How sport affects digestion

Heartburn, flatulence, diarrhea or constipation — complaints that many of us know only too well. People who don’t exercise suffer more often from constipation, while endurance athletes usually suffer from diarrhea or stomach pain. In addition to all sorts of home remedies, moderate physical exercise helps with many of these digestive complaints — but often the sport itself can also trigger such complaints. How does exercise really affect our digestion? What can exercise do and how much exercise is ideal to positively influence digestion?

What happens in the intestine during sport

Digestion is a process that occurs under resting conditions. “Exercise” is considered a deliberate activation of the muscles that results in short-term effects (for minutes or hours). “Physical activity,” on the other hand, describes repetitive periods of exercise that lead to long-term effects (for days, weeks, months, or years). The gut is not an athletic organ in that sense, nor can it become accustomed to exercise-induced stress — as, for example, our muscles can.

How exercise affects the gastrointestinal tract depends primarily on exercise intensity. Although heartburn, chest pain, nausea, vomiting, abdominal cramps, side stitching and diarrhea are not uncommon during strenuous exercise sessions, there is sufficient evidence to suggest that physical activity as a whole can protect against conditions such as gallstone disease, intestinal diverticulosis (diverticular disease) and even colon cancer. Unpleasant constipation (medically known as constipation) can also be counteracted with sufficient exercise.

What mechanisms are thought to be responsible for these effects? It is likely that altered intestinal blood flow, intestinal motility due to body movement, neuroendocrine changes, and mechanical effects are involved in these effects. According to surveys, 30 to 50% of respondents suffer from digestive symptoms during strenuous endurance runs, which are often misinterpreted as a result of improper food and fluid intake. In fact, they are simply a physiological reaction of the intestines to physical exertion.

When the body is in motion, much of the blood flow is diverted away from the gastrointestinal tract and toward the active muscles as well as the lungs. These changes in blood supply, nerve activity, circulating hormones, neurotransmitters, and metabolic waste products result in a throttling of intestinal motility as well as nutrient and fluid absorption and release in the gut. At 70% exercise intensity, Rowell and colleagues found a 60–70% decrease in intestinal blood flow. At maximal exercise intensity, intestinal blood flow may be reduced by as much as 80%.

Critical blood supply to the intestine under extreme exercise conditions, as well as increased body temperature, fluid deficiency, sugar and oxygen deficiency, or a combination of all of these factors, result in intestinal cells not being adequately supplied. Due to the lack of oxygen and nutrient supply via the blood, metabolic waste products are no longer removed from the intestine, which can lead to the death of these cells. The waste products remain in the stomach and intestines, causing significant gastric reflux, reduced nutrient absorption, and maldigestion due to disturbances in the intestinal flora. The overall reduced intestinal transit time can result in diarrhea, intestinal cramps and urge to defecate.

Although these complaints are unpleasant, they are nevertheless of a temporary nature and do not affect the athlete’s health in the long term.

How can such complaints be counteracted even during strenuous training sessions?

Results of field and laboratory studies show that ingestion of high-fiber, high-fat, and high-protein foods before exercise and highly hypertonic drinks can further aggravate stomach pain, vomiting, reflux, or heartburn after exercise. Iso- or hypotonic sports drinks containing carbohydrates have been shown not to cause this aggravation for intense performances lasting 45–60 min or longer and are therefore particularly useful.

Target amounts of carbohydrate consumed before exercise should be about 30–60 g per planned hour of exercise, and drinking should be adjusted according to thirst to about 0.4–0.8 liters per hour.

Moderate exercise results in a less dramatic decrease in intestinal blood flow and has been shown to be beneficial for intestinal health. Regular physical activity at relatively low intensity can reduce colorectal cancer risk by up to 50%. The primary hypothesized mechanism is that physical activity reduces intestinal transit time, thus shortening the contact time between the intestinal mucosa and the often harmful substances in the food pulp. Runners, in particular, have been shown to have a better “stool pattern” (less solid stool, higher frequency, higher stool weight) in contrast to inactive individuals. The underlying mechanisms are unclear, but a beneficial effect on colonic motility, decreased blood flow to the bowel, biomechanical bouncing of the bowel during running, compression of the colon by abdominal muscles, and increased food and fluid intake are thought to result from increased energy expenditure.

Other mechanisms associated with increased risk of colon cancer, such as impaired immune function, an unhealthy diet (large amounts of alcohol or fat, low amounts of fiber), or an elevated body mass index can be counteracted equally favorably by physical activity.

In addition, moderate-intensity physical activity may protect against inflammation by inducing changes in the activity of macrophages (phagocytes), natural killer cells, and regulatory cytokines. However, heavy physical exertion can lead to an opposite effect: high-intensity exercise, especially when not proportional to the level of exercise, causes oxidative stress through the production of free radicals, muscle damage, systemic inflammation by reducing natural killer cells, exuberant immune responses, and psychological stress, as well as decreased performance, fatigue, insomnia, and anxiety.

Which type of sport best promotes digestion?

The stronger the upward and downward movements, the stronger the effects on the digestive system. The effects on the stomach and intestines are almost twice as pronounced with running than with other endurance sports such as cycling or swimming.

How to manage digestive problems caused by sports?

The negative effects of high-intensity workouts on the immune system and digestion in athletes provide the basis for the current popularity of antioxidant-containing sports supplements. A key role among antioxidants is played by so-called polyphenols (e.g., berry extracts), which are commonly found in dietary supplements. For example, pomegranate polyphenols are known for their anti-inflammatory and anti-infective effects. Antioxidants restore regular intestinal flora after inflammatory stimuli by helping to scavenge free radicals. However, buying special supplements is nonsensical — fresh berries or teas show the same effect.

Not to be neglected is the influence of proteins on the composition and functionality of the intestinal flora. Athletes in particular are exposed to multiple stresses through regular, intense and/or long training routines; to promote protein synthesis and the resulting training adaptations, they need adequate protein intake. Protein sources with different amino acid compositions may have different effects on the gut microbiota. Some studies suggest that plant proteins are more beneficial to gut bacteria than animal proteins. Protein should be consumed before and after performance and regularly throughout the day to ensure an efficient supply of essential amino acids.

Regular (intestinal) exercise

Physical activity affects the gastrointestinal tract in a variety of ways. On the one hand, extreme physical exertion can cause many gastrointestinal symptoms; on the other hand, daily, 30-minute moderate physical activity reduces the risk of digestive disorders and chronic gastrointestinal symptoms. These effects are probably mediated by changes in intestinal blood flow and movement, but also by purely mechanical factors and hormonal changes. Regular physical activity is the best prerequisite for regular and regular digestion — that’s for sure.

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