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Dear DrSugar,
Sometimes after a rigorous cardio workout (one hour, doing intervals for the first half hour and steady pace for the second half), I come home and start feeling queasy and weak, then need to lie down. Sometimes I feel this way for an hour or more postworkout, well after I've showered and eaten. There have been a few times when I've lost my stomach as well. But it usually goes away after an hour or so of resting/lying down. What's going on? I try to be well hydrated and have a little snack a bit before working out, but sometimes, it doesn't seem to help.
— Workout Weary Wendy

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Feeling queasy or weak following or during rigorous exercise could be due to a variety of factors; however, lactic acidosis is the most likely cause of your symptoms. Lactic acid is produced by muscles during anaerobic metabolism. Lactic acidosis most commonly occurs after long, rigorous exercise like you’ve described. Common symptoms of lactic acidosis include nausea, weakness, fast heart rate, hyperventilation, and sometimes even anxiety. Lactic acidosis can be more of a problem when coupled with dehydration, prolonged exercise, starvation, alcohol use, and strenuous exercise when not in excellent physical condition.

Preventing lactic acidosis can be difficult if your goal is prolonged anaerobic exercise such as sprints or intervals. The first step is to maintain adequate hydration before, during, and after your workouts. Another way to avoid this problem is to separate your aerobic and anaerobic workouts on alternate days by jogging Mondays, Wednesdays, and Fridays and doing sprints, intervals, and/or weights Tuesdays and Thursdays.

The other possible cause of your symptoms is low blood sugar, which can also cause weakness or queasiness during or after exercise. To prevent low blood sugar you can avoid prolonged exercise on an empty stomach, before breakfast, or before dinner if you didn’t eat an adequate lunch. And, of course, always avoid starvation diets. They don’t work and they are dangerous. Sometimes these post-exercise symptoms can be related to an underlying metabolic problem such as diabetes or liver disease, therefore it is highly recommended that you see a physician if the problem persists.

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