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Exercise's Impact on the Gut
Much of what is known about exercise and digestive function comes from studies on athletes. Physical activity influences gut function depending on exercise type and exercise intensity. Mechanisms to consider range from pressure alterations in the gut resulting from exertion to blood-flow changes, hormonal impacts, and mechanical damage. Exercise shunts blood away from the gastrointestinal tract toward the working muscles. This is mediated by the degree of sympathetic activity elicited. Higher-intensity activity generates much larger effects over lower-intensity activity in this regard. This reduction in blood flow can be substantial. Research has shown splanchnic blood flow is reduced by as much as 80% with exercise greater than 70% of VO2 max (82% of maximum heart rate), which is at the upper end of aerobic exercise.1 This represents a significant challenge to the digestive tract and has been shown to lead to increased gut permeability (i.e., leaky gut), disrupt GI microflora, and generate intraluminal endotoxins.2 Obviously, these are consequences that any practitioner would want to avoid and is usually working to correct. These responses are aggravated further with dehydration, exercising in hot conditions, and concomitant use of NSAIDs.3 This is an important consideration since many "health-conscious" patients seeking help for digestive distress often engage in strenuous exercise patterns that may be detrimental or contraindicated while undergoing treatment.
In addition to ischemic effects, activity can also cause damage through mechanical mechanisms including increased intra-abdominal pressure and visceral shear forces through bouncing. The mode of exercise is important in this regard. Weight training, cycling, and contact sports can greatly impact pressure in the abdominal cavity, while long-distance running is known to cause abdominal distress and induce bleeding through repetitive organ jostling. Gastrointestinal symptoms occur in 20% to 50% of all athletes and are especially prevalent in runners.2,6 These mechanistic considerations manifest clinically by disrupting gastrointestinal motility. This can lead to increased risk of GERD in the case of cycling or weight training, while running leads to decreased small bowel transit time and increased colonic peristalsis.2,4,5
The most complicated aspect of exercise and digestive function has to do with signaling molecules related to gut homeostasis. These are intricate mechanisms involving the neurological, immunological, and endocrinological systems. Exercise is registered by the body as a stressor and activates the HPA-axis, leading to a host of downstream hormonal adjustments. Cortisol is a key hormone regulator in this regard and is known to be highly responsive to activity. Cortisol is tightly tied to both intensity and duration of activity and has influence over gut immune function through secretory IgA.7 Prolonged stress with elevated cortisol is known to lower secretory IgA, leaving the gut and respiratory tract susceptible to infections. This issue frequently manifests after acute long-duration exercise such as running a marathon, or repeated high-intensity or long-duration exercise over months (overtraining).10,11 Interestingly, though, chronic exercise training of moderate intensity has been shown to improve levels of secretory IgA.12 Another consideration is that skeletal muscle and gut mucosa compete heavily for resources. Glutamine is a key amino acid used in exercise and is also a key component involved in gut integrity. Under catabolic states, the muscle will be broken down to liberate glutamine. Glutamine is a preferred fuel of lymphocytes, hepatocytes, and especially the enterocytes lining the gut.8 Intensive exercise is known to disrupt glutamine concentrations and negatively impact the gut immune complex.9
Exercise and Food Allergy
The exercise–gut connection seems to be most severe in those who already have compromised gut function. As described above, exercise is believed to increase small bowel permeability in susceptible individuals. This is an undesirable effect when trying to help repair gut inflammation, food allergy, or "leaky gut" conditions. Food-dependent exercise-induced anaphylaxis (FDEIA) is a rare condition wherein the combination of a food along with exercise induces allergic-type reactions, including anaphylaxis. Matsuo et al. in 2005 showed that this effect is indeed mediated by increased gut permeability. Subjects sensitive to the wheat protein gliadin were challenged with wheat alone, exercise alone, or wheat and exercise in combination. ELISA testing was utilized to quantify serum gliadin levels. The wheat and exercise challenges alone saw little change, while the exercise and wheat combination saw a large rise in gliadin levels in the blood, which was associated with physical symptoms.13 In this same study, subjects without known reactivity to wheat were analyzed as well. The healthy subjects too saw a large increase in gliadin protein in the blood, although physical symptoms did not occur. It appears that exercise increases intestinal absorption of potential allergens in both normal and reactive individuals. This is essential knowledge for practitioners to have when evaluating exercise in patients with complaints suggestive of food sensitivity or allergy.
Given an understanding of exercise and its impact on the digestive system, it is important for practitioners to know the type of exercise and corresponding intensity to prescribe. Based on the above data, it may be advisable for patients undergoing gut-restoration programs and allergy eliminations protocols to avoid both higher-intensity and longer-duration exercise sessions. Intense weight training, cycling intervals, and long-distance running all could negatively affect digestive function. Refraining from these activities during treatment may speed digestive healing. At the same time, there are gentler exercise modalities. Low-intensity activities such as tai chi, walking, and restorative yoga would seem to be advisable. All have been shown to favorably affect immune function and psychological well-being.14-16 In addition, cortisol rhythms are positively affected in each, attesting to their lower-intensity nature and restorative functions.14-16 Based on these measures, three to five sessions of 30 to 60 minutes each of walking, tai chi, or yoga per week would seem to be a good option for exercise in those with compromised gut function.
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