During the aging process, physical fitness will reduce by 40-80%, usually caused by metabolic disorders or chronic disease that often occur in adults (1). Age is one aspect that affects fitness levels driven by physiological, psychosocial and environmental factors (2). This decrease in physical fitness is caused by excessive oxidative stress. Under these conditions, the body needs additional or exogenous antioxidants. One source of exogenous antioxidant is rosella (Hibiscus sabdariffa L.), which contains high levels of anthocyanins and flavonoids, especially in the flower petals (3). Anthocyanins have been proven to have effects on human body systems, one of which is the cardiovascular system: Research in New Zealand demonstrated that the anthocyanin content in blackcurrants can increase vasodilation and cardiac output (4). This can increase peripheral blood flow and potentially improve exercise performance. Anthocyanin compounds also influence muscle regeneration by accelerating the myogenesis process, protecting mitochondria, muscle protein synthesis and protecting against muscle protein degradation (5). A systemic review concluded that H. sabdariffa inhibits LDL oxidation and prevent atherosclerosis (6).

To the best of our knowledge, there are no previous studies on the effect of anthocyanins in H. sabdariffa on physical fitness (comprising flexibility and musculoskeletal and cardiorespiratory fitness). Therefore, the present study aimed to investigate whether the anthocyanins in H. sabdariffa influence strength, balance, left and right hand grip, vertical jump and VO_2max (maximal oxygen consumption), which are the parameters of physical fitness.

All subjects received rosella tea; nutritional content is shown in Table I. Subject characteristics are presented in Table II. HST had significantly improved strength (24.9 to 27.3 kg; P=0.043), balance (23.3 to 42.2 sec; P=0.006), right (31.8 to 35.1 kg; P<0.001) and left hand grip (29.8 to 31.6 kg; P=0.016), vertical jump (38.6 to 41.1 m/sec; P=0.014) and VO_2max (31.1 to 34.3 ml/kg/min; P=0.021; Table III).

HST that mostly contain water and calcium level, a moderate level of anthocyanin and a small amount of fat, carbohydrate, and vitamin C. The part with the most anthocyanin levels is the rosella calyx. There are two major anthocyanins in rosella, namely delphinidin-3-xylose glucoside or hibiscin and cyanidin-3-xyloside glucoside or gossypicyanin (19). Anthocyanins are natural water-soluble pigments and serve as antitumor, antioxidants and anticarcinogenic. At the cancer formation stage, anthocyanins play a role in various mechanisms, that is inhibiting cellular transformation, cell proliferation and signaling pathways (20). These compounds are derivatives of flavonoids found in phenolics (19).

Anthocyanins improve the performance and maintenance of skeletal muscle by increasing the synthesis of skeletal muscle proteins by several mechanisms (5). First, anthocyanins are directly involved in the PI3K/Akt signaling pathway. Activation of PI3K/Akt signaling activates mTOR, which plays a role in initiating protein synthesis in muscle. Second, anthocyanins are directly involved in the expression of IGF-1, which plays a role in inhibiting C-reactive protein (CRP). CRP inhibits protein synthesis by inhibiting mTOR. Third, anthocyanins serve as angiotensin-converting enzyme inhibitors to improve endothelial function and angiogenesis, increase levels of IGF-1, initiate glucose uptake and inhibit the release of pro-inflammatory molecules such as IL-6(21). The present study showed difference in results from the left hand (29.8 to 31.6 kg; P=0.016) and the right hand (31.8 to 35.1 kg; P<0.001). This can be because all subjects were right hand-dominant. The dominant hand has higher strength than the non-dominant hand (22).

The vestibular system has a function in detecting changes in the position of the head and also maintaining body balance. One part of the vestibular system is the vestibular hair cell, which serves as a mechanosensory receptor. Anthocyanins protect vestibular hair cells from damage by inhibiting reactive oxygen species production and caspase-3 activation (23).

When jumping, muscles and nerves work together to produce a force and speed, which is called power. This neuromuscular cooperation is called the stretch-shortening cycle. This cycle consists of three phases. The first phase, eccentric, occurs when there is stretching of the target muscle, which causes an increase in energy storage in muscles and tendons. When a muscle lengthens, there is feedback from the central nervous system to produce active movement. This phase is amortization. As a result, the muscles will contract to prevent overstretching (concentric phase) (24).

As aforementioned, anthocyanins increase muscle performance by increasing protein synthesis in muscles. In addition, anthocyanins have a neuroprotective effect by decreasing neuroinflammation and promoting neuronal autophagy to decrease neuronal damage (25). Therefore, anthocyanins can increase power in vertical jumps increasing performance of muscles and nerves.

Maximum oxygen intake (VO_2max) is limited by the cardiorespiratory ability to deliver oxygen supply when the muscles contract (26). VO_2max is affected by the ability of the heart to pump, blood flow and the ability of skeletal muscles to capture and use oxygen.

Anthocyanins have an antioxidant and anti-inflammatory effect to improve vasodilatory function and blood flow. Blood that flows well can increase tissue perfusion and uptake of oxygen in skeletal muscle (27). In addition, anthocyanins enhance mitochondrial biogenesis and metabolism. Mitochondria have the function of using oxygen to produce energy (5).

In the present study, 200 ml HST given to healthy adults for 30 days in the morning and evening significantly improved physical fitness parameters including strength, balance, hand grip, vertical jump and VO_2max. HST, which contains anthocyanins provides antioxidant and neuroprotective effects and improves muscle function. However, there are several shortcomings and limitations, such as the heterogeneous subjects, small sample size, absence of a normal control group and subject physical activities that may affect physical fitness result. In addition, further research is needed to prove whether the levels of physical activity are related to increasing physical fitness when giving HST in healthy adults.