In current production, isotonic beverages consist of a natural or artificial source of electrolyte salts (29), mainly ions of sodium, potassium, magnesium, chloride, and calcium (31). Sodium is the main cation in the extracellular space, and a large quantity of this cation can be lost by sweating (32). Salt is added to sports beverages in order to provide sufficient quantities of sodium and keep up its concentration and the volume of plasma (33). Sodium plays a major role in the adjustment of the balance of water in the body and transport of carbohydrates and proteins to tissues (34), increases the intestinal absorption of carbohydrates (19, 35), aids the active working muscle to contract and relax (36), and prevents hyponatraemia (28), which is the lack of sodium in the body caused by loss of sodium due to sweating, diarrhea, and vomiting (34).

Potassium creates and preserves constant muscle contraction and nerve impulse, prevents clotting of blood and keeps its pH level, helps in the storage of carbohydrates in the muscles (36), and sets the intracellular water content by its intervention in the synthetic processes of proteins and carbohydrates (26).

Chloride plays an important role in the safety of osmotic pressure and acid-base balance of the body and is necessary for the gastric juice (26, 36).

According to (28), isotonic drinks should contain 0.5–0.7 g/L of sodium and 6–9% carbohydrates, and the concentration of sodium should be increased to 0.7–1.2 g/L during an exercise in the heat and for more than 1 h. However, (34) obtained a level of 342 mg/L sodium in isotonic drinks. In another study by (25), the results of the mineral analysis of a commercial isotonic drink showed a sodium concentration of 45.753 mg/100 ml, potassium 7.526 mg/100 ml, calcium 0.327 mg/100 ml, and magnesium 0.326 mg/100 ml.

During intense exercise, muscle glycogen stores and blood glucose decrease which requires a continuous source of carbohydrates to provide energy and avoid fatigue. Isotonic drinks are composed of carbohydrates as a source of energy in the form of mono- and polysaccharides (glucose, fructose, and maltodextrin) (29) with a recommended concentration of 6–9 % (28). According to (37), the intake of carbohydrates before exercise slows down and prevents homeostatic disturbances, gives an adequate plasmatic volume from the start of the exercise, and contributes to offset the loss of carbohydrate stores, enhancing performances during exercises (35).

We have given in Table 1 the main composition of some isotonic, energy, and fruit juices available in the market in order to give information on the composition of isotonic drinks and compare their compositions with other types of beverages. It is clearly observed that there is a difference between the three groups of beverages. The concentration of carbohydrates and salt differs among the six isotonic drinks. carbohydrate concentrations in isotonic drinks (4.4 ± 0.78 g/100 ml) are lower than the concentrations present in energy drinks and fruit juices. They are ranging from 2.9 to 5 g/100 ml in isotonic drinks, from 9.9 to 17 g/100 ml in fruit juices, and from 11 to 16 g/100 ml in energy drinks. Caloric values are also higher in energy drinks and fruit juices compared to isotonic drinks. Organic grape juice has high carbohydrate concentration among fruit juices (17 g/100 ml). Thus, in our previous study (7), to obtain a healthy beverage from grape juice, we diluted the sugar content to 40–50 g/L with mineral water, because it cannot be considered a healthy beverage if it contains a high concentration of sugars. Therefore, a significant way to improve the impact of grape isotonic juices on health is dilution of sugars to 40–50 g/L with mineral water, which also improves the glycemic index. On the other hand, research shows that simple carbohydrates (sucrose, glucose, and fructose) are most effective in stimulating fast absorption and promoting carbohydrate oxidation. Thus, the amount and type of carbohydrates used in a sports drink are important to optimize the potential of the drink and improve performance (37). In addition, it was mentioned that consumption of carbohydrates with a low glycemic index (GI) before physical exercises has a beneficial effect, because they perform a slow release of glucose into the blood after digestion, providing a sustained source of energy to contracting muscles, maintaining muscle glycogen, and improving performances. They also promote a low insulin response that is beneficial for substrate metabolism, because a high level of insulin inhibits fat lipolysis and oxidation (38). Also, a comparison of sugar, salt, and calorie values in the same group (isotonic, energy, or fruit beverages) demonstrates that there is not a large difference between isotonic beverages composition unlike in the group of energy drinks and fruit juices. However, salt concentrations are higher in isotonic drinks (0.08 ± 0.02 g/100 ml) and absent in fruit juices. Proteins, fats, and fibers are missing in both isotonic and energy drinks, and they are present in fruit juices with a mean value 0.5 ± 0.16 g/100 ml. Stimulants (caffeine and guarana) are found only in energy drinks.

Accordingly, optimization of carbohydrates and salt concentration for performance is an interest of sport nutritionists and drink manufacturers (39). Higher concentration provides more carbohydrate and salt but minimizes the rate of gastric emptying and can consequently delay the rate of delivery of fluids (19, 35). The needed osmolality value (270–330 mOsm/kg of water) (29) defined by the European Food Safety Authority (EFSA) for isotonic drinks (6) which is the same osmolality found in the human body, leads to prevention of tiredness and increase performances after ingestion of the beverage (12) because of fast absorption of water and ions, which is the main purpose of isotonic drink consumption, to replenish the fluids lost during physical exercises (31).

New dietary guidelines and health professionals are interested in developing foods with lower sugar content or with alternative sweetener sources due to the multiple diseases associated with sugar intake such as obesity, diabetes, cardiovascular disorders, and cholesterol (40). Zhang et al. (33) mentioned that grape juice sports drinks do not need to use sweeteners and acidulants. Grape juice was chosen for designing an isotonic beverage according to its known bioactivity and nutritional composition. Grape juice is a beverage extracted from different grape varieties, mainly the Vitis vinifera, Vitis labrusca, and Vitis rotundifolia species (41), by different technological processes (hot press, cold press, and hot break) (42). Its consumption is increasing worldwide, because of its sensory characteristics and nutritional value (43). The United States, Spain, China, Italy, France, Turkey, and Chile are the top producers of grape juice (41). Grape juice contains water, high concentration of sugars and organic acids (42, 44, 45), and minerals, phenolic compounds, and other nutrients such as vitamins, proteins, fatty acids, and amino acids (46, 47). Carbohydrates are found in the form of fructose and glucose (48). The main organic acids in grape juice are tartaric, malic, and citric acids (41), In addition, these acids are used as indicators of microbiological alterations in the beverage because of their impact on its stability (49). Phenolic compounds are the most abundant compounds followed by sugars and acids (41). They play a major role in prevention of various diseases caused by oxidative stress (50). The phenolic compounds found in grape juice are those extracted from grape skins and seeds (42). They are classified into flavonoids such as flavanols, flavonols, and anthocyanins, and non-flavonoids mainly phenolic acids and stilbene (49, 51). Numerous studies have identified and quantified the phenolic content of different varieties and cultivars of grape juice (46, 49, 52–55). The phenolic content differed among various grape juices. Thus, researchers have indicated that the content and profile of phenolic compounds are dependent on grape varieties, species, (43) technology of juice preparation (49), geographical origin, ripeness, type of soil, sunlight exposure, method used for quantification (41), farming system of grapes (organic, conventional, and biodynamic) (56), and culture conditions. Grape tissues in the pulp are rich in phenolic acids, and the skin is rich in flavonoids (57). However, anthocyanins are the main phenolic compounds in red grape juices, while flavan-3-ols are more abundant in white grape juices (42). It is mentioned that most phenolic compounds in white grapes belong to the non-flavonoid group, including mainly phenolic acids such as gallic, vanillic, syringic, protocatechuic, and ellagic acids, flavonoids such as flavanols mainly catechin, epicatechin, procyanidins, and flavonols mainly quercetin and other aglycones (44).

The interest of consumers and the food industry in polyphenols has been growing, because there is a relationship between their intake and prevention of various diseases (50, 54). Accordingly biological activities of polyphenols in grape juice are linked to their antioxidant, anti-inflammation, anticancer, anti-aging, antimicrobial, and cardioprotective properties (58). They can prevent platelet aggregation, LDL, DNA (59), lipid, protein (53), and membrane damage oxidation (57), reduce adhesion molecule expression and limit inflammations (60), which block cellular events predisposing atherosclerosis (61), enhance the regulation of blood pressure and vascular reactivity, reduce serum cholesterol and triglycerides (60), and improve memory function in older adults (62). They also help to prevent obesity and diabetes by inhibiting specific enzymes (52). Phenolic compounds improve antioxidant activity by scavenging reactive oxygen and nitrogen molecules, chelating redox-active transition minerals, collaborating with other antioxidants, stimulating antioxidant enzymes and proteins, inhibiting pro-oxidant enzymes, and modulating transcription factors redox-sensitive (60). In Moreno-Montoro et al. (57), it was mentioned that catechin and gallic acid act as free radical scavengers, and that epicatechin has an antibacterial activity. In addition to their antioxidant capacity, gallic, caffeic, and chlorogenic acids act as venous dilators (49). Resveratrol plays a beneficial role in protection against various neoplasias, cardiovascular and neurodegenerative disorders, and viral infections as well as helps to retard body aging and reduces the incidence of heart and muscle diseases (56). Quercetin and its derivatives have shown anti-inflammation and anticarcinogenic properties when used in the treatment of some types of cancer (44).

During extended and intense exercises such as marathon and ultramarathon races, athletes exhibit severe physiological stress that appears as muscle microtrauma, oxidative stress, gastrointestinal dysfunction, or inflammation (63). According to (64), during long and extenuating physical exercises, oxidative stress is caused owing to a considerable increase of reactive oxygen species (ROS) promoting an imbalance in antioxidant capacity in the body, which leads to protein modification, lipid oxidation, DNA damage (65), inflammation (66), and chronic diseases including cancer and neurological and cardiovascular diseases (67). Grape and grape derivative products are a source of polyphenols (63) which are known of high antioxidant activities (68) and can be beneficial against oxidative damage (69). In addition, the carbohydrate content is useful for glycogen deposition and improvements of practice during long term exercises (64). Therefore, many researchers have studied the beneficial effect of grape juice related to improve performances during physical exercises. Table 2 shows some experimental studies carried out and their consequences, which have demonstrated that grape juice was able to improve performances and antioxidant activity, protect against oxidative damage, and reduce inflammation. The figure in the graphical abstract shows the contribution of grape juice in the development of an effective isotonic drink and its impact on athletes' performances.

Another interesting characteristic of grape juice is its great sensory characteristic. The phenolic compounds in grape juice are responsible for its sensory properties (color, flavor, and taste) (42). The phenolic acids affect the organoleptic properties of grape juice (49) and ensure a low pH value, which provides equilibrium between sweet and sour tastes (42). Accordingly, organoleptic properties are an important factor when choosing food products. Color is the most important characteristic used when choosing beverages (76). In addition, color is used by consumers as an indicator of juice quality (77), because there is a strong relationship between color and flavor, as consumers can be able to expect the flavor through the color of food products (78–80). Nowadays, replacement of synthetic dyes with natural colorant is a challenge for the food industry (76). The attractive orange, red, and purple colors and water solubility of anthocyanins allow their integration into aqueous food systems and use as natural colorants (81, 82). They also participate in multiple chemical reactions such as copigmentation and formation of polymeric pigments that contribute to color changes (77). For grape juice, cyanidin, peonidin, delphinidin, petunidin, and malvidin are the main anthocyanins responsible for the red color (83). Their stability is influenced by pH, temperature, oxygen, light, presence of ascorbic acid and metal ions, and high concentration of sugar (42, 82, 84). On the other hand, consumption of anthocyanins promotes health benefits by reducing the risk of cancer, inflammation, neuronal and cardiovascular diseases, diabetes, obesity, and cognitive function disorders (85) owing to their antioxidant anti-cardiovascular, anticancer, anti-inflammatory, anti-thrombotic, anti-ulcer, anti-allergenic, and anti-coagulant activities as well their immunomodulatory, vasodilatory, and analgesic activities (86).