I know a few people who would vomit after or during training (various running exercises). My trainer said it was due to the lactate level in the blood, though I can't find a source to corroborate this.
According to the reference from Wikipedia's entry on exercise-induced nausea the duration of exercise and intensity (aerobic vs anaerobic) and prior food or drink consumption are the main factors contributing to this effect. I gather it is not a very common phenomenon. It seems like a basic reflex to get rid of food so the muscles and/or brain can get more blood.
So is a high lactate level and a non-empty stomach all there is to this? If so I think it would be more common. I'm guessing there is a psychological element too? Can anyone explain the triggers/causes better?
Consider the following scenario: a group of 30 people of a similar age perform intensive exercise. 2 people feel really nauseous afterwards. Assuming that hydration is not a cause, what mechanism(s) explain(s) why these people feel sicker?
I think assuming that only pH is a factor is flawed. The wikipedia article I mentioned explicitly states that food intake affects nausea. I can also corroborate this with experiences of many of the people (friends/family) I asked. The article also states that nausea is a consequence of bloodflow to the stomach/gut being restricted (implying the pH recepter does not directly induce nausea). So I'm guessing this flow restriction follows from the pH mechanism in the brain?
By piecing together this info I then get this order of events:
1 - Exercise reduces pH
2 - Brain pH receptor reduces blood flow to stomach
3 - Stomach receptors trigger nausea when blood flow is low (stronger signal if stomach is fuller)
This theory seems to cover both claims of pH (in the answer below) and food intake (in the wiki article). But it is just conjecture on my part. I could imagine a 4th step where a person has less nausea if they produce more endorfins during exercise, or more when they have a psycological tendency to throw up whenever nauseous. Or maybe I'm wrong about pH affecting blood flow to the stomach.
So, barring hydration-related causes, is it just blood pH and stomach contents that determine the nausea felt after some forms of exercise?
And if pH is a factor (as stated by an answer), how much does sensitivity depend on 1)their fitness/condition to counter the pH decrease and 2) their physical response to a low pH--e.g. maybe a genetic factor makes some people more sensitive?