Lactobreath: A Study to Diagnose Lactose Intolerance Using Breath Markers

Inclusion criteria: * Men and women * Swiss and non-Swiss living in the Zurich area (if necessary, beyond Zürich in Switzerland), * Ability/desire to provide informed consent and partake in the procedures of the study * Aged 18-65 years at screening * Agreement to refrain from all other treatments and products used for dairy intolerance (e.g., Lactaid® dietary supplements) during study involvement * Willing to return for all study visits and complete all study-related procedures, including fasting before and during the intervention * Able to understand and provide written informed consent in English and/or German. Exclusion criteria: * Allergic to milk * Currently pregnant * Currently lactating * Cigarette smoking or other use of tobacco or nicotine-containing products within 3 months of screening * Diagnosed with any of the following disorders known to be associated with abnormal GI motility: gastroparesis, amyloidosis, neuromuscular diseases (including Parkinson's disease), collagen vascular diseases, alcoholism, uremia, malnutrition, or untreated hypothyroidism * History of surgery that alters normal GI tract function, including but not limited to: GI bypass surgery, bariatric surgery, gastric banding, vagotomy, fundoplication, pyloroplasty (N.B. history of uncomplicated abdominal or GI surgeries such as removal of an appendix \>12 months before screening will not be excluded) * Suspected obscure GI bleeding * Past or present: organ transplant, chronic pancreatitis, pancreatic insufficiency, symptomatic biliary disease, coeliac disease, diverticular disease, inflammatory bowel disease, strictures (suspected or known), fistulas, or any GI obstruction, gastroparesis, history of gastric bezoar or any other medical condition with symptoms that could confound collection of adverse events * Diabetes mellitus * Congestive heart failure * Human immunodeficiency virus, hepatitis B, or hepatitis C * Body mass index \> 35 kg/m2 * Swallowing disorders or dysphagia to food or pills * Presence of implantable or portable electro-mechanical medical devices (e.g. pacemakers) * Recent bowel preparation for endoscopic or radiologic investigation within 4 weeks of screening (e.g., colonoscopy preparation) * Chronic antacid and/or proton pump inhibitor use * Recent use of systemic antibiotics, defined as use within 2 months prior to screening * History of ethanol (alcohol) and/or drug abuse in the past 12 months * Patients with severe irritable bowel syndrome (IBS) (i.e., IBS Symptom Severity Score \>400) * Dietary restrictions including vegan or vegetarian diet. * Any other conditions/issues noted by the study staff and/or Principal Investigator that would impact participation and/or protocol compliance. * Previous enrollment in another clinical trial within the last 3 months. * Results of the screening test showing GI symptoms in response to lactose ingestion and genetic LP.