The search identified 36,535 unique citations, of which 1,108 were potentially relevant; at this point, the team elected to limit to systematic reviews (Figure 1). A second screen identified 337 citations for full-text review, of which 54 were included (Table 1). A list of excluded studies with reasons is provided in Additional file 2.
Included reviews reported on several types of interventions, with some reporting separate results for more than one intervention type. Most reviews focused on exercise or PA only (n = 46) (21-66), while others included exercise with nutritional supplements (n = 8) (67-74). No reviews included group-based nutrition interventions alone. Exercise or PA interventions were categorized as resistance exercise (n = 11) (21, 30, 34, 35, 38, 46, 56, 61-63, 75), aerobic exercise (n = 4) (22, 24, 28, 51), combined aerobic and resistance exercise (n = 9) (23, 29, 34, 36, 44, 46, 50, 63, 75), general PA (n = 9) (32, 33, 35, 40, 45, 49, 52, 54, 66), mind-body exercise (e.g., Tai Chi, yoga, Pilates) (n = 8) (26, 27, 35, 43, 47, 55, 58, 59), dance (n = 5) (31, 37, 48, 57, 63), and other (n = 10) (25, 34, 35, 39, 42, 53, 63-65, 76). Nutritional supplements included protein (n = 4) (69, 72-74), creatine (n = 2) (67, 71), vitamin D (n = 1) (68), or dairy (n = 1) (70). Meta-analyses were undertaken in 34 reviews (21, 22, 26-30, 32-35, 40, 42, 43, 46, 48, 51, 53, 54, 56, 59, 61-63, 65, 66, 68, 70-75, 77), 19 reviews presented findings narratively (23, 24, 31, 37-39, 44, 45, 47, 49, 50, 52, 55, 57, 58, 64, 67, 69, 76), and one performed a network meta-analysis (25). Total sample sizes ranged from 153-28,523 when reported. Participants ranged from 42-98 years old, with most reviews only including studies with participants aged 60 and older. No reviews extracted data on material deprivation, low income, or immigrant populations.
Eligible reviews included 1,170 primary studies, of which 853 were unique (27.1% overlap across reviews, although some duplicates were included in reviews focused on different intervention types). Reviews with the most overlap by intervention type were exercise with nutritional supplements (36.2% overlap), dance (31.9% overlap), and resistance exercise (26.7% overlap). Single studies were published between 1983-2020 (range 5 to 99 studies per review). Of these, 82% were randomized controlled trials and 18% were quasi-experimental, observational, or not reported.
Methodological Quality of Included Reviews
Methodological quality of the reviews was variable (summary in Figure 2, full assessment in Additional file 3), with one review (35) rated as having high confidence in findings. The confidence for the remaining reviews were moderate (n = 21) (24, 25, 30, 32, 33, 39, 51, 52, 55, 56, 59, 62-65, 68-71, 76, 77), low (n = 14) (26, 29, 31, 40, 42, 43, 46, 49, 54, 57, 58, 66, 67, 72), and critically low (n = 18) (21-23, 27, 28, 34, 37, 38, 44, 45, 47, 48, 50, 53, 61, 73-75). Most reviews did not report protocol registration, describe an adequate search strategy, justify excluded studies, or incorporate risk of bias in interpreting review findings.
Findings of Reviews
A summary of findings by intervention type and outcome category, alongside review quality is listed in Table 2, with summary of certainty of evidence (GRADE) in Figure 3.
The effect of interventions on aerobic capacity was reported in 13 reviews (Additional file 4). Across reviews, 97 studies were reported, of which 92 were unique (overlap, 5.2% across intervention types). Aerobic capacity was most often assessed using measured or predicted maximal or peak oxygen consumption (VO2 peak). Based on high-certainty evidence, a combination of aerobic and resistance training results in meaningful improvements in aerobic capacity in older adults. Based on moderate certainty evidence, aerobic exercise probably results in improvements in aerobic capacity. Dance interventions may result in increased aerobic capacity, although this is based on low-certainty evidence and findings may change as more information becomes available. Interventions that combined exercise with nutritional supplements may make little to no difference in aerobic capacity of older adults, although this is based on low-certainty evidence. The evidence is very uncertain about the effect of mind-body exercise or resistance exercise alone on aerobic capacity in older adults (very low certainty evidence). Other exercise types, including aquatic exercise and a combination of PA and cognitive training were also examined for their effect on aerobic capacity. No included reviews explored the effect of general PA interventions on aerobic capacity.
In total, 44 reviews reported on 516 single studies, of which 372 were unique (27.9% overlap across intervention types). Physical function was typically assessed using the Timed Up and Go test, chair stands, gait speed, and six-minute walk test; often findings from single studies were compiled into a composite score for self-reported and/or measured physical function within meta-analyses (Additional file 5). Based on moderate certainty evidence, interventions that included a combination of aerobic and resistance training, interventions focused on general PA, and mind-body exercise interventions are all likely to result in improvements in physical function in older adults. Resistance training and dance interventions may also increase physical function (low certainty evidence). Low certainty evidence suggests that aerobic exercise interventions and exercise combined with nutritional supplements may have little to no impact on physical function.
In total, 25 reviews reported on 217 single studies, of which 181 were unique (16.6% overlap). Static and dynamic balance tests (e.g., single-leg stance, Berg Balance Scale) and composite balance measures were used across reviews (Additional file 6). High certainty evidence suggests that participation in mind-body exercise interventions increases balance in older adults. General PA interventions and interventions that combined aerobic and resistance training are also likely to result in improvements in balance in older adults, based on moderate-certainty evidence. Dance interventions may improve balance; however, this is based on low certainty evidence. Also based on low certainty evidence, resistance training and aerobic exercise alone may result in little to no change in balance. No included reviews explored the effects of exercise and nutritional supplements on balance.
Falls and safety
Number of falls, risk of falling, and fall-related injuries were measured across 10 reviews including 88 single studies, 78 of which were unique (11.4% overlap). Interventions that combined aerobic and resistance exercise and interventions focused on general PA are likely to result in a small reduction in the risk of falls or fall-related injuries in older adults, based on moderate certainty evidence (Additional file 7). Based on low-certainty evidence, mind-body exercises may have little to no meaningful effect on fall risk, although these findings may change as more data are available. Dance interventions may reduce falls, but the evidence is of very low certainty. Also based on very low certainty evidence, aerobic training alone and resistance training alone may have little to no effect on falls risk. No reviews reported the risk of falls within interventions that combined exercise and nutrition.
Within reviews reporting muscle strength outcomes, 34 reviews reported on 369 single studies, of which 292 were unique (20.9% overlap). Various measures were reported, including handgrip strength, upper body strength, lower body strength, muscle mass, and overall muscle strength (Additional file 8). Both resistance exercise interventions and general PA interventions likely increase upper and lower body strength (moderate certainty evidence). Aerobic exercise alone, combined aerobic and resistance exercise, mind-body exercise, and dance interventions may result in improvements in muscle strength, however this is based on low certainty evidence and findings may change as more data become available. Also based on low certainty evidence, interventions that combined exercise with nutritional supplements may not improve muscle strength.
Health-related quality of life and self-reported wellbeing
In total, 14 reviews reported health-related quality of life and self-reported wellbeing outcomes (Additional file 9). Given the variation in constructs measured within this domain (e.g., activities of daily living, quality of life (SF-36), perceived mental health) and limited number of reviews for each outcome type, these results were not incorporated into the overall summary of findings using GRADE.