Female life stages including premenopausal, perimenopausal, postmenopausal, are characterized by changing concentrations and patterns of hormones. The premenopause stage represents the reproductive years with normal menstrual cycles lasting between 21-35 days. The perimenopause stage is the beginning of the menopause transition and can last up to 10 years. During this transition, menstrual cycles become longer and irregular in length (7 days or longer). After 12 consecutive months with no period, the transition is complete into the postmenopause stage.

Hormones that change across the lifespan are known to influence systems that regulate thirst, fluid balance, and sodium balance. Yet, fluid balance across life stages remains poorly understood, with no direct comparisons across all three life stages. The purpose of this study is to evaluate the effect of female life stage on rehydration with water and electrolyte solution after overnight fast and fluid restriction

Inclusion Criteria:

• • Sex assigned female at birth.

  • Recreationally active (engaged in light to moderate-intensity, intermittent, or steady-state exercise at least 3 days per week for at least 30 min at a time)

  • Age 18-60 years, within the following life stage ranges specified below:

  • Premenopausal:

    • 18-35 years
    • Self-report regular spontaneous menstrual cycle length (21-35 days) for the past 2 menstrual cycles
    • Menstrual cycle length < 7 days variation in consecutive cycles

  • Perimenopausal:

    • Perimenopause: 35-55 years
    • If tracking cycles: self reported cycle irregularity (≥7 day difference from usual cycle length) for the past 2 menstrual cycles, not due to known pathology
    • If not tracking cycles: clinician diagnosis

  • Postmenopausal:

    • Postmenopause: 45-60 years
    • If tracking cycles: ≥12 consecutive months of spontaneous amenorrhea not due to pregnancy, lactation, or pathology
    • If not tracking cycles: clinician diagnosis

  • Healthy, with no known renal, cardiovascular, metabolic, or endocrine disorders that affect hydration or fluid regulation.

  • Not currently taking medications known to influence fluid balance (e.g., diuretics, laxatives, weight loss agents, or other relevant medications) or the hypothalamus-pituitary-gonadal axis (e.g., ovulation induction agents, gonadotropins, IVF stimulation medications, or other relevant medications)

    o See appendix 13.8 for list of medications, drugs, and/or medical conditions that may impair cognitive functioning, alter physiological responses relevant to the study outcomes, or increase the risk associated with participation.

  • Not receive any fertility treatments within the past 6 months

  • No hormonal contraceptives use within the past 6 months

  • If using a paraguard copper IUD, it must be inserted for ≥ 3 months (if applicable)

  • BMI ≤ 30 kg/m²

  • Are not participating in a weight loss program

  • Are not allergic to adhesives, stevia, and strawberry lime flavoring

  • Willing to avoid strenuous activity and alcohol consumption 24 hours prior to all visits

  • Willing to fast overnight (~12 hours) prior to the medical screening

  • For visit 2 and 4:

    • Willing to fast overnight (~12 hours)
    • Willing to consume a minimum of 2.7 L of water 24 hours prior to visits 2 and 4, then 500 mL 2 hours before the start of the visits 2 and 4

  • For visits 3 and 5:

  • Willing to restrict all fluids (~18 hours)

  • Willing to limit fluid consumption to 1 L the morning/afternoon prior to the start of the fluid restriction

  • Willing to restrict food (~12 hours) the day prior to each visit Willing to eat a consistent diet the day prior

  • Fluent in English (able to read, understand, and provide informed consent).

  • Willing and able to provide informed consent to participate in the study and comply with all study procedures and scheduled visits

Exclusion Criteria:

• • Currently pregnant or breastfeeding

  • < 6 months since giving birth (even if experiencing 2 regular menstrual cycles in a row)
  • Smoking within the last 5 years
  • History of adverse events (e.g., fainting) with blood draws
  • Participants with large amount of internal metal (e.g., spinal rod) will be excluded from the DXA scan only (Brandenton)
  • Participant has participated in a clinical trial within the past 30 days
  • Participant has participated in any PepsiCo trial within past 3 months
  • Participant has a condition or is taking medication that the investigator believes would interfere with his/her ability to provide informed consent, comply with the study protocol, which might confound the interpretation of the study results, or put the person at undue risk
  • Participant is employed by, or has a parent, guardian, or other immediate family member employed by a company that manufactures any products that compete with any Gatorade product. If participant is unsure if a company would be considered a competitor to Gatorade, they will be asked to please let the study investigator know the name of the other company and the nature of their relationship to that company before they sign the informed consent.