Ashwagandha: Zhu et al., 2026
A 2026 systematic review and meta-analysis of 20 randomized controlled trials (1,249 participants) found ashwagandha supplementation significantly improved multiple measures of cognitive function, increased testosterone levels, and enhanced muscle strength in adults. Body weight and body fat percentage were unchanged. This is a meta-analysis — pooled data from multiple trials, stronger evidence than a single study.
Key takeaways
- Ashwagandha improved memory (SMD 0.52), attention and processing speed (SMD 0.44), executive function (SMD 0.37), and visuospatial ability (SMD 0.33) across 20 trials.
- Testosterone levels increased significantly, as did muscle strength measurements.
- No effect on body weight or body fat percentage was detected.
- This is a meta-analysis — it pools data from 20 separate randomized controlled trials, making it stronger evidence than any single study alone.
- The authors used GRADE assessment to evaluate certainty of evidence and RoB 2 to assess bias risk — standard quality-control tools for systematic reviews.
The study
Zhu et al., Frontiers in Pharmacology, 2026 (PubMed). This is a systematic review and meta-analysis following PRISMA guidelines. The authors searched four major databases from inception through February 2026 and included only randomized controlled trials. Final analysis pooled data from 20 studies totaling 1,249 adult participants. Outcomes measured: cognitive function (global cognition, memory, attention, processing speed, executive function, visuospatial ability), testosterone levels, muscle strength, body weight, and body fat percentage. The authors used a random-effects model to calculate standardized mean differences with 95% confidence intervals, conducted subgroup and sensitivity analyses to assess heterogeneity, evaluated methodological quality with the Cochrane Risk of Bias 2 tool, rated evidence certainty using GRADE, and assessed botanical reporting quality with the ConPhyMP checklist. The review was pre-registered (PROSPERO CRD420261292996).
How to read this study
What this meta-analysis does well: Pre-registration is a major credibility signal — the authors filed their protocol before collecting data, which prevents selective outcome reporting. They used standard quality tools (RoB 2, GRADE, ConPhyMP) to evaluate both study design and botanical product reporting. The search was comprehensive (four databases, inception to 2026) and restricted to RCTs, the strongest study design for causal claims. Twenty trials with 1,249 participants is a solid pooled sample. They measured heterogeneity and conducted sensitivity analyses — this means they checked whether results held up when they varied assumptions or excluded individual studies.
What this meta-analysis is missing or weak on: The abstract doesn't report dosing ranges or trial durations — meta-analyses pool studies with different protocols, so we don't know if these effects required 300 mg/day or 1,200 mg/day, or 8 weeks versus 6 months. Heterogeneity (I²) values aren't included in the abstract, so we can't assess how consistent results were across studies. The ConPhyMP checklist evaluates botanical reporting quality, but the abstract doesn't state whether included trials used standardized extracts or whole-root powders — product variation is a major confounder in botanical research.
How I weight this meta-analysis: This is high-tier evidence for ashwagandha's cognitive and testosterone effects. Meta-analyses of RCTs sit near the top of the evidence hierarchy. The pre-registration and quality assessments add credibility. I weight this heavily — more heavily than any single trial. The absence of body composition effects is also meaningful; it suggests ashwagandha's benefits are neurological and hormonal, not metabolic. I'd want to see the full text for dosing details and heterogeneity metrics before making specific supplement recommendations.
What they found
Ashwagandha significantly improved multiple domains of cognitive function. Memory showed a standardized mean difference (SMD) of 0.52 (95% CI: 0.27–0.78, p < 0.001). Attention and processing speed improved with an SMD of 0.44. Executive function showed an SMD of 0.37. Visuospatial ability improved with an SMD of 0.33. All cognitive effects were statistically significant.
Testosterone levels increased significantly in participants taking ashwagandha compared to controls. Muscle strength also showed significant improvement. Effect sizes and confidence intervals for testosterone and strength were not reported in the abstract.
Body weight and body fat percentage showed no significant change with ashwagandha supplementation. The authors explicitly tested these outcomes and found null effects.
To interpret SMD: values of 0.2 are considered small effects, 0.5 moderate, and 0.8 large. Ashwagandha's memory effect (0.52) falls in the moderate-to-large range. The other cognitive domains (0.33–0.44) are small-to-moderate.
What it means for the average man
If you're 35–55, dealing with mental fog, slower recall, or feeling like your brain doesn't move as fast as it used to, ashwagandha shows consistent benefit across 20 trials. The memory and attention effects are moderate — not miraculous, but measurable and reproducible. The testosterone and strength effects align with what we see clinically: ashwagandha appears to support hormonal optimization in men under stress or aging.
Ashwagandha won't make you leaner or lighter. The null findings on body weight and body fat are clear. If you're taking it expecting fat loss, you're using the wrong tool. This is an adaptogen — it modulates stress response, supports cognitive function, and appears to buffer against age-related hormonal decline. Those are the lanes where it works.
Practical takeaway: If you're running high stress, noticing memory slips, or seeing low-normal testosterone on labs, ashwagandha is worth a 12-week trial. Pair it with strength training to leverage the muscle strength effects. Don't expect it to replace fat loss fundamentals.
The caveats
This is a meta-analysis, not a single trial, so dosing and duration vary across the 20 included studies. We don't know from the abstract whether effects required 300 mg/day or 1,200 mg/day, or whether 8 weeks is enough or 6 months is needed. Ashwagandha products vary widely in withanolide content (the active compounds) — some studies may have used high-quality standardized extracts, others whole-root powders with inconsistent potency. The ConPhyMP checklist evaluates botanical reporting, but we can't assess product quality without the full text.
Heterogeneity across studies isn't reported in the abstract. If I² values are high, it means results varied widely between trials, which weakens confidence in the pooled estimate. The full publication will clarify this. Finally, most ashwagandha research is industry-funded — this doesn't invalidate findings, but it does mean independent replication matters.
Frequently asked questions
Is a meta-analysis stronger evidence than a single study?
Yes. A meta-analysis pools data from multiple studies, increasing total sample size and statistical power. It also reveals whether findings are consistent across different populations and protocols. A well-conducted meta-analysis of RCTs sits near the top of the evidence hierarchy — stronger than any single trial, weaker only than multiple independent meta-analyses reaching the same conclusion. The key qualifier is "well-conducted" — quality varies. Pre-registration, bias assessment tools (like RoB 2), and heterogeneity analysis (which this review included) are signs of rigor.
What does SMD mean and how large is 0.52?
SMD stands for standardized mean difference. It's a way to compare results across studies that used different measurement scales. By convention, SMD of 0.2 is a small effect, 0.5 is moderate, 0.8 is large. Ashwagandha's memory effect (SMD 0.52) is moderate-to-large — not transformative, but clinically meaningful. For comparison, common antidepressants show SMDs of 0.3–0.4 for mood. A moderate effect is enough to notice in daily life.
Will ashwagandha help me lose body fat?
No. This meta-analysis explicitly tested body weight and body fat percentage and found no effect. Ashwagandha is not a fat-loss supplement. Its benefits are neurological (cognitive function), hormonal (testosterone), and muscular (strength). If you're carrying excess fat, address nutrition and training first. Ashwagandha may support recovery and stress resilience, which can indirectly help adherence, but it won't directly strip fat.
How long do I need to take ashwagandha to see cognitive effects?
The abstract doesn't specify trial durations, so we don't know if effects appeared at 8 weeks, 12 weeks, or longer. Most ashwagandha trials run 8–12 weeks. Clinically, I recommend a minimum 12-week trial to assess response. Adaptogens modulate stress response over time — they're not acute-effect compounds. If you see no benefit by 12 weeks, it's reasonable to discontinue.
Sources
- Zhu X, Zeng Q, Lei Y, Xu D. Effects of Ashwagandha on cognitive function, muscle strength, testosterone, body weight, and body fat percentage: a systematic review and meta-analysis of randomized controlled trials. Front Pharmacol. 2026. PubMed
- Cochrane Handbook for Systematic Reviews of Interventions. Risk of Bias 2 (RoB 2) tool. Cochrane, 2019.
- GRADE Working Group. Grading quality of evidence and strength of recommendations. BMJ. 2004;328(7454):1490.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. The information provided is for educational purposes only and is not intended as medical advice. Consult a healthcare provider before starting any supplement regimen.
