The trial enrolled 41 middle-aged and older adults, randomly assigning them to receive either a daily supplement delivering roughly 700 milligrams of polyphenols or an identical placebo. After a 30-day loading phase without exercise, participants began a carefully supervised 12-week programme: two full-body resistance training sessions per week, plus a third session combining lower-body resistance work with 7 to 9 minutes of high-intensity intervals on a cycle ergometer. Loads progressed from moderate to high intensity throughout, with lower-body exercises moving from approximately 12 to 6 repetition maximum and upper-body work from 15 to 10. Before and after the intervention, researchers took biopsies from the vastus lateralis, the large quadriceps muscle on the side of the thigh, and sequenced the RNA within.

The sequencing pipeline was demanding. Of 79 muscle samples collected, 76 passed quality control, yielding a final dataset of 38 participants — 17 in the polyphenol group and 21 on placebo. The team aligned trimmed reads to the human reference genome GRCh38 and applied a stringent statistical framework, adjusting every model for age, sex and, crucially, the estimated proportion of slow- and fast-twitch muscle fibres in each sample. That last adjustment proved essential. Fibre-type composition influenced the expression of 633 genes in the dataset, far more than age, which affected only nine. Sex was an even stronger confounder, shaping 1,040 genes. Without accounting for these factors, apparent training effects could easily have reflected baseline biological differences between individuals rather than true responses to exercise.

When the researchers pooled all participants and compared pre- and post-training samples, they found 60 transcripts that reached statistical significance after correction for multiple testing. But only seven of those also exceeded a fold-change threshold of greater than twofold — the combined statistical and effect-size criterion the team used to flag changes of potentially larger biological magnitude. Among those seven, six were upregulated and one was downregulated. The largest increases appeared in CLLU1 and PRND, while the most pronounced decrease occurred in MYH1, the gene encoding the myosin heavy chain IIx isoform that powers the fastest, most glycolytic muscle fibres.

That MYH1 signal is biologically intriguing. Exercise programmes blending resistance and endurance components are known to nudge muscle toward a more oxidative phenotype, often involving transitions from type IIx toward type IIa fast fibres. Yet the histological analyses previously performed in this same cohort showed that training enlarged type II fibres overall without changing the relative distribution of type I and type II fibres, and did not separately quantify the IIa and IIx subtypes. The authors therefore interpret the MYH1 downregulation as a transcript-level change related to the fast contractile phenotype, not as direct evidence of a fibre-type switch. Similarly, the upregulation of LOX, which encodes lysyl oxidase — an enzyme that cross-links collagen and elastin in the extracellular matrix — fits with the well-documented role of exercise in remodelling the connective tissue scaffolding that transmits force within muscle. But without coordinated enrichment of broader matrix-related pathways, the researchers caution that LOX stands as an individual transcript-level signal rather than proof of systematic extracellular matrix remodelling.

Perhaps the most striking aspect of the findings is what was absent. Despite the robust physiological adaptations documented in this cohort in earlier analyses — gains in muscle mass, strength, aerobic capacity and mitochondrial respiratory capacity — the transcriptional response was predominantly modest in magnitude and failed to converge on any coherent pathway-level signature. No robust enrichment emerged from gene ontology, Reactome or KEGG pathway analyses. This is not as paradoxical as it may seem. Accumulating multi-omic evidence, including large-scale consortium studies, indicates that chronic exercise adaptations in human muscle are typically distributed across many genes with small effect sizes — a coordinated fine-tuning of biological systems rather than a dramatic transcriptional overhaul. Recent single-nucleus RNA sequencing studies have reported similarly limited numbers of differentially expressed genes even with cell-type-specific resolution.

The polyphenol story ended in near silence. At baseline, before any training began, the supplement and placebo groups were nearly indistinguishable at the transcriptomic level, with only three FDR-significant transcripts — and just one, the oxidative stress response gene HMOX1, exceeding the fold-change threshold, and even that with a modest effect. The group-by-time interaction analysis, designed to detect whether the supplement changed how muscle responded to training, identified 14 FDR-significant transcripts, of which eight met the combined criterion. But inspection of individual expression profiles revealed low or sparse expression and marked inter-individual variability rather than consistent within-subject changes. The most biologically interpretable candidate, KERA, which encodes the extracellular matrix proteoglycan keratocan, was expressed at very low levels and appeared without any accompanying response in other matrix-related genes. Heatmaps of top-ranked genes showed no clear clustering by group or timepoint, and no significantly enriched pathways emerged in KEGG analysis.

These molecular null results align seamlessly with the broader picture from the same trial. Polyphenol supplementation had previously failed to augment training-induced improvements in muscle mass, strength and aerobic capacity, did not enhance mitochondrial bioenergetics or redox balance beyond what training alone achieved, did not alter the inflammatory response to exercise, and left skeletal muscle accumulations of advanced glycation end-products unchanged. Taken together, the authors conclude that polyphenol supplementation did not meaningfully modulate the skeletal muscle transcriptional response to concurrent training. For a supplement category often marketed on the premise of priming redox-sensitive and inflammatory signalling pathways, the message is pointed: any modulatory effects, if they exist, are too subtle to register at the level of muscle gene expression or physiological function.

The study also carries methodological lessons for the field. By incorporating in silico estimates of fibre-type composition — derived from a validated gene-signature matrix using the FibeRtypeR tool — as a covariate, the researchers reduced the risk of mistaking baseline phenotypic heterogeneity for intervention effects, a confound that many human muscle transcriptomics studies have ignored. The authors acknowledge limitations, however: the sample size limits statistical power for detecting small but real changes; bulk RNA sequencing cannot resolve cell-type-specific adaptations; transcriptomics misses post-transcriptional and translational regulation, including ribosome biogenesis, a central driver of muscle hypertrophy whose signals are largely invisible to mRNA sequencing; and relatively low RNA quality and a modest 10-nanogram input may have reduced detection sensitivity. The stringent fibre-type adjustment itself likely suppressed the number of detectable differentially expressed genes.

For ageing adults, the practical takeaway remains firmly anchored in the sweat, not the supplement. Twelve weeks of combined resistance training and high-intensity intervals produced measurable, if molecularly subtle, remodelling in the transcriptome of ageing muscle — touching genes of the extracellular matrix and the contractile machinery — while delivering well-documented gains in strength, fitness and mitochondrial function. The polyphenol extract, whatever its merits elsewhere, added no detectable molecular edge. As the population ages and sarcopenia looms as a major driver of frailty and loss of independence, the trial reinforces a simple hierarchy: progressive mechanical loading of muscle remains the most reliable lever we have, and no berry extract has yet been shown to turn its dials further.

Subject of Research: Skeletal muscle transcriptomic responses to concurrent resistance and high-intensity interval training with and without polyphenol supplementation in ageing adults

Article Title: Skeletal Muscle Transcriptomic Responses to Concurrent Training With Polyphenols in Ageing Adults: A Randomized Trial

Article References: Guadalupe‐Grau, A., Gudra, D., Lismane, D., Raudeniece, J., Akuratere, G., Viksne, K., Gailite, L., Vilne, B., Pivovarova‐Ramich, O., Flensted‐Jensen, M., Dela, F., & Reihmane, D. (2026). Skeletal Muscle Transcriptomic Responses to Concurrent Training With Polyphenols in Ageing Adults: A Randomized Trial. Journal of Cachexia, Sarcopenia and Muscle, 17(5), Article e70391. https://doi.org/10.1002/jcsm.70391

Image Credits: AI Generated

DOI: 10.1002/jcsm.70391

Keywords: skeletal muscle, ageing, transcriptomics, RNA sequencing, resistance training, HIIT, polyphenols, randomized trial, extracellular matrix, MYH1, mitochondria, sarcopenia