Introduction

Fermented foods are part of the diverse food cultures found in various nations and areas around the world [1]. Fermentation has been used for centuries to extend the shelf life of food and has been linked to several health benefits [2,3,4]. They have received a lot of attention for their natural, nutritive, and functional qualities that support health [5]. In Korea, kimchi is an ethnic food, consumed daily with every meal [6], and an adult consumes about 50–200 g of kimchi per day on average [7].

The vegetables most frequently used to make kimchi are baechu cabbages (Brassica rapa) and radishes (Raphanus raphanistrum); however, other vegetables including cucumbers, spring onions, and other plants are also widely used, resulting in hundreds of different kimchi being consumed in Korea [6, 8]. The fermentation of kimchi involves numerous microorganisms, especially lactic acid bacteria (LAB), and the microbial composition of kimchi differs based on the type and amount of ingredients being used in the making of kimchi. Among many microorganisms associated with the fermentation of kimchi, LAB are one of the predominant species with probiotic properties [7, 9]. LAB that are commonly present and representative in kimchi include species of genera Lactobacillus, Leuconostoc, and Weissella [8, 10].

Many studies have reported the beneficial effects of kimchi consumption. Kimchi has been found to exhibit anti-inflammatory properties [11, 12], ameliorate cancer cachexia [13], induce apoptosis and prevent colon cancer [14,15,16], prevent atherosclerosis [17] and hepatic damage caused by high cholesterol [18], improve general metabolic parameters [19, 20], fasting blood glucose and cholesterol [21], improve cognitive impairments [22], enhance immunity [23] and protect against atopic dermatitis [24]. With many beneficial effects, kimchi has been considered as a type of medicinal food [25]. Therefore, the purpose of this review was to systematically evaluate all randomized controlled studies related to kimchi and to evaluate the effectiveness of kimchi for health in general.

Methods

This review was registered on PROSPERO: International prospective register of systematic reviews (CRD42018087375). The review was performed and reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR).

Search strategy

Three English and four Korean databases were searched for data retrieval. The databases searched in English were PubMed/Medline, Embase, and Cochrane Library; the databases searched in Korean were DBpia, Korean Studies Information Service System (KISS), ScienceOn (formerly, National Digital Science Library (NDSL)), and Oriental Medicine Advanced Searching Integrated System (OASIS). The search was done from their inception until November 7, 2022. The search strategy included various terms of different kimchi types, fermented cabbage, and fermented vegetable.

Inclusion criteria and outcomes

Only randomized controlled trials (RCTs) were included as they are regarded as the reference standard to scientifically and rigorously test the hypothesis on the effectiveness of interventions [26]. Other types of clinical studies such as cohort, cross-sectional, case reports or retrospective clinical studies were excluded. As this review did not focus on any specific population group, any participants with or without health conditions were eligible for inclusion. All RCTs that used kimchi or kimchi-derived probiotics as an intervention regardless of their comparators were included. Outcome measures for this review were any health-related outcomes, and additional outcomes were quality of life and adverse events.

Study selection and data extraction

Two review authors (ES and LA) independently performed the screening and selection of the searched records. All titles and abstracts were screened for eligibility. Full-text retrieval of included studies, data extraction, and data tabulation were also performed independently by two review authors. From the included studies, information regarding study design, participants, disease type, interventions, and outcomes were extracted. Any discrepancies were discussed with the third author (MSL).

Results

Literature search

Searches from seven electronic databases and two trial registries identified 15,085 records, resulting in 8,551 records to be screened for inclusion after the removal of duplicates. The titles and abstracts were screened based on the inclusion criteria. The full-text of 111 records was then retrieved for further assessment, of which 100 records were excluded. A total of 11 studies were included in the scoping review (Fig. 1). The characteristics of the included studies are tabulated in Table 1.

Fig. 1
figure 1

PRISMA flow diagram illustrating the screening process

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Table 1 Summary of randomized controlled trials investigating the health benefits of kimchi
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Study characteristics

Eleven randomized controlled trials (RCTs) were included in this review, with 638 participants enrolled in total and 608 participants completing the trials. Most of the trials were conducted in Korea with the trial duration ranging from 7 days to 12 weeks. Five trials [21, 27,28,29,30] studied on healthy subjects, two trials [31, 32] studied on obese subjects, one trial [33] studied on both healthy and obese subjects, one trial [34] studied on cancer patients, one trial [35] studied on prediabetic subjects, and one trial [36] studied on irritable bowel syndrome patients. In terms of intervention, five trials [21, 28, 29, 31, 36] used kimchi consumption, two trials [27, 33] used kimchi supplement, and four trials [30, 32, 34, 35] used kimchi-derived probiotics. In terms of control, seven [27, 30, 32,33,34,35] used placebo, four [21, 29, 31, 36] used other types of kimchi, and one [28] used non-kimchi diet (Table 1).

Outcome assessment

Kimchi on healthy subjects

In a clinical trial by Choi et al. [21], high amount consumption of kimchi compared to low amount consumption of kimchi improved serum lipid profiles and fasting glucose levels. Concentrations of fasting blood glucose (FBG), total glucose, total cholesterol (TC) and low-density lipoprotein (LDL)-C decreased in both groups. FBG was reduced in the high kimchi consumption group from 80.7 ± 5.4 mg/dL to 75.1 ± 6.0 mg/dL (p < 0.001), which showed a significant decrease compared to the low-intake group (p = 0.003).

Another trial by Kim and Park [29] investigated the effects of kimchi consumption (standard kimchi and functional kimchi) on healthy subjects and found improvements in dietary fiber intake. The functional kimchi group showed significant improvements in body fat percentage, skeletal muscle mass, TC, triglycerides (TG), LDL-C, adiponectin, and interleukin (IL)-6 (p < 0.05), and a significant improvement in HDL-C (p < 0.01), while the standard kimchi group showed significant improvements in LDL-C and adiponectin only (p < 0.05). The trial also revealed an increase in beneficial bacteria such as Faecalibacterium and Bifidobacterium and a decrease in harmful bacteria such as Clostridium and Escherichia coli).

A trial by Han [30] investigated effects of kimchi-derived probiotics (Lactobacillus plantarum) on skin health of young healthy subjects in comparison with placebo. Skin pH was decreased significantly in the kimchi group from 5.18 ± 0.07 to 4.80 ± 0.06 at 12 weeks and showed a significant change compared to the control group (p = 0.025). The epidermal level of lactate (percent change) in the kimchi group was increased by 25.56 ± 13.65% while the control group showed 9.76 ± 9.70% decrease (p < 0.05). Epidermal levels of FAA were not changed in both groups, but those of FFA were lower in the kimchi group (p = 0.029).

In a trial involving healthy subjects and kimchi supplement by Choi et al. [27], kimchi supplement in comparison with placebo (rice powder pills) showed beneficial effects on controlling serum lipid profiles. The plasma TG concentration decreased from 115.2 ± 57.7 to 97.3 ± 52.3 mg/dL in the kimchi supplement group and the group’s average change in TG was 16.8% (p < 0.05), while plasma TG concentration increased from 98.5 ± 34.9 to 105.7 ± 32.9 mg/dL in the control group with the group’s average change in TG being 9.8% increase.

In another kimchi supplement trial by Song et al. [33], effects of kimchi supplement were investigated in comparison with placebo or no intervention. Kimchi was found to reduced body mass index (BMI), body fat percentage, and systolic blood pressure. TG was decreased in the kimchi supplement group by − 15.8 ± 10.7%, while TG was increased in the control group by 9.8 ± 15.8%. LDL/LDL-C was also decreased in the kimchi supplement group by − 6.7 ± 17.1%. Body weight, BMI, body fat mass, and body fat percentage decreased in all intervention groups compared to the control group.

However, in a trial by Lee et al. [28], the kimchi diet group in comparison with the non-kimchi diet did not show positive immunomodulatory effects. Between the two groups, lymphocyte subsets, pro-inflammatory cytokines, anti-inflammatory cytokines, and immunoglobulins did not show significant improvements.

Kimchi on obese subjects

In a trial by Han et al. [31], effects of fermented kimchi were compared with fresh kimchi. Waist circumferences and body fat percentage showed a significant decrease in the fresh kimchi group (p < 0.05), while HDL-C showed a significant improvement in the fermented kimchi group (p < 0.05). However, no significant changes were seen in clinical parameters between both groups. Fermented kimchi was found to influence metabolic pathways and immunity.

In a trial by Lim et al. [37], kimchi-derived probiotics (Lactobacillus sakei) in comparison with placebo were investigated. The trial found to reduce body fat mass by 0.2 kg in the kimchi group, while it increased by 0.6 kg in the placebo group. Waist circumference was significantly reduced in the kimchi group than in the placebo group (p = 0.013). Adverse events, including gastrointestinal discomfort, were mild.

Others

In a trial by Yoon et al. [34] effects of kimchi on bowel function and quality of life in rectal cancer patients were investigated, and found no significant effect of kimchi-derived probiotics (Lactobacillus plantarum) in comparison with the placebo.

In a trial involving prediabetic subjects, Oh et al. [35] studied effects of kimchi-derived probiotics (Lactobacillus plantarum) compared with placebo. The 2h-PPG (p = 0.045) and HbA1c (p = 0.013) levels in the kimchi group were significantly reduced. No serious adverse effects were reported.

In a trial by Kim et al. [36], effects of kimchi on irritable bowel syndrome were compared by using kimchi with different properties: standard kimchi, Lactobacillus plantarum (nF1) added standard kimchi, and functional kimchi (Viscum album (mistletoe) extract added with Lactobacillus plantarum). For IBS symptoms, all three types of kimchi groups had significant improvements in abdominal pain or inconvenience (p < 0.001), desperation (p < 0.001), incomplete evacuation (p < 0.001), and bloating (p < 0.001). However, there were no significant differences between the kimchi groups for improvement of overall IBS symptoms. For serum inflammatory cytokine levels, all three kimchi groups had significant improvements in tumor necrosis factor (TNF)-α (p < 0.001). For other inflammatory factors, nF1 kimchi group and functional kimchi group showed significant improvements in IL-4 (p < 0.001), IL-10 (p < 0.001), IL-12 (p < 0.01).

Discussion

Kimchi is a traditional Korean fermented vegetable dish that is stored and preserved in a special way. There are various pickled vegetable foods all over the world, but kimchi differs from other salted vegetables as they are first salted and then seasoned and fermented secondarily [38]. Korean kimchi was selected as the world’s top five healthiest foods along with Spanish olive oil, Japanese bean products, Greek yogurt, and Indian lentils. The reason for the selection of kimchi as a super food was based on the fact that it is rich in lactic acid bacteria, fiber, and various minerals and vitamins, which are beneficial for health and for cancer prevention [39].

Currently, various studies report the effectiveness of kimchi in improving overall health. A controlled clinical trial, [40] which was excluded from this review because it was not randomized controlled trial, involved 12 young female adults, 6 participants in each group. The trial interventions were low consumption of kimchi (15 g/day) and high consumption of kimchi (150 g/day) for 7 days, and reported the decrease in potentially harmful microorganism (such as Listeria and Clostridium, Enterobacter, Prevotella, and Shigella) percentage in the high-consumption group. There were 34 species of intestinal microorganisms whose percentage changes between the two groups were significantly different (p < 0.05). Thus, kimchi consumption was found to influence the formation of intestinal microbiota. Also, the functionality of kimchi as a probiotic is expected to improve with the increase in the percentage of kimchi LAB in the intestine.

One crossover study [19] excluded from this review investigated the effects of fermented kimchi on body weight and metabolic parameters in 22 overweight and obese patients in 2 sets of 4-week interventions with 2-week washout period. The clinical study found that the fermented kimchi group showed significant improvements in the waist-hip ratio, fasting blood glucose, total cholesterol, body fat percentage, systolic blood pressure, and diastolic blood pressure (p < 0.05) in the fermented kimchi group compared to those in the fresh kimchi group. Even though fresh kimchi also showed significant improvement from initial value to final value in terms of body weight, BMI, body fat percentage, TG, E-selectin, and adiponectin (p < 0.05), fermentation of kimchi was found to provide more positive effects.

Similarly, another crossover study [20] of 2 sets of 8-week interventions with 4-week washout period investigated the effects of fresh and fermented kimchi in 21 participants with prediabetes. The parameters associated with prediabetes such as hemoglobin A1C (HbA1c), fasting insulin, insulin resistance, and Matsuda index (whole-body insulin sensitivity index) showed significant improvements within the group (p < 0.05) in both the fresh kimchi group and the fermented kimchi group. For quantitative insulin sensitivity check index and disposition index, only the fermented kimchi group showed improvements compared to before intervention (p < 0.05). Overall, the fermented kimchi group showed better effects on insulin resistance and insulin sensitivity than the fresh kimchi group.

In terms of anticancer and cancer prevention effects reported by previous studies, kimchi was found to have inhibitory effects of cancer cell growth for gastric cancer (AGS cell and KATOIII), lung cancer (A549 cell), colon cancer (HT-29 cell and HCT-116 cell), breast cancer (MCF-7 cell), liver cancer (HepG2 cell) and uterine cancer (Hela cell) [41]. Even though there are numerous cell and animal studies exploring anti-inflammation and anticancer effects of kimchi, clinical trial data on such effects are currently not available yet.

In this review, the included RCTs reported on lipid-lowering effects, colon health improvement, and anti-obesity effects of kimchi, although there were differences depending on the amount of kimchi consumed and the fermentation stage. Despite the differences in the types and amount of kimchi consumed, all kimchi consumption in general was found to have benefits in improving health. Other types of clinical studies have similar results which also support the effects of kimchi on body fat and serum lipid profiles [41].

For lipid-lowering effect, various animal studies support the effects of kimchi by inducing hyperlipidemia with a high-fat or high-cholesterol diet. In particular, the lipid-lowering effect of kimchi was reported by experimenting on obese rats and diabetic rats [42,43,44]. In other experiments with rabbits, the beneficial effects of kimchi were supported by demonstrating lipid inhibitory effect and changes of lipid content in various tissues [17, 45]. In a cross-sectional study, the correlation between kimchi intake and lipid indicators showed a positive correlation with HDL cholesterol and a negative correlation with LDL cholesterol [46]. This further confirms the positive effects of kimchi consumption in improving serum lipid profiles.

Despite beneficial effects of kimchi, there were concerns about consuming salted vegetables that some studies addressed kimchi’s association with hypertension. However, a cross-sectional study [47] explored kimchi’s effects on hypertension among 20,114 participants using the Korea National Health and Nutrition Examination Survey (KNHANES) data and found that consumption of kimchi was not associated with increased prevalence of hypertension (odds ratio: 0.87; 95% CI 0.70–1.08). In a community-based cohort study of 12-year follow-up [48], 5,932 participants were included and it also concluded that consuming kimchi was not linked to a higher risk of hypertension.

Kimchi is considered as probiotic food. Kimchi is made by fermentation process with many bacteria, but pathogenic and putrefactive bacteria are suppressed, leaving probiotic LAB as dominant one remaining [49]. Kimchi involves its ingredients to go through fermentation, but different LAB strains are found in different stages of kimchi fermentation and in different kimchi samples. Kimchi microorganisms vary depending on the ingredients, methods, and environment (such as acidity and temperature), and thus, its functionality may differ from kimchi to kimchi. Therefore, certain kimchi may have certain LAB strain that may work better on specific disease which can be investigated further for custom-made kimchi.

Various components and compounds found in kimchi provide health benefits. Kimchi has important nutritional and functional properties as kimchi includes vitamins, minerals, dietary fibers, probiotics, capsaicin, gingerol, chlorophyll, allyl compounds, benzyl isothiocyanate, indole compounds, thiocyanate, and beta-sitosterol [50]. Also, previous cell studies concluded that the kimchi LAB strains were sensitive to antimicrobial agents such as erythromycin, ampicillin, chloramphenicol, and benzylpenicillin [51] and that the LAB were not only safe for human consumption but also met the functional criteria [52]. Numerous studies support the beneficial effects of LAB, including anticancer effects and immune-stimulating effects [53,54,55,56,57].

There were several limitations in this review. The search for this review was restricted to English and Korean databases only, which may have resulted in several non-English and non-Korean studies to be overlooked. Also, the Korean terms of various types of kimchi are expressed in many different ways in English romanization letters, and due to lack of standardized terms, there are possibilities of missing studies.

Conclusion

Numerous clinical studies reported on the positive effects of kimchi. In particular, this review found that kimchi interventions may be safe and effective treatment option for the treatment of general health, obesity, and irritable bowel syndrome. However, questions have been raised about its health functionality due to the lack of adequate trials. In the future, research that can verify the conflicting results on the health benefits of kimchi should be conducted rigorously to provide the scientific basis for the benefits of kimchi.