Participation will take place over two visits to the Clinical Exercise Research Center (CERC) at the University of Southern California Health Sciences Campus.

On the first visit, height, weight, and body composition by bioelectrical impedance are recorded. Questionnaires and withdrawal of a sample of blood (1 tablespoon) are also completed. The intervention begins the day following the first visit. The intervention protocol consists of three weeks of consuming either a placebo capsule or a 5000 IU Vitamin D capsule daily and completing a daily survey.

The second visit to CERC immediately follows the 3-weeks intervention. In this visit, the same blood sampling protocol will be completed. The assignment of the intervention will be randomized (i.e., rolling a die), and participants are blinded to group assignment.

High-dose Vitamin D supplementation significantly increases testosterone in men, but research about Vitamin D's effect on testosterone in young, healthy females is limited [1]. However, testosterone is a critical hormone for many physiologic functions in females including maintenance of reproductive health, cognitive function, body composition, mental health, bone density, muscle strength, and much more. Thus, this study aims to investigate the effect of Vitamin D supplementation on testosterone and gonadotropin concentrations in premenopausal, healthy females. The study investigators hypothesize that Vitamin D supplementation will result in a significant increase in circulating Vitamin D [25(OH)D], testosterone, and sex hormone modulators (i.e., gonadotropins and SHBG) concentrations in pre-menopausal females and that the magnitude of testosterone increase will be inversely related to fat mass and BMI. 20 young, healthy females will be randomized to either a treatment or placebo group for a three-week intervention, where the control group will take 5000 IU of Vitamin D orally, daily and the placebo group will take a placebo capsule orally, daily. Circulating Vitamin D, testosterone, gonadotropin, and SHBG concentrations will be assessed via serum pre- and post-intervention. Two-tailed unpaired t-test will be conducted to assess the relative change in total testosterone and free testosterone levels between the groups. Additionally, correlation analyses will be performed for correlations between body composition metrics and 25(OH)D, testosterone, and sex hormone modulator concentrations.

Inclusion Criteria:

• Assigned female at birth
• 18 to 35 years of age
• No chronic diseases affecting the uptake of Vitamin D or causing Vitamin D sensitivity (e.g., sarcoidosis, tuberculosis, Wegener's granulomatosis, other forms of vasculitis, inflammatory bowel diseases, etc.) [21]
• Completion and signature of the informed consent document
• Non-pregnant or trying to become pregnant
• No supplementation with Vitamin D within three months of enrollment
• No documented gynecological disease (e.g., PCOS, endometriosis, etc.)
• Weight greater than or equal to 110 lbs

Exclusion Criteria:

• Pregnant
• Supplementation with Vitamin D within three months of enrollment
• Taking exogenous hormones
• Documented gynecological disease (e.g., PCOS, endometriosis, etc.)
• Suffering from chronic diseases affecting the uptake of Vitamin D or causing Vitamin D sensitivity (e.g., sarcoidosis, tuberculosis, Wegener's granulomatosis, other forms of vasculitis, inflammatory bowel diseases, etc.) [21]
• Weight less than 110 lbs

Single blinded intervention. Those measuring the blood samples' hormonal will also be blinded to the intervention group but will not have a role in data analysis.