Fasting and Caloric Restriction Leave Reproductive Hormones Largely Intact
A meta-analysis of 15 RCTs finds fasting and caloric restriction have minimal impact on reproductive hormones, suggesting endocrinological safety for metabolic health strategies.
Summary
Many people worry that intermittent fasting or cutting calories could disrupt hormones like testosterone or estrogen. This systematic review and meta-analysis pooled data from 15 randomized controlled trials involving 954 adults to find out. The results were largely reassuring: neither fasting nor caloric restriction produced consistent, statistically significant changes in reproductive hormones including testosterone, estradiol, FSH, LH, SHBG, or DHEA-S. One exception was free testosterone, which showed a small but statistically significant decrease with fasting, though this came from a limited number of small trials. The authors conclude that these dietary strategies appear hormonally safe for most adults pursuing weight management or metabolic health goals, while calling for larger, longer-term studies to clarify effects in specific subgroups.
Detailed Summary
Fasting protocols and caloric restriction are among the most widely studied dietary interventions for improving metabolic health, body composition, and longevity-related biomarkers. A persistent concern, however, is whether these strategies might disrupt the endocrine system — particularly reproductive hormones that influence energy, mood, bone density, cardiovascular health, and healthy aging.
This systematic review and meta-analysis set out to provide a rigorous, quantitative answer. Researchers from Iran University of Medical Sciences searched PubMed, Scopus, and Google Scholar for randomized controlled trials published up to April 2025. They included 15 trials involving 954 adult participants undergoing alternate-day fasting, time-restricted eating (TRE), full-day fasting, or 20–30% caloric restriction. Outcomes included testosterone, free testosterone, SHBG, DHEA-S, FSH, LH, estradiol, estrogen, AMH, prolactin, and progesterone. Evidence certainty was graded using the GRADE framework.
The headline finding is reassuring: both fasting and caloric restriction produced largely neutral effects across reproductive hormones. Total testosterone was unaffected by fasting (WMD: −0.88 ng/mL; p = 0.24) and caloric restriction (WMD: −0.02 ng/mL; p = 0.65). No significant effects emerged for estradiol, FSH, LH, SHBG, DHEA-S, or other hormones measured. The one notable exception was free testosterone, which declined significantly with fasting (WMD: −4.9 pg/mL; p = 0.02), though this finding rested on a small number of underpowered trials.
For clinicians and health-conscious individuals, the practical implication is that standard fasting and caloric restriction protocols are unlikely to cause meaningful reproductive hormone disruption in healthy adults. These strategies can be pursued for metabolic or longevity goals without strong concern for hormonal harm.
Caveats are important. The evidence base remains small — 15 trials and under 1,000 participants — with significant heterogeneity in several analyses. Symptom changes were not assessed, so statistical neutrality does not guarantee clinical neutrality. Sex-specific, age-specific, and condition-specific subgroup analyses were limited. Larger, longer, and more targeted trials are needed before definitive conclusions can be drawn for women with PCOS, perimenopausal women, or men with low baseline testosterone.
Key Findings
- Fasting showed no significant effect on total testosterone (WMD: −0.88 ng/mL; p = 0.24) across 15 RCTs.
- Caloric restriction (20–30% energy reduction) did not significantly alter total testosterone, estradiol, FSH, or LH.
- Free testosterone declined significantly with fasting (WMD: −4.9 pg/mL; p = 0.02), but evidence came from few small trials.
- No significant effects detected for SHBG, DHEA-S, AMH, prolactin, or progesterone with either intervention.
- Overall, fasting and caloric restriction appear endocrinologically safe for adults pursuing metabolic health goals.
Methodology
Systematic review and meta-analysis of 15 parallel or crossover RCTs (n = 954 adults) using random-effects models. Interventions included alternate-day fasting, TRE, full-day fasting, and 20–30% caloric restriction. Evidence certainty was evaluated via the GRADE framework; literature search covered PubMed, Scopus, and Google Scholar through April 2025.
Study Limitations
The meta-analysis included only 15 trials with under 1,000 participants total, limiting statistical power for subgroup analyses. Significant heterogeneity was present in several hormone outcomes, and symptom-level clinical effects were not assessed. Summary is based on the abstract only, as the full text was not available for review.
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