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How To Avoid GI Issues During Endurance Training

By Edited Nov 13, 2013 0 0

GI Distress And The Endurance Athlete

What You Need To Know To Prevent GI Distress


GI Distress And The Triathlete


One of the constant realities of being a competitive athlete is that at some point in your training or racing career you will experience significant GI distress.  By that I mean dehydration, delayed gastric emptying which results in nausea and possibly vomiting, abdominal pain and diarrhea.  

Occasionally athletes, especially triathletes and marathoners, may experience bloody diarrhea which is a result of the gut not getting enough blood flow (ischemia).

Training and competing in hot and humid environments has a profound effect on limiting sweat evaporation that results in muscle glycogen (energy) depletion and is a significant risk factor for heat-related illness.

There is data that shows that lack of adequate blood supply (underperfusion) of the gut leads to damage to the lining of the intestine which can lead to blood loss and the invasion of bacteria from the gut into the bloodstream.



The GI complaints caused by exercise occur because of redistribution of blood flow which shunts blood from the organs to the skeletal muscle, heart lung and skin. Common symptoms include dizziness, nauseousness, stomach or intestinal cramps, vomiting and diarrhea and this occurs anywhere from 30-50%


The most severe symptoms usually include vomiting diarrhea and occur mainly during running.  Scientists believe that the up and down movement of the intestines and stomach contributes to this phenomenon.  Also consuming beverages having a very high concentration (osmolarity) have also been linked to a variety of unpleasant GI sensations and distress.


Hyponatremia or low sodium has been occasionally reported among slow competitors in triathlon and most likely is caused by prolonged sweating and very high intake of water or other low sodium drinks.



Exercise-induced dehydration


There are number of normal responses to exercise including changes in the endocrine system and hormone levels.  As athletes exercise the frequency and contraction forces of the heart are increased, the tone of the blood vessels particularly in the intestinal area and kidneys as well as non-contracting muscles and veins are increased.  This is how the output of the heart is improved; the constriction of blood vessels in these non-exercising areas that results in redistribution in favor of the working skeletal muscles and skin.


Blood flow to the skin is dramatically increased to facilitate loss of heat through sweating and evaporation.  It's normal to lose approximately 2-3% of body mass during routine intermittent high intensity exercises particularly when the ambient temperature is high.

We know that thirst as triggered one a person his artery approximate 5% dehydrated. This can be worsened by suppression of the thirst response by stress hormones.

The heart rate rises and additional 3-5 beats per minute for every 1% body weight loss due to dehydration.  This has a negative effect on endurance performance by increasing use expenditure of muscle glycogen and also increasing plasma lactate levels(feel the burn in your muscles).


The concept of cardiovascular drift relates to increasing heart rate at a given level exertion.  The reduced ability to transport heat to the skin and periphery of the body results in increased core temperature.


During exercise heat production is increased up to 20 times that over rest and will potentially cause a core temperature arise 1°C every 5 minutes.  The body has multiple thermoregulatory adjustments is quite adept by utilizing multiple mechanisms to cause heat dissipation.


Once the outside temperature rises above 20°C, the bulk of heat loss occurs from a perspiration in sweat.  Exercising in hot dry environments results in 90% of heat loss through evaporation alone.  The massive evaporation of sweat could potentially lead to dehydration that will increase the internal body temperature further.


There are several types of common heat illness including heat edema, heat cramps, heat syncope and heat exhaustion.  The most severe form of heat illness is heat stroke.  

The body’s ability to dissipate and regulate temperature is lost causing damage to tissues.  Heat stroke has a constellation of clinical findings and results in damage to multiple organs.  Marked core temperature elevations of greater than 40°C can be seen in heat stroke.

Unless emergency medical attention is soft and aggressive cooling measures undertaken the mortality rate can exceed 10%.


Blood flow to the gut and liver will be reduced by almost 80% when athlete exercises at 70% of the VO2 max.  The decreased blood flow changes nutrient absorption and slows the movement of food through the intestinal tract.  This is one reason why 30-50% of endurance athletes, particularly marathon runners and long distance triathlon competitors will experience GI distress.


Once in athlete exercises at intensity of over 70% of the VO2 max gastric emptying is also delayed.  This is significantly associated with exercise-induced nauseousness. Contributing to this includes exercise-induced dehydration and possibly thermal effects related to increased body core temperature.  


From reviewing the literature, we know that risk factors for exercise-induced GI symptoms include dehydration where the body weight loss is greater than 4% of the pre-exercise weight, being female or younger, high intensity exercise, vertical impact sports and medication use including aspirin.


Sports Drinks

Oral rehydration solutions and sports drinks are designed to facilitate rapid fluid delivery and recovery after exercise.  The addition of sodium and carbohydrates to these fluids assists absorption of water and allows more efficient fluid replacement than just water alone.  


The maximum rate of intestinal absorption is 0.5 L per hour (from studies involving cycling at 85% of VO2 max).  These studies also noted that almost 1 L per and C. 0.9 L) remained in the stomach and intestine at the end of exercise.  This data was obtained by radioisotope labeling of water.  The intake of large volumes of water are unlikely to be advantageous because there is no enhancement performance and the absorption is limited by the above rate.


Glucose is actively transported across the intestinal membrane and is aided by the addition of sodium.  This is why the addition of sodium to most carbohydrate sources and sports drinks are widely recommended.  


Post exercise rehydration as best achieved by consuming beverages that have a higher sodium content  (being greater than 60 mmol) and also consuming a volume equivalent to 150% of the body mass loss during exercise.


There is evidence that blending different and carbohydrates may result in increased gastric absorption and fluid delivery when compared to consuming glucose only.  Studies have shown benefit of glucose and fructose combinations although this is somewhat controversial.


Combining different sources of carbohydrate to produce a rehydration drink with up to 5% total carbohydrates will provide sufficient fuel for muscle performance during activity.  Never go higher than 10% because this is associated with an increased risk of cramping, diarrhea and nausea due to the high osmolar load of carbohydrate concentrations exceeding 10%.


Hypertonic solutions such as these delays transport in the intestine because water instead flows into the intestine due to osmotic effects.  Balance must be between the goal of maintaining hydration and providing energy substrate to the working muscle with the realization that slow gastric emptying will occur during strenuous training.


Consuming highly concentrated beverages will slow movement of substances through the GI track even further.


Current nutritional recommendations include drinking during exercise to prevent excessive dehydration and excessive changes in the electrolyte balance.  Also maintaining an adequate carbohydrate consumption to provide an energy source for muscle.  The general recommendation is that sports drinks have carbohydrateconcentrations around 5% and no more than 8%.


Severe Allergy To Training??

Another interesting but serious condition related to training is FDEIA. This condition results in severe allergic symptoms related to exercise.  FDEIA= Food Dependent Exercise Induced Anaphylaxis. 


We know that specific foods can cause allergic symptoms after exercise.  We also know that taking aspirin along with exercise increases the permeability of the intestinal tract, which is believed to allow allergy causing (antigens) substance and certain foods to enter the bloodstream and result in the onset of anaphylaxis during or soon after exercise.


This is an IgE mediated reaction which means that it can be rapid in onset and life threatening in a matter of minutes. FDEIA happens only after consumption of a food ALLERGY if it is followed by vigorous activity within a few hours of consuming the food.  It's also dependent on the amount of food intake.


 Hyponatremia - Low Sodium

Low Sodium (hyponatremia) is usually do to excessive water intake and has been reported in long-distance runner's and long distance triathletes.  The symptoms include those that are very similar to dehydration which includes weakness fainting mental confusion.  Symptoms are not usually seen until sodium concentrations get down to the 126-130 mmol per liter level.


The confusion lies in that because symptoms of hyponatremia are very similar to that of dehydration, this condition is often misdiagnosed in endurance athletes.

 The treatment for dehydration is oral and IV fluids. However, such a treatment for an athlete already low in sodium could be catastrophic.  


Hyponatremia in the endurance athlete usually occurs due to excessive fluid overload and intake of a higher volume of fluids than the sweat rate. This causes the sodium to be diluted out in the blood.

 Prevent this by avoiding over hydration and avoid consuming just plain water (medically called free water).  

Many athletes use salt tablets when competing in iron distance events.  The sports medicine data shows no evidence that intake of salt tablets at a rate of 700 mg per hour is any better than fluid replacement alone with a balanced sports drink



The general recommendations:

1)   Take in  .5 liter per hour of a sports drink

2)   Consume electrolyte/sports drinks during and after prolonged exercise

3)   Keep the carbohydrate concentration between 5-8%

4)   Combining carbohydrate sources may increase carbohydrate oxygenation and increase energy production


TriDoc7 - Triathlon A Physician's Perspective




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