Intestinal permeability in athletes: Zonulin-Independent Mechanisms Revealed Original paper

What was studied?

This original research study investigated the relationship between intestinal permeability and stool zonulin concentrations in athletes undergoing intense physical training. The researchers aimed to determine whether increased intestinal permeability (IIP) in athletes is associated with elevated zonulin—a protein that modulates intestinal tight junctions—or whether other mechanisms might be responsible. The study compared intestinal permeability using the lactulose/mannitol (L/M) differential sugar absorption test (where lactulose is absorbed paracellularly and mannitol transcellularly; an L/M ratio ≥0.035 indicates IIP) with stool zonulin concentrations (measured by competitive ELISA, with ≥30 ng/mL considered elevated). The researchers examined three groups: professional athletes (PRO), amateur athletes (AMA), and non-athlete controls (CTR), and analyzed the coexistence of increased stool zonulin (ISZ) and IIP across these populations.

Who was studied?

The study enrolled 38 healthy male volunteers aged 19-43 years (median 26 years) recruited from three distinct populations in Poland. The control group (CTR) consisted of 9 men with typical non-athletic physical activity. The amateur athlete group (AMA) comprised 9 players recruited from a local American football team. The professional athlete group (PRO) included 20 mixed martial arts (MMA) fighters tested during the peak of their competition season. All participants were asymptomatic, free from acute or chronic illnesses, receiving no long-term medical treatment, and medically certified as fit for intense athletic training. Key exclusion criteria included antibiotic or nonsteroidal anti-inflammatory drug (NSAID) use within one month before the trial. Anthropometric measurements were performed using a medical Body Composition Analyzer (mBCA 515), assessing parameters including height, weight, body composition, fat mass, fat-free mass, skeletal muscle mass, and body water content. Stool samples for zonulin assay were collected one day before the L/M test, and all participants provided written informed consent. The study received approval from the Pomeranian Medical University Bioethics Committee (KB-0012/05/16).

Most important findings

The study revealed that professional athletic activity is significantly associated with intestinal permeability in athletes, confirming previous research. However, the relationship between zonulin release and intestinal permeability proved more complex than anticipated. The key findings are summarized below:

The L/M ratio was significantly higher in PRO compared to CTR ($p=0.013$p=0.013), confirming that intense training increases intestinal permeability. Strikingly, when participants were categorized into four groups based on ISZ and IIP status (ISZ-/IIP+, ISZ+/IIP+, ISZ+/IIP-, ISZ-/IIP-), significant differences emerged between CTR and PRO ($p=0.014$p=0.014), but no statistically significant association was found between ISZ and IIP within the PRO group ($R_s=0.105,p>0.05$Rs​=0.105,p>0.05). This indicates that intestinal permeability in athletes can occur independently of elevated zonulin. In fact, 35% of professional athletes exhibited IIP without ISZ, while 15% showed ISZ without IIP. Within the PRO group, positive correlations were observed between stool zonulin concentration and waist diameter ($R^2=0.467,p<0.05$R2=0.467,p<0.05) and total body water ($R^2=0.484,p<0.05$R2=0.484,p<0.05), suggesting potential metabolic or endocrine associations. The control group showed no ISZ at all, yet 44% demonstrated IIP, indicating that factors other than zonulin contribute to permeability even in non-athletes.

Key implications

This study provides clinically valuable insights into the pathophysiology of intestinal permeability in athletes and has important implications for monitoring and management. The finding that increased intestinal permeability frequently occurs without elevated stool zonulin in professional athletes suggests that exercise-induced hyperthermia, ischemia-reperfusion injury to the gut, mechanical stress, or other factors may directly damage the intestinal epithelial barrier independent of the zonulin pathway. This challenges the assumption that zonulin is the universal mediator of intestinal permeability and indicates that monitoring strategies for athletes should not rely solely on zonulin measurements. The differential sugar absorption test (L/M test) may better reflect functional permeability status in this population. For clinicians working with athletes, these findings suggest that interventions targeting zonulin (such as gluten avoidance) may not fully address exercise-induced permeability issues. Instead, strategies that support overall gut health—including proper hydration, antioxidant support, glutamine supplementation, and careful NSAID avoidance—may be more appropriate. The correlations between stool zonulin and both waist diameter and total body water in professional athletes raise intriguing questions about potential connections between body composition, metabolic health, and gut barrier function that warrant further investigation. This study underscores that intestinal permeability in athletes is multifactorial, and a nuanced approach to assessment and intervention is necessary.