[trux]

Well, I am not an expert, but it is an interesting topic so I googled it up, and found indeed some interesting information.

Tony is certainly right that the nausea and diarrhea are primarily caused by the lactic acidosis. Blood shift (hence reduced blood flow in the digestive tract), and strong diaphragmatic contractions during prolonged apnea likely aggravate the situation further.

The tolerance to lactic acid increases with training, but if you experience such problems since a long time with no improvement, there may be some additional factors involved. Dehydration or over-hydration are the most obvious ones (more about it for example here: Puking After Working Out).

You can also ask your physician to test your liver - it is the liver that breaks down acid lactic, so if it is not all right, you can indeed have more problems than others do. You can also ask the doctor to do a resting lactic acid test, to see if the resting level is not pathologically increased. More about it here: Lactic Acid

I found some excellent threads in a martial art forum. Read through them, there is some excellent info and good advices. Especially the guy with the nick MikeMartial posts excellent information:
Why Do We Puke When Exhausted? - Sherdog Mixed Martial Arts Forums
Anything to reduce WO nausea? - Sherdog Mixed Martial Arts Forums

Some quotes from the threads:

The Gastrointestinal System and Exercise
It's well established that intense exercise (e.g., >85% VO2 max) shunts blood away from the GI tract and slows down digestion and absorption. However, at exercise intensities that ultra cyclists use, digestion is not impaired, provided hydration is maintained.
...
The sensation of nausea and subsequent retching/vomiting is controlled by the vomiting centers in the reticular formations of the medulla (in your brainstem). It receives signals from four sources:
1. the chemoreceptor trigger zone (see below)
2. visceral nerves from the GI tract (ate something bad, stomach flu, food poisoning, etc)
3. visceral nerves from outside the GI tract (e.g. gall bladder stone causing you to vomit, heart attack causing intense nausea, etc)
4. nerves from the extramedullary centers in the brain (e.g. bad odors, fear, motion sickness, brain injury)

What I want to focus on is #1, the chemoreceptor trigger zone. This "zone" is located in your brainstem and gets activated whenever an abnormal systemic, whole-body event is happening, like hypoxia, diabetic ketoacidosis, uremia, vomit-inducing drugs, AND IN THIS CASE, lactic acidosis.
...
"The anaerobic threshold (AT) is the exercise intensity at which lactate starts to accumulate in the blood stream. This happens when it is produced faster than it can be removed (****bolized). This point is sometimes referred to as the lactate threshold, or the onset of blood lactate accumulation (OBLA). When exercising below the AT intensity any lactate produced by the muscles is removed by the body without it building up.

The anaerobic threshold is a useful measure for deciding exercise intensity for training and racing in endurance sports (e.g. distance running, cycling, rowing, swimming and cross country skiing), and can be increased greatly with training."

They contain also some advices how to reduce the nausea:

Hydrating might have a slight effect in prolonging the duration of exercise time before experiencing nausea (fluid increase may reduce concentration of lactic acid in the bloodstream slightly), but if you're exercising past your anaerobic threshold, hydration will only be a temporary measure before the inevitable happens.
...
NaHCO3. I can sell you some, but it'll cost you, and I have to disguise it in the mail....

Seriously, try some sodium bicarb, aka baking soda. There's a few things floating around on it, and for something like 20 rep hell programs, it just may do the trick to buffer that excess lactic acid.

More files on the topic are for example here:
Ultracycling: The Puke Files
Puking After Working Out
Post Workout Nausea - MESO-Rx
lactic acid nausea OR diarrhea - Google Search
puking exercise - Google Search


PS: could someone click below on the link "add tags" and add the tags "nausea" and "diarrhea"? I already exhausted my limit of 2 tags for this thread. Also, moderators should consider changing the thread title to something more describing the topic. I think it is an important topic, and many freedivers may be interested in it. The current title won't attract a lot of attention among most people. Something like "nausea & diarrhea during training" may be better suitable. And moving it to the Freediving Science subforum may be a good idea too.