Obesity is a major public health problem in both developed and developing countries. The number of overweight and obese adults is at a record high, with 39–40% of adults being overweight or obese, and the incidence of these conditions is increasing in adolescents (Ruiz et al., 2019; Thompson et al., 2022). It is well known that being overweight or obese harms people's health, and consequently leads to numerous diseases, such as diabetes and cardiovascular diseases (Guo et al., 2021). Despite a global focus on reducing obesity, many people are still overweight or obese. There are many strategies for obesity treatment, including behavior therapy, dietary changes, physical activity, pharmacotherapies, and bariatric surgery etc. (Cerdo et al., 2019). However, bariatric surgery is not available or suitable for the majority of people who are suffering from overweight or obesity, and for some individuals the pharmacotherapies are costly and may have adverse effects (Golden, 2017; Nguyen and Varela, 2017; Bluher, 2019). Nevertheless, a number of strategies are recommended to reduce the increasing prevalence of obesity, which included regular physical activity, and the changes in dietary and lifestyle habits (Manna and Jain, 2015). Evidence is increasing from studies that plant-based dietary patterns, which are rich in fruits, vegetables, and whole grains, are valuable in preventing various chronic diseases, including obesity (Medina-Remon et al., 2018).

Banana (including plantains) is cultivated in tropical, subtropical and contains several bioactive compounds, such as phenolics, carotenoids, biogenic amines and phytosterols (Anyasi et al., 2013). Utilization of banana as an ingredient in different food products exerts a beneficial effect on health (Singh et al., 2016). There are high levels of resistant starch in unripe bananas, and the dry fruit contains more than 50% resistant starch content. Studies have shown that unripe banana flour helps to reduce the risk of non-communicable diseases, such as obesity, hyperglycaemia, and hyperlipidaemia (Hoffmann Sardá et al., 2016). Previous studies have shown that banana flour (rich in resistant starch) and banana resistant starch were effective in alleviating obesity (Tan and Wang, 2018; Fu et al., 2021). However, the anti-obesity properties vary depending on the banana cultivated groups. Two cultivated groups namely Cavendish (Musa AAA) and Dajiao (Musa ABB), were previously reported to have anti-obesity properties (Tan, 2019).

Plantain (Musa spp. AAB), is a special type of cooking/starchy banana, which is widely growing in African and Latin American countries. Plantain flour is an alternative source of indigestible carbohydrates (Garcia-Valle et al., 2019); in addition, it is also recognized as the source with the highest resistant starch (RS) content (Almanza-benitez et al., 2015). It has been reported that plantain flour can be blended with common flours, such as rice, wheat, and maize, to prepare bread (Juarez-Garcia et al., 2006), gluten-free pasta (Patiño-Rodríguez et al., 2018; Patino-Rodriguez et al., 2019), and cookies (García-Solís et al., 2018; Sanchez-Rivera et al., 2019). In addition to being an energy source in many people's daily diet, plantain flour can also improve gastrointestinal disease symptoms, lower blood sugar, and alleviate obesity and diabetes (Falcomer et al., 2019). Studies have shown that gluten-free snacks with high dietary fiber content and low predicted glycemic index can be developed using the unripe plantain flours as the main ingredient, and these gluten-free snacks may also be used as an alternative to reduce excess weight and obesity problems in the general population and celiac community (Flores-Silva et al., 2015). In addition, amelioration of acute pancreatitis by unripe plantain could play a key role in its management of diabetes and related complications, which indicates the potential of unripe plantain in the management of renal and liver complications arising from diabetes mellitus (Eleazu and Okafor, 2015). Therefore, plantain has considerable potential for use in banana food development.

Detailed data on the anti-obesity properties of unripe plantain flour (UPF), including its influence on body weight, food intake, and biochemical indexes, have been previously obtained (Han et al., 2021) (Supplementary Table 1). Here, the effects of UPF on gut microorganisms and a correlation analysis along with transcriptomic data from the liver fat metabolic pathway are reported. Results from this research on the anti-obesity properties of UPF and its mechanisms of action provide fundamental data on a natural food associated with weight loss and will benefit health food development.

Obesity is a metabolic disease that manifests as an increase in weight due to the accumulation of body fat (Nguyen and El-Serag, 2010). UPF contains high levels of resistant starch. In addition, our former studies have shown that resistant starch isolated from bananas can help to alleviate obesity (Fu et al., 2021). Therefore, UPF can be used a health food material with anti-obesity properties. The effects of UPF on the intestinal flora and liver transcriptome were examined here to explore the mechanisms underlying the anti-obesity properties of UPF.

Here, a UPF-supplemented diet was shown to improve dyslipidemia and repair the liver injury induced by an HFD (Han et al., 2021) (Supplementary Table 1). This could possibly be attributed to the effect of the resistant starch and dietary fiber found in UPF on lipid metabolism, studies have shown that dietary RS can reduce serum total cholesterol and triacylglycerol concentrations and fat accretion, and the increasing fiber intake can lower blood pressure and serum cholesterol levels (de Deckere et al., 1993; Nugent, 2005; Anderson et al., 2009). Obesity increases the free fatty acid levels in blood plasma and inhibits glucose transport, leading to insulin resistance (Felber and Gola, 2002). Insulin resistance is a condition in which cells become resistant to the effects of insulin, with disruption in glucose absorption, metabolism, and storage (Kahn and Flier, 2000). Hence, excess insulin is secreted, which causes hyperinsulinemia. In this research, the insulin level of the OC group was significantly higher in the OC group than the NC group. Insulin levels decreased after UPF intervention, indicating that insulin sensitivity was increased while insulin resistance was reduced.

The gut microbiota produces a large number of enzymes that hydrolyze various complex polysaccharides. In particular, resistant starch is fermented by gut microbiota in the large intestine, which produces short-chain fatty acids (SCFAs) and provides nutrients to the gut microbiota (Cherbuy et al., 2019). Obesity causes disorder in the intestinal flora and further impacts body metabolism. Here, the UPF intervention led to structural changes in the gut microbial community and increased the cluster diversity as indicated by the increase in the Chao1 and ACE index values (p < 0.05).

At the phylum level, the relative abundance of Proteobacteria increased with UPF feeding. At the genus level, the relative abundances of Blautia and Parasutterella were high in the OC group. The relative abundance of Allobaculum increased in the LD group (p < 0.05), whereas those of Romboutsia (p < 0.01) and Staphylococcus (p < 0.05) increased in the MD group. Interestingly, Fusicatenibacter was not detected in the NC group but was enriched in the OC group. The abundance of Fusicatenibacter decreased (p < 0.01) after the UPF diet intervention. Bacteroides was the most abundant probiotic in the HD group (5.66%).

Studies have shown that Blautia is positively related to weight, including waist circumference, body mass index, and blood fat levels, which includes measures of low-density lipoprotein (LDL), total triglyceride (TG), and total cholesterol (TC) (Zeng et al., 2019). The increase in the relative abundance of Parasutterella could be a potential indicator of malnutrition and metabolic abnormality associated with obesity (Lee et al., 2020). Allobaculum species are involved in SCFA production. Plantain feeding led to the enrichment of SCFA-producing microorganisms (Zhang, X. et al., 2015). Romboutsia is negatively related to body weight, fasting blood glucose, and insulin (Liu et al., 2018). It uses glucose to produce acetic acid and iso-butyric acid, which is beneficial for fat loss. In one study, Staphylococcus was shown to have a high relative abundance in thin adults in a comparison of its abundance in the intestinal flora of fat and thin adults, indicating that the enrichment of Staphylococcus may have anti-obesity properties (Vasana et al., 2018).

Fusicatenibacter showed a positive correlation with lipid metabolic indexes (TC and TG) (Huang et al., 2020). The UPF intervention significantly reduced the abundance of Fusicatenibacter, indicating that UPF had a beneficial influence on lipid metabolism. Similar to previous investigations (Amabebe et al., 2020), the gut microbiota enriched by an HFD were predominated by Firmicutes (Clostridium), Prevotella, and Methanobrevibacter, while probiotics such Bacteroides, Bifidobacterium, Lactobacillus, and Akkermansia were absent.

The relative abundance of Bacteroides in the HD group was increased, indicating that UPF had the capacity to increase probiotic proliferation. During the microbial fermentation of RS in the colon, lactic acid, acetate, propionate, and butyrate are generated (Bojarczuk et al., 2022). Data suggest that propionic acid and acetic acid can lower the pH in the large intestine. A low pH in the intestinal environment promotes the proliferation of the beneficial bacteria that make up the intestinal microbiota as well as protect it from the growth of pathogenic microorganisms (Shen et al., 2017). Moreover, RS can be used as a substrate for the growth of probiotic microorganisms because it almost completely passes through the small intestine (Sajilata et al., 2006). In summary, UPF can reduce the abundances of Blautia, Parasutterella, and Fusicatenibacter while increasing the abundance of beneficial bacteria such as Allobaculum, Romboutsia, Staphylococcus, and Bacteroides.

Spearman correlation analysis revealed the relationships between the intestinal flora and metabolic indexes. The mechanism underlying the anti-obesity properties of UPF is likely as follows: the depression of blood sugar levels may result from the decreased relative abundance of Parasutterella, a decrease in the abundances of Blautia and Fusicatenibacter may cause insulin abatement, and an increase in the abundance of Bacteroides may lead to adiponectin augmentation. Through the regulation of gut microorganisms, UPF alleviates the disorder of sugar and fat metabolism associated with obesity. Moreover, it improves hormone secretion in the host.

Pparaa and Cpt1a expression was higher in the UPF intervention groups than that in the OC group. PPARα plays a key role in fatty acid metabolism (Ricote and Glass, 2007). Mitochondrial β oxidation, microparticle ω oxidation, and peroxidase β oxidation are the three pathways of fatty acid metabolism. PPARα participates in the metabolic regulation all these pathways (Pawlak et al., 2015). Mitochondria are the main sites of fatty acid oxidation in animal cells. When fatty acids enter mitochondria, a transport medium is needed; this role is played by carnitine palmitoyl transferase (CPT1), of which CPT1A is an hypotype (McGarry and Brown, 1997). Fatty acids are restricted from entering the mitochondria if CPT1A activity is low. If this occurs, the fatty acids are converted to free fatty acids (FFAs). However, excessive FFA levels lead to insulin resistance and the accumulation of triglycerides in the liver (Levin et al., 2007). The possible regulatory mechanisms of UPF include the increased expression of the Pparaa and Cpt1a genes, which could accelerate fatty acid oxidation and decrease the triglyceride content.

AMPK, described as a “cellular energy receptor”, is a key enzyme (Woods et al., 2003). Activated AMPK can upregulate the expression of downstream PGC1α (Woods et al., 2003). In the glucose metabolic process, PGC1α can promote the expression of glucose transporter 4 (GLUT4) and subsequently increase the ability of the body to utilize glucose (Jäger et al., 2007; Chopra et al., 2012). In this study (Table 5), the expression of the AMPK gene (Prkaa1, Prkab1, and Prkaa2) was downregulated in the OC group compared with the NC group. UPF supplementation contributed to an increase in the expression of AMPK (Prkaa1, Prkab1, and Prkaa2) and PGC1α (Ppargc1a), thereby reducing glucose absorption. These effects assisted in the abatement of blood sugar and the loss of body fat and weight.

We analyzed the mechanisms underlying the anti-obesity properties of UPF. The mechanism of regulation lipid metabolism by UPF (rich in RS) was likely related to the RS fermentation in the large intestine and the production of SCFAs. SCFAs turned out to promote the release of the L-cells to secret the putative satiety hormones such as intestinal peptide YY (PYY), glucagon-like peptide-1 (GLP-1) hormones, suggesting that RS might augment satiety (Zhang, L. et al., 2015). On the other hand, RS in the diet may assist in the management of body weight via its potential effects on delaying the delivery of glucose as fuel with subsequent fat utilization, which may increase fat oxidation to promote fat mobilization (Birt et al., 2013). However, it should be noted that the UPF was a mixture; thus, elucidating the specific substance responsible for the observed effects is a matter worthy of further exploration.

In this study, the UPF intervention caused changes in the hormone levels by decreasing insulin levels, and increased the diversity of gut microbes and modulate the gut microbiota composition (such as the improvement in the levels of Allobaculum, Romboutsia, Staphylococcus, and Bacteroides, as well as the decrement in the levels of Fusicatenibacter). Importantly, UPF supplementation promoted glucolipid metabolism through PPAR signaling pathway (upregulation of Pparaa and Cpt1a expression) and AMPK signaling pathway (upregulation of Prkaa1, Prkab1, Prkaa2, and Ppargc1a) mechanism, improved glycemic control, and decreased insulin resistance, suggesting that the anti-obesity properties of UPF might at least partially be mediated by these genes and pathways. Hence, UPF could be a potential food ingredient for the prevention of obesity-induced by a high-fat diet and the consumption of UPF could potentially aid in improving metabolic health.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found below: https://www.ncbi.nlm.nih.gov/-SRR21422979-SRR21423008 and https://www.ncbi.nlm.nih.gov/geo/-GSM6574159-GSM6574176.

The animal study was reviewed and approved by the Experimental Animal Ethics Review Committee of South China Agricultural University (SCXK 2019-0048). Ethical care and use of the laboratory animals were ensured according to the Guidelines for Animal Experimentation in Animal Research Laboratories.

Funding acquisition: JW and OS. Methodology: JF, JX, and ST. Project administration: GY, JW, and OS. Writing—original draft: JF and JX. Writing—review and editing: JF, QS, and OS. All authors contributed to the article and approved the submitted version.

This project was financially supported by National Key R&D Program of China (2019YFD1000203), Science Foundation of Guangdong Province (2018A030313026), GDAAS (R2020PY-JX002), the earmarked fund for CARS (CARS-31-01), Laboratory of Lingnan Modern Agriculture Project (NT2021004), Maoming Branch Grant (No. 2021TDQD003), and the Fundamental Research Funds for the Central Universities (2022ZYGXZR102).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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