The use of hormonal birth control (HBC) by athletes is common, but its relationship with athletic performance is complex and highly individualized. Hormonal contraceptives, such as the combined oral contraceptive pill, introduce synthetic hormones (like ethinyl estradiol and progestins) that override the body’s natural hormonal rhythm. This intervention affects various physiological systems, including metabolism and cardiovascular function. Research findings on the impact of HBC on competitive performance are often mixed and inconclusive, making it difficult to draw universal conclusions. The effect varies dramatically depending on the specific type and dose of synthetic hormones used.
Hormonal Mechanisms and Performance Variables
HBC introduces a steady dose of synthetic hormones that can influence physiological metrics important for competition. In endurance sports, a primary concern is the effect on aerobic capacity, often measured by maximal oxygen uptake (VO2 max). Some studies, particularly older ones, reported a small reduction in VO2 max (up to 5%) in women using combined oral contraceptives (COCs) compared to naturally cycling women. This potential reduction is linked to the synthetic estrogen component, which can slightly increase plasma volume and affect oxygen transport efficiency or impact thermoregulation.
However, many modern studies find only a minimal reduction in peak aerobic performance, suggesting that for the majority of athletes, COCs do not significantly hinder endurance capacity. The synthetic progestin component may also contribute to a slight increase in ventilation rate, though this effect is generally not significant enough to impair performance. The impact on strength and power is also debated, as the synthetic hormones can reduce the body’s free testosterone levels and insulin-like growth factor I (IGF-I), both of which are involved in muscle growth.
This hormonal environment may create a less anabolic state, potentially leading to lower gains in lean muscle mass during resistance training. For example, some athletes using progestin-containing COCs have shown less gain in lean mass over a 10-week strength protocol compared to non-users. Despite this difference in muscle adaptation, studies often report no significant difference in overall strength output, suggesting the effect is subtle and most relevant to athletes focused on maximizing hypertrophy. The synthetic hormones also appear linked to higher levels of the stress hormone cortisol after exercise, which can slow recovery and potentially contribute to a catabolic state over time.
Influence on Body Composition and Energy Metabolism
Hormonal contraceptives can alter an athlete’s physical structure and how the body processes fuel, indirectly affecting performance. A common concern is weight gain; however, most modern low-dose COCs do not cause a significant increase in fat mass for the average user. Any initial weight increase is typically temporary water retention or bloating, linked to the synthetic estrogen component’s effect on the renin-angiotensin-aldosterone system.
Certain progestin-only methods, such as the depot medroxyprogesterone acetate (DMPA) injection, have a clearer association with increased fat mass and decreased lean body mass over time. This shift in body composition is a significant consideration for athletes in weight-sensitive sports. HBC also influences energy substrate utilization during exercise, which is crucial for endurance athletes.
The presence of synthetic hormones can shift the body’s fuel preference during sub-maximal exercise. COCs may slightly reduce the body’s reliance on carbohydrate stores (glycogen), favoring fat oxidation instead. This change could theoretically spare glycogen for later use, but it may also be associated with a higher perceived effort during exercise. The metabolic effect is highly dependent on the specific hormone doses and the athlete’s training status, with some studies showing minimal to no change in fuel use.
Managing the Menstrual Cycle and Training Adaptations
For many female athletes, using HBC is a practical tool for gaining control and predictability over training and competition schedules. Suppressing the natural cycle can eliminate debilitating symptoms like severe menstrual cramps (dysmenorrhea) or hormonally-driven migraines that interrupt high-quality training. This stability allows athletes to avoid performance dips and discomfort associated with natural menstrual cycle phases, offering a consistent physical state throughout the month.
The use of HBC, however, can mask important physiological signals, particularly those related to Relative Energy Deficiency in Sport (RED-S). Since the bleeding experienced on the pill is a withdrawal bleed and not a true menstrual period, an athlete may not realize they have low energy availability, which is a significant health concern. HBC use may also be associated with side effects that impact training consistency, such as mood changes, increased fatigue, or greater perception of muscle soreness after unaccustomed exercise.
A potentially beneficial side effect for some athletes is the influence on injury risk. The natural menstrual cycle involves fluctuations of hormones like Relaxin-2, which temporarily increase ligament laxity, potentially raising the risk of injuries like anterior cruciate ligament (ACL) tears during the late follicular and ovulatory phases. By maintaining a steady, lower level of hormones, some forms of HBC may offer a protective effect by reducing this natural ligament laxity, helping to stabilize joints during intense training and competition.