A pre-registered PRISMA/SWiM synthesis of 248 studies (100 RCTs, 62 cohorts) in adults 60 and older found the most consistent benefit for sarcopenia from protein or leucine and vitamin D at 800 IU/day or more. A beneficial direction of effect appeared in 84% of single-nutrient RCTs, 75% of whole-diet trials and 78% of gut-targeted trials. Observational data linked Mediterranean and MIND-style diets, higher protein quality and fibre to lower sarcopenia incidence. Gut-muscle interventions, via short-chain fatty acids and Akkermansia and Faecalibacterium enrichment, were favourable but low-certainty.
The protein and vitamin D signal fits earlier meta-analyses, but this review's real value is its hierarchy of certainty. Be wary of the headline percentages: they count trials pointing in a favourable direction, not effect sizes, and the authors say so. Vote-counting without pooled estimates cannot tell you how much grip strength or lean mass improves, and it is vulnerable to publication bias and to small trials with mixed outcome measures. Dietary-pattern evidence is mostly observational, so healthy-user confounding is a live concern. The gut-microbiome results are best treated as hypothesis-generating. Practically, the findings reinforce what geriatric guidance already suggests: adequate protein spread across meals, correcting vitamin D insufficiency, and eating a plant-forward pattern, ideally alongside resistance training, which this nutrition-focused review does not centre. An incremental, confirmatory synthesis with useful transparency about certainty, not a shift in practice.