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DIET, EXERCISE, AND THE GUT

Steps to a Healthier Gut

Manore, Melinda M. Ph.D., R.D., CSSD, FACSM

doi: 10.1249/FIT.0000000000000421
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  • Encourage clients to include whole fruits, vegetables, grains, and beans and legumes in their diet over fruit juices, sweetened beverages, energy drinks, and more processed foods.
  • Encourage the inclusion of fermented and higher fiber foods in the diet to assure a healthy gut. Select healthy foods first over supplements.

Melinda M. Manore, Ph.D., R.D., CSSD, FACSM,is a professor of Nutrition, School of Biological and Population Health Sciences, College of Public Health and Human Sciences, at Oregon State University. Her research focuses on the role of diet and exercise for health, energy balance, and performance in active individuals.

Disclosure:The author declares no conflict of interest and does not have any financial disclosures.

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INTRODUCTION

It is not uncommon for active individuals to experience gastrointestinal (GI) distress, upper respiratory symptoms (URS), and other illnesses associated with sport participation. Research in endurance athletes shows that 30% to 50% report some type of GI distress associated with exercise (1). In addition, URS are common among athletes. In the three months before the 2016 summer Olympics, URS were the most common illness (48%) reported by 132 athletes participating in the games, especially those involved in prolonged, intense activity and during competitions (2). Of those reporting URS, 95% reported being unable to participate in sport. In this same sample, 21% reported GI distress during the same period, with 79% of these athletes being unable to participate in sport. You don’t have to be an Olympic athlete to have GI symptoms associated with exercise. Even recreational and fitness athletes experience GI issues or concerns associated with training or competition. No one wants to run to the bathroom, throw up, or quit a sporting event because of GI distress. Although GI symptoms associated with sport are typically mild to moderate, when they occur, they can derail performance or prevent participation in exercise and sports.

It is well established that gut health is closely linked to immune health and the prevention of illness (3). Thus, a healthy intestinal microbiota (defined in the following section) translates into a healthier immune system and decreased susceptibility to illness or disease (3). Part of having a healthy gut means having a healthy microbiota. In the following section, we address key steps to help you maintain a healthy gut, improve your overall immune system, and reduce your risk of illness.

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STEPS TO A HEALTHIER GUT

Healthy Gut Microbiota. The human gut contains thousands of bacterial species plus many other microorganisms that are collectively known as the gut microbiota (4). The number of microorganisms inhabiting the gut plays a significant role in regulating host immunity, protecting against pathogens, strengthening gut integrity, synthesizing vitamins, and producing energy, especially short chain fatty acids (SCFA). The SCFA (propionate, butyrate, and acetate) are produced when nondigestible carbohydrates (e.g., dietary fiber) or other food components are metabolized by the gut bacteria (4). These SCFA are now recognized as key metabolic and immune mediators and have an anti-inflammatory role. Thus, anything that disrupts the gut microbiota, such as pathogens or antibiotics, has the potential to significantly impact the health of the host.

How do we keep our gut microbiota healthy? Diet has a major impact on the gut microbiota. The ability of the gut bacteria to metabolize indigestible proteins and carbohydrates, especially those found in dietary fiber, contributes to the health of the gut microbiota. Thus, extremes in dietary behaviors, such as high animal-based diet versus plant-based diet, can result in a wide range of alterations in the gut microbiota in humans. Low-fiber diets will mean lower production of SCFA and the benefits these metabolites play in providing energy to the gut and keeping it healthy. The specific dietary factors that play a role in improving gut health are discussed as follows.

How do we keep our gut microbiota healthy? Diet has a major impact on the gut microbiota. The ability of the gut bacteria to metabolize indigestible proteins and carbohydrates, especially those found in dietary fiber, contributes to the health of the gut microbiota. Thus, extremes in dietary behaviors, such as high animal-based diet versus plant-based diet, can result in a wide range of alterations in the gut microbiota in humans.

Healthy Diet. There are a number of dietary steps you can take to help maintain a healthy gut.

  1. Probiotics. One way to maintain the beneficial bacteria in the gut is to consume foods that contain probiotics. The World Health Organization (5) has defined a probiotic as a “live microorganism which, when administered in adequate amounts, confer a health benefit to the host.” There are thousands of fermented foods consumed worldwide by every culture. These foods are generally defined as foods or beverages made through a controlled microbial growth (e.g., bacteria are added to the product) and enzymatic conversions of major or minor food components to simpler components (e.g., enzymatic digestion of lactose to its simple sugars) (6). Some of the more common fermented foods in the American diet include dairy foods, such as yogurt and kefir. These foods contain the bacteria necessary to begin the fermentation process, but also can contain additional healthy bacteria added to increase the health benefit of the product. A variety of other fermented foods contain healthy bacteria such as kimchi or sauerkraut, kombucha, tempeh, miso, natto, cheese, sour cream, cultured buttermilk, wine, beer, sourdough bread, olives, pickled vegetables, salami, and soy sauce. Tempe, miso, and natto are made from fermented soybeans, whereas kimchi is a fermented cabbage popular in Korea, and kombucha is a fermented sweet tea. When selecting fermented foods, remember they can be high in added salt and sugar, so read the labels. The processing of these products also is important because exposure to high temperatures will kill the healthy bacteria. If you have never tried some of these fermented foods, make a point of stepping out and trying something new.
  2. Prebiotics. The bacteria of the gut need to be maintained; thus, they require their own sources of energy and nutrients. The prebiotic concept was developed when researchers realized the health benefit of nondigestible carbohydrates (e.g., dietary fibers) on the gut. Thus, a probiotic adds healthy bacteria to the gut, whereas a prebiotic provides the energy and nutrition (e.g., dietary fiber) the bacteria need for growth. However, it is important to remember that although all prebiotics are fiber, not all fibers are prebiotics. To be considered a prebiotic, a fiber must be resistant to absorption, be fermentable in the gut, and provide nourishment to the gut bacteria. These indigestible carbohydrates reach the colon and undergo various degrees of degradation or fermentation by the gut bacteria. Fibers with a higher degree of fermentation produce energy and other byproducts, such as SCFA, which enhance microbial growth and gut health and improve overall immune function (7,8). Thus, the high-fiber foods recommended in the next section, especially those soluble and fermentable fibers, are considered prebiotics.
  3. High-fiber foods. Based on national survey data, adults in the United States consume only half the recommend adequate intake for dietary fiber per day (men = 38 g/d women = 25 g/d) (8). Dietary fiber recommendations can be normalized based on energy intake, so that 14 g of fiber per 1000 kcals will meet the recommended intake for most adults consuming adequate energy. Dietary fibers are primarily made up of a large class of nondigestible carbohydrates, which can be further subdivided by their characteristics such as solubility, fermentability, and viscosity (9). Foods most beneficial to the gut bacteria are those that contain fibers that are soluble (oats, barley, beans, peas, and fruits like apple and citrus) and fermentable (fruits and vegetables). Thus, foods such as whole grains, fruits, and vegetables, and beans and legumes, contain a variety of fibers in varying quantities.

To meet the dietary fiber recommendations, an individual needs to replace low-fiber foods with those higher in fiber. This means consuming whole fruits and vegetables instead of fruit and vegetable juices, selecting whole grains (e.g., brown rice, quinoa, faro, oats, barley, wheat) instead of lower fiber options like white bread, rice, and pasta and eating more nuts, beans, and legumes. See Table 1 for a list of high-fiber food options. Select breads, pasta, cereals, and crackers that provide at least 2 to 3 g of fiber per serving. Learning to read food labels will help you identify those foods higher in fiber. Eat beans and legumes more frequently because they provide the highest amount of fiber per serving.

TABLE 1

TABLE 1

  1. Foods to limit: There are a number of foods that individuals will say they “like,” but the foods do not “like” them. These foods seem to bother some people and cause GI distress. For example, individuals may complain of chocolate, citrus foods, spicy foods, coffee, or alcohol causing GI distress. For others, foods or beverages containing high fructose, such as sweetened beverages and sugar-free candy or gums can cause gas due to poor fructose absorption. Carbonated drinks can cause bloating and gas. Finally, highly processed foods that are frequently high in sugar and/or fat provide little benefit to the gut microbiota. Adding more nutrient- and fiber-dense foods to the diet will improve gut health.

To meet the dietary fiber recommendations, an individual needs to replace low-fiber foods with those higher in fiber. This means consuming whole fruits and vegetables instead of fruit and vegetable juices, selecting whole grains (e.g., brown rice, quinoa, faro, oats, barley, wheat) instead of lower fiber options like white bread, rice, and pasta and eating more nuts, beans, and legumes.

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OTHER GUT ISSUES

There are many other reasons an individual may have GI distress. Some foods and beverages, such as alcohol, caffeine, or compounds in these products, can irritate the gut causing GI distress or increasing esophageal reflux. If the frequency of reflux is high, the condition is called gastroesophageal reflex disease (GERD) (Table 2). GI distress may be due to a particular food that is not well tolerated because of the inability to digest a specific food component (e.g., gluten or lactose intolerance) or having an allergy to a food component (e.g., allergy to milk protein). There can be more severe gut issues, such as irritable bowel syndrome (IBS) or Celiac disease. Any of these issues can disrupt health and derail an athlete’s ability to exercise and train optimally. Table 2 lists some of these GI issues, their definitions, diet and lifestyle approaches for management, and links to more information. Until these gut issues are identified and appropriate dietary or medical measures taken, being physically active can be difficult. Many of these issues require seeking medical help and visiting a dietitian for dietary advice to help relieve or improve symptoms.

TABLE 2

TABLE 2

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SUMMARY

The gut is the largest immune organ in the body. Thus, part of staying healthy is providing the gut with the energy and nutrients needed. Keep the gut healthy by consuming fermented foods that contain healthy bacteria (probiotics), such as yogurt, kefir or fermented soy products, and eating a diet high in unprocessed, high-fiber foods (prebiotics) (whole fruits, vegetables, grains, and legumes). These foods provide indigestible carbohydrates that help maintain the gut bacteria and produce byproducts that help keep the body healthy.

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BRIDGING THE GAP

We now know that the gut is our largest immune organ. Thus, keeping the gut healthy will help keep us healthy and resistant to illness and disease. Diet plays a major role in maintaining a healthy gut by providing both prebiotics and probiotics. Dietary steps to a healthy gut include consuming whole grains, fruits, and vegetables and beans/legumes, which are high in dietary fiber and are used by the gut bacteria for energy and nutrients. The gut bacteria also produce byproducts important to our health. The inclusion of fermented foods in the diet will help maintain the concentration of healthy bacteria in the gut.

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References

1. Lis DM, Stellingwerff T, Kitic CM, Fell JW, Ahuja KDK. Low FODMAP: a preliminary strategy to reduce gastrointestinal distress in athletes. Med Sci Sports Exerc. 2018;50(1):116–23. doi: 10.1249/mss.0000000000001419. PubMed PMID: 00005768-201801000-00015.
2. Drew M, Vlahovich N, Hughes D, et al. Prevalence of illness, poor mental health and sleep quality and low energy availability prior to the 2016 Summer Olympic Games. Br J Sports Med. 2018;52(1):47–53. doi: 10.1136/bjsports-2017-098208.
3. West NP, Colbey C, Vider J, Cox AJ. Nutrition strategies for gut health and immune function—what do we know and what are the gaps? Sports Science Exchange. 2017;28(179):1–5.
4. Thursby E, Juge N. Introduction to the human gut microbiota. Biochem J. 2017;474(11):1823–36. doi: 10.1042/BCJ20160510. PubMed PMID: 28512250.
5. World Health Organization. Probiotics in food. Health and nutritional properties and guidelines for evaluation. 2001.
6. Marco ML, Heeney D, Binda S, et al. Health benefits of fermented foods: microbiota and beyond. Curr Opin Biotechnol. 2017;44:94–102. doi: https://doi.org/10.1016/j.copbio.2016.11.010.
7. Gibson GR, Scott KP, Rastall RA, et al. Dietary prebiotics: current status and new definition. Food Sci Technol Bull Funct Foods. 2010;7(1):1–19.
8. Sweat W, Manore MM. Dietary fiber: simple steps for managing weight and improving health. ACSMs Health Fit J. 2015;19(1):9–16.
9. Institute of Medicine (IOM); Food and Nutrition Board. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty acids, Cholesterol, Protein, and Amino Acids. Washington (D.C.): The National Academies Press; 2005.
Keywords:

Gut Microbiota; Prebiotic; Probiotic; Dietary Fiber; Gastrointestinal Distress

© 2018 American College of Sports Medicine.