Effects of an Elderberry-Based Functional Beverage on Exercise Performance, Muscle Structure, and the Gut-Muscle Axis

In modern times, the increasing prevalence of sedentary lifestyles, age-related decline in muscle function, and gut dysbiosis have been shown to negatively affect both metabolic health and muscular performance. These problems are not only observed in the elderly population but also in physically active young individuals, manifesting as exercise-induced fatigue, reduced performance, and impaired immune function. Sustaining muscle performance is closely related not only to physical fitness but also to maintaining gut microbiota balance and controlling inflammation.

The health effects of functional foods have become a focal point in multidisciplinary research in recent years. In this context, developing functional beverages rich in bioactive compounds such as polyphenols, flavonoids, and anthocyanins offers new research opportunities, particularly in the fields of exercise physiology and immune response. This study aims to evaluate the effects of an anthocyanin-enriched functional beverage derived from Sambucus nigra L. (elderberry) on muscle performance, fatigue, and the gut-muscle axis in healthy individuals participating in a combined exercise protocol.

Elderberry is a bioactive source noted for its high anthocyanin content, showing antioxidant and anti-inflammatory effects, and its potential to positively modulate gut microbiota. Since anthocyanins are only minimally absorbed in the small intestine, a large proportion reaches the colon, where they are fermented by the gut microbiota into various phenolic acids. This fermentation process may support the proliferation of beneficial species such as Bifidobacterium and Lactobacillus, while reducing pathogenic species such as Clostridium perfringens. Furthermore, elderberry polyphenols may promote the growth of anti-inflammatory species like Akkermansia muciniphila and Faecalibacterium prausnitzii. These microbial changes-especially the increase in short-chain fatty acids (SCFAs), particularly butyrate-have been associated with improved immune response, gut barrier integrity, and muscle metabolism. In vivo studies have shown that anthocyanin supplementation enhances microbial diversity and stimulates SCFA production, thereby helping to suppress inflammation. Within the so-called "gut-muscle axis," SCFAs-especially butyrate-play roles in muscle protein synthesis, mitochondrial function, and inflammation regulation. Some animal studies have also shown that modulation of microbiota composition via probiotics and polyphenol supplementation positively affects muscle mass and endurance.

Therefore, the researchers hypothesize that the elderberry-based beverage may reduce exercise-induced muscle fatigue and enhance performance not only through systemic antioxidant mechanisms but also by regulating gut microbiota. Consequently, this study will also assess how the microbiota changes induced by the beverage affect muscle metabolism.

A combined exercise protocol (2 days of resistance + 1 day of endurance) has been chosen, as it is an effective model for stimulating both cardiorespiratory capacity and muscle function. The literature highlights that endurance training improves mitochondrial biogenesis, oxidative capacity, and lactate clearance, whereas resistance training increases muscular strength and hypertrophy. It has been demonstrated that combined protocols are more effective for improving metabolic health and functional capacity.

To evaluate not only general muscle hypertrophy but also fiber-type-specific physiological adaptations, three muscle groups will be assessed: quadriceps, soleus (Type I fiber dominant), and gastrocnemius (Type II fiber dominant). These muscles were selected based on their well-defined morphological and metabolic profiles in the literature, allowing for indirect assessment of anthocyanins and polyphenols on mitochondrial function and oxidative stress. This makes the study one of the few human-based interventions that systematically investigates the effects of functional foods on exercise physiology with a muscle fiber-type-specific approach.

This study is among the few examples in the literature that evaluate functional beverages not only at the bioactive compound level but also in the context of exercise physiology and microbiota interaction. Most current research focuses on the short-term effects of polyphenol supplementation or isolated microbiota outcomes. However, this project aims to evaluate the holistic effects of a polyphenol/anthocyanin-rich functional beverage on both performance physiology and gut microbiota. Furthermore, the mediating role of the gut-muscle axis will be explored through quantitative measurements. This approach introduces a new methodological framework for both bioactive food development and microbiota-based exercise science. To date, there has been no human-based randomized controlled trial combining exercise, microbiota, and functional beverage interventions in the same design.