Gut feeling

We have not one, but two brains. Look down for the second one. Yes, your gut. An organ system that is both fascinating and annoying at the same time - who doesn't recognise the slowly emerging feeling that the course of your long run will need to include a portable toilet? Or that your stomach refuses to take in one more gel? You're certainly not alone -  in long-distance events, prevalence of gastrointestinal (GI) distress is thought to affect 30-50% of participants, and up to 93% of long-distance triathletes.

From top to bottom - the gastrointestinal system.

That second brain, however fascinating, is one of the reasons for the well-known gastrointestinal (GI) problems athletes encounter. This time I'll explain why, what other causes for GI problems exist, and what you can do to prevent that ‘breaking the wind’ no longer means your training buddies are happy to draft off you.

The 'second brain' refers to a special complex of nerves in the lining of the gut, called the enteric nervous system. It is connected with the brain through the autonomous nervous system, the part of our nervous system that we have no conscious control over and that, amongst others, makes sure our inner organs exchange information with our brain.

The gut is not only controlled by the brain, but - and this is so special about the enteric nerve complex - it also controls itself, via reflexes. Signals also travel from the gut up to the brain, telling it how it’s doing. To make matters even more complex, different hormones from the blood can influence the gut – and even the gut itself excretes hormones into the blood, giving other organs a status update. All signals can both be stimulatory (increasing gut activity) or inhibitory (slowing down gut activity).

Yes, it’s complicated. No wonder the gut reacts to emotions. Good when it comes to butterflies before competition (or when in love) – bad when the gut turns ‘angry’.

Nervous control of the gut not only by the brain, but also by the gut itself (blue dots are markers for the enteric nervous system).

So how does that relate to GI problems before, during or after exercise and in particular, races?

Before a race, the mental excitement and anticipation of competition can trigger the whole autonomous nervous system, including the nerves to the gut. An excess of stimulatory signals to the gut can cause 'urgency', cramps or even diarrhea.

During exercise, there are many different factors at play:

Reduced blood flow

The main physiological cause of GI problems is thought to be a reduced blood flow to the gut, also called splanchnic hypoperfusion. Strenuous exercise causes the release of noradrenalin from the gut's nervous system, causing constriction of the gut's blood vessels. This  redistribution of blood flow, away from the digestive organs and to the organs that need oxygen and energy the most (=muscles, heart, brain), decreases blood flow in the intestinal system by 20-50%. As the gut's cells also need blood, oxygen and nutrients, this hypoperfusion can result in gut cells being damaged. With damaged cells, the barrier function of the GI tract is compromised, making it easier for bacteria and other potentially harmful bugs to move from the gut into the bloodstream (this, by the way, might be one of the many explanations for being more sensitive to illness directly following intensive training).

Also, absorption and digestion of nutrients from the gut is more difficult when blood flow is restricted and cells are damaged. It is not hard to imagine that poor absorption and digestion of food can lead to abdominal pain, cramps and diarrhea - though it must be noted that hard evidence and full understanding of the problems is still lacking.

Mechanical trauma

The repetitive high-impact 'jostling' of the gut during running is another physiological cause for GI distress. The bumping movement of running mechanically increases the gut’s contractions, pushing food faster through the gut. In combination with the hypoperfusion I mentioned above, food pushes through the gut fast and without proper absorption or digestion. The mechanical stress can also damage the gut cells, even leading to intestinal bleeding.

There seems to be an effect of posture in itself, too. On the bike complaints are, in contrast to running, often more limited to the upper gastrointestinal tract. This is likely due to the increased pressure on the abdomen.

Upper versus lower abdominal problems. From Oliviera and Jeukendrup, SSE, 2013.

Nutrition

A wrong choice of nutrition can trigger or exacerbate problems. Some are obvious, others aren't.

Food items high in fiber, fat, protein and fructose are known to induce GI problems. So are hypertonic sport drinks. Lactose-containing dairy is another common pitfall, as even mild lactose intolerance can trigger GI irritation and distress. Probiotics, dairy products that contain extra bacteria, might cause bloating and diarrhea, so these are best avoided the days prior to competition. The timing of caffeine intake deserves some special attention too, as one needs to carefully balance its mental effects with its toilet effects. Adequate fluid intake is important, as severe dehydration decreases blood volume and rises core temperature, which hampers nutrient absorption and affects GI motility, respectively. Intolerance to food components, such as lactose, gluten or milk protein, are sure to make you run for the bathroom as well.

Other

A (worrying) number of athletes use pain killers such as NSAIDs (ibuprofen, naproxen, aspirin) to relieve existing or anticipated pain, which are thought to increase the risk of upper GI complications.

After a race, the damage done to the gut cells might trigger intestinal bleeding. Though not uncommon, it is not unharmful. Amongst others, the damaged cells let bacteria and other harmful components slip into the blood stream easier, and your immune system will have a hard time fighting these off - energy that is better spent on recovery.

Though there is currently limited scientific evidence to support these guidelines, the following might help you run to the finish line rather than the toilet:

- Avoid dairy products and foods high in fiber, protein, lactose and fat 24-48 hours before competition. Dairy products can be exchanged for alternatives such as lactose-free milk, soy-, almond- or rice-milk. Artificial sweeteners can have a laxative effect, too.

- Stay well hydrated. Start the training or race well-hydrated and ensure an adequate intake of fluid during exercise. In a triathlon, make full use of the tummy-friendly bike leg to drink sufficient amounts of sports drinks (never just water) to replenish the fluids lost by sweat and evaporation and load up for the run. Remember that the rapid evaporation of sweat by riding wind (even a mild breeze) might mask fluid loss rates!

- Avoid sport drinks with only fructose as fructose is not as easily absorbed by the gut. The combination of glucose and fructose does not seem to elicit any problems, on the other hand.

- Avoid hypertonic drinks (pre/during/post exercise). Here, it is about finding the right balance between energy supply, liquid intake and osmolarity. A low concentrated sport drink has low osmolality and is less likely to trigger GI distress, but also has low energy and sodium content. (For those who wish to refresh their memory from biology and physics class ages ago - osmolarity is a measure for the number of particles in liquid. (Over)simplified, it’s a measure of how concentrated the sports drink is. A sports drink that is hypertonic means its osmolarity is higher than in your blood, whereas a hypotonic drink has an osmolarity lower than your blood. Thus, you can determine the osmolarity of the drink itself by making it more or less concentrated)

- A history of GI problems is a risk factor for more problems. If you know you are sensitive (and you wouldn't be the only one), take the time to figure out what works best for you.

- Make sure you are properly trained for the race. The higher the relative intensity, the more restricted blood flow to the gut will be. Things go from bad to worse once you end up in a down-ward spiral of feeling nauseous, having side aches or diarrhea, and not being unable to drink and eat as a result of that. Low blood sugar (hypoglycemia), dehydration and severe damage to your gut cells are all possible consequences that are best avoided.

- There is a scientific debate whether one can actually 'train' the gut to be less sensitive. Some say you can, others say you can't. In effect, it does no harm to practice new nutrition strategies many times before race day to determine what does or doesn't work. Get used to the brand of sports nutrition that the organisation of the race will give you on race day. Determine fluid loss rates by weighing before and after hard training sessions in a climate comparable to the expected race weather, and make sure you don't lose more than 2% of your body weight.

- So obvious it is easily forgotten: When travelling to foreign races, postpone experimenting with the local cuisine to after the race and pay extra attention to the freshness and ingredients of food. If you can't read the ingredients list of an unknown product, don't eat it. Don't show your cultural awareness by eating sausages with undefined content, prepared with an unknown level of hygiene and predictable level of bacteria. Be careful drinking water from the tap - the water may be pure and bacteriafree but high levels of chlorine can also upset your stomach. Choose your restaurant with your eyes, nose and brain rather than your stomach.

All this said, some level of GI discomfort seems unavoidable, given the fact that even well-trained, well-prepared elite athletes often experience problems. However, regular heavy damage to the gut should be certainly be avoided as it might make the gut more sensitive to food intolerances during every day life. The gut is a fascinating organ, but when it's not happy, nobody is happy.