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DISCLOSURES
Sports Science Insights is a consulting firm that helps companies
make informed decisions about issues related to exercise science
and sports nutrition.
Bob Murray founded SSI in 2008 after retiring as director of the
Gatorade Sports Science Institute (1985-2008).
SSI’s current clients include companies large and small with
interests in exercise science and sports nutrition.
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No Guts, No Glory:
GI Function During Exercise
Bob Murray, PhD, FACSM
Sports Science Insights
Crystal Lake, IL
bob@sportsscienceinsights.com
www.sportsscienceinsights.com
Incidence of GI Discomfort & Distress
Up to 70% of endurance athletes report
repeated incidents of GI discomfort or distress
during training & competition.
The brain can upset the gut &
the gut can upset the brain.
Enteric Nervous System
✓ second brain
✓ autonomous
✓ rich afferent output
✓ 100 million neurons
✓ 90% of serotonin
✓ 50% of dopamine
GI Discomfort & Distress
DISCOMFORT
Acid reflux
Heartburn
Bloating
Sloshing
Fullness
Dull abdominal pain
Side stitch
Eructation
Flatulence
Nausea
DISTRESS
Piercing abdominal pain
Vomiting
Diarrhea
Bloody stool
GI Discomfort & Distress
#1 complaint in race medical tents
Too much food or fluid
Physical
discomfort
Slowed
gastric
emptying
Osmotic
dumping
or
Fluid
overload
Fullness
Bloating
Sloshing
Burping
Nausea
Vomiting
GI Discomfort & Distress
#1 complaint in race medical tents
Too little food or fluid
Hypoglycemia
Hypovolemia
Slowed gastric
emptying
Reduced
splanchnic
blood flow
GI ischemia
Hypoxia,
ATP depletion,
acidosis
Hyperthermia
Mucosal injury
Increased
membrane
permeability
Loss of barrier
function
Endotoxemia
60-min cycling @ 70% MWC
Splanchnic hypoperfusion with
mild exercise increased markers
of intestinal epithelial cell damage.
Why so many GI complaints?
Bob Murray, PhD, FACSM
Gatorade Sports Science Institute
Barrington, IL
Athletes Do Dumb Stuff
Bob Murray, PhD, FACSM
Gatorade Sports Science Institute
Barrington, IL
Oops!
“I calculated my energy needs for the entire Ironman – about
8,200 calories – and tried to eat that much during the race.”
“I didn’t drink much so I could be lighter on the hills.”
“I’m sure my vomiting was because I only ate one
cheeseburger on the bike rather than my usual two.”
“The night before the race I drank 8 quarts of water to make
certain I was well hydrated.”
“I spoke to Tim DeBoom the day before the race, found out
what he ate and drank, and tried to do the same.”
Derek Clayton
1969 World Record: 2:08:33 (Antwerp)
For 48-h post-race: vomited black
mucous, defecated black diarrhea,
urinated blood clots
Never drank during a race
Hypovolemia = splanchnic ischemia
Rob de Castella
1981 World Record: 2:08:15 (Fukuoka)
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The Gut is an Athletic Organ
Goal: Get 8+ L water, 800+ g CHO, and 8+ g Na+
into the body over 8 - 17 hours.
Challenge: Accomplish this without impeding
gastric emptying or absorption of water & solute.
Gastric emptying
Absorption of water & solute
Practical tips
Experiments and Observations on the Gastric Juice
and the Physiology of Digestion (1833)
William Beaumont (1785-1853)
Alexis St. Marten attended to (and experimented on)
by Dr. William Beaumont
Gastric Emptying
Slowed by dehydration, hyperthermia, & too many calories
Responds to training
Gastric Emptying
Slowed by dehydration, hyperthermia, & too many calories
Responds to training
66 hotdogs and buns in 10 minutes
Gastric filling and emptying
Contraction of proximal stomach
Increased antral motility
Relaxation of pylorus
Gastric emptying during rest or exercise
% initial volume
100
50
20% CHO
12% CHO
0-6% CHO
0
20
10
Time (min)
30
300 ml pre-ex; 200 ml every 15 min
Gastric volume (ml)
300
150
200 ml
300 ml
0
30
15
Time (min)
45
Rapid Gastric Emptying
Comfortably large volume (trainable)
Low energy content (< 400 kcal/h)
Well hydrated & cool
= gastric emptying > 20 ml/min
Absorption of Water & Solute
Microvilli of human small intestine
- Enormous surface area for absorption
- Susceptible to overload & injury
Lumen
SGLT1
H 2O
H2O GLUT5
fructose
(galactose)
2 Na+
sucrose
H2O, Na+, K+, Cl-
glucose
H 2O
fructose glucose
fructose
H 2O
ATP
(passive diffusion)
Na+
Na+
GLUT2
glucose, galactose, fructose
H 2O
AQP
glucose, galactose, fructose
Solvent drag
(galactose)
K+
Paracellular
transport
glucose
2 Na+
K+
???
DRTS
H 2O
H2O, Na+, K+, Cl-
Plasma
From Lambert P, X Shi, R Murray. The Gastrointestinal System. ACSM’s Advanced Exercise Physiology, 2nd edition. Baltimore: Lippincott Williams & Wilkins, 2012.
Increased HR = Decreased Splanchnic Blood Flow
60-min cycling @ 70% MWC
Splanchnic hypoperfusion with
mild exercise increased markers
of intestinal epithelial cell damage.
Gut Facts
Microvilli have low PO2, so are susceptible to ischemia.
Gut can withstand 12 h of 75% reduction in SpBF
without histological changes.
Severe reductions in SpBF can lead to GI lesions,
increased permeability, hemorrhagic gastritis,
hematochezia, ischemic colitis, endotoxemia.
Some athletes may have a hyper-reflexive decrease in
SpBF whereas most others do not.
SpBF is higher in athletes at any workload and that
may improve barrier function.
Gut Adaptations with Training
Increased tolerance for eating and drinking during exercise
Increased skill at eating and drinking during exercise
Increased gastric capacity for food and fluid
Improved timing & content of intake
Increased gastric emptying rate
Up-regulated transport proteins
Greater SpBF at any workload
Faster oro-cecal transit times
Proliferation of microvilli
PRACTICAL TIPS
Avoid the 3 B’s: Bloating, Burping, & Barfing
For persistently symptomatic athletes
✓ Get fit
✓ Get acclimatized
✓ Reduce exercise intensity
✓ Stay hydrated & fueled
✓ High-fat foods post-ex
✓ Stay cool
✓ Eat foods high in nitrate
✓ Drink/eat small quantities frequently ✓ Try glutamine, citrulline, or arginine
✓ Don’t over-do it
✓ Calorie intake between 200-400 kcal/h
✓ Fluid intake to minimize weight loss
✓ Train to eat & drink
✓ Eat a high-carb diet
✓ Avoid high-fiber foods before exercise
✓ Start exercise with empty bladder & colon
✓ Start exercise with some fluid in the stomach
✓ Minimize/avoid NSAIDs
• Brouns F, E Beckers. Is the gut an athletic organ? Sports Med 15:242-257, 1993.
• Murray R. Training the gut for competition. Curr Sports Med Reports 5:161-164, 2006.
• van Wijck K et al. Physiology and pathophysiology of splanchnic hypoperfusion and intestinal injury during exercise:
strategies for evaluation and prevention. Am J Gastrointest Liver Physiol 303:G155-G168, 2012.
Thank you!