The purpose of this research study is to determine if the cold-induced change in fat burning during exercise is different between females and males.

It is well characterized that females rely more on fat oxidation compared to men during standardized exercise. Exposure to cold during exercise increases fat oxidation in humans. It has been shown that the biological sex differences in substrate oxidation observed during exercise persist in the cold. However, it is unclear if the degree of the effect of cold on substrate oxidation during exercise is different between females and males.

The purpose of this study is to determine if the change in maximal fat oxidation that occurs in cold compared to room temperature environments is different between females and males.

The hypotheses are 1) females will have greater maximal fat oxidation than males in both conditions, and 2) the change in maximal fat oxidation between the control and cold conditions will be the same in females and males.

Inclusion Criteria:

• Age 18-40 years (inclusive)
• Weight stable (no change in body mass greater than 5 lbs. within the previous 6 months.
• Willing to abstain from caffeine and alcohol for 24-hours prior to two different study visits
• Competency in English as informally assessed by comprehension of the Informed Consent. This study involves maximal exercise testing; for safety reasons, the ability for clear and rapid communication will be necessary between the research participants and the investigators.
• Regular exercise (i.e., at least 150 minutes of moderate-vigorous exercise per week)

Exclusion Criteria:

• Use of hormonal contraceptives
• History of autonomic, cardio-pulmonary, and/or metabolic disease (including heart failure, hypertension, arrhythmia, vascular disease, and/or diabetes)
• Pregnancy or breast feeding
• Habitual use of tobacco/nicotine products or recreational drugs (2 or more uses within the previous month)