of lactose-free solutions. Most studies investigated individuals with proven lactose ma