The purpose of this study is to evaluate the ability of a wearable Smart Underwear prototype device to quantify diet-induced changes in gut microbial hydrogen sulfide (H₂S) production. The core design is a single-site, 2-period, crossover feeding study with 6-day diet periods and an approximately 11-day washout period. Participants are fed each of two isocaloric diets designed to contrast gut microbial H₂S production (i.e., a high cysteine vs. low cysteine diet), in a random order.

H₂S is a circulating signaling molecule that plays numerous roles in human physiology. H₂S produced exogenously by the gut microbiota influences human health by modulating systemic H₂S bioavailability in a diet-dependent manner. Several lines of evidence suggest excessive gut microbial H₂S production may be etiologically involved in a wide range of diseases, from colorectal cancer to ulcerative colitis. However, research on the health-relevant effects of gut microbially-produced H₂S has been constrained by technical limitations.

Gap: Current techniques for measuring gut microbial H₂S production are imprecise, invasive, and have limited temporal resolution, hindering the ability to deconvolute how gut microbial H₂S production both affects human health and is influenced by factors such as diet. Gaseous rectal effluent (flatus) is an ideal, yet untapped biospecimen to measure gut microbial H₂S production.

Preliminary Evidence: To enable real time measurements of gut microbial H₂S production in flatus, the investigators are developing a wearable device called "Smart Underwear". The Smart Underwear device is fundamentally different from previous methods used to measure gut microbial H₂S production. The Smart Underwear device uses an array of metal oxide gas sensors coupled with a multi-layer filtration system to selectively measure H₂S by comparing the signal from filtered and unfiltered sensors. The Smart Underwear sensor array is adhered to the outside of a participant's underwear adjacent to the perineum. Through in vitro and human wearing validation, the investigators have demonstrated the feasibility of the Smart Underwear v1 prototype to measure gut microbial H₂S production in flatus and have shown that the device has potential utility in human cohort studies as a non-invasive device.

The long-term goal of this study is to develop a validated Smart Underwear device that measures real time gut microbial hydrogen sulfide (H₂S) production which is responsive to dietary changes and capable of predicting of pathophysiological outcomes.

The investigators will validate the Smart Underwear prototype device using structured dietary modulation of H₂S production.

Aim: Conduct a randomized controlled feeding trial to evaluate the ability of the Smart Underwear device to quantify diet-induced changes in gut microbial H₂S production.

Hypothesis: The investigators hypothesize that the Smart Underwear device will detect higher H₂S concentrations in flatus during a high cysteine dietary intervention compared to a low cysteine dietary intervention.

Methods: The investigators will employ a randomized crossover feeding trial with two arms designed to contrast gut microbial H₂S production by modulating dietary cysteine while controlling other dietary factors (total calories and nutrients). The investigators have conducted extensive background research to determine that modulation of dietary cysteine is the most effective means of producing a diet-induced contrast in H₂S production. Participants will wear the Smart Underwear device during the dietary interventions allowing for a rigorous evaluation of the devices ability to detect diet-induced alterations of gut microbial H₂S production in flatus.

The inclusion/exclusion criteria will identify generally healthy volunteers that are able to ingest the intervention diets. Exclusion criteria based on diseases, medications, allergies, and specific dietary requirements will minimize the risks of adverse reactions to the intervention diets.

Inclusion Criteria:

• Generally healthy volunteers defined as having no major known health conditions (e.g., diabetes, cancer, hypertension, etc.).
• Normal bowel movements, with approximately 1 bowel movement reported per day
• Willing to discuss flatus
• Ages >18yrs
• Willing to complete the entire study protocol, i.e. eating all of the food that is provided and completing all required measurements.

Exclusion Criteria:

• Self-report or other evidence of diabetes, other endocrine/metabolic abnormality, dyslipidemia, morbid obesity, severe hypertension, chronic kidney disease, liver disease, pulmonary disease, gastrointestinal, and cardiovascular diseases
• Chronic medications for any of the above conditions
• Food allergy that interferes with ability to complete the study
• Food preferences, intolerance, or dietary requirements that would interfere with diet adherence
• Planned dietary changes during the study period
• Lack of appropriate food refrigeration and preparation equipment (e.g.- oven or microwave)
• Pregnancy or planned pregnancy in the next month
• Physical measurements: BMI > 35 kg/m2

The investigators and the outcomes assessors will not know which order of diets the participant has been assigned. The participant will not be explicitly told which diet he or she is receiving during each period, but by the nature of the differences between the high cystine and low cystine diets, the participant may be able to guess based on the menus.