Fresh mare milk with particular composition, hypoallergenicity, and nutraceutical properties (10), the raw material of koumiss, contains many nutrients essential for the human body. These include proteins, fats, lactose, galactose, vitamins, enzymes, and minerals, etc. (Table 1). The nutrients in mare milk are comparable to those found in human breast milk (HBM) and can be used as a substitute for infant formula (1, 11). Additionally, the contents of essential amino acids, unsaturated fatty acids, and lactose which can promote calcium absorption among humans, are greater in mare milk than in other domestic animal milks (12). Furthermore, fresh mare milk is more easily digested and absorbed by the human body. This is because digestible whey protein constitutes approximately 40% of the total protein, more than twice as much as found in cow milk. The indigestible casein is also relatively lower in contents (13). The lower fat content and smaller fat droplets in fresh mare milk are more easily absorbed by the human body (14, 15).

Fresh mare milk is rich in vitamins C, A, E, D, B₁, B₂, B₁₂, and calcium, phosphorus, and other mineral elements, of which the ratio of calcium and phosphorus is 2:1 which is very similar to HBM (16). Mare milk also contains large numbers of essential trace elements such as Zn, Cu, V, Cr, Ni, Co, Mo, and other elements not essential but beneficial to the human body, such as Sr., Rb, Ba, and Li (11). An additional twelve rare elements that positively affect the human body have also been found in mare milk. The contents of rare elements from high to low are Sc, Ce, Nd, La, Y, Sm, Eu, Pr, Gd, Yb, Er, and Ho (11). Additionally, the content of lysozyme in mare milk is twice that of HBM and promotes recovery from gastric ulcers and upper respiratory infections, as well as healing wounds and postoperative scars. The combination of lysozyme and lactoferrin in mare milk is also a type of natural anti-infection substance with bactericidal effects (5, 17, 18).

The abundant nutrients and bioactive compounds found in koumiss are also metabolized by probiotic lactic acid bacteria, yeast, and other microorganisms in addition to those found in fresh mare milk. Unique tastes and high nutritional values in koumiss are the results of enzyme catalysis by lactic acid bacteria and yeast, such as the metabolism of carbohydrates and amino acids and the biosynthesis of fatty acids, which all benefit the pancreas and promote digestion (11, 12, 19). Gastrointestinal motility disorders can also be relieved because the acid amines and peptones converted from casein and albumin in koumiss can be absorbed easily and quickly by patients (13). Whey proteins rather than caseins are major proteins in the koumiss (20). Additionally, a variety of volatile organic compounds in koumiss, including lactic acid, ethanol, carbon dioxide, and other substances converted from lactose by lactic acid bacteria and yeast would prove beneficial for consumers who are lactose intolerant (Table 1) (3, 12). Essential fatty acids, such as α-linoleic acid and linolenic acid, are higher in koumiss than in other unsaturated fatty acids and can also be produced through natural fermentation (21).

Furthermore, organic acids, aromatic compounds, extracellular polysaccharides, peptides, enzymes, bacteriocins, and other special nutrients are also produced during natural fermentation. The organic acids and bacteriocins inhibit harmful microorganisms, and bioactive peptides have excellent anticancer and antioxidant properties (5, 12). Therefore, the high nutritional values and rich functional components identified in koumiss are primarily the results of the original fresh mare milk and microbiological metabolites found in it (Table 1) (11, 22).

Numerous probiotics found in koumiss, especially lactic acid bacteria, can directly regulate the intestinal flora balance and microenvironment. As a result, the probiotics and their metabolites can improve specific immunopotentiation (23–25). Lacticaseibacillus casei Zhang isolated from koumiss can effectively inhibit the inflammatory response caused by polyinosinic:polycytidylic acid (Poly I:C). Poly I:C is often used as an analog double-stranded RNA (dsRNA) virus to simulate the viral infection process in macrophage cells (RAW264.7) (26). Research has shown that L. casei Zhang in koumiss can prevent or mitigate inflammatory reactions by reducing the amount of tumor necrosis factor-α (TNF-α) in serum (27, 28). Additionally, L. casei Zhang contributes to immunopotentiation by increasing the level of total immunoglobulin G (IgG) in serum and the content of interleukin-2 (IL-2) in the spleen. This induces the activation of T and B lymphocytes, enhances NK cell activity, and increases the activity of monocytes and macrophages against tumor cells or bacteria (Figure 1) (29–31). L. casei Zhang can stimulate the activation of T helper cells and NK cells, which then secrete the proinflammatory cytokine interferon γ (IFN-γ). IFN-γ can bind to the transmembrane glycoprotein receptor IFN-γR, inducing macrophage cells and T lymphocytes to flow to the inflammatory site and enhance immune reactions (28, 32, 33). Another strain isolated from koumiss, Lactobacillus acidophilus NCFM, can increase the murine dendritic cell expression of interferon β (IFN-β), interleukin-12 (IL-12), and interleukin-10 (IL-10). These observations demonstrate the strong antiviral ability of koumiss (26, 34).

Drinking koumiss which contains L. casei Zhang can promote the transcription of toll-like receptor 3 (TLR3) for double-stranded ribonucleic acid (dsRNA) in the pattern recognition receptor family. This is located on the surface of epithelial cells. Consequently, TLR3 upregulates the expression of proinflammatory factors by recognizing ligands and activates the nuclear factor kappa B (NF-κB) signaling pathway, thereby effectively promoting immune responses (26, 35). Additionally, the transcription of the immune receptor TLR9 by L. casei Zhang present in koumiss can activate plasmacytoid dendritic cells (pDCs) and B lymphocytes, thus enhancing immunological effects when the human body is stimulated by harmful bacteria carrying common unmethylated CpG motifs in their genomes (26, 36, 37).

Consuming koumiss probiotics has been shown to increase the expression of peroxisome proliferator-activated receptor α/β/δ (PPAR-α/β/δ) in mice kidney proximal tubule cells, thereby enhancing immune responses (38, 39). Among them, fatty acids β-oxidation (FAO) in renal proximal tubules is promoted by high-expression of PPAR-α in acute kidney injury (AKI) disease induced by cisplatin (CP) or ischemia/reperfusion (I/R). Thus, the lipoperoxides, including 4-hydroxy-2-hexenal, would be reduced by probiotics in koumiss, which help alleviate or hinder oxidative stress-mediated AKI. This also prevents cell apoptosis and proximal tubular cell death from occurring (40–42). Therefore, probiotics in koumiss can prevent kidney damage and protect kidney function (43, 44).

Saccharomyces cerevisiae isolated from koumiss can secrete antibacterial compounds, the main ingredients of which include citric acid, ascorbic acid, lactic acid, malic acid, and killer toxins (45, 46). These antibacterial compounds can effectively increase the concentration of immunoglobulin A (IgA), which can rapidly initiate the humoral immune response, and increase the number of beneficial bacteria while reducing the number of harmful bacteria or viruses in the gastrointestinal tract, together strengthening the resistance to various diseases (47–49). These antibacterial compounds in koumiss can also increase the concentrations of CD3+ and mature T lymphocytes in peripheral lymphoid organs to enhance the immune function of lymphoid organs or can decrease the concentrations of CD8+ in T lymphocytes to regulate the ratio of CD4+/CD8+ (50, 51). Humoral immunity is increased by antibacterial compounds and would be increased when infected by Escherichia coli O8 (51, 52). The antibacterial compounds can increase the numbers of Bifidobacterium and inhibit extraintestinal pathogenic E. coli in the cecum. Additionally, they can regulate the pH of the intestine and inhibit harmful bacteria by producing acetic acid. This helps maintain a normal microflora structure in the cecum, promoting physical health and preventing various diseases (51). Therefore, koumiss consumption can effectively strengthen the body’s immunocompetence and anti-inflammatory responses.

The probiotics found in koumiss have been shown to strengthen the immune system, suppress tumor growth, and prevent the accumulation of carcinogenic compounds. Lactic acid bacteria is considered the dominant probiotic involved in the process of koumiss fermentation (53, 54). Additionally, it has been found that lactic acid bacteria isolated from koumiss collected in Xinjiang, China effectively reduced the number of colon cancer HT-29 cells. When Limosilactobacillus reuteri (Lactobacillus reuteri) BCRC14625 (1.0 × 10⁹ CFU/mL) was cultured together with HT-29 cells, there was a significant increase in lactate dehydrogenase (LDH) activity (p < 0.05), resulting in damage to the cell membrane. In contrast, L. reuteri BCRC14625 has been observed inducing HT-29 cancer cells to secrete nitric oxide (NO), leading to apoptosis of the cells. Therefore, koumiss may be both a potential preventive and therapeutic agent for treating colon cancer (55–57).

Long-term consumption of koumiss had been shown to reduce Bacteroides uniformis concentrations, which are responsible for degrading isoflavones in vivo (p = 0.016). This results in a greater concentration of isoflavones that can bind to estrogen receptors (ERα and ERβ) as estrogen-like compounds in patients. Therefore, the level of estrogen in the body can be regulated to prevent hormone-induced breast cancer and other cancers (Figure 2) (58–60). Drinking koumiss can potentially serve as an early predictor of gastric cancer, while also relieving the symptoms of chronic atrophic gastritis. Furthermore, consuming koumiss can significantly reduce blood platelet counts, which in turn, may prevent inflammation and cancer (60–63).

Interferon γ (IFN- γ) levels can be increased greatly in vivo by L. casei Zhang isolated from koumiss (28). IFN-γ plays an important role in the antiproliferation of ovarian cancer, rectal cancer, and hepatocellular carcinoma. High doses of IFN-γ have been shown to promote the transcription and synthesis of protease caspase 3 and caspase 7. These activate the apoptosis process of cancer cells by initiating Janus kinase-signal transducers and activators of the transcription 1-caspase (JAK-STAT1-caspase) signal (32, 64, 65). Research has shown that IFN-γ can downregulate the expression of vascular endothelial growth factor A (VEGFA), which in turn disturbs the proliferation and survival of endothelial cells (66). This leads to blocking angiogenesis in the tumor microenvironment, ultimately preventing tumor growth. IFN-γ can also block the interleukin 8-chemokine receptor CXCR2 (CXCL8-CXCR2) axis, which prevents the timely transportation of CXCR2+ CD68+ immunosuppressive macrophages to the tumor microenvironment (TME). This enhances the therapeutic effects of programmed cell death protein 1 (PD-1) blockade therapy for pancreatic cancer (67).

Consuming koumiss can also significantly reduce the levels of lithocholic acid and bile acid in feces, which can help prevent intrahepatic cholestasis and reduce liver cancer incidence (68–71). These findings demonstrate that the probiotics found in koumiss should be effective in reducing the incidence rates of tumors and cancers.

The results of high throughput sequencing have shown that lactic acid bacteria is the dominant flora present in koumiss. Through assimilation of lactic acid bacteria, cholesterol in the cell membrane can be bound to the phospholipid tail, upper-phospholipid, and polar head regions, and thereby be converted to coprostanol and reduce cholesterol levels in vivo (60, 72–74). The binding of lactic acid bacteria with bile acids can inhibit its absorption by the small intestine and facilitate its excretion in feces. Endogenous cholesterol is then sequentially converted into bile acid, ultimately leading to a reduction in cholesterol levels (74, 75). Additionally, the contents of short-chain fatty acids (SCFAs) such as propionate and butyrate in the intestine increases in the presence of three lactic acid bacteria strains (Lactobacillus helveticus MG9-2, Lactiplantibacillus plantarum LIP-1, and Limosilactobacillus fermentum E7301) isolated from koumiss. The activity of pyruvate dehydrogenase in the liver is effectively restrained thereby leading to the inhibition of fatty acid synthesis and ultimately resulting in a reduction in the concentration of cholesterol in serum and the liver (74, 76). lactic acid bacteria in koumiss can also effectively increase high-density lipoprotein in the blood, which can reduce or block the flow of cholesterol into the liver (77, 78).

Lacticaseibacillus casei in koumiss collected from Xinjiang has been shown to secrete bile salt hydrolase, which catalyzes the deconjugation of bile salts in enterohepatic circulation (55). This leads to the release of glycine or taurine groups from the steroid’s core molecular structure, converting conjugated bile acid into free bile acid that is excreted in feces. As a result, the bile acid content in the body is reduced (Figure 3) (79, 80). Additionally, the expression of the farnesoid x receptor is significantly downregulated, while that of cholesterol 7-alpha hydroxylase (CYP7A1) is upregulated, both of which can accelerate the conversion of bile acid from endogenous cholesterol in the liver. This ultimately helps maintain normal metabolic activities (Figure 3) (81–83).

A total of seven metabolites naturally found in koumiss have been identified in the feces of patients with hyperlipidemia who had been drinking koumiss for an extended period. These metabolites include stearic acid, sphingosine, tyrosine, α-tocotrienol, γ-tocotrienol, butyric acid, and butyrate (84). Stearic acid, a saturated fatty acid, can increase the oxidation rate of low-density lipoprotein (LDL) cholesterol in the blood after being converted to oleic acid (85). Sphingosine is bound to cholesterol to form a complex that can limit intestinal cholesterol absorption. Sphingosine can also attenuate the affinity of transporter Niemann Pick C1 like 1 (NPC1L1) toward cholesterol (86). Additionally, tyrosine can be converted to adrenaline, which further promotes the process of lipid hydrolysis in vivo (87), and α-tocotrienol and γ-tocotrienol downregulate the expression of HMG CoA reductase to inhibit the synthesis of endogenous cholesterol and linoleic acid promotes β-oxidation, thus reducing the synthesis of endogenous triglycerides (TGs). Butyrate accelerates the hydrolysis of fatty acids, leading to a reduction in the concentration of TGs in the blood, and can also block the transportation of Very Low-Density Lipoprotein Cholesterol (VLDL-C) from the liver to the blood (84). Therefore, consuming koumiss can help reduce cholesterol through multiple processes.

Koumiss is a functional drink that can be used to treat chronic gastritis. Studies have shown that when mice consumed koumiss, their intestinal numbers of Eubacterium rectale and Faecalibacterium prausnitzii significantly increased (60). These two intestinal microorganisms secrete butyric acid, which can regulate intestinal flora disorders and effectively promote gut content isolation and hypodermis among intestinal epithelial cells, thus stabilizing colon cells. These actions can inhibit the growth of cancer cells and reduce inflammation, making koumiss an effective treatment for chronic gastritis caused by Prevotella copri (enterotype 2) in the gut (Figure 4) (60, 88).

Koumiss had also been shown to be effective in treating gastrointestinal inflammation caused by Salmonella typhimurium ATCC 14028 (89). Consuming koumiss can boost the relative abundance of Akkermania muciniphila and Lachnospiraceae in the intestine. Autonomic control of systemic glucose metabolism and energy balance of A. muciniphila can inhibit the growth and reproduction of Toxoplasma gondii in the intestinal tract. SCFAs produced by Lachnospiraceae not only stimulated the proliferation of colon epithelial cells and reduce colonic transit time but also provide nutrients for the colon mucosa, preventing mucosal atrophy. By maintaining primal gastrointestinal morphology and normal physiological functioning, koumiss provides an effective treatment option (90–92).

A large number of Lacticaseibacillus paracasei and abundant paracaseins are also found in koumiss (Figure 5). Some substances, such as short-chain or long-chain fatty acids produced by L. paracasei can effectively inhibit the reproduction of pathogenic microorganisms (Figure 5). Paracaseins can induce endogenous mucin secretion, promote the expression of tight junction proteins, and inhibit the transmission of the NF-kB MLCK signal pathways (Figure 5). As a result, the intestinal damage caused by E. coli can be reduced and the intestinal mucosal barrier can be strengthened to effectively treat diarrhea (Figure 5) (93).

Consumption of koumiss can increase the numbers of Vibrio desulfovibrio in mouse guts and generate more H₂S. This can lead to the disordered oxidation pathway of butyric acid in cells, resulting in DNA chain breakage, inhibition of ATP-dependent K⁺ channels, and the release of cytochrome C which can help reduce both pulmonary and extrapulmonary tuberculosis in hosts (94, 95). It has also been reported that drinking koumiss can prevent tuberculosis caused by the pathogen Mycobacterium tuberculosis H37RV (89).

Consuming koumiss has been shown to effectively repair glomerular damage and prevent chronic liver and kidney diseases by increasing platelet-derived growth factor c (PDGF-c) and platelet-derived growth factor receptor alpha (PDGFR-α) expression (38, 96). Additionally, L. casei Zhang in koumiss downregulates the expression of toll-like receptors 4 (TLR4) in the liver, promotes the production of TNF-α and oxidative stress responses, and effectively protects the liver (97). Furthermore, koumiss contains angiotensin-converting enzyme inhibitors, which can reduce Angiotensin II (Ang II) generation and reduce hypertension (98–100).