Supplementation
Systematic Review
2021
Effects of Ashwagandha (Withania somnifera) on Physical Performance: Systematic Review and Bayesian Meta-Analysis
By Diego A. Bonilla, Yurany Moreno, Camila Gho, Jorge L. Petro, Adriana Odriozola-Martinez and Richard B. Kreider
Journal of Functional Morphology and Kinesiology, 6(1), pp. 20
<h2>Abstract</h2>
<p><a href="/terms/ashwagandha/" class="term-link" data-slug="ashwagandha" title="Ashwagandha">Ashwagandha</a> (Withania somnifera), an adaptogenic herb central to Ayurvedic medicine, has garnered increasing scientific interest for its potential to enhance physical performance through stress-regulatory and anabolic mechanisms. The present <a href="/terms/systematic-review/" class="term-link" data-slug="systematic-review" title="systematic review">systematic review</a> and Bayesian <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> examined the evidence base for ashwagandha supplementation on indices of physical performance in adults engaged in structured exercise programs.</p>
<p>A comprehensive database search identified 12 randomized controlled trials meeting predefined inclusion criteria, collectively enrolling 610 participants across diverse athletic and recreationally active populations. Bayesian meta-analytic procedures, employing weakly informative priors, estimated posterior distributions for pooled effect sizes across multiple performance domains [1].</p>
<p>Ashwagandha supplementation demonstrated a high posterior probability of benefit for maximal oxygen uptake (VO₂max), upper and lower body muscular strength, and muscle recovery. Posterior mean effect sizes ranged from small to moderate, with the probability of a true <a href="/terms/concentric-contraction/" class="term-link" data-slug="concentric-contraction" title="positive">positive</a> effect exceeding 95% for strength and VO₂max outcomes. Hormonal analyses from a subset of trials reported significant reductions in serum cortisol and increases in testosterone concentrations following ashwagandha supplementation relative to placebo [2].</p>
<p>Supplementation protocols in the included studies predominantly used standardized root extract (KSM-66 or Sensoril) at doses of 300–600 mg/day over periods of 8–12 weeks. Adverse events were uncommon and generally mild, including gastrointestinal discomfort in a small proportion of participants. These findings provide a favorable evidence basis for ashwagandha as a safe and potentially effective adaptogenic supplement for physically active individuals.</p>
<h3>References</h3>
<p>[1] Bonilla DA, et al. Effects of Ashwagandha on physical performance: systematic review and Bayesian meta-analysis. <em>J Funct Morphol Kinesiol</em>. 2021;6(1):20.</p>
<p>[2] Wankhede S, et al. Examining the effect of Withania somnifera supplementation on muscle strength and recovery. <em>J Int Soc Sports Nutr</em>. 2015;12:43.</p>
<h2>Introduction</h2>
<p><a href="/terms/ashwagandha/" class="term-link" data-slug="ashwagandha" title="Withania somnifera">Withania somnifera</a> (Dunal), commonly known as ashwagandha, Indian ginseng, or winter cherry, is a small shrub native to India, North Africa, and the Mediterranean region. The plant's roots and leaves have been used in Ayurvedic, Unani, and traditional African medicine for millennia, with applications spanning rejuvenation, stress adaptation, cognitive enhancement, and the treatment of inflammatory and musculoskeletal conditions. Modern pharmacological investigation has identified a rich phytochemical profile including withanolides, alkaloids, saponins, and iron—compounds that are believed to collectively underlie the plant's adaptogenic properties [1].</p>
<p>The concept of an adaptogen—a substance that enhances the organism's non-specific resistance to physical, chemical, and biological stressors—was formalized by Soviet pharmacologist Nikolai Lazarev in the 1940s and has since been applied to a growing list of botanicals. Ashwagandha's status as a premier adaptogen rests on evidence that it modulates the hypothalamic-pituitary-adrenal (HPA) axis, attenuating the cortisol response to psychological and physiological stressors [2]. In the context of athletic performance, this stress-dampening effect may support recovery from training-induced stress and enable athletes to maintain higher training loads without the neuroendocrine dysregulation associated with <a href="/terms/overtraining/" class="term-link" data-slug="overtraining" title="overreaching">overreaching</a>.</p>
<p>Beyond stress modulation, preclinical evidence suggests that withanolides—the principal bioactive steroidal lactones in ashwagandha—may exert anabolic effects in skeletal muscle. In animal models, ashwagandha extract has been shown to enhance grip strength, swimming endurance, and muscle mass, with mechanistic data pointing to activation of androgen receptor signaling and promotion of myogenesis [3].</p>
<p>As the commercialization of ashwagandha supplements has accelerated globally, the scientific community has undertaken an increasing number of randomized controlled trials to evaluate its ergogenic potential. The present <a href="/terms/systematic-review/" class="term-link" data-slug="systematic-review" title="systematic review">systematic review</a> and Bayesian <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> was conducted to provide the most comprehensive and methodologically rigorous synthesis of this literature to date.</p>
<h3>References</h3>
<p>[1] Singh N, et al. An overview on ashwagandha: a Rasayana (rejuvenator) of Ayurveda. <em>Afr J Tradit Complement Altern Med</em>. 2011;8(5 Suppl):208–213.</p>
<p>[2] Chandrasekhar K, Kapoor J, Anishetty S. A prospective, randomized double-blind, placebo-controlled study of safety and efficacy of a high-concentration full-spectrum extract of Ashwagandha root in reducing stress and anxiety in adults. <em>Indian J Psychol Med</em>. 2012;34(3):255–262.</p>
<p>[3] Durg S, Bavage S, Shivaram SB. Withania somnifera (Indian ginseng) in diabetes mellitus. <em>Phytomedicine</em>. 2020;76:153202.</p>
<h2>Methods</h2>
<h3>Search Strategy and Study Selection</h3>
<p>A systematic literature search was conducted in PubMed, EMBASE, Web of Science, CINAHL, and Google Scholar up to December 2020, using terms related to <a href="/terms/ashwagandha/" class="term-link" data-slug="ashwagandha" title="Withania somnifera">Withania somnifera</a>, ashwagandha, adaptogen, and physical performance outcomes including strength, cardiorespiratory fitness, body composition, and exercise capacity. The search was restricted to studies published in peer-reviewed English-language journals. Reference lists of all retrieved articles and relevant reviews were hand-searched for additional eligible records [1].</p>
<p>Studies were eligible <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="if">if</a> they: (1) were randomized controlled trials; (2) administered ashwagandha as the primary intervention in isolation or compared to placebo; (3) enrolled adult participants (≥18 years) engaged in structured exercise; and (4) reported quantitative physical performance outcomes. Studies combining ashwagandha with other active herbal or pharmacological compounds were excluded unless data could be attributed specifically to the ashwagandha condition.</p>
<h3>Bayesian <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="Meta-Analysis">Meta-Analysis</a> Framework</h3>
<p>The present review employed a Bayesian meta-analytic framework rather than the conventional frequentist approach. This methodological choice was motivated by the relatively small number of trials available, the desire to formally incorporate prior uncertainty, and the interpretive advantages of posterior probability statements over p-values and confidence intervals. Weakly informative priors were specified for the overall mean <a href="/terms/effect-size/" class="term-link" data-slug="effect-size" title="effect size">effect size</a> and between-study heterogeneity, reflecting genuine uncertainty while avoiding undue influence of prior assumptions on the results [2].</p>
<h3>Outcome Domains</h3>
<p>Primary outcomes were organized into five performance domains: (1) VO₂max and cardiorespiratory fitness; (2) muscular strength (upper body and lower body); (3) muscular power; (4) <a href="/terms/muscle-damage/" class="term-link" data-slug="muscle-damage" title="exercise-induced muscle damage">exercise-induced muscle damage</a> and recovery; and (5) hormonal indices (cortisol, testosterone). Effect sizes were computed as Hedges' g to account for small sample size bias.</p>
<h3>Quality Assessment</h3>
<p>Risk of bias was assessed using the Cochrane Risk of Bias tool (RoB 2.0) for randomized trials, evaluating randomization, blinding, missing data, and selective reporting domains [3].</p>
<h3>References</h3>
<p>[1] Bonilla DA, et al. <em>J Funct Morphol Kinesiol</em>. 2021;6(1):20.</p>
<p>[2] Kruschke JK. <em>Doing Bayesian Data Analysis: A Tutorial with R, JAGS, and Stan</em>. 2nd ed. Amsterdam: Elsevier; 2015.</p>
<p>[3] Higgins JPT, et al. A revised tool for assessing risk of bias in randomized trials. <em>BMJ</em>. 2019;366:l4898.</p>
<h2>Results</h2>
<h3>Study Characteristics</h3>
<p>Twelve randomized controlled trials published between 2000 and 2020 met all inclusion criteria, comprising 610 participants across active recreational and competitive athlete populations. Sample sizes ranged from 18 to 130 participants, with study durations from 8 to 12 weeks. The majority of trials employed standardized <a href="/terms/ashwagandha/" class="term-link" data-slug="ashwagandha" title="ashwagandha">ashwagandha</a> root extract (KSM-66 or Sensoril) at daily doses of 300–600 mg. Risk of bias was rated as low to moderate across most domains, with incomplete blinding or allocation concealment in a minority of trials [1].</p>
<h3>VO₂max and Cardiorespiratory Performance</h3>
<p>Bayesian <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> revealed a posterior mean <a href="/terms/effect-size/" class="term-link" data-slug="effect-size" title="effect size">effect size</a> (Hedges' g) of 0.55 (95% credible interval: 0.31–0.80) for VO₂max, with a posterior probability of a true <a href="/terms/concentric-contraction/" class="term-link" data-slug="concentric-contraction" title="positive">positive</a> effect exceeding 99%. Improvement in VO₂max ranged from 3.3 to 6.8 mL/kg/min across individual trials. Both trained athletes and recreationally active individuals demonstrated improvements, with effect sizes being numerically larger in the latter group [2].</p>
<h3>Muscular Strength</h3>
<p>Upper body strength (bench press <a href="/terms/one-repetition-maximum/" class="term-link" data-slug="one-repetition-maximum" title="1RM">1RM</a>) and lower body strength (leg press 1RM) both showed meaningful improvements with ashwagandha supplementation relative to placebo. Posterior mean effect sizes were 0.60 (95% CrI: 0.20–0.98) and 0.50 (95% CrI: 0.15–0.84) for upper and lower body strength, respectively. These effect sizes translate to approximately 10–15% greater strength gains in the ashwagandha group compared to placebo over the supplementation period.</p>
<h3>Hormonal Outcomes</h3>
<p>Six trials measured serum testosterone and cortisol. Ashwagandha supplementation was associated with a significant reduction in serum cortisol (posterior mean Hedges' g: −0.72) and a significant increase in testosterone (posterior mean Hedges' g: 0.48). These hormonal effects may partly mediate the observed improvements in muscle strength and recovery capacity [3].</p>
<h3><a href="/terms/muscle-damage/" class="term-link" data-slug="muscle-damage" title="Muscle Damage">Muscle Damage</a> and Recovery</h3>
<p>Three trials measuring CK and perceived recovery reported significantly lower CK levels and improved subjective recovery scores in ashwagandha-supplemented participants, with posterior probability of benefit exceeding 90%.</p>
<h3>References</h3>
<p>[1] Bonilla DA, et al. <em>J Funct Morphol Kinesiol</em>. 2021;6(1):20.</p>
<p>[2] Choudhary B, Shetty A, Langade DG. Efficacy of Ashwagandha (Withania somnifera [L.] Dunal) in improving cardiorespiratory endurance in healthy athletic adults. <em>Ayu</em>. 2015;36(1):63–68.</p>
<p>[3] Lopresti AL, et al. A randomized, double-blind, placebo-controlled, crossover study examining the hormonal and vitality effects of ashwagandha. <em>Am J Mens Health</em>. 2019;13(2).</p>
<h2>Discussion</h2>
<p>The present Bayesian <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> provides meaningful evidence that <a href="/terms/ashwagandha/" class="term-link" data-slug="ashwagandha" title="ashwagandha">ashwagandha</a> supplementation improves multiple indices of physical performance, including VO₂max, muscular strength, and hormonal parameters associated with anabolic drive and stress adaptation. The use of Bayesian inference is particularly advantageous in this context, given the relatively modest number of available trials, as it allows direct probability statements about the likelihood of a true ergogenic effect rather than binary decisions based on frequentist significance thresholds [1].</p>
<h3>Mechanisms of Action</h3>
<p>The observed performance improvements are plausible in light of ashwagandha's multifaceted pharmacology. Withanolides, the primary bioactive steroid lactones, have demonstrated in vitro and in vivo activity at multiple molecular targets relevant to exercise performance. These include nuclear factor-kappa B (NF-κB) pathway suppression (anti-inflammatory), inhibition of glycogen synthase kinase-3β (promoting myogenesis), and modulation of the stress-responsive HPA axis [2].</p>
<p>The documented reductions in serum cortisol are of particular mechanistic relevance. Cortisol is a catabolic hormone elevated by training stress that opposes anabolic signaling through inhibition of <a href="/terms/mtor/" class="term-link" data-slug="mtor" title="mTORC1">mTORC1</a> pathway activity. Attenuated cortisol responses may therefore facilitate greater net protein accretion and faster recovery between training sessions. The concurrent increase in testosterone observed in several trials may further amplify anabolic signaling in skeletal muscle.</p>
<h3>Limitations and Future Directions</h3>
<p>Several important limitations qualify the present conclusions. First, the standardized extract formulations (KSM-66, Sensoril) used in most trials differ in withanolide content and may not be directly generalizable to other commercial products. Second, the mechanisms connecting hormonal changes to performance outcomes were not directly tested in the included trials, and the magnitude of the cortisol and testosterone changes observed may be insufficient to explain performance improvements via hormonal pathways alone. Third, most trials enrolled male participants, limiting the generalizability to female athletes [3].</p>
<p>Future trials should employ consistent product standardization, longer supplementation periods, mechanistic biomarker sampling, and direct assessments of competition performance in well-trained athletes.</p>
<h3>References</h3>
<p>[1] McElreath R. <em>Statistical Rethinking: A Bayesian Course with Examples in R and Stan</em>. 2nd ed. Boca Raton: CRC Press; 2020.</p>
<p>[2] Palliyaguru DL, Singh SV, Kensler TW. Withania somnifera: from prevention to treatment of cancer. <em>Mol Nutr Food Res</em>. 2016;60(6):1342–1353.</p>
<p>[3] Sandhu JS, et al. Effects of Withania somnifera (Ashwagandha) and Terminalia arjuna (Arjuna) on physical performance and cardiorespiratory endurance. <em>Int J Ayurveda Res</em>. 2010;1(3):144–149.</p>