By Dietitian Laura Tilt
Regular exercise is one way to care for your mind and body - including your gut microbiome. But, when it comes to exercising, the type and intensity can vary its impact. Here’s what you need to know - including how to fuel for the exercise you do.
So How Does Exercise Impact the Gut Microbiome?
Let’s start from the top. Some studies have shown that exercise can shorten transit time (meaning the time it takes for food to go from entry to exit!), which may be down to changes in blood flow, hormones that affect motility or simply the movement of the gut that occurs with exercise. This is why exercise is often recommended as a self-help measure for constipation.
There’s also evidence that moderate exercise (like walking or cycling) can help move gas through the intestines in both adults with no gut discomforts and those who experience bloating.
Moving on down to the large intestine, studies show a relationship between regular exercise and a lower risk of colon (bowel) cancer. Some experts think this association is down to the fact that exercise reduces the amount of time that potentially harmful substances stay in contact with the lining of your gut. Others believe exercise helps to maintain the health of the cells lining your gut. Whichever it is, the protective effects appear consistent.
Then there’s your gut microbes, most of which live in your large intestine. Research has found that exercise can alter both the numbers and diversity of microbes in the gut. In particular, it seems to enhance the numbers of bacteria which produce butyrate - a compound (known as a short-chain fatty acid) which helps to keep the lining of your gut healthy. An increase in diversity is a positive thing too, because it represents a more resilient and capable microbiome.
Scientists aren’t completely sure why exercise has these positive effects on gut microbes. Some studies have shown that exercise alters the gene expression of immune cells lining the gut, increasing the amount of anti-inflammatory compounds they manufacture. These immune cells live in very close proximity to your microbes, and the compounds they produce foster the right conditions for beneficial microbes to thrive.
Other research suggests the benefits are down to changes in gut motility or the increased circulation of bile acids (which impact microbial communities) or changes in lactate (produced by muscles during exercise) which can change the pH of the gut, and thus control the growth of less friendly bacteria.
But - Intensity Matters
From the research available we can conclude that moderate intensity exercise (think the type you can do whilst talking but not singing - a bike ride, brisk walking, gentle jog or gardening for example) seems to benefit the gut and its microbes.
However, as exercise intensity increases towards all-out effort (working at 80% or more of your maximum heart rate), there is an increasing trend towards gut symptoms like nausea, diarrhoea and stomach pain. In a 2008 study of over 1200 athletes, 45% reported at least one gut symptom, but depending on the sport studied, the prevalence runs to as much as 93%. Long distance runners and ultra-endurance athletes are particularly vulnerable, and in some cases gut discomforts result in athletes abandoning a race.
Why does this happen? Whilst there are a number of factors at play, the main one is reduced blood flow to the gut. During strenuous exercise, blood flow is redistributed from the gut to the working muscles, lungs and skin. At high intensities blood flow to the gut can be reduced by as much as 80%, and this is believed to contribute to gut discomforts like nausea and abdominal pain. Other causes include the mechanical effects of exercise (particularly running, which is high impact) and nutrient intake, with some types of foods more likely to cause GI distress when consumed before or during exercise.
So, if you’re someone who competes or trains regularly, what strategies can you use to help reduce the chance of discomfort?
- Taking adequate rest days
- Fuel and hydrate properly for the training you do - both dehydration and nutrient availability can impact the gut
- Train your gut by practicing taking on fluid (and carbohydrate) during training. If you’re unsure about how to fuel, a sports dietitian or nutritionist can help
- Take care with NSAIDs (non-steroidal anti-inflammatory drugs) like ibuprofen and aspirin, as these types of painkillers have been linked with an increased likelihood of gut symptoms
- If you are prone to gut symptoms, reducing fibre the day before a race can help, as can avoiding caffeine and spicy or high fat foods
If you do notice high intensity exercise aggravates your gut, try stepping back on the intensity and see what happens - you might find exercising at a lower heart rate more comfortable. This is particularly relevant if you have a history of gut symptoms, as studies suggest this makes you more vulnerable.
Foods to eat to help fuel exercise
When it comes to fuelling exercise - particularly endurance or high intensity exercise, carbohydrates are king. That’s because they’re easily converted into a compound known as ATP, which is the preferred energy source for both muscles and brain during exercise. A high carbohydrate diet can delay fatigue and improve performance for anyone training regularly.
The amount of carbohydrate needed will vary with your body size and training you do. If you are active, basing your meals on starchy carbohydrates like oats, cereal, pasta, rice, potatoes and bread is a good start. Try to have a higher carb meal before and after training to give your body sufficient fuel for exercise and recovery.
Protein is important too, as it provides the body with the building blocks needed to repair and synthesize muscle tissue. If you’re someone who is regularly doing strength or endurance training your protein requirements will be higher than someone who isn’t training (somewhere between 1.2 and 2g per kilo of bodyweight), but this can easily be achieved by including a high protein food with each meal (e.g. eggs, meat, fish, tofu, dairy or soy milk) and after a workout.
When to eat before exercise
A meal or snack containing carbohydrate before exercise will help top up your fuel stores, but aim to leave 2-4 hours between eating and exercising to avoid cramps and discomfort. Meals high in fat are also best avoided as these take longer to digest.
If you need something to top up your energy levels, try a snack like a banana, a small bowl of cereal and milk, a piece of toast with honey or jam 1-2 hours before you exercise.
When to eat during exercise
When exercising for an hour or more, consuming some carbohydrate will help to top up your blood glucose levels and delay fatigue. As a general guide, 30-60 grams of carbohydrate per hour is recommended - what you choose will be determined by the type of exercise you do and how long you’re doing it for. Sports drinks and carbohydrate gels are convenient and often used by runners, but on a bike you might be able to manage solid food like a banana or a muesli-type bar. If you’re training for less than an hour you generally won’t need to take on any carbohydrates if you’ve fuelled well before, but don’t forget to hydrate in all cases.
When to eat after exercise
After exercise the goal is to help replenish your fuel stores and support muscle repair so that you recover well in time for your next training session. Recovery nutrition also accelerates training adaptations, and supports immune function.
So what to eat? The formula is simple - a good portion of carbohydrate-rich food, some protein (20 ish grams) and some fluid to rehydrate. Depending on what time of day it is, this might be a sweet or savoury meal or snack. Ideas include:
- a chicken and salad roll with a banana
- a bowl of muesli with yoghurt and berries
- beans on toast with some cheese
- Spaghetti bolognese
- Egg noodles with tofu and veggies
- scrambled eggs on toast with tomatoes
...plus a drink on the side. Or if you fancy trying something new, I’ve created this delicious one-pot salmon biryani which contains the all-important carbohydrate and protein combo. You can also switch up the salmon for tofu if you’re wanting a plant-powered option.
References
- Oettlé GJ. Effect of moderate exercise on bowel habit. Gut. 1991;32(8):941–4.
- Villoria A, Serra J, Azpiroz F, Malagelada JR. Physical activity and intestinal gas clearance in patients with bloating. Am J Gastroenterol. 2006;
- Mitchell CM, Davy BM, Hulver MW, Neilson AP, Bennett BJ, Davy KP. Does Exercise Alter Gut Microbial Composition? A Systematic Review. Med Sci Sports Exerc. 2019;51(1):160–7.
- Mailing LJ, Allen JM, Buford TW, Fields CJ, Woods JA. Exercise and the Gut Microbiome: A Review of the Evidence, Potential Mechanisms, and Implications for Human Health. Exerc Sport Sci Rev. 2019;47(2):75–85.
- De Oliveira EP, Burini RC, Jeukendrup A. Gastrointestinal complaints during exercise: Prevalence, etiology, and nutritional recommendations. Sport Med. 2014;44(SUPPL.1):79–85.
- Van Wijck K, Lenaerts K, Grootjans J, Wijnands KAP, Poeze M, van Loon LJC, et al. Physiology and pathophysiology of splanchnic hypoperfusion and intestinal injury during exercise: Strategies for evaluation and prevention. Am J Physiol - Gastrointest Liver Physiol. 2012;303(2).
