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.
- 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.