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A few sources online, like this one, say that an athlete should fast for 4 hours and abstain from caffeine before a standard lab V02 max test in which a mask measures air inhalation and expiration. By what physiological mechanism would food or caffeine affect results in a V02 max test?

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For caffeine, it is a mild stimulant. Especially if you are not used to drinking caffeine, it can artificially elevate your heart rate, and give a false reading. That is why the advisory against alcohol and smoking/nicotine, because those can alter your heart rate as well.

I could not find specific reasons for the fasting, and a few say to eat up to 3 hours before the test rather than 4. Based on previous readings, when your body is digesting it is more active than it is in a resting state, so my assumption would be that the testing site wants to avoid anything that could throw off the test. (RMR testing has similar restrictions listed).

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  • I wonder why heart rate or digestion would matter, though -- do those affect air expiration?
    – half-pass
    Commented Sep 2, 2021 at 20:35
  • Maybe the thing about eating is just that they don't want people throwing up all over their fancy equipment.
    – Nobody
    Commented Sep 2, 2021 at 21:09
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A VO2 test essentially tests your heart and lung capacity. The lung surrounds the mediastinum and the heart and stomach are the two main organs lying within it.

Therefore, as VO2 tests make your heart and lung (to be more precise: the diaphragm and other breathing muscles) work at peak power and their volume changes are at their maximum, there is immense pressure going on in this region. A full stomach, therefore, means that they cannot work at their full potential.

Another problem is that this pressure at high-intensity exercise leads to gastric emptying delay, which is a main cause for exercise-induced nausea.

Long story short, if you want to see your full potential and don't like throwing up, your stomach should be relatively empty for the test.

As of caffeine, it's like @JohnP said: it elevates your base heart rate and is (or rather because it is) a vasodilator. Therefore, it influences test results.

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Anything that causes the body to burn energy other than the nrg spent during the test is counter productive. You want to be fresh and fueled but not digesting at the time of the test as this burns energy

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    You become tired with a full stomach because digestion is linked to parasympathetic neural activity. If you force your body into (sympathetic) athletic activity, digestion will essentially stop.
    – Philip Klöcking
    Commented Jul 18 at 10:29

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