The goal of this clinical trial is to learn about the processes occurring in the kidneys while under heat stress in healthy volunteers. The main questions it aims to answer are:

• How do the chemicals produced by the body change under conditions of higher versus lower heat stress?
• What role does a specific area of the body's metabolism, known as NAD+ metabolism, play in the body's response to heat stress, and can this response be modified by taking vitamin B3?

To answer these questions, researchers will compare the chemical changes in each participant under progressively higher levels of heat stress, and while taking either vitamin B3 or a placebo.

This clinical trial will occur in two stages. Participants may choose participate in stage 1 only, stage 2 only, or both parts of this clinical trial.

During stage 1, participants will exercise using a stationary rowing ergometer in a hot and humid environmental chamber for three sessions, each session separated by about a week.

• Each session, they will be asked to work out at a progressively higher intensity with climate conditions kept the same in the chamber.
• More intense exercise produces greater heat stress, resulting in lower, moderate, and higher levels of heat stress exposure across the three sessions.
• Researchers will see how chemicals in the blood and urine, along with physical measurements like heart rate and body temperature, change across these different levels of heat stress.

During stage 2, participants will exercise using the same rowing ergometer in the same environmental chamber. They will do this for two sessions, each separated by about a week.

• One of the sessions each participant will take vitamin B3, and the other session each participant will take placebo.
• Heat stress exposure will be the same each session.
• Researchers will again see how chemicals in the blood and urine, along with physical measurements like heart rate and body temperature, differ between sessions with vitamin B3 and sessions with placebo

Inclusion Criteria:

• Healthy volunteer
• Any race
• Estimated glomerular filtration rate greater than 90 ml/min/1.73m2
• Urine albumin/creatinine ratio less than 30mg/g
• Nonsmoker
• No regular dietary supplements, particularly vitamin B3
• Physically fit, defined as having a VO2 max of between 35 and 60mL/kg/min

Exclusion Criteria:

• Medical condition preventing safe participation in exercise during heat
• Allergy to Vitamin B3
• Severe food allergies or dietary restrictions that would preclude eating the planned study diet without major modifications

De-identified individual participant data will be stored in the Harvard Dataverse Repository.

Data availability will begin at the time of an associated publication or the end of the performance period, whichever comes first. Data will continue to be available for ten years after completion of study activities.

Researchers may request controlled-access data using standard processes at the repository.