## 1 Introduction  
Functional foods contain one or more specific compounds that have a functional effect on improving the health and well-being of the consumer. These beneficial components can be naturally increased in the food or intentionally added during the production process to produce health effects such as regulating metabolic activities and fitness and improving digestive systems, heart, vessels, etc. [1]. Probiotic products are one of the most common types of functional foods. In recent years, there has been an increasing effort to use probiotic microorganisms to produce various foods in which they have been marketed, including probiotic yogurts, cheese, and fermented beverages [2]. Reactive oxygen species (ROS) are reactive chemical individuals incorporating oxygen atoms with an unpaired electron (radical) or O-O bond and can participate in chemical reactions [3]. ROS have crucial roles in the basic biological processes in the body. These molecules participate in oxidation and reduction reactions (in the respiratory chain) and the removal of toxins, renew energy (ATP), enable oxygen transport by hemoglobin, and activate cytochrome P450 and phagocytosis of microorganisms. However, their overproduction can lead to free radical reactions, resulting in damage to lipids, proteins, carbohydrates, and nucleic acids. The quick and easy diffusion of ROS and their ability to react with multiple nonspecific components of cells may lead to disturbances of many biological processes, resulting in the development of many diseases [4]. Antioxidant compounds in foods play significant roles as health-protecting factors. They can deactivate free radicals, which can cause cell and tissue damage. These types of damage cause malfunctioning of cells or cell death. Epidemiological studies have shown that antioxidants can prevent the development of degenerative diseases such as cancer, coronary heart diseases, obesity, type 2 diabetes, hypertension, premature aging, and inflammatory diseases [5]. Yogurt is the best-known nutritional carrier for the efficient transfer of beneficial microbes into the body [6]. Recently, there has been an intensified demand for a new range of dairy products such as cheese, various lactic acid bacteria drinks, combined probiotic (fermented) milk products, and several types of yogurts, including symbiotic yogurt containing both probiotics and prebiotics [7; 8]. Lactic acid bacteria also produce abundant bactericidal proteins in dairy foods [9; 10]. Symbiotic yogurt has become increasingly popular as a type of functional food that beneficially affects human health conditions [11; 12]. Tibetan kefir grains (TKGs) contain a complex symbiotic diversity of microorganisms, including lactic acid bacteria (LAB), yeast, and acetic acid bacteria. They are a unique natural dairy starter that originated in Tibet, China [13]. The milk is fermented into Tibetan kefir, which has been shown to be an impressive functional food with health benefits including anti-inflammatory effects [14], cholesterol-lowering ability [15], and antioxidant activities [16]. During the fermentation process, yeasts and LAB interact with each other. Yeasts provide vitamins, amino acids, and other essential growth factors for bacteria, and bacterial metabolic end-products are used as energy sources by yeast [17; 18]. Native bacteria are not probiotics unless they are isolated, purified, and proven beneficial to health when introduced in animal models in vivo. Prebiotics are a nondigestible part of food, which may serve as nutritional supplements for probiotic microorganisms to enhance their survival chances and implantation in the host intestinal tract [19]. Thus, prebiotics cause particular changes in the composition and activity of the gastrointestinal microflora that benefit the host's well-being and health [20]. Probiotics may provide a potentially promising approach to preventing microbial dysbiosis [21]. However, synbiotics can better influence lipid profiles and protect against colorectal cancer than probiotics or prebiotics alone [22].