Abstract

Bananas are a well-balanced typical prebiotic food rich in dietary fiber, oligosaccharides, vitamins, and minerals. However, evidence supporting the stress-relieving effects of banana consumption in daily life is scarce. Therefore, we investigated the effects of banana-intake on stress-related markers, such as the intestinal environment, biochemical markers, autonomic balance, and mood status, in an open, randomized, parallel-group controlled trial using 20 healthy adult volunteers divided into a banana-intake group (n = 13 subjects, two bananas daily for 2 weeks) and a non-intake control group (n = 7, no banana-intake for 2 weeks). We measured the intestinal environmental marker urinary indoxyl sulfate, stress markers (salivary cortisol and chromogranin A), autonomic nervous system activity markers (heart rate, natural logarithm of low-frequency power [LnLF], natural logarithm of high-frequency power [LnHF], and LnLF/LnHF ratio), and mood status before and after the 2-week experimental period. We assessed the rate of change before and after banana-intake for all parameters by performing comparisons between the banana-intake and the non-intake control group as well as between the effective and ineffective groups within the banana-intake group. There was no significant difference in the rate of change for all parameters before and after banana-intake compared with the non-intake group. However, approximately 50% of the banana-intake group showed decreased urinary indoxyl sulfate; decreased cortisol and chromogranin A levels; decreased heartbeat and LnLF power, increased LnHF power, and decreased LnLF/LnHF ratio; a decrease in the five negative subscales, an increase in the two positive subscales, and a decrease in total mood disorder score. Our findings suggest that banana-intake for 2 weeks improves the intestinal environment, leads to predominant parasympathetic activity, and provides stress relief and psychological stability in approximately 50% of healthy adults.

1. Introduction

The sophistication and diversification of modern social structures are changing our surrounding social environment. Consequently, whether we like it or not, we are exposed to various physical and psychological stresses in our daily lives. Excessive stress induces an imbalance in homeostatic functions of the autonomic nervous system, endocrine system, and immune system, which are essential for health, and causes physical and psychological symptoms, such as digestive disorders, depression, anxiety, headache, and decreased attention span ¹.

Continued mental stress may lead to tension that causes intestinal upsets, such as diarrhea, constipation, and abdominal pain. On the other hand, continued intestinal upsets cause mental stress ^{2, 3, 4}. These experiences and symptoms are due to the gut–brain axis that communicates and interacts bidirectionally between the brain and the gut ². Since intestinal flora are deeply involved in the gut–brain axis, the concept of the microbiota–gut–brain axis, which newly adds intestinal flora to the gut–brain axis, is strongly supported ^{5, 6, 7}. These interactions are associated with the etiology and pathophysiology of disorders such as inflammatory bowel disease, which is a typical intestinal disease, as well as a wide range of psychiatric and neurological disorders, such as stress, anxiety, depression, Parkinson disease, and autism spectrum disorders ^{5, 6, 7}. Furthermore, a link between the worsening of the condition of new coronavirus patients and dysbiosis, which is an abnormal or imbalanced gut microbiota, has been reported ⁸.

The concept of the microbiota–gut–brain axis suggests that mental disorders, such as stress symptoms, can be improved by correcting dysbiosis and adjusting the intestinal environment ^{5, 6, 7}. The ingestion of probiotics (intestinal bacteria that have beneficial effects or products and foods containing them) and prebiotics (compounds that induce the growth and activity of beneficial intestinal bacteria, such as dietary fiber and oligosaccharides) has been actively attempted to improve dysbiosis. For example, daily intake of the probiotics Lactobacillus and Bifidobacterium in humans was reported to significantly reduce the stress marker salivary cortisol and relieve psychological distress, such as depression, anxiety, anger, and hostility ^{9, 10, 11}. It was also suggested that the daily intake of prebiotics and oligosaccharides lowers salivary cortisol levels and improves anxiety and depression ¹². These findings imply that probiotic or prebiotic intake improves stress-related symptoms by regulating the gut microbiota ¹³. Furthermore, short-chain fatty acids produced from dietary fiber and oligosaccharides decomposed by intestinal bacteria have been suggested as being involved in these regulatory mechanisms ^{14, 15}.

Bananas are generally considered a well-balanced typical prebiotic food that is rich in dietary fiber, oligosaccharides, vitamins, and minerals ^{16, 17, 18, 19}. Various beneficial effects have been reported, such as improvement in intestinal bacterial flora balance, short-chain fatty acid production, body mass index reduction, lowering of blood pressure, bowel movement promotion, antioxidant effects, and improvements in mental state ^{16, 17, 18, 19, 20, 21}. However, to our knowledge, no scientific studies have investigated the effects of banana-intake on the stresses of daily life. Therefore, this study aimed to clarify the effects of banana-intake as a prebiotic on stress, which is a problem in modern society. We examined the following stress-related markers in healthy Japanese adult volunteers: the intestinal environmental marker urinary indoxyl sulfate ²²; the biochemical stress markers salivary cortisol and chromogranin A ^{3, 23}; autonomic balance markers, such as heart rate (beats per min [BPM]) and heart rate variability (HRV) power spectra ^{24, 25}; and mood states according to the Profile of Mood States Second Edition (POMS2) ^{3, 26, 27}.

2. Materials and Methods

We enrolled 20 healthy Japanese adult male and female volunteers aged 20‒60 years in our study, all of whom submitted written informed consent. The exclusion criteria were those taking medications that may affect the study outcomes, those at risk of developing research-related allergies, and those who were considered inappropriate for participation due to other reasons. These criteria were judged by a medical doctor. All study procedures complied with the Helsinki Declaration and the Ethical Guidelines for Medical and Health Research Involving Human Subjects and were approved by the Ethics Committee of Medical Corp. Koyokai (KEC-2021-10).

We used highland-cultivated bananas (Cavendish, Dole Sweetio banana, edible portion approximately 60 g per banana) supplied by Dole Japan, Inc. (Tokyo, Japan) for this study. The nutritional components per 100 g of edible portion listed in the Standard Tables of Food Composition in Japan-2020 (8th revised edition) ²⁸ were as follows ¹⁷: energy, 110.64 kcal; protein, 1.54 g; fat, 0.26 g; carbohydrate, 28.66 g; total dietary fiber, 1.3 g; moisture, 88.61 g; vitamin A, 6 μg; α-carotene, 54.48 μg; β-carotene, 42.72 μg; vitamin B1, 0.06 mg; vitamin B2, 0.05 mg; vitamin B6, 0.41 mg; vitamin C, 19 mg; potassium, 430 mg; calcium, 7 mg; magnesium, 38 mg; zinc, 0.2 mg; glutamate, 0.2 mg; and γ-aminobutyric acid (GABA), 10.52 mg.

We performed an open, randomized, parallel-group comparative study on 20 healthy adult Japanese volunteers assigned to either the banana-intake group (n = 13, aged 38.54 ± 3.80 years) or the non-intake control group (n = 7, aged 39.86 ± 2.71 years). Subjects in the banana-intake group ate two bananas (edible portion, approximately 120 g/day) daily for 2 weeks, and those in the non-intake control group did not eat bananas during the same period. Before and after the 2-week experimental period, we measured the intestinal environmental marker indoxyl sulfate, stress markers, autonomic nervous system activity markers, and mood status at Kobayashi Clinic Tokyo (Tokyo, Japan). All samples, as well as the HRV and mood data, were collected between 10:00 and 13:00. After completing the reception process at the hospital, all subjects rested for 30 min to adjust to the environment and then answered the mood questionnaire, followed by autonomic nerve activity analysis, saliva collection, and urine collection, in sequence.

Urine samples (approximately 5 mL) collected from all subjects were stored at 4°C and sent to the contract research institute Healthcare Systems Co., Ltd. (Nagoya, Japan) for measuring indoxyl sulfate levels. The urine samples were centrifuged at 1,500 ×g and 4 °C for 15 min before determining the level of indoxyl sulfate using a QuantiChrom Indole Assay Kit (BioAssay Systems, Hayward, CA, USA).

We measured cortisol and chromogranin A levels in all participants according to our previously reported procedures ³. Saliva (approximately 1 mL) passively pooled in the mouth was collected using a saliva collection aid (Salimetrics LLC, Carlsbad, CA, USA) and immediately frozen and stored at −20°C until analysis.

On the day of the assay, the frozen saliva samples were thawed at room temperature, followed by centrifugation at 1,500 ×g at room temperature for 15 min. The supernatant was used to measure cortisol and chromogranin A concentrations with a Salivary Cortisol Enzyme Immunoassay Kit (Salimetrics LLC) and a YK070 Human Chromogranin A EIA Kit (Yanaihara Institute Inc., Shizuoka, Japan), respectively.

To assess autonomic function, we measured BPM and HRV power spectra (low-frequency power [LF], high-frequency power [HF], and LF/HF ratios) in a private room using an HRV accelerated photoplethysmograph analyzer (SA-3000Plus, Tokyo Iken Co., Ltd., Tokyo, Japan). A photoplethysmography sensor connected to the analyzer was placed on the right index finger of the subject, and all parameters were measured for 3 min after resting in a sitting position. The low-frequency component LF reflects both sympathetic and parasympathetic nerve function. The high-frequency component HF is generated by parasympathetic nerve activity and used as an index of parasympathetic nerve function. The LF/HF ratio is used to assess relative sympathetic nerve activity ²⁵. In this study, we converted the parameter data to natural logarithms of low-frequency power (LnLF) and natural logarithms of high-frequency power (LnHF) to analyze the data closer to the normal distribution ²⁹. We also calculated the LnLF/LnHF ratio.

The mood profiles of all subjects were analyzed using the POMS2 questionnaire ^{3, 25}. This mood inventory comprises 65 items that assess the following seven different subscales (moods): anger–hostility (AH), confusion–bewilderment (CB), depression–dejection (DD), fatigue–inertia (FI), tension–anxiety (TA), vigor–activity (VA), and friendliness (F). Subjects were asked to indicate their mood states on a five-point Likert scale ranging from 0 (not at all) to 4 (extremely). The sum of the scores was calculated for each subscale. The total mood disorder (TMD) score was calculated using the individual total scores of all subscales, except subscale F, as follows: TMD = (AH + CB + DD + FI + TA) − VA. The individual subscales and TMD scores were evaluated using standardized scores (T-scores) that were converted from the individual total subscale scores and TMD scores using a conversion table.

Each parameter value measured after 2 weeks in the banana-intake and non-intake control groups was converted to the change rate of each pre-value. Each change rate in the banana-intake group was finally expressed using the control group as 100%. Additionally, for all parameters, the subjects in the banana-intake group were divided into an effective group (those who improved, even a little, before and after the experimental period) and an ineffective group (those who did not improve at all). The change rate of each parameter in the effective group was finally expressed using the ineffective group as 100%. All values were represented as the mean ± standard error of the mean (SEM). Statistical analysis was performed using the Mann–Whitney U test in SPSS version 9.3.0 (IBM Corporation, Armonk, NY, USA). P values < 0.05 were considered statistically significant.

3. Results

Figure 1 shows the effect of banana-intake on urinary indoxyl sulfate. The change rate of indoxyl sulfate levels in the banana-intake group decreased compared with the non-intake control group (P = 0.175), although this was not statistically significant (Figure 1A). However, when the banana-intake subjects (n = 13) were divided into ineffective and effective groups, six (46%) were classified into the ineffective group and seven (54%) into the effective group (Figure 1B). The indoxyl sulfate change rate in the effective group was significantly lower (P < 0.01) than that in the ineffective group (Figure 1C), indicating that banana-intake was effective in reducing indoxyl sulfate in 54% of banana-intake subjects.

Figure 2 shows the effects of banana-intake on salivary stress markers cortisol and chromogranin A. There was no significant difference in the change rate of cortisol (Figure 2-1A) and chromogranin A (Figure 2-2A) between the banana-intake and the non-intake control groups. The banana-intake subjects (n = 13) were then classified into an ineffective (46%) or effective (54%) group (Figure 2-1B and Figure 2-2B). Both cortisol and chromogranin A change rates in the effective group were significantly lower (P < 0.01) than those in the ineffective group (Figures 2-1C and Figure 2-2C), indicating that banana-intake was effective in reducing stress markers in 54% of banana-intake subjects.

Figure 3 shows the effect of banana-intake on BPM. No significant difference was observed in the change rate of BPM in the banana-intake group compared with the non-intake control group (Figure 3A). When the banana-intake subjects (n = 13) were divided into ineffective and effective groups, seven (54%) were classified into the ineffective group and six (46%) into the effective group (Figure 3B). The BPM change rate in the effective group was significantly lower (P < 0.01) than that in the ineffective group (Figure 3C), indicating that 46% of banana-intake subjects had a slower heart rate.

Figure 4 shows the effects of banana-intake on the HRV power spectra LnLF, LnHF, and LnLF/LnHF. No significant difference was detected in the change rates of these parameters between the banana-intake and non-intake control groups (Figure 4-1A, Figure 4-2A, and Figure 4-3A). The banana-intake subjects were then divided into ineffective and effective groups. For LnLF, nine subjects (69%) were classified into the ineffective group and four (31%) into the effective group (Figure 4-1B). The LnLF change rate in the effective group was significantly lower (P < 0.01) than that in the ineffective group (Figure 4-1C). For LnHF, six subjects (46%) were classified into the ineffective group and seven (54%) into the effective group (Figure 4-2B). The LnHF change rate in the effective group was significantly higher (P < 0.01) than that in the ineffective group (Figure 4-2C). For LnLF/LnHF, nine subjects (69%) were classified into the ineffective group and four (31%) into the effective group (Figure 4-3B). The LnLF/LnHF change rate in the effective group was significantly lower (P < 0.01) than that in the ineffective group (Figure 4-3C). These results showed that parasympathetic activity was increased in 54% of banana-intake subjects, and sympathetic activity decreased in 31%.

Table 1 shows the effects of banana-intake on the seven mood subscales evaluated by the POMS2 questionnaire. The change rate of CB was significantly lower in the banana-intake than in the non-intake control group, but there were no significant differences between the banana-intake and non-intake control groups for the other mood subscales. When the banana-intake subjects were divided into ineffective and effective groups for all subscales, five to seven subjects (38%–54%) were classified into the ineffective group and six to eight (46%‒62%) into the effective group. The change rates of negative mood subscales AH, CB, DD, FI, and TA in the effective group were significantly lower (P < 0.01) than those in the ineffective control group. Conversely, the change rates of positive mood subscales VA and F in the effective group were significantly higher (P < 0.01) than those in the ineffective control group.

Figure 5 shows the effect of banana-intake on TMD, which was calculated from six subscale scores. No significant difference was observed in the change rate of TMD in the banana-intake group compared with the non-intake control group (Figure 5A). When the banana-intake subjects were divided into ineffective and effective groups, five (38%) were classified into the ineffective group and eight (62%) into the effective group (Figure 5B). The TMD change rate in the effective group was significantly lower (P < 0.01) than that in the ineffective group (Figure 5C).

Collectively, the results of the seven subscales and TMD showed an improvement in negative mood states in 46%‒62% of banana-intake subjects.

4. Discussion

In this study, we demonstrated that banana-intake as a prebiotic improved intestinal environment, led to predominant parasympathetic activity, and provided stress relief and psychological stability in approximately 50% of the banana-intake subjects.

Approximately 10% of the intestinal flora in healthy humans are harmful bacteria, including indole-producing bacteria such as Escherichia coli, which produce the putrefactive substance indole in the intestine. Indole is then absorbed from the intestine, metabolized to indoxyl sulfate in the liver, and excreted in the urine ²². In this study, we measured urinary indoxyl sulfate levels as a marker to assess the intestinal environment and found that it decreased in 54% of banana-intake subjects (Figure 1). The indigestible carbohydrates abundant in bananas are a food source for probiotic bacteria (such as Lactobacillus sp.), which promote the growth of the probiotics themselves ^{18, 20}. Oligosaccharides derived from bananas increase fecal viable counts of Lactobacillus sp. and decrease Enterobacteriaceae ²⁰. Banana powder improves the human gut microbiota, promotes the growth of Bifidobacterium and Bacteroidetes, and leads to the production of beneficial short-chain fatty acids ¹⁹. A recent randomized, parallel-group study investigating the effects of long-term (4 weeks) banana-intake on human intestinal flora in 28 healthy adults reported a significant reduction in Akkermansia (a bacterium involved in preventing obesity, diabetes, and heart disease, while also promoting inflammation in multiple sclerosis), Bilophila (a hydrogen sulfide-producing bacterium that causes infection depending on the intestinal environment), Megasphaera (a gingivitis-inducing bacterium), and Ruminococcus (a bacterium detected in bacteremia associated with septic arthritis and intestinal lesions) in the banana-intake group. On the other hand, Phascolarctobacterium (a bacterium that produces short-chain fatty acids, such as propionic acid) decreased significantly in the non-intake group, but this decrease was suppressed in the banana-intake group ¹⁶. These results, showing an increase in some beneficial flora and a decrease in some undesirable flora, suggest that banana-intake alters intestinal flora composition. Therefore, the decrease in urinary indoxyl sulfate levels in the banana-intake subjects in this study is thought to be due to an improvement in the intestinal environment, such as correction of the imbalance of intestinal bacteria, due to banana ingestion.

We assessed the stress status of our study subjects by measuring salivary cortisol and chromogranin A levels, which have been used as stress biomarkers for the hypothalamic–pituitary–adrenal (HPA) axis and the sympathetic–adrenomedullary system, respectively, in many stress-related clinical studies ^{3, 23}. In our present study, salivary cortisol and chromogranin A were decreased in 54% of banana-intake subjects (Figure 2). Microbiota disturbances are closely associated with psychiatric disorders, such as stress ^{5, 6, 7}. Thus, improving the intestinal environment, including the intestinal microbiota, can improve stress symptoms ⁷. In fact, probiotic or prebiotic intake has been reported to alleviate stress-related symptoms by modulating the gut microbiota ¹³. For example, daily intake of probiotics, such as Lactobacillus and Bifidobacterium, improves dysbiosis, significantly reduces salivary cortisol, and relieves psychological distress, such as depression, anxiety, anger, and hostility ^{9, 10, 11}. Daily intake of prebiotics, such as oligosaccharides, reduces salivary cortisol levels and improves anxiety and depression ¹². In our study, urinary indoxyl sulfate levels decreased in 54% of banana-intake subjects (Figure 1). Supporting this result, the levels of stress markers cortisol and chromogranin A decreased in the same proportion (54%) of banana-intake subjects. These findings suggest that as a prebiotic, bananas may exert a stress-reducing effect by improving the intestinal environment.

We evaluated autonomic balance according to heart rate (BPM) and HRV power spectral analysis. In a stressed or tense state, the sympathetic nervous system becomes dominant, and the heart rate increases. Conversely, in a relaxed state, the parasympathetic nervous system becomes dominant, and the heart rate decreases ²⁴. HRV power spectral analysis is widely used to quantify the regulatory state of autonomic nervous system activity ²⁵. This technique partitions the total variance of HRV into its frequency components, typically identifying two main peaks: LF (0.04–0.15 Hz) and HF (0.15–0.4 Hz). The LF peak reflects both sympathetic and parasympathetic activities, while the HF peak reflects parasympathetic activity. Therefore, the ratio of LF to HF (LF/HF) reflects the balance between sympathetic and parasympathetic activity. In other words, a higher LF/HF ratio indicates sympathetic dominance, and a lower ratio indicates parasympathetic dominance ²⁵. As shown in Figure 3, heart rate slowed in 46% of banana-intake subjects, suggesting suppressed sympathetic activity or predominance of parasympathetic activity. HRV power spectrum analysis (Figure 4) showed that LnLF was low in 31% of banana-intake subjects, LnHF was high in 54% of banana-intake subjects, and the LnLF/LnHF ratio was low in 31% of banana-intake subjects. These results indicated the predominance of parasympathetic activity in 54% of banana-intake subjects and the inhibition of sympathetic activity in 31% of banana-intake subjects, suggesting the predominance of parasympathetic activity over sympathetic activity in 31%‒54% of banana-intake subjects. It was recently reported that the gut microbiota balance controls the HPA axis response; oral administration of the probiotics Bifidobacteria and lactic acid bacteria to mice transmit intestinal information to the hypothalamus, which is the center of the autonomic nervous system and the endocrine system, via afferent vagal nerves distributed in large numbers in the intestine ⁷. This suggests that gut bacteria may affect the hypothalamus, a region that integrates the HPA axis and autonomic pathways. Additionally, intestinal bacteria produce various physiologically active substances, such as short-chain fatty acids, GABA, and polyamines, which directly and indirectly affect the nervous system ^{14, 15}. Therefore, our findings regarding the effects of banana-intake on autonomic activity (Figure 3) may reflect direct or indirect responses mediated by the microbiota or microbiota–gut–brain axis ^{4, 5, 6, 7}.

Regarding mental state, Masuda et al. ¹⁷ used a visual analog scale to compare fatigue and tension before and after 4 weeks of banana-intake (120 g/day) and reported a significant improvement. In our study, we used the POMS2 questionnaire, which is suitable for evaluating changes in mood over time (hours or days), to assess mood states ²⁶. POMS2 is used in various fields, such as medicine, mental health, sports, and exercise, and has the advantage of being able to simultaneously measure multiple mood states with one questionnaire ^{3, 27}. As shown in Table 1, negative moods (AH, CB, DD, FI, and TA) were suppressed, and positive moods (VA and F) were enhanced in 46%‒62% of banana-intake subjects. These findings were also supported by the TMD scores, which improved in 62% of banana-intake subjects. The POMS2 analysis consists of seven subscales and a TMD score. Our findings suggest that banana-intake improves negative mood states in approximately half (46%–62%) of banana-intake subjects. As already discussed, banana-intake improved the intestinal environment, stress state, and autonomic balance at almost the same rate as the mood state. As a prebiotic, banana-intake may improve stress and autonomic balance by modulating the gut microbiota, thereby alleviating stress-related symptoms. Additionally, bananas contain dietary fiber as well as amino acids, such as tryptophan, GABA, and histidine ^{30, 31, 32}. Tryptophan is an essential amino acid and precursor of serotonin that stabilizes mental state by synthesizing serotonin in the brain ^{30, 33}. The oral intake of GABA alleviates stress induced by mental tasks ³⁴. The antioxidant effect of histidine is effective in relieving fatigue ³². Thus, supplementation of these amino acids by ingesting bananas may also contribute to a stable mental state.

A limitation of our study is its small sample size, which may be the reason we did not detect any significant differences between the banana-intake group and the non-intake control group. Fortunately, a comparison of the effective and ineffective groups in the banana-intake subjects revealed that banana-intake improved the intestinal environment, led to the predominance of parasympathetic activity, and provided stress relief and psychological stability in approximately 50% of the banana-intake subjects. We also divided the subjects in the non-intake group into effective and ineffective groups in the same manner as the banana-intake subjects, but no significant differences were observed between the two groups for all parameters (data not shown). This suggests that the improvements observed in the banana-intake subjects are banana-intake-specific.

5. Conclusions

We demonstrated that banana-intake for 2 weeks improved the intestinal environment, led to a predominance of parasympathetic activity, and provided stress relief and psychological stability in approximately 50% of healthy adult volunteers. Our findings suggest that banana-intake may alleviate stress-related reactions in people living in stressful modern societies. We believe that our results will form the basis of future studies pursuing the usefulness of bananas as a prebiotic health food. However, well-controlled clinical trials with large sample sizes are necessary to conclusively demonstrate the anti-stress effects of prebiotic bananas.

Acknowledgements

The authors thank Dole Japan, Inc. for providing highland-cultivated bananas (Cavendish, Dole Sweetio banana) for this study. The authors also thank Enago (www.enago.jp) for the English language review.

Statement of Competing Interests

The authors have no competing interests.

List of Abbreviations

AH: anger–hostility, BPM: beats per min, CB: confusion–bewilderment, DD: depression–dejection, F: friendliness, FI: fatigue–inertiA, GABA: γ-aminobutyric acid, HF: high-frequency power, HPA: hypothalamic–pituitary–adrenal, HRV: heart rate variability, LF: low-frequency power, LnHF: natural logarithm of high-frequency power; LnLF: natural logarithm of low-frequency power, POMS2: Profile of Mood States Second Edition, SEM: standard error of the mean, TA: tension–anxiety, TMD: total mood disorder, VA: vigor–activity.

References