Strength of human evidence
Which amino acids have the studies behind them.
Twenty-five compounds, sixty-eight outcomes, one grade each, and the paper the grade rests on. Bubbles rise with the strength of the human evidence. Size is public interest, not effectiveness. Null results, mixed results, and harm signals are shown, not hidden.
Preview. Grades are draft (rubric 0.2-draft) and awaiting independent review by a licensed professional before publication. Citations were re-resolved against PubMed on 2026-09-15.
Benefit shown Mixed results No effect or untested Harm signalBubble size = public interest, not effectiveness
- StrongMultiple RCTs or a systematic review or meta-analysis, consistent direction
- GoodMultiple RCTs or a meta-analysis with some inconsistency
- PromisingOne or two RCTs, or consistent smaller trials
- SlightPilot, observational, or animal data only
- No evidence or conflictingNo human trial found, or trials disagree
Tap a bubble to see the population, dose, and the study behind the grade.
Full table: every entry, its grade, and its citation (68 shown)
| Compound | Outcome | Evidence | Direction | Population | Dose studied | Source |
|---|---|---|---|---|---|---|
| Leucine | Muscle protein synthesis and lean mass in older adults | Good | Mixed results | Older adults (healthy and sarcopenic) | Varied across trials; not pooled in abstract | meta-analysis, 2015 (PMID 25234223) |
| Leucine | Muscle mass and strength with resistance training (young adults) | No evidence or conflicting | No effect or untested | Healthy untrained young adults | 3 g/day (post-training); 6 g/day in recovery trial | RCT, 2017 (PMID 28444456) |
| BCAAs (leucine, isoleucine, valine) | Acute muscle protein synthesis after resistance exercise | Slight | Mixed results | Resistance-trained young men | 5.6 g BCAA post-exercise | RCT, 2017 (PMID 28638350) |
| BCAAs (leucine, isoleucine, valine) | Muscle mass and strength gains with resistance training | No evidence or conflicting | No effect or untested | Postmenopausal women; adults 50+ | 9 g/day | RCT, 2021 (PMID 33279662) |
| BCAAs (leucine, isoleucine, valine) | Delayed-onset muscle soreness / exercise-induced muscle damage | Promising | Benefit shown | Healthy adults, mostly recreationally active | >200 mg/kg/day for >10 days, taken before damaging exercise | meta-analysis, 2019 (PMID 30938579) |
| Essential amino acid blend (EAA) | Muscle protein synthesis / anabolic response to feeding | Good | Benefit shown | Healthy young and older adults | 3.2-18 g EAA per dose | RCT, 2003 (PMID 12885705) |
| Protein supplement (whey/casein/soy; comparator) | Muscle mass and strength gains with resistance training | Strong | Benefit shown | Healthy adults performing resistance training | Total protein up to ~1.6 g/kg/day | meta-analysis, 2018 (PMID 28698222) |
| Creatine | Strength performance (lower and upper limb) | Strong | Benefit shown | Healthy adults of varied age and training status | Varied; meta-regression found effect independent of dose and loading | meta-analysis, 2015 (PMID 25946994) |
| Creatine | Lean tissue mass and strength with resistance training in older adults | Strong | Benefit shown | Older adults (57-70 y) performing resistance training | <=5 g/day or >5 g/day, with or without loading; 7-52 weeks | meta-analysis, 2017 (PMID 29138605) |
| Creatine | Memory / cognition | Promising | Mixed results | Healthy adults; benefit confined to 66-76 y subgroup | ~2.2-20 g/day for 5 days to 24 weeks | meta-analysis, 2023 (PMID 35984306) |
| HMB (beta-hydroxy-beta-methylbutyrate) | Strength and body composition in trained athletes / young adults | No evidence or conflicting | No effect or untested | Trained/competitive athletes; young adults 18-45 | Typically 3 g/day (subgroups by dose showed no difference) | meta-analysis, 2018 (PMID 29249685) |
| HMB (beta-hydroxy-beta-methylbutyrate) | Muscle mass preservation in older adults / bed rest | Good | Benefit shown | Adults 65+; healthy older adults during bed rest | 3 g/day | meta-analysis, 2015 (PMID 26169182) |
| Beta-alanine | High-intensity exercise capacity (0.5-10 min efforts) | Strong | Benefit shown | Healthy adults, trained and untrained | Chronic ingestion; total dose did not moderate effect in meta-regression | meta-analysis, 2017 (PMID 27797728) |
| Citrulline / citrulline malate | High-intensity strength and power performance | Promising | Mixed results | Healthy trained men and women | >=3 g citrulline (commonly 8 g citrulline malate) 30-60 min pre-exercise | meta-analysis, 2019 (PMID 30895562) |
| Citrulline | Blood pressure | No evidence or conflicting | No effect or untested | Adults in clinical trials (mixed weight status) | Not pooled in abstract | meta-analysis, 2018 (PMID 30284051) |
| Arginine | Blood pressure | Strong | Benefit shown | Adults, normotensive and hypertensive | 4-24 g/day; >=4 g/day effective for SBP | meta-analysis, 2022 (PMID 34967840) |
| Arginine | Endothelial function (flow-mediated dilation) | Good | Mixed results | Adults; benefit limited to those with impaired baseline FMD | Short-term oral arginine (doses varied) | meta-analysis, 2009 (PMID 19056561) |
| Arginine | Recovery after acute myocardial infarction (safety signal) | No evidence or conflicting | Harm signal | Patients after first ST-elevation myocardial infarction | 3 g three times daily (9 g/day) for 6 months | RCT, 2006 (PMID 16391217) |
| Arginine | Erectile dysfunction (mild to moderate) | Good | Benefit shown | Men with mild to moderate erectile dysfunction | 1,500-5,000 mg/day | meta-analysis, 2019 (PMID 30770070) |
| Arginine (and arginine + HMB + glutamine) | Wound collagen deposition / healing | Promising | Benefit shown | Healthy volunteers, including elderly (65+ / 70+) | 17-25 g/day free arginine; or 14 g arginine + 3 g HMB + 14 g glutamine | RCT, 1993 (PMID 8342121) |
| Arginine (in immunonutrition formulas with omega-3 and nucleotides) | Post-surgical infections and length of stay (GI cancer surgery) | Good | Benefit shown | Patients undergoing elective gastrointestinal cancer surgery | Commercial immunonutrition formulas (arginine dose varies by product) | meta-analysis, 2012 (PMID 22317969) |
| Glutamine | Mortality and recovery in critical illness and severe burns | No evidence or conflicting | Harm signal | Mechanically ventilated ICU patients with multi-organ failure; severe burn patients | IV + enteral glutamine (REDOXS); 0.5 g/kg/day enteral (burns) | RCT, 2013 (PMID 23594003) |
| Glutamine | Post-infectious diarrhoea-predominant IBS symptoms | Promising | Benefit shown | Adults with post-infectious IBS-D and increased intestinal permeability | 5 g three times daily for 8 weeks | RCT, 2019 (PMID 30108163) |
| Glutamine | Exercise-induced intestinal permeability (running in heat) | Slight | Benefit shown | Recreationally active men | 0.25, 0.5 or 0.9 g/kg fat-free mass, 2 h pre-exercise | RCT, 2017 (PMID 29058112) |
| Glutamine | Self-reported infections after prolonged exhaustive exercise | Slight | Benefit shown | Marathon/ultramarathon runners and elite rowers | Two glutamine drinks (immediately and 2 h post-exercise; dose not in abstract) | RCT, 1996 (PMID 8803512) |
| HMB + arginine + glutamine | Fat-free mass in cancer cachexia | Promising | Benefit shown | Advanced (stage IV) cancer patients with weight loss | 3 g HMB + 14 g arginine + 14 g glutamine per day | RCT, 2002 (PMID 11975938) |
| Glycine | Sleep quality and next-day fatigue | Promising | Benefit shown | Healthy adults with sleep complaints or acute sleep restriction | 3 g before bedtime | RCT, 2012 (PMID 22529837) |
| Tryptophan | Sleep quality | Promising | Mixed results | Adults (mixed healthy and sleep-disturbed) | >=1 g/day | meta-analysis, 2022 (PMID 33942088) |
| Tryptophan | Mood and anxiety in healthy adults | Promising | Mixed results | Healthy adults | 0.14-3 g/day | systematic review, 2021 (PMID 32272859) |
| 5-HTP (and tryptophan) | Depression | Slight | Benefit shown | Adults with unipolar depression or dysthymia | Varied; not pooled | meta-analysis, 2020 (PMID 31504850) |
| L-theanine | Stress and anxiety | Promising | Benefit shown | Healthy adults exposed to stress | 200-400 mg/day | systematic review, 2020 (PMID 31758301) |
| N-acetylcysteine (NAC) | Depressive symptoms (adjunctive, psychiatric populations) | Good | Benefit shown | Adults with depressive symptoms in bipolar disorder, MDD, trichotillomania, heavy smoking | Not pooled in abstract | meta-analysis, 2016 (PMID 27137430) |
| N-acetylcysteine (NAC) | Influenza-like illness frequency and severity | Promising | Benefit shown | Adults, 78% aged 65+, without chronic respiratory disease | 600 mg twice daily for 6 months | RCT, 1997 (PMID 9230243) |
| N-acetylcysteine (NAC) | Liver protection after acetaminophen overdose | Strong | Benefit shown | Patients with acetaminophen overdose | 140 mg/kg loading then 70 mg/kg every 4 h x17 doses (medical protocol) | cohort, 1988 (PMID 3059186) |
| N-acetylcysteine (NAC) | Liver enzymes in non-alcoholic fatty liver disease | Slight | Benefit shown | Adults with NAFLD | Not stated in abstract | RCT, 2010 (PMID 22308119) |
| Glycine | Non-alcoholic fatty liver disease | Slight | Benefit shown | Mice with diet-induced NASH; human liver tissue (observational) | Not applicable (preclinical) | cohort, 2020 (PMID 33268508) |
| BCAAs (leucine, isoleucine, valine) | Hepatic encephalopathy in cirrhosis | Good | Benefit shown | Adults with cirrhosis and overt or covert hepatic encephalopathy | Oral (10 trials) or intravenous (8 trials) BCAA; doses varied | meta-analysis, 2026 (PMID 41542879) |
| Tyrosine | Cognitive performance under acute stress | Promising | Benefit shown | Healthy adults under acute psychological/physical stress (e.g. military cadets) | 2 g per dose, five daily doses (cadet trial) | systematic review, 2015 (PMID 26424423) |
| Lysine | Prevention of herpes simplex labialis (cold sores) | No evidence or conflicting | Mixed results | Immunocompetent adults with recurrent herpes simplex | 1 g three times daily (3 g/day) in the positive 1987 trial | systematic review, 2015 (PMID 26252373) |
| Lysine | Anxiety and stress response | Slight | Benefit shown | Families in economically weak communities consuming wheat as staple (NW Syria) | Lysine fortification of wheat flour (dose not in abstract) | RCT, 2004 (PMID 15159538) |
| Taurine | Blood pressure | Good | Benefit shown | Adults at risk of or with metabolic syndrome; mixed health status | 0.5-6 g/day | meta-analysis, 2024 (PMID 38755142) |
| Taurine | Endurance exercise performance | Promising | Benefit shown | Healthy adults, varied training status | 1-6 g single dose or up to 2 weeks | meta-analysis, 2018 (PMID 29546641) |
| Taurine | Heart failure symptoms and function | Slight | Benefit shown | Patients with congestive heart failure (NYHA II-III) | 500 mg three times daily (2017); 1985 dose not in abstract | RCT, 1985 (PMID 3888464) |
| Taurine | Glycemic control and lipids in overweight/obesity | Good | Benefit shown | Adults with overweight or obesity; adults with metabolic-syndrome risk | 0.5-6 g/day; 3 g/day most effective | meta-analysis, 2024 (PMID 39796489) |
| Taurine | Aging / healthspan | Slight | Benefit shown | Mice, monkeys; humans observational | Not applicable to humans (animal dosing) | cohort, 2023 (PMID 37289866) |
| L-carnitine | Secondary prevention after myocardial infarction | Promising | Mixed results | Patients with acute myocardial infarction | Not pooled in abstract | meta-analysis, 2013 (PMID 23597877) |
| L-carnitine | Chronic heart failure (cardiac function and symptoms) | Good | Benefit shown | Patients with chronic heart failure | Not pooled in abstract | meta-analysis, 2017 (PMID 28497060) |
| L-carnitine | Body weight and fat mass | Good | Benefit shown | Adults, especially with overweight/obesity | 2,000 mg/day (maximum effect in dose-response) | meta-analysis, 2020 (PMID 32359762) |
| L-carnitine | Glycemic control and lipids in type 2 diabetes | Good | Benefit shown | Adults with type 2 diabetes | 1-4 g/day (U-shaped BMI effect, largest at 2 g/day) | meta-analysis, 2024 (PMID 38594107) |
| L-carnitine | Physical and mental fatigue in the very old | Slight | Benefit shown | Centenarians with fatigue after slight activity | 2 g levocarnitine once daily | RCT, 2007 (PMID 18065594) |
| Leucine-enriched whey protein + vitamin D (PROVIDE formula) | Sarcopenia measures in older adults | Good | Mixed results | Sarcopenic, primarily independent-living older adults (SPPB 4-9, low muscle mass index) | Twice-daily supplement (each serving: leucine-enriched whey protein + vitamin D; product-specific) | RCT, 2015 (PMID 26170041) |
| Essential amino acid blend (EAA) | Lean body mass and function in older adults (including bed rest) | Promising | Benefit shown | Older adults (67-71 y), including bed-rest and glucose-intolerant subjects | 15 g/day; 11 g EAA + arginine twice daily; 16.5 g EAA + 30 g carbohydrate three times daily | RCT, 2009 (PMID 19208731) |
| Dietary protein (expert position papers) | Recommended protein intake to maintain muscle in adults 65+ | Good | Benefit shown | Adults over 65 | 1.0-1.2 g protein/kg/day (healthy); 1.2-1.5 g/kg/day (acute or chronic illness) | position paper, 2013 (PMID 23867520) |
| Higher-protein diet (comparator; indirect evidence for the GLP-1 segment) | Lean-mass preservation during caloric restriction | Strong | Benefit shown | Adults and older adults undergoing diet-induced weight loss (no GLP-1 therapy) | >=25% energy from protein or >=1.0 g/kg/day; up to 2.4 g/kg/day with intense training | meta-analysis, 2012 (PMID 23097268) |
| Whey protein + leucine + vitamin D supplement | Muscle preservation during intentional weight loss in obese older adults | Promising | Benefit shown | Obese older adults (mean 63 y, BMI 33) on a hypocaloric diet with resistance training | 21 g protein per serving, 10 servings/week (leucine- and vitamin D-enriched) | RCT, 2015 (PMID 25646324) |
| Essential amino acid blend (EAA) + whey meal replacement | Lean-mass preservation during caloric restriction in obese elderly | Slight | Mixed results | Elderly obese adults on an 1200 kcal/day diet | 800 kcal/day EAA/whey meal replacements (EAA dose not in abstract) | RCT, 2012 (PMID 23231757) |
| Essential amino acids / protein supplementation | Lean-mass preservation during GLP-1 medication | No evidence or conflicting | No effect or untested | Adults with obesity on GLP-1 medication | None tested directly; reviews suggest 1.2-2.0 g protein/kg/day by extrapolation | meta-analysis, 2026 (PMID 42321502) |
| Protein / amino acid supplementation | Body composition after bariatric surgery | Promising | Mixed results | Adults after metabolic/bariatric surgery | Daily protein supplement (products and doses varied; pilot reached ~25% of energy from protein) | meta-analysis, 2025 (PMID 40312971) |
| Histidine | Insulin resistance and inflammation in obese women with metabolic syndrome | Slight | Benefit shown | Obese women (33-51 y) with metabolic syndrome | 4 g/day for 12 weeks | RCT, 2013 (PMID 23361591) |
| Methionine | Metabolic health (supplementation) | No evidence or conflicting | No effect or untested | Obese adults with metabolic syndrome (restriction study) | Restriction to 2 mg/kg/day vs 35 mg/kg/day (not supplementation) | RCT, 2011 (PMID 21346062) |
| Threonine | Any health outcome (supplementation) | No evidence or conflicting | No effect or untested | Healthy adult men (mean age 43) | 0-12 g/day for 4 weeks (safety only) | RCT, 2025 (PMID 40419835) |
| Collagen peptides / hydrolysate | Activity-related joint pain and joint function | Good | Benefit shown | Recreational and varsity athletes with activity-related joint pain | 10-15 g/day | systematic review, 2021 (PMID 34491424) |
| Collagen peptides | Fat-free mass and strength with resistance training in sarcopenic older men | Promising | Benefit shown | Elderly men (mean 72 y) with sarcopenia performing resistance training | 15 g/day post-exercise | RCT, 2015 (PMID 26353786) |
| Hydrolyzed collagen | Skin hydration, elasticity and wrinkles | Good | Benefit shown | Adults 20-70 y, 95% women | Varied; 90 days of supplementation | meta-analysis, 2021 (PMID 33742704) |
| Gelatin / collagen (with vitamin C) | Collagen synthesis for tendon/ligament injury prevention | Slight | Benefit shown | Healthy young men | 5 or 15 g gelatin with vitamin C, 1 h before exercise, three times daily | RCT, 2017 (PMID 27852613) |
| BCAAs (leucine, isoleucine, valine) | Fatigue biomarkers (cortisol, lactate) in exercise recovery | Slight | Mixed results | Trained and untrained adults | Not pooled in abstract | meta-analysis, 2024 (PMID 36177954) |
| BCAAs (leucine, isoleucine, valine) | Insulin resistance (metabolic safety signal) | Slight | Harm signal | Obese vs lean humans (observational); rats (intervention) | Not applicable (dietary BCAA supplementation in rats) | cohort, 2009 (PMID 19356713) |
| Arginine | Glycemic control | Slight | Mixed results | Adults in clinical trials (diabetic and non-diabetic) | >6.5 g/day for <=12.8 weeks in the responsive subgroup | meta-analysis, 2020 (PMID 32561357) |
How the grades work
Five levels. One rule each.
Grades describe the strength of human evidence for a compound and an outcome in the population studied. They are not a rating of POWER9®, and a grade for a compound does not transfer to a product that contains it at a different dose.
- StrongMultiple RCTs or a systematic review or meta-analysis, consistent direction
- GoodMultiple RCTs or a meta-analysis with some inconsistency
- PromisingOne or two RCTs, or consistent smaller trials
- SlightPilot, observational, or animal data only
- No evidence or conflictingNo human trial found, or trials disagree
Compounds IMPOWER LIFE does not sell are included. Negative findings about compounds it does sell are included. Bubble size = relative interest, not effectiveness: 70% English Wikipedia pageviews (Aug 2025-Aug 2026, user agent only) + 30% PubMed title/abstract mention count, log-scaled 10-100.