What factors affect exercise induced nausea?

Question

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?

Edit:

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?

Answer 1

The simple answer is "metabolic lactic acidosis".

According to Wikipedia, the duration of exercise and intensity (aerobic vs anaerobic) and prior food or drink consumption are the main factors contributing to this effect.

Actually, the Wiki articles doesn't quite say that (?).

Prior food/drink would determine the glucose and glycogen - the stored form of glucose - available. Also, hydration is important.

The very, very, very small study (12 volunteers) linked by Wikipedia concludes that

nausea [was] greater during exercise at fasting state and immediately after eating [a hamburger patty] than those without exercise (p<0.05 during low-intensity exercise, and p<0.01 during high-intensity exercise). Immediately after eating, scores for nausea were greater during high-intensity exercise than during low-intensity exercise (p<0.05). During high-intensity exercise, scores for nausea were greater immediately after eating than without eating (p<0.05).

Normally, muscles using glucose as an energy source produce a 3-carbon molecule called pyruvic acid (or pyruvate). In the presence of oxygen, the pyruvate is further metabolized into CO₂, which is carried by the blood and exhaled from the lungs.

Exercising beyond a certain point causes muscles to burn glucose under anaerobic (no oxygen) conditions (the blood cannot deliver oxygen to the muscles quickly enough.) The result is that pyruvate is converted into lactate (lactic acid) instead of CO₂ which is released from the muscle cells into the bloodstream, where it accumulates over time. Since lactate is a relatively strong acid, it results in a decreased pH (an increase in free hydrogen ions, or H+) of the blood. This is called lactic acidosis.

Normally, H+ in the blood is efficiently handled by a buffer, bicarbonate.

The direction of the equation in the case of acidosis is driven to the left by the concentration of hydrogen ions. Normally, the CO₂ that is formed is simply exhaled by increasing the respiratory rate. This decreases the H* concentration and is called compensatory respiratory alkalosis.

There is respiratory compensation whereby the body increases breathing to expel H+ in the form of CO2.

In the presence of excess H+, the body tries to compensate metabolically by increasing HCO₃-, which takes time. With continued exercise, the body cannot 'make' enough HCO₃- to handle all the H+, and the blood's pH falls.

This is the situation which results in nausea and vomiting if serious enough.

_{Introduction to Acid-Base Disorders <- click through}