Selected aspects of the habitual diet and levels of intestinal permeability markers of Polish alpinists

Study Participants

The study group consisted of 17 male mountaineers from Poland between the ages of 23 and 40 (mean 30.29±5.8 years) with a body weight of 74.96±5.03 kg and a height of 180.47±8.36 cm. A body mass index (BMI) was calculated from the height (cm) and weight (kg) data and was 22.83±2.1 kg/m2. The study participants took part in mountain expeditions at least once a year, during which they stayed at an altitude of over 3,000 meters for at least three weeks. Individuals qualified for the study were members of high-mountain clubs affiliated with the Polish Mountaineering Association.

The alpinists participating in the study were characterized by long climbing experience (10 ± 5 years) in sport climbing and mountaineering. The average climbing level of participants, as measured by the IRCRA (The International Rock Climbing Research Association) scale, was 20.69 ± 2.8, which according to the proposed criteria, places them in the advanced (level 3) category [13]. The climbers spent an average of 8 ± 3 hours per week training.

Before the start of the study, all participants underwent a consultation with a sports medicine physician and were deemed eligible to participate based on the results of the previous standard medical tests, including ECG, and blood and urine analyses. The presence of chronic diseases and age over 45 were disqualifying factors for participation in the project. The Bioethics Committee of the Cracow Regional Medical Chamber approved the research project (67/KBL/OIL/2021), which was conducted in accordance with the Declaration of Helsinki. After learning about the risks and benefits of participating in the study, all climbers gave written informed consent to participate in this study.

Study Design

Statistical analysis

The Shapiro-Wilk test was performed indicating non-normality of the distribution. Basic statistical measures in the form median (Me) and quartiles (Q1, Q3) were used to characterize the received results. Correlations between quantitative variables were analyzed using the Spearman-rho correlation coefficient. The significance level was taken as p≤0.05. The statistical analysis package JASP (University of Amsterdam, Amsterdam, The Netherlands; version: 0.19.3) was used for statistical analyses.

A binary classification approach was used to assess the implementation of nutritional standards. For each nutrient, a value of 1 was assigned if the subject's intake of the nutrient met the recommended standard, and 0 if it did not. Then, based on these data, the percentage of subjects who met the standards for each nutrient was calculated. The analysis was carried out using an Excel spreadsheet (Microsoft Corporation, Washington, DC, USA). The median intake of macronutrients (protein, fat, total carbohydrates), dietary fiber, simple carbohydrates, saturated fatty acids, and micronutrients from 3 days was referred to recommendations for athletes [15–20]. The results were presented in the form of percentages, which made it possible to assess the degree to which the study group implemented the standards.

Main Findings

The present study evaluated the habitual dietary intake and selected markers of intestinal permeability in a group of Polish alpinists. More than half of the alpinists surveyed maintained a negative energy balance, indicating a notable discrepancy between energy demand and intake, and suggesting a risk of chronic low energy availability (LEA). Low energy intake can result in loss of muscle mass and bone density, as well as an increased risk of fatigue, injury, and illness [21]. The failure to meet energy requirements may exacerbate gut barrier dysfunction [22]. Inadequate energy supply relative to demand can potentially have a negative impact on recovery, immune function and overall performance during training, as well as during planned high-altitude activities [23, 24]. The results of another study involving sport climbers also indicate an inadequate dietary energy intake. Overall, 77.5% of the climbers studied did not meet the predicted energy requirements necessary to maintain a “moderate” level of physical activity [25]. Individuals engaged in climbing and alpine activities are at risk of energy restriction and/or low energy availability, evidenced by suboptimal energy intakes, and a lack of adjustment of energy intake in relation to exercise volume.

Carbohydrate (CHO) intake was adequate in the majority of study participants (70.59%), aligning with the recommendations for athletes engaged in endurance sports [17]. However, dietary fiber intake was insufficient in almost a quarter of respondents. Dietary fiber can significantly influence the composition, diversity, and richness of the microbiome, offering a wide array of substrates for fermentation processes conducted by particular microbial species that possess the requisite enzymatic capabilities to break down these intricate carbohydrates [26]. Low intake of dietary fiber and resistant starch may lead to decreased bowel movements resulting in decreased bowel function, and also decreased the diversity of gut microbiota [27]. Given the role of dietary fiber in supporting gut microbiota diversity and intestinal barrier function, this may have implications for gut health, especially in the context of exposure to hypoxic conditions and physical stress. A proper intestinal barrier is crucial for mountaineers, as hypobaric hypoxia can increase permeability and lead to gastrointestinal distress [28]. Maintaining gut health before expeditions helps prevent symptoms like nausea and diarrhea, ensuring better performance and overall well-being at high altitudes [29].

All mountaineers met the requirements for protein and fats with their diets, with 23.53% of them exceeding the normative values for fats and 5.88% for protein. These findings hold physiological relevance when considering the impact of macronutrient intake on intestinal permeability - a key factor in maintaining gut barrier integrity under environmental and physical stressors. Excessive supply of protein with the diet, though observed in a minority, is less directly implicated in intestinal permeability but may modulate gut microbial activity in ways that affect mucosal health. High protein intake increases the delivery of undigested proteins to the colon, where they undergo microbial fermentation. This process produces potentially harmful metabolites such as ammonia, hydrogen sulfide (H2S), and phenolic compounds, which have cytotoxic and pro-inflammatory properties. These metabolites can impair epithelial cell respiration, damage DNA, and compromise tight junction integrity, contributing to gut barrier dysfunction [30, 31]. Currently, an increasing body of evidence suggests that the ingestion of excessive dietary lipids can differentially augment intestinal permeability. For instance, dietary lipids influence the expression and localization of tight junctions (TJs), promote a transition towards barrier-compromising hydrophobic bile acids, and additionally provoke oxidative stress and apoptosis in intestinal epithelial cells (IECs) [32]. Another study shows that 6-day ketogenic low-CHO, high-fat dietary intervention combined with a high-fat preexercise meal resulted in greater exercise-associated perturbations to markers of intestinal integrity than a high CHO diet and even low energy availability [33].

The median intake of saturated fatty acids (SFA) with the mountaineers' diet was 11.32%, exceeding the established recommendations. Current guidelines from the European Food Safety Authority advise that SFA intake should be as low as possible [16]. In contrast, the position statement of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine [17] recommends limiting SFA intake to less than 10% of total energy intake. However, the correlation between SFA intake and fecal zonulin and alpha-1-antitrypsin levels was not statistically significant. The intake of monounsaturated and polyunsaturated fatty acids in the mountaineers’ diets slightly exceeded that of saturated fatty acids. Available scientific evidence indicates that a diet with high nutrient density - rich in polyphenols, omega-3 fatty acids, dietary fiber, and probiotics - has a beneficial effect on the composition and function of the intestinal microbiota. In contrast, a diet rich in highly processed foods, saturated fatty acids, and simple sugars is associated with adverse changes in the gut microbiota [34].

Micronutrient analysis revealed dietary inadequacies in selenium, iodine, and vitamins D and K - nutrients essential for immune function, antioxidant defense, and bone health. Emerging evidence suggests that deficiencies in these micronutrients may contribute to increased intestinal permeability, often referred to as "leaky gut", which can lead to systemic inflammation and impaired immune defense - factors particularly detrimental during high-altitude exposure. Vitamin D, beyond its well-established role in calcium homeostasis and immune regulation, has been shown to modulate the expression of tight junction proteins (e.g., occludin and claudins) in intestinal epithelial cells [35]. Its deficiency has been associated with compromised mucosal barrier function and increased translocation of endotoxins into the bloodstream, which may exacerbate systemic inflammation [35]. Given the important role of vitamin D in immune function and its association with an increased risk of anemia when deficient [36], it is essential to monitor serum vitamin D levels before an expedition and adjust supplementation accordingly. Selenium, a crucial cofactor for antioxidant enzymes such as glutathione peroxidase, also plays a protective role in gut health by counteracting oxidative stress-induced epithelial damage [37]. Inadequate selenium intake may impair redox balance and increased epithelial permeability, thereby contributing to greater susceptibility to infections and reduced tolerance to hypoxic stress [38]. Similarly, iodine is a critical micronutrient for thyroid hormone synthesis, which affects metabolic rate and energy balance, but also modulates mucosal immunity and epithelial turnover. Subclinical hypothyroidism, often observed with low iodine intake, can impair gastrointestinal motility and barrier maintenance [39]. Vitamin K has also been implicated in the regulation of inflammation and mucosal immunity. By downregulating expression of cytokines such as TNF-α and IL-6, vitamin K may help mitigate inflammation-induced disruption of tight junction proteins (e.g., occludin, claudin-1, zonula occludens 1), which are critical to the structural integrity of the gut barier [40]. The observed deficiencies in the habitual diets of the studied alpinists raise concerns about suboptimal gut resilience and systemic immune competence prior to and during expeditions.

Referring to the levels of the analysed intestinal permeability markers, elevated levels of zonulin (28.40 (14; 193.7) ng/ml) were found in the studied group of alpinists, with concentrations exceeding the reference values in approximately 35.29%. The results may indicate a disturbed intestinal microbiota and a potential state of dysbiosis, associated with increased permeability of the intestinal barrier. Similar trends of increased intestinal permeability and elevated levels of relevant markers in athletes have also been found in other studies. Increased intestinal permeability and elevated levels of zonulin have been observed in Polish athletes [41], and increased levels of circulating zonulin (tight junction modulator), occludin (tight junction protein) and LPS translocation have been confirmed in elite Italian soccer players [42]. In a study involving Finnish runners, a significant increase in intestinal permeability (as determined by zonulin and LPS levels) was observed after 90 minutes of intense exercise [43]. Elevated levels of zonulin were also observed in Polish triathletes after completing a race [44]. Changes in markers of muscle cell damage correlated strongly with changes in zonulin concentration at different stages of the study [44]. The elevated zonulin levels observed in some alpinists may suggest disturbances in the gut microbiota and increased intestinal permeability; however, our analyses did not show significant correlations with nutrient intake. This interpretation should be treated with caution and requires confirmation in studies with larger groups and the use of additional permeability markers.

Another less common marker of intestinal permeability is fecal alpha-1-antitrypsin, the levels of which were normal in the group of alpinists studied. AAT is a protease inhibitor that would be increased during the disruption of the mucosal barrier and increased permeability [8]. It was shown that AAT reduces the production of pro-inflammatory cytokines, inhibits apoptosis and inflammatory reactions [45]. Several studies reported that AAT activity is increased post-exercise [46–48]. Another study reported post-exercise increase of both IL-6 concentration and AAT activity, which may confirm that the release of pro-inflammatory cytokines stimulates the release of acute-phase proteins [49]. There is a lack of studies in athletes in which intestinal permeability was determined using alpha-1-antitrypsin. The anti-inflammatory and immunomodulatory properties of AAT can be used therapeutically to treat pulmonary, infectious diseases and autoimmune diseases [50]. In the studied group, alpha-1-antitrypsin levels remained within the normal range. Possible explanations include insufficient intensity of training stimuli to induce an increase in this marker, or substantial inter-individual variability that may have masked subtle changes. Further studies with larger samples are needed, including additional markers of inflammation and intestinal barrier integrity. Recent research also highlights the need to assess physical activity and fitness in different populations, including non-traditional groups such as esports players. For example, a study assessing physical activity, exercise capacity and fitness level of Polish esports players revealed generally low levels of physical fitness compared with athletic populations [51].

Limitations

The limitations of the study include the limited sample size and lack of a control group and limited statistical power. Moreover, the assessment of intestinal permeability was limited to selected fecal biomarkers, which may not capture the full complexity of gut barrier dynamics. Importantly, other established markers of intestinal integrity, such as lipopolysaccharide (LPS), intestinal fatty acid binding protein (I-FABP), or occludin, were not included.

Another limitation is the lack of detailed validation data regarding the 3-day food diaries, which may be subject to potential reporting errors. Furthermore, possible confounding variables, such as vitamin D supplementation or previous gastrointestinal complaints, were not considered. Future studies should address these limitations, including a more comprehensive set of intestinal permeability markers, validated dietary assessment methods, and the evaluation of additional determinants of gut microbiota as well as psychological aspects.