Study archive · page 39

A critical appraisal of 15 published reviews concluded that the short-term association between airborne particles and cardiopulmonary mortality is valid and satisfies most standard criteria for causality, with effects observed even where air quality standards were respected.

⚠What it shows: A 2001 critical assessment of the arguments in 15 existing reviews of ecological time-series studies, not a pooled analysis of individual data; time-series designs compare days rather than people, and the exposure landscape has changed since publication.Meta-analysis · J Air Waste Manag Assoc

Across five periods in Beijing between 1990 and 2017, concentrations of SO2, NO2, PM10 and PM2.5 fell by up to 42%, 10%, 33% and 15%, and the estimated short-term mortality effect of each pollutant shrank alongside them: PM2.5's effect on cardiovascular and respiratory deaths falling by up to 0.33% and 0.13%.

⚠What it shows: Longitudinal comparative study in one city: published effect estimates for 1990-2013 pooled by random-effects meta-analysis and combined with newly analysed 2015-2017 daily data. The design is ecological, so parallel changes in healthcare, smoking or population structure cannot be excluded.Meta-analysis · Chemosphere

In a meta-analysis of cohort studies with individual-level data, each 1 µg/m3 increase in long-term PM2.5 exposure was associated with a 4% higher risk of SARS-CoV-2 infection, an 11% higher risk of COVID-19 hospitalisation and a 9% higher risk of COVID-19 death; NO2 was associated with hospitalisation and death, while ozone showed no association with any outcome.

⚠What it shows: Systematic review and random-effects meta-analysis restricted to cohort studies with individual-level data, pooled only where more than three studies existed per exposure-outcome pair. Observational throughout, so residual confounding remains possible, and country of study explained much of the heterogeneity between results.Meta-analysis · Environ Pollut

Across 12 counties in Hubei, China, covering 42 739 stroke deaths between 2009 and 2012, cold spells were associated with a pooled stroke mortality risk of 1.180 at lags of 3-14 days, with the effect appearing after a delay of 2-3 days and lasting about 10 days, while heatwaves carried a pooled risk of 1.114 concentrated in the first 0-2 days.

⚠What it shows: Observational multivariate meta-analysis pooling daily mortality and weather records from 12 counties in one Chinese province over four years; ecological time-series design with no individual data, and county-level estimates varied widely, from 0.968 to 1.523 for cold spells.Meta-analysis · Zhonghua Liu Xing Bing Xue Za Zhi

Pooling 58 observational studies, extreme cold carried the highest all-cause mortality risk at a relative risk of 1.478, above heatwaves at 1.280 and extreme heat at 1.145; heatwaves were also associated with more hospitalisations (1.060) and emergency department visits (1.047).

⚠What it shows: Systematic review and meta-analysis of 58 observational studies published between 2000 and 2024, so all estimates are associations from population records rather than controlled comparisons; definitions of extreme cold, extreme heat and heatwave vary between studies, and few studies covered length of stay or costs.Meta-analysis · Int J Hyg Environ Health

In a meta-analysis of 28 studies, days combining heat with high ozone were associated with a 17% higher overall mortality risk than days without co-exposure, with elevated cardiovascular and respiratory mortality and increased risks across age and gender groups and in urban areas.

⚠What it shows: Systematic review of 78 studies with 28 pooled by random-effects meta-analysis; all observational, with the authors flagging risk of bias from inadequate confounder adjustment and unclear handling of missing data, plus inconsistent definitions of what counts as a compound heat-ozone event.Meta-analysis · Environ Pollut

Across 20 randomised trials in older adults, probiotic supplementation showed a small positive trend on cognitive test scores and a reduction in inflammatory markers, but neither reached statistical significance. Heterogeneity between trials was high enough that no true overall effect size could be determined.

⚠What it shows: Meta-analysis of 20 randomised controlled trials in older adults; both pooled effects had confidence intervals crossing zero, and the authors state that high heterogeneity prevents determining a true overall effect size.Meta-analysis · Benef Microbes

Combined aerobic and resistance training improved global cognition in healthy adults aged 65 and over compared with doing nothing, but a meta-analysis of 12 randomised trials found it was no better than active comparison conditions.

⚠What it shows: Meta-analysis of 12 RCTs (2557 healthy adults aged 65+); the overall pooled effect was non-significant, benefit appeared only against passive controls, and heterogeneity was moderate.Meta-analysis · Exp Gerontol

In adults with depression, regular exercise reduced rumination and improved cognitive shifting in a meta-analysis of randomised trials, with benefits tied to specific doses: moderate-intensity aerobic sessions for rumination and high-intensity aerobic training for shifting. Inhibition and updating did not improve significantly.

⚠What it shows: Meta-analysis pooling few RCTs per outcome (5-9 studies, 260-832 participants); dose effects come from subgroup analyses, and the authors themselves call the findings preliminary and tentative.Meta-analysis · Psychol Sport Exerc

In a systematic review of ten moderate-to-high quality trials in knee osteoarthritis, quadriceps strengthening reduced pain and improved function, and outperformed proprioceptive training. No one strengthening method beat the others.

⚠What it shows: Systematic review of ten trials selected from 1,128 records, synthesised narratively rather than pooled statistically; exercise protocols, comparators and add-on treatments varied widely, and all participants had diagnosed knee osteoarthritis rather than healthy knees.Systematic review · Rheumatol Int

Pooling 183 studies of people with knee osteoarthritis, walking produced the greatest knee loading and stair ascent the most pain, while sit-to-stand was lowest on both; during walking, higher knee loading was correlated with less pain (r = -0.507).

⚠What it shows: Systematic review and meta-analysis of 183 studies, but the loading differences across activities were mostly non-significant apart from sit-to-stand, the populations and methods were highly heterogeneous, and many studies measured only one activity: usually walking. The loading-pain correlation is cross-study association, not cause.Meta-analysis · Clin Biomech (Bristol)

A meta-analysis of 24 studies (1,972 participants, 71.46% female) found that low-intensity physical activity, Baduanjin, tai chi, walking and general activity, significantly reduced knee osteoarthritis pain (d = -0.65) and stiffness (d = -0.71) and improved physical function (d = -0.58) in middle-aged and older adults.

⚠What it shows: Pooled from 24 trials of mixed quality (assessed with the Cochrane risk-of-bias tool) in middle-aged and older adults with knee osteoarthritis, mostly women; exercise trials cannot blind participants, so effect sizes for self-reported pain may be flattered, and the review does not establish which modality is best.Meta-analysis · J Aging Phys Act

A meta-analysis of 14 randomised trials (763 women across reproductive stages) found that adding protein, amino acids or creatine to exercise produced no significant improvement in skeletal muscle mass, appendicular lean mass, fat-free mass, bone mineral content or bone mineral density; only bench press (g = 0.279) and handgrip strength (g = 0.412) improved significantly, with no increase in adverse events.

⚠What it shows: Pooled from just 14 RCTs totalling 763 women, lumping together whole proteins, amino acids and creatine monohydrate, very different compounds, so heterogeneity is high and the null may hide effects of specific supplements. The authors note that trials were mostly shorter than 12 months and lacked site-specific bone measurements, and no head-to-head comparisons exist.Meta-analysis · Int J Med Sci

A Cochrane review of five randomised trials (305 women) found very low quality evidence that continuous oestrogen delivered by patch or implant plus progestogen produced a small to moderate improvement in overall premenstrual syndrome symptom scores (SMD -0.34), while one small crossover trial suggested luteal-phase oral oestrogen was ineffective and might aggravate symptoms.

⚠What it shows: Only five RCTs and 305 women in total, with Cochrane rating every comparison very low quality because of attrition bias, imprecision and heterogeneity; the oral luteal-phase finding rests on a single crossover trial of 11 women. No included trial looked beyond 2-8 months, so long-term risks such as endometrial or breast cancer were never assessed.Meta-analysis · Cochrane Database Syst Rev

A systematic review and meta-analysis of 11 randomised trials in women with genitourinary syndrome of menopause found that fractional CO2 laser was associated with improved sexual function, including less pain during sexual activity (standardised mean difference 0.51), and better urinary function including less leakage and urinary frequency (standardised mean difference 0.51), but not with improved vaginal epithelium quality.

⚠What it shows: Eleven RCTs pooled, but with mixed comparators, sham laser, topical hormonal treatment or a lubricant gel, and only short-term outcomes, so durability is unknown. The authors themselves state the clinical significance of the changes is unclear, and women with any history of cancer or prior laser treatment were excluded.Meta-analysis · Obstet Gynecol

Pooling five randomised trials in older adults (74 doing photobiomodulation plus resistance training, 72 doing sham), adding light therapy to periodised lifting did not significantly improve maximal strength (SMD 0.26, 95% CI -0.08 to 0.59; p = 0.14). A small benefit could not be ruled out.

⚠What it shows: Systematic review of 11 RCTs (n = 456), but only five trials with 146 participants in total reported one-repetition maximum and were pooled; devices, wavelengths and dosing varied, and the authors say a small effect cannot be excluded.Meta-analysis · Photobiomodul Photomed Laser Surg

In a meta-analysis of seven randomised trials in adults with low bone mass, combining exercise with antiresorptive drugs increased lumbar spine bone mineral density significantly more than the drugs alone (SMD 0.55, 95% CI 0.36 to 0.75; p < 0.00001).

⚠What it shows: Meta-analysis of seven RCTs in adults with low bone mass; most trials scored 3 on the Jadad quality scale, and the authors call for longer trials with larger samples. Only lumbar spine bone density was measured, not fractures, and not other skeletal sites.Meta-analysis · Osteoporos Int

Pooling randomised trials, probiotic supplementation produced small increases in lumbar spine areal bone mineral density (+0.010) and hip aBMD (+0.022 g/cm2), with no significant effect on femoral neck density or on the bone turnover markers P1NP and CTX. Most included trials were in postmenopausal women.

⚠What it shows: Systematic review and meta-analysis of randomised trials (16 comparisons for lumbar spine, 9 for hip, 7 for femoral neck), mostly in postmenopausal women, with moderate-to-high heterogeneity throughout; the hip effect became non-significant when key trials were removed, and no fracture outcomes were pooled.Meta-analysis · Osteoporos Int

A critical review with meta-analysis found that vitamin D's apparent protection against acute respiratory infections was confined to small trials: the summary odds ratio was 0.69 in trials of 25 to fewer than 248 patients but 0.98 in trials of 248 to 16,000 patients (interaction p = 0.0001), with strong evidence of publication bias.

⚠What it shows: A meta-analysis of previously published randomised trials, not new experimental data; the acute respiratory infection trials were re-analysed from an earlier pooled dataset, and the analysis splits trials by size, which can be confounded by differences in populations, doses and outcome definitions. It shows the benefit signal is fragile, not that vitamin D is useless for people who are deficient.Meta-analysis · PLoS One

An umbrella review pooling meta-analyses of observational studies linked higher dietary fibre intake to a 22% lower cancer risk and 17% lower cancer mortality, with the strongest inverse association for digestive tract cancers and a smaller one for breast cancer. Observational data cannot establish cause.

⚠What it shows: Umbrella review and meta-meta-analysis of 25 papers and 28 reports, all built on observational epidemiology: people who eat more fibre differ in many other ways, and no randomised trial evidence is included, so causation is not established.Meta-analysis · Crit Rev Food Sci Nutr

A meta-analysis of 13 randomised trials found oropharyngeal (mouth-and-throat) muscle training and respiratory muscle training were associated with reductions in obstructive sleep apnoea severity, while conventional resistance training was not. Daytime sleepiness showed no consistent, clinically meaningful improvement with any modality.

⚠What it shows: Meta-analysis of 13 small RCTs totalling 429 participants; substantial heterogeneity in protocols, durations and participants, and daytime sleepiness did not consistently improve.Meta-analysis · Respir Med

A Bayesian network meta-analysis of 93 randomised trials in 4,446 adults with overweight or obesity found that all six exercise types tested improved VO2max, with high-intensity interval training ranked highest and resistance training alone ranked least effective.

⚠What it shows: Network meta-analysis of 93 randomised trials with 4,446 adults living with overweight or obesity; rankings come from indirect statistical comparison rather than head-to-head trials, and the pooled effect sizes are unusually large, which suggests variation in how VO2max was measured across studies.Meta-analysis · J Sports Sci

Across 11 randomised trials in 546 adults with prediabetes, prebiotic supplementation did not significantly change fasting glucose, insulin, HbA1c, HOMA-IR or blood lipids, and did not shift BMI or waist circumference. The single significant result was a reduction in body fat percentage, rated very low certainty.

⚠What it shows: Systematic review and meta-analysis of 11 randomised trials totalling 546 adults with prediabetes; the authors themselves rate certainty in the pooled estimates as very low for most outcomes, and the one positive finding sits on a wide confidence interval.Meta-analysis · J Nutr

A Cochrane review of six randomised trials with 599 adults found that exercising shortly before an influenza vaccination was neither beneficial nor harmful, with no significant difference in outcomes between people who exercised and those who did not. The one trial reporting actual influenza cases also found no significant difference.

⚠What it shows: Cochrane review of six small RCTs (599 adults aged 18-80, study size 46-158), rated low quality: five of six trials had at least four unclear risk-of-bias domains, most measured biochemical rather than participant-centred outcomes, and no trial reported influenza complications.Meta-analysis · Cochrane Database Syst Rev