The majority of vitamin D research has been done in non-athletic populations, particularly older populations, to analyze muscle weakness, pain, balance, and fractures. It has been reported that over a billion people worldwide are vitamin D deficient, including 36-70% of the young adult population. Previous literature also suggests that, even among healthy athletes, over 50% of subjects sampled had inadequate or insufficient vitamin D levels. Vitamin D deficiency can occur in young women, including pregnant women, and the risk of deficiency is even higher with advancing age in a woman's lifecycle. It is known that preserving skeletal muscle (SM) function is critical for women of all ages to prevent sarcopenia. Two factors in preserving SM are protein intake and resistance training. Relatively unknown are the actions of Vitamin D on SM function. Vitamin D deficiency can have a negative effect on SM function . However, additional research is needed to investigate the increase in SM strength when the serum concentration of vitamin D is improved. It is possible that an athlete may require an increased intake of vitamin D to assure adequate availability and storage for optimal performance; however, is known that food sources of vitamin D are limited in sufficient quantity to meet these requirements. Athletes who are insufficient will require a supplement of vitamin D3 for up to 5000 IU/day for at least eight weeks, to potentially reach optimal levels, then 1000-2000 IU/day for maintenance.

The investigators seek to determine if women with a vitamin D serum concentration lower than 50 nmol/L and who regularly resistance train will improve strength performance outcomes when taking a daily supplement of 5000 IU of vitamin D3 over an 8-week period, as compared to the same demographic of women who do not take a daily supplement of vitamin D3. The participants will receive "Nature's Bounty Vitamin D3, 5,000 IU, soft gels" and the PI will be providing the supplement and the placebo. A pre-intervention data collection and screening (PRE) will be performed involving blood draw, food log, a body composition test (using Dual-energy X-ray absorptiometry [DEXA]), and a BioDex testing to determine maximal muscle strength. Following PRE testing, the participants will be divided into one of two groups: 1) taking a vitamin D3 supplement (n=20) or, 2) taking a placebo. The participants will be instructed to continue their resistance training regimen. Training will be logged and turned in to the PI weekly. The participants will perform a post-intervention data collection (POST), which includes the same test from PRE. Variables of interest will include changes in: 1) blood level of vitamin D, 2) muscle and fat mass, and 3) leg strength/power during the BioDex test.

Inclusion criteria will include:

Participant will need to answer "yes" to the following items:

1. female
2. not pregnant
3. between the ages of 19-55 years old
4. a non-smoker
5. currently resistance training (i.e., lifting weights) for more than 2 days/week for a minimum of > 24 months
6. free of any known overt cardiovascular or metabolic disease
7. not an active user of tanning beds
8. not consuming now or haven't consumed within the last 6 months, a vitamin D or multivitamin supplement or a form of a supplement with vitamin D added
9. not consuming now or haven't consumed within the last 2 months, agents that affect hormones (testosterone boosters, growth hormone boosters, etc.) (does not include birth control/oral contraceptives)
10. not allergic to rubbing alcohol

Exclusion criteria will include:

1) Outside of study inclusion parameters

Double-blind masking; neither the investigator nor the participants will know, the assigned intervention placement.