- Exercise increases microbial diversity independent of diet; the microbiota of athletes may be related to dietary protein content.
- Microbiota alterations as a result of exercise are more substantial in earlier life compared with later life.
- Exercise capacity may be influenced by the presence of a diverse microbiota.
- High-fat diets increase intestinal inflammation; exercise reduces this inflammation and may improve gut epithelial integrity.
Humans live in symbiosis with clusters of microbes in various parts of the body ranging from the skin, gut, oral cavity, vagina, and other areas exposed to the environment. This community can weigh up to 2 kg and are composed of roughly 100 trillion microorganisms, including 1000 different species of known bacteria with more than 3 million genes (4). These bacterial communities are primary constituents of the microbiome that encompasses the complete genetic potential of a bacterial population as well as the products of the microbiota (microbial taxa) and host environment. The majority of the microbiota is harbored in the large intestine and undergoes stages during the host's life cycle with the most dynamic time being infancy. It is recognized that one third of our gut microbiota is common to most humans whereas the other two thirds are specific to the individual (34). As a result, the microbiota can provide a personal identity; however, it then becomes difficult to define a healthy microbiota. Despite this, it is generally agreed that the characteristics of a healthy microbiota include community stability and increased species diversity.
Balance in the gut regulates dietary energy harvest as well as the metabolism of microbial and host-derived chemicals. The gut also plays a key role in immune modulation because 70% of the cellular constituents of the entire immune system are found at this site. Likewise, all parts of the gut immune system are influenced by the microbiota, from B cell maturation and the development of the gut mucosal immune system to the prevention of pathogen intrusion (for review of this, see (35)). Thus, any perturbations in the microbiota may interrupt intestinal homeostasis. As such, accumulating evidence suggests that the imbalance between the abundance of beneficial and harmful organisms, or dysbiosis, contributes to the development of chronic diseases such as obesity, type 2 diabetes (2,7,27,40), inflammatory bowel disease (39), and cancer (9,23). It is believed that the westernization of society, increased hygiene, dietary changes, reductions in physical activity, and increased antibiotic use have contributed to changes in the gut microbiota. Studies in the mid-2000s reported that changes in the two major phyla Firmicutes:Bacteriodetes, specifically an increase in that ratio, coincided with obesity (27,40). Furthermore, studies examining insulin resistance have shown that germ-free mice are insulin sensitive but can become insulin resistant after microbiota transplantation from an obese donor (2). The reverse also has been shown in humans when insulin-resistant individuals received a fecal transplant from healthy donors, becoming insulin sensitive within 6 wk of transplantation (43). These findings suggest that the microbiota may work as a virulence factor in driving insulin resistance and obesity.
Interestingly, potent microbial byproducts have shown to challenge gut epithelial integrity and exacerbate the immune response contributing to the onset of chronic diseases as well. Specifically, endotoxin or lipopolysacharride (LPS) is a microbial byproduct released during the lysing of the Gram-negative bacteria (Bacteroidetes) and has shown to induce inflammation that precedes obesity and diabetes development (7,8). LPS has been postulated to alter epithelial membrane integrity creating a leaky gut, a term coined by Gummesson who related lower gastrointestinal (GI) leakiness to adiposity, producing low-grade systemic inflammation as a result of metabolic endotoxemia (26). Cani et al. (8) also have observed that this phenomenon is associated with diabetes and obesity arising from dysbiosis. Alternatively, we have shown that both diet and exercise seem to modulate the expression of key proteins involved in the maintenance of epithelial membrane integrity through tight junctions, as well as the inflammatory status of the gut (6).
Therefore, the most common contributor to microbial changes is human behavior, through diet and exercise. Exercise is known to exert a beneficial role in energy homeostasis and regulation, and recent evidence suggests this may be through increasing microbial diversity (1,6,15,32). We and others have shown that exercise can enhance microbial diversity in the presence of a high-fat diet (HFD), prevent weight gain, and improve body composition as indicated by decreased fat mass (6,15). The effects of exercise are not limited to increasing diversity, however. Exercise also is known to reduce inflammatory mediators, increase antioxidant enzymes, and decrease tumor necrosis factor (TNF)-α expression in intestinal lymphocytes (17,33). Furthermore, exercise can prevent morphological changes in intestinal villi, regulate tight-junction proteins, and increase butyrate production (6,31). In this article, we propose that exercise can promote intestinal health and microbial diversity thereby reducing the risk for chronic disease (Figure).
EXERCISE AND ITS INTERACTION WITH THE MICROBIOTA
Exercise represents a cornerstone in the primary prevention of at least 35 chronic conditions (5). However, during the past 2 decades, considerable knowledge has accumulated concerning the significance of exercise as the first-line treatment of several chronic diseases. Few environmental factors, outside of diet, exert an influence across a range of physiological factors like exercise does. To date, interest in understanding the interactions between exercise and the gut microbiota has increased in popularity, but the mechanisms thereof still remain largely understudied. Of those studies that have investigated these interactions, the majority have used murine models in combination with some dietary treatment. Consequently, the inclusion of dietary treatments makes isolating the influence of exercise difficult. Before we summarize findings regarding the influence of exercise on the microbiota, a clever study examining the effect of the microbiota on exercise should be discussed. Hsu et al. (21) aimed to examine antioxidant enzyme activity and endurance exercise time in mice lacking gut microbiota, germ free (GF), after an exhaustive exercise challenge. In addition, they sought to examine the effect of colonization of GF mice with Bacteroides fragilis (BF) on exercise performance. The GF and BF mice were compared with specific pathogen-free (SPF) mice, which are simply animals with a normal microbiota but lack pathogenic bacteria that would cause disease or inflammation, such as Helicobacter pylori. Using a swim time to exhaustion test, their results showed that having one bacterium (BF) was better for exercise than having none (GF), but that having a complete microbiota (SPF) was best. Specifically, time to exhaustion times for SPF, GF, and BF were 81.6, 39.0, and 67.2 min, respectively. This was coupled with a decrease in antioxidant enzymes, including serum and liver glutathione peroxidase and serum catalase, observed in the GF and BF compared with the SPF mice. The authors concluded that different microbiota status affects exercise performance and it may depend on glutathione peroxidase and catalase activity. So although most experimental questions have focused on how exercise influences the microbiota, it is interesting to note that exercise performance is strongly influenced by the microbiota itself and, to a greater degree, by one that seems to be diverse and free from pathogenic bacteria.
Based on the aforementioned findings, it begs the question as to what exercise does to the microbiota. Does exercise training induce a more diverse microbiota? Does exercise reduce the number of pathogenic bacteria? And the simple answer that we will explore more is, yes: exercise does produce a more diverse microbiota and does seem to reduce pathogenic bacterial communities and increase good communities. This represents a cycle whereby exercise manifests a microbiota that enhances the colonization of more health-promoting bacteria, reduces the burden of pathogenic bacteria, and improves exercise performance. Furthermore, the increased diversity and decrease in pathogenic species also represents a microbiota that is associated with a reduced risk of obesity and type 2 diabetes. Once again, suggesting that another mechanism by which exercise reduces disease risk is through beneficial changes in the microbiota.
The increase in microbial diversity as a result of exercise and the positive health outcomes that follow may be further mediated by microbiota-derived metabolites, particularly in regard to colon health. Specifically, short-chain fatty acids (SCFA) are one of the major end products of microbial fermentation in the gut. These organic acids are absorbed through regional mucosas of the digestive tract (36) and have been shown to modulate host energy homeostasis through interactions between chemosensory enteroendocrine cells (24). Principal SCFA are acetate, propionate, and butyrate in which all are important sources of carbon and energy for host tissues (36). Of note, butyrate is known to influence proliferation, differentiation, and survival of colonocytes and is found to be higher in healthy individuals compared with patients with colon cancer (44). Likewise, exercise has shown to be a potent modulator of SCFA composition, exerting a particular influence on butyrate concentrations. For example, an early study by Matsumoto et al. (31) employed a free-wheel running protocol on male Wistar rats to evaluate the gut microbiota and SCFA composition. After a 5-wk experimental period, animals were sacrificed after a 4-h fast and cecal contents were analyzed. There were no significant differences in food efficiency and body weight gain between the groups (control vs running on standard laboratory chow diet). The exercise group ran 3530 ± 950 m·d. They also noted heavier cecal tissue with more contents in the exercise group compared with the sedentary group. Although this study used a novice sequencing technique for assessing the microbiota, they found that the exercise group was notably different from the sedentary group. In addition, the SCFA butyrate was significantly higher in the exercise group. This is an important initial finding because it is among one of the first to report that exercise microbial communities differ from sedentary ones.
Our study (6) and another (3) support these results because it was found that exercise increased a number of butyrate-producing bacteria and colonic butyrate concentrations, respectively. The major difference being that Basterfield and Mathers (3) showed butyrate was highest in those animals exposed to treadmill running compared with free-wheel run, which showed no increase compared with sedentary controls, contrasting the Matsumoto findings. The Basterfield findings also contrast Cook et al. (11), which found significant differences between treadmill-run and free-whee-run animals. Specifically, the forced treadmill-run animals, when exposed to dextran sodium sulfate (DSS) to induce colitis, showed an increased incidence in morbidity as well as a greater incidence of diarrhea and mortality. Furthermore, forced treadmill running increased proinflammatory cytokine expression, induced more necrosis and inflammation as seen in histopathology, and increased hepatic acute-phase serum amyloid A in systemic circulation. In contrast, free-wheel-run animals had an attenuation of inflammation and were protected from DSS-induced morbidity.
A follow-up study (1) observed that forced treadmill-run animals had a greater microbiota phylum-level diversity compared with free-wheel-run and sedentary animals. Specifically, the Tenericutes and Proteobacteria phyla were enriched with treadmill exercise. It is interesting to note the differences in treadmill protocol with the Cook and Allen study using 5 d·wk 40 min·day at 8–12 m·min, 5% grade for 30 sessions, whereas Basterfield used 5 d·wk for 30–60 min at 18–21 m·min on a 5% grade for 10–12 wk. These findings suggest a few things: 1) exercise does seem to benefit butyrate levels in exercising animals, 2) exercise also produces a unique microbiota, and 3) the type of exercise (treadmill vs free-wheel run) using an animal model to explore exercise-microbiota hypotheses may need to account for other measures being done in the study considering the variety and benefit of responses that can be seen for each modality. Finally, with the scarcity of human trials in this area, resources should focus on determining if the response in humans is similar to those of animals, specifically with regard to exercise and butyrate concentrations.
To date, there has been one study that has looked at exercise in humans and its influence on the microbiota (10) and one proposed protocol examining exercise and a low-carbohydrate diet on prediabetic nonalcoholic fatty liver disease in postmenopausal women and middle-aged men (30). The earlier study by Clarke examined professional rugby players and compared them with high and low body mass index (BMI) sedentary counterparts. They controlled for physical size, age, and sex. Questionnaires were used to collect data on physical activity (EPIC-Norfolk questionnaire), especially of the control groups, and dietary intake (food frequency questionnaire). Stool samples were analyzed and the results stated clearly that the gut microbiota of elite athletes is more diverse than that of controls. Several taxa were significantly higher in the athletes including Ruminococcaceae, Succinivibrionaceae, Succinivibrio, Prevotellaceae, and Akkermansiaceae. The increase seen in diversity was compared with both control groups. Furthermore, the authors noted that the athletes' protein consumption accounted for more of the total energy intake compared with controls. Follow-up correlations seem to suggest that the protein intake may drive the diversity seen in athletes. Although these correlations are promising, it begs the question if the increased protein intake itself elicits the changes in the microbiota, or do the changes in muscle mass that may be accompanied by the protein intake contribute as well. Although this association with protein intake seems novel, microbial diversity may be completely unrelated to protein intake. It is well documented that many metabolic adaptations occur with increased muscle mass as a result of exercise training. This may be a factor influencing the microbiota because the host becomes more dependent on fuel to keep those muscles working efficiently.
The aforementioned study by Clarke focused on the elite athletes who ranged in age from approximately 23 to 35 yr. Would these changes be seen in older athletes or can exercise influence the microbiota as we age? To date, there seems to be one animal study that looked at this and concluded that exercise is more effective at altering the microbiota in younger, not older, animals. Mika et al. (32) studied this question using juvenile (postnatal day 20) versus adult (postnatal day 70) male F344 rats. Rats were allowed access to running wheels for 6 wk and then locked for 25 d. Fecal samples were collected after 3 d of exercise, 6 wk of exercise, and 25 d after the wheels were locked. After sample preparation and analysis, results showed first that wheel running increased during the course of the 6 wk and distance ran was not influenced by age. Alpha and beta diversity revealed that the juveniles had a distinct clustering of microbial communities that was different from the adults. Phyla-level analysis revealed that juvenile-onset exercise increased the relative abundance of Euryarchaeota and Bacteroidetes and decreased Firmicutes and Proteobacteria. Furthermore, genera-level analysis also revealed that more genera were impacted by juvenile-onset exercise (6) versus adult-onset exercise (3). Some potential reasons for this could be animal strain, housing, age differences, or even exercise protocol.
The aforementioned studies highlight a particular area of novelty related to the proposed conceptual model. It can be said that the microbiome influences host physiology similar to an endocrine organ by releasing compounds into the interstitial tissue, and these are then picked up by blood or lymph to influence target tissues. As previously alluded to, SCFA are a good example of this. These SCFA can be transported across the gut lumen and to effector organs. A primary target site is the brain, giving rise to the term brain-gut axis when describing these interactions. For example, SCFA can modulate secretion of neurotransmitters such as neuropeptide YY and act as an energy source for neurons (24). Furthermore, changes in SCFA have been noted in patients with colon cancer along with changes in microbiota. Specifically, butyrate was significantly higher in healthy individuals whereas acetate was significantly lower in healthy patients; propionate also was lower in healthy patients although not statistically significant. Interestingly, they found several microbes that were much higher in healthy individuals compared with patients who had cancer, e.g., Prevotella corpi, Lachnospira pectinoschiza, Dialister pneumosintes, and others to be much higher in patients who had cancer compared with healthy individuals, e.g., Akkermansia muciniphilia, Acidaminobacter. It is plausible that those microbes in patients with cancer (A. muciniphilia) degrade butyrate, depriving colonic mucosa and epithelial cells a fuel source and compromising colon health (44). These types of hypotheses need to be explored to help make more firm connections to how exercise may be used to manipulate microbes and thereby improve health.
EXERCISE AND DIETARY MODIFICATION ON MICROBIOTA OUTCOMES
Obesity and other inflammatory-based diseases often are linked to an HFD and lack of exercise. As such, dietary modifications and the initiation of an exercise program can be efficacious treatments to combat these diseases. The Cochrane Review, to date, has been one of the most comprehensive reviews on the effects of diet and exercise for the treatment of overweight/obese individuals (37). They reported that exercise alone induced significant weight loss, whereas exercise combined with a restricted diet and dietary counseling was more effective. High-intensity physical activity was more effective than moderate activity. Although the importance of exercise for weight loss assessed by body weight or BMI remains controversial, physical training leads to a reduction in fat mass and abdominal obesity, in addition to counteracting loss of muscle mass during dieting. Strong evidence exists that exercise is important for preventing weight gain in general as well as for maintaining body weight after weight loss.
To date, studies examining the interaction between HFD and exercise agree that exercise can induce a unique and diverse microbiota. These beneficial changes to the microbiota have been seen both in studies that report changes in the Firmicutes-to-Bacteroidetes (F:B) ratio (13,15) and those that do not (6,22,45). However, these results highlight a critical issue currently in the field. There seems to be no standard for experimental protocols. Studies use both 45% and 60% HFD compared with a control diet; some studies use rats whereas others use mice, and these mice typically are from various vendors; and finally, some use the treadmill whereas others a running wheel.
An excellent example of this is illustrated by our study and another that used similar methods with one showing a distinct reduction in the F:B ratio (15) whereas we did not (6). Briefly, Evans et al. (15) used 5-wk old male mice (Jackson Labs) and randomly assigned them to one of the same four groups: low fat/sedentary (LF/Sed); low fat/exercise (LF/Ex); high fat/sedentary (HF/Sed); and high fat/exercise (HF/Ex) for 12 wk. The Evans study used the 60% HFD whereas we used the 45%; however, both sets of animals in the two studies had access to a running wheel. Similar to our findings, their results showed that exercise prevented weight gain and adiposity elevated by the HFD. Not surprising, exercise also normalized glucose tolerance in the animals fed the HFD. As previously mentioned, there was a significant decrease in the F:B ratio and they also reported an increase in the families Clostridiaceae, Lachnospiraceae, and Ruminococcaeae in which some species associated with these families are known producers of butyrate. The differences observed in our study are likely due to different methodologies. For instance, these differences could be related to the 2 different sources of mice (Jackson Laboratories and Taconic Laboratories), different percentage fat in the diet (60% vs 45%), the differences in DNA extraction techniques (bead beating/12 h at 55°C vs 5 freeze/thaws and immediate extraction), or the use of different primers (group specific vs universal primers) for amplifying the target 16S rRNA genes. In addition, in our study the microbiota was analyzed at the genus level rather than the phylum/class or family level, which showed exercise increased Faecalibacterium prausnitzi, Clostridium species, and Allobaculum species, which are known to produce butyrate; exercise influencing butyrate-producing bacteria is the commonality between the two studies.
Denou et al. (13) examined high-intensity interval training (HIIT) and regional microbiota distributions. Specifically, they asked if HIIT alters the microbiota after the establishment of obesity along various segments of the gut including the duodenum, jejunum and ileum of the small intestine, and cecum and colon of the large intestine. The mice were bred on-site and exercise was done on a treadmill so that intensity could be easily manipulated during the session. Mice were fed a 45% HFD for 6 wk to induce obesity, which was supported by glucose and insulin intolerance. HIIT improved insulin tolerance independent of adiposity and altered the microbiota in a segment-dependent manner. Exercise increased overall diversity in the HFD-fed mice, specifically in the phyla Bacteroidetes in both the cecum and colon. Furthermore, the F:B ratio significantly decreased in the cecum with a similar trend in the colon with specific changes to the bacteriodales genus. Finally, they did note differences between acute and chronic exercise microbiota adaptations, whereas acutely, Lactococcus decreased after exercise training but that this change was not persistent after 1 wk. Lactococcus are known as homofermentors meaning that they produce a single product, lactic acid in this case, as the major or only product of glucose fermentation.
Although the overall results suggest that exercise has a robust impact on microbial diversity, direct comparisons between studies are difficult due to inconsistent methods. Furthermore, simply reporting what the gut microbes are as a result of sequencing is exciting but limited because we do not yet fully understand their role in human health. They are part of an ecosystem where they must act independently to carry out their specific roles, but also as part of a community to benefit the host. Until functional roles of these communities are more fully understood, interpretation of the results and their relation to human health and metabolism is incomplete. This, in particular, highlights exciting areas of research that focus on what bacteria are doing rather than who is present to elucidate microbiota-host interactions. Furthermore, this emerging area will enhance the conceptual model immensely as we distinguish which microbiota-derived molecules drive key phenotypes, at what concentrations do these molecules then become physiologically relevant and influence host physiology, and which experimental systems are appropriate for testing the activity of a molecule deemed relevant.
EXERCISE AND INTESTINAL TIGHT JUNCTIONS
The intestine is the primary site for absorption of nutrients, fluids, and electrolytes. Furthermore, specialized proteins make up the mucosal layer and adhere neighboring epithelial cells and enterocytes to one another through tight junctions forming an impermeable barrier. Maintenance of this barrier is key in preserving intestinal homeostasis but the integrity of which remains vulnerable to changes in host behavior. Of note, altered intestinal integrity has been associated with HFD and breakdown of tight junction proteins, occludin, and zona-occludin-1 (ZO-1) (8,14,16,25). Regulation of this barrier by tight junction proteins is dynamic and represents a balance between the pore pathway and leak pathway. The pore pathway regulates the permeability of the tight junction to ions and small molecules, whereas the leak pathway regulates the permeability of macromolecules (28). The pore pathway seems to be dependent on the cadherins and claudins whereas the leak pathway is dependent on the ZO-1 and the occludins (41,42). Because exercise has typically been reported to be beneficial to the intestine, there are some exceptions.
Research on exercise and barrier function suggests that the more strenuous the exercise the greater barrier disruption (46). This can be influenced by two separate pathways depending on the “insult” to the tissue leading to increased endotoxin. These pathways are reviewed by Zuhl et al. (46) but briefly, long-duration exercise in the heat induces a disruption involving novel protein kinase C (heat stress pathway), or intestinal ischemia causing the production of hydrogen peroxide leading to increased permeability (ischemic pathway). Although there is abundant literature on exercise heat stress and intestinal barrier function, less has been done relating exercise to barrier function in diseases such as obesity and diabetes. As previously mentioned, one of the mechanisms by which obesity and diabetes can be perpetuated is through the lysing of Gram-negative bacteria, releasing endotoxin leading to cause a leaky gut. Typically, under normal physiological states, small amounts of endotoxin are released; however, this is rapidly removed by monocytes, in particular the resident Kupffer cells in the liver. Lira et al. (29) showed that endotoxin levels correlated positively with a sedentary lifestyle and negatively in highly trained subjects. From experience, we can say that measuring endotoxin in the laboratory is difficult because it requires a high level of near-sterility to avoid contamination. The Lira study did use a commercially available kit, which could be problematic. In addition, they report using highly trained subjects without reporting V˙O2max values, so their training status cannot be validated. It would be nice to see additional studies in this area to gain a consensus on endotoxin levels in athletes.
In our study (6), we used 36 6-wk-old C57BL/6NTac male mice that were fed a normal diet or HFD (45% fat) for 12 wk and randomly assigned to exercise (running wheels) or sedentary groups. After 12 wk, the animals were sacrificed and duodenum/ileum tissues were fixed for immunohistochemistry for occludin and E-cadherin. Occludins are integral membrane proteins crucial for tight junctions, and the cadherins are involved in cell-to-cell adhesion as well as communication. Our results showed that high-fat-fed animals had reduced E-cadherin expression, whereas the opposite was true for occludin. The occludin response differs from previous published data, suggesting impaired barrier function with HFD (8,12). One animal in the obese-exercised group had influenced our thoughts on these results. This animal's exercise volume was only 50% of the other animals in the group and weighed 48 g at sacrifice. We observed that this mouse's COX-2 and occludin expression was upregulated compared with the other mice, and E-cadherin expression also was upregulated compared with the animals that exercised normally. These data suggest that the increased expression of occludin may not imply intact tight junctions but may be compensatory expression due to inflammation (COX-2)-induced damage. The influence of intense exercise on barrier function, as previously mentioned, suggests increased permeability. However, the exercise protocol in our studies is not considered strenuous. Thus, investigation of blood flow changes, particularly in the obese gut, would be informative due to the scarcity of literature in this area. As previously mentioned, claudins play a critical role in barrier function by sealing neighbor epithelial cells and these were not examined. Information about their expression may help to fully understand the intricate relationship of the epithelial barrier.
This area of research is in great need of investigation as evidenced by the small body of research examining permeability, exercise, and disease state. In the disease state, exercise seems to promote intestinal epithelial integrity, which may reduce endotoxin (LPS) release in accordance to our conceptual model. A decrease in endotoxin could theoretically reduce the activation of systemic proinflammatory mediators such as TNF-α and NF-kB resulting in an overall decrease in adipose tissue and systemic inflammation. Furthermore, elucidating agents that can protect the barrier during intense exercise to decrease permeability may be beneficial as well.
EXERCISE, GUT IMMUNITY, AND INFLAMMATION
As previously alluded to, the gut plays a key role in immune regulation. This GI-associated lymphoid tissue is responsible for the recognition of pathogens; the recognition and tolerance of self-antigens and commensal bacteria; and the sensitization and desensitization of foods. The overwhelming literature suggests chronic exercise can influence inflammatory status in chronic disease by reducing it. Although less is known about the gut, accumulating evidence seems to suggest that exercise can reduce gut inflammation. Our laboratory has recently shown that HFD can alter small intestine morphology. Specifically, the low-fat-fed sedentary animals had normal villous histology, exhibiting a single layer of epithelium covering the villi and lamina propria (6). The HFD sedentary animals' villi were the same height (~300 μm tall) whereas the width was twice that of the low-fat-fed sedentary animals (95 vs 44 μm, respectively). As a result, HFD sedentary animals' villi were crowded. Major causes of villi widening include an increase in inflammatory cells present in lymph and plasma cells and an increase in fat cells. In exercised animals fed either an HFD or low-fat diet, villi were histologically normal with open lumens. The villi were well formed and thicker than their sedentary counterparts, which was primarily due to vessel dilation. In the HFD exercise group, the plasmacystoid and lymphocytic infiltrate was absent in contrast to their sedentary counterparts. These results suggest that exercise prevented the morphological changes that were associated with high-fat feedings and reduced inflammatory infiltrate. Furthermore, COX-2 expression was significantly elevated in HFD sedentary animals compared with the low-fat-fed counterparts. Exercise had a reduced expression in both groups (high- and low-fat fed) suggesting that those animals who consumed an HFD but exercised were protected from inflammation.
Despite these findings, the emerging mechanisms underlying the protective effects of exercise still need investigation. To date, a series of studies by Hoffman-Goetz (17–20,33,38) have used animal models to investigate the influence of exercise on intestinal lymphocytes. These studies have noted that exercise can increase antioxidant enzymes (glutathione peroxidase and catalase), antiinflammatory cytokines (interleukin (IL)-10), and antiapoptotic proteins (Bcl-2) in intestinal lymphocytes. It also was observed that exercise decreased TNF-α, proapoptotic proteins (caspase 3 and 7), and the proinflammatory cytokine IL-17, supporting that exercise can modulate the intestinal immune response. Unfortunately, human research in this area is limited, and results from such studies would be helpful in establishing exercise prescriptions for individuals with GI disorders.
The conceptual model proposed is a work in progress, and many mechanisms need to be understood to make firm connections between changes in the microbiome and physiological outcomes. Furthermore, human studies are a must to replicate results seen in animal models, and some standardization among animal research is in dire need to be able to effectively compare results.
The authors thank our collaborators for their help and guidance with experiments: Lee Kerkhof, Max Häggblom, Lora McGuinness, Laurie Joseph, and Stan Lightfoot. We also thank Dipak Sarkar for graciously allowing us to use his running wheels for our experiments. Finally, thank you to Barry Braun for helping us with this manuscript and guidance during the process.
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