The Impact of Acute Exercise in the Heat on Breast Milk Production and Composition in Lactating Women

This study will be a randomized, cross-over design with four study visits: 1) an initial visit where the participant can see the study equipment, ask questions, and fill out a brief questionnaire to determine whether they are eligible to participant; 2) baseline and metabolic heat production testing; 3) acute exercise in the hot condition [experimental trial 1 (HOT): 36C, 40% relative humidity], and 4) acute exercise in the temperate, thermoneutral condition [experimental trial 2 (TEMP): 20C, 20% relative humidity]. Data collection will occur at the Health and Human Performance Laboratory (HHPL) in the Department of Health Sciences.

Visit 1: Initial Visit The participant will come to the lab for an initial visit. During this brief visit (<30 min), the participant can ask questions about the study, see the study equipment, and will be asked to complete a questionnaire to determine their eligibility.

Visit 2 (Baseline and Metabolic Heat Production Testing):

The baseline testing will collect participant anthropometric data, which includes height, body mass, and body composition (dual-energy X-ray absorptiometry, DXA) scan and a stepwise metabolic heat production test. Participants will report to the laboratory having abstained from caffeine, alcohol, and food (12, 24, and 4 hours, respectively). Upon arrival, participants will provide a small urine sample in a sterile urine cup to measure hydration status and for a pregnancy test. Researchers will measure hydration status using a refractometer. If the pregnancy test is positive, the PI will inform the participant that the test is positive and will not perform the scan. The participant will also no longer be eligible to continue with the study. After hydration assessment, nude body mass and height will be collected and the participant's body composition will measured using DXA. To obtain a nude body mass, a private curtain-changing area with a body weight scale will be provided to ensure privacy. Only the participant will be able to enter. The scale reading will come outside the private curtain area for the researchers to see. The DXA software will generate lean mass, fat mass, and body fat percentage. To calculate exercise intensity based on a fixed metabolic heat production (per unit body mass), participants will undergo a 24-min incremental treadmill test in an environmental chamber. Participants will perform the test in the same environmental conditions as the HOT trial (36C, 40% relative humidity; described below). Participants will don a heart rate (HR) monitor before testing. The treadmill test will consist of 6 x 4-min stages and progressively increase speed (2.5mph, 3.0mph, 3.5mph, 4.0mph, 4.5mph, and 5.0mph). The treadmill grade will be set at 2% to account for terrain and wind resistance. Oxygen consumption and respiratory exchange were collected continuously (30-sec average) using a Hans Rudolph metabolic mask (Hans Rudolph Inc., Shawnee, KS, USA) attached to a hose and delivered to the metabolic cart system (Parvo Medics Truemax 2400 Metabolic Measurement System, Consentius Technologies, Sandy, UT). These variables will be used to calculate metabolic heat production using partitional calorimetry (Cramer & Jay, 2019). An exercise intensity of 8 W/kg was selected to elicit a brisk walking intensity. The metabolic heat production test is a standard procedure used in the thermal physiology literature to perform an unbiased comparison of thermoregulatory responses between groups (Jay & Cramer 2015).

Visit 3 and 4: Experimental Trials (HOT & TEMP). Experimental trials (HOT and TEMP) will consist of a 60 min treadmill exercise protocol at a fixed metabolic heat production per unit body mass (8 W/kg). Exercise will be performed in an environmental chamber. HR, mean skin temperature (Tsk), core temperature (Tcore), sweat loss, and fluid intake will be measured throughout the exercise protocol. Breast milk composition will be measured through milk expression using a manual expression breast pump 24-hour before (post exercise), one hour-post exercise and 24 hours post-exercise. Breastmilk samples will not be collected at the laboratory. Participants will be asked to collect samples at home and will be provided written instructions on the breastmilk collection procedure (described below). Participants will log their breast milk production for 24 hours before and after the exercise protocol using a supplied infant scale.