An athlete's ability to reach maximal performance is a direct result of physical performance, stress tolerance, and immune function. The gastrointestinal (GI) tract is also part of the system that regulates adaptation and regeneration of the athlete.
For over a decade at CF, hurling has been unofficially viewed as somewhat of a badge of honor with a pet name, photos, and T-shirts to commemorate the event. However, vomiting is at least an abuse to the sensitive esophageal lining highly intolerant of acid baths and possibly, when a frequent event, causing inflammatory damage to that important sphincter separating the stomach from the esophagus.  Damage of the lower esophageal sphincter muscle leads to reflux and consistent irritation of the esophagus leads to at least inflammation and at worst esophageal ulcers.
Additionally, intense exercise causes decreased blood flow (ischemia) to the gastric mucosa–swimmers have known this for as long as I can remember. The result may be nausea, which at some point may progress to vomiting, stomach cramps and diarrhea—more than just impaired digestion. These symptoms can be severely and even critically compounded when hot weather and dehydration are thrown into the mix.
Without sufficient blood supply the GI tract simply can’t function as designed, rejects its stomach contents, and in some cases the contents of the intestinal tract (diarrhea). The ischemia achieved by athletes in long endurance events also compromises the intestinal barrier, contributing to and compounding the GI symptoms.1
The intestinal barrier. The stomach and intestines are lined with a protective barrier formed by an intricate combination of membranes, junctions, mucus, and immunological factors. Different types of stress can breach this barrier, causing increased permeability of the gut lining and allowing entry of harmful bacterial toxins into the blood stream. This distressing concatenation of events may in turn cause inflammation and systemic complications as well as the nausea, vomiting, bloating, bloody diarrhea, and cramping seen in up to half of all participants in endurance events.1 CrossFit WODs are an intimate union of anaerobic and aerobic lasting about 20 minutes. All the same, because of the intensity we may dip slightly into the GI effects of the long endurance events.
Dial it back. As you will see in a report from Dr. Lon Kilgore, our bodies can adjust to this challenge---the annoying nausea, vomiting, and diarrhea, if we back off intensity when the first symptoms of nausea appear.
Just back off a bit and see if you’re OK. Don’t push it to meet Pukie; he’s not worth it.
Crossfitters don’t quit, but we can dial it down. If we respond early enough, this may be all it takes to restore adequate blood supply to the stomach. However, if the symptoms persist, take a break and lie down before it progresses further. Wait until your system recovers, then finish the workout. If vomiting is inevitable, we have all seen that we do recover with no lasting damage. We eventually adapt, improve VO2 max, and get stronger.
Physiology of adaptation. More is appearing lately in the scientific literature about the impact of exercise on the GI tract, but very little is said about how our adaptation to GI ischemia takes place. One of many suggestions makes a lot of sense and is very compelling.
There is an area in the brain that detects toxins in the blood and acts as a vomit inducing center among other functions. Some drugs, for example, morphine, activate this area. It is called the area postrema and it’s located in the medulla oblongota. Studies in animals have shown that this area can be desensitized.  Unfortunately, the effects of GI ischemia, desaturated blood oxygen, or lactic acid on the area postrema, to my knowledge, have not yet been addressed.
A SCIENTIFIC EXPERIMENT BY ONE OF OUR OWN
What we do have, however, is an email from Lon Kilgore Ph.D, who did a study in his laboratory at Midwestern State University, Department of Kinesiology involving his own performance of 4 consecutive weeks, 5 days per week of multi-modal CrossFit workouts to evaluate the affect on VO2 max. Dr. Kilgore very generously took time to read this article. His response is in the following two emails.
" I did a 4 week pilot study of blood oxygen desaturation to 91% or lower via CF training and its relationship to improving VO2 max .
The first workout out off the blocks was Cindy (used as a bench mark) and within 5 minutes I experienced relatively profound nausea. My strength allowed me to work hugely faster than my oxygen handling systems could and thus---severe vascular shunting away from the viscera and nausea. Five minutes later I was back finishing the workout but at a slower pace for sure.
Two days later Diane (as Rx'd) kicked my butt about 7 minutes in for the same reason, 225 lb. deadlifts and handstand pushups are easy but doing so many so rapidly caused a repeat session of nausea and a dismally slow time.
desaturated to 89-91% throughout those two sessions. What I think is
happening is that by the time you are experiencing nausea, you have
driven your O2 saturation too low (worked too hard) causing profound shunting away from the GI tract.
You need to push just shy of that point, to the edge of nausea, to get the maximal results, but pushing on into full on nausea and Pukie-town slows down the work rate and makes you miss the target workload - and thereby the fitness gain possible from that workout.
It is inevitable that people will miss the mark and get nausea and ralph their guts out every now and then. And that should be a learning experience, they should learn about the sensations and the signals their body is sending them so they can push the envelope with more accuracy and avoid spending valuable training time rolling on the floor nauseas or praying to the porcelain god.
After I figured it out (took two sessions of nausea), I knew when to start taking breaks rather than just brute forcing it. And it paid off, 33.4% increase in VO2max in four weeks."
Email #2 at my request:
Daily log of nausea
Days 1 and 3: Cindy and Diane. Huge and nasty nausea on days 1 and 3. The kind you lay on the bathroom floor with. The kind reminiscent of a severe college all night bender.
Day 2: Not bad on day 2 (easier workout)
Day 4: Day 4 not bad either.
A running repeat day and kettlebell swings I believe and there was a
little nausea controlled by the trainer (Justin) telling me to back off.
After that it never reappeared.
<The “Justin” in Lon’s email is Justin Lascek, head of CrossFit Wichita Falls at Mark Rippitoe’s Wichita Falls Athletic Club.>
Hydration. I can’t close without at least a word on hydration and its affect on digestion. An elegant study out of The Netherlands has found that dehydration causes a delay in the emptying of the stomach. This in turn may result in exercise-induced nausea. It’s not a good idea to arrive for our workout in a dehydrated or NEVER an over-hydrated state (a dangerous condition). A very brilliant physician once told me: “Let thirst be your guide and pay attention to its prompts.”
- Never give up on your CrossFit workouts because of nausea.
- When nausea occurs, back off the intensity or if needs be lay down until you recover and then finish the workout. You will adapt and improve VO2 max; thanks to Dr. Kilgore, we have it on very good authority!
- While progressing in your personal nausea-abatement program, don’t skip your planned workouts.
- Never arrive for workout right after a meal, dehydrated or overhydrated. We have a very easy way to tell when we need fluids. It’s called thirst.
Lon Kilgore Ph.D, deserves everyone’s thanks for sharing his very personal experience in a respected scientific study involving CrossFit WODs and VO2 max. His observations on nausea are a gift. Thank you, Lon, for being a very brave and strong guinea pig.
Big credit is due Ronnie Boose at CrossFit North Santa Cruz for his inspiration with a photo and an excellent post on their web site (June 9). Thanks Ronnie, you really launched this one—been waiting years for an opening!
Denver also deserves credit and acknowledgement for taking me one step further. Thank you, Denver!
Thanks to Kelly Starrett Ph.D for suggesting the area postrema. I hadn’t heretofore known about that fascinating spot. Love that word!
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 Kawamura O, et al: Endoscopic ultrasonographic abnormalities and lower esophageal sphincter function in reflux esophagitis. Digestive diseases and sciences, 1995, vol. 40, no3, pp. 598-605 (18 ref.) Gunma univ. school medicine, first dep. internal medicine, Maebashi, Gunma 371, JAPON
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 Masaru SoriMmachia, Minoru Wakamoriam Norio Akaikeb: Excitatory effect of ATP on rat area postrema neurons. Purinergic Signalling, Vol 2(3),2006.
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2000 Dec;83(6):578-84. Department of Gastroenterology, University Hospital Maastricht, The Netherlands. M.vanNieuwenhoven@HB.UNIMAAS.NL