You might hate doing sit-ups, but in most cases, they shouldn't make you feel nauseated. If you feel sick to your stomach during or after a strenuous round of the exercise, it's likely not the sit-ups itself that's making you ill, but rather a related cause. If you can figure out what's making you nauseated, then you can take measures to avoid it next time you're engaged in an intense core workout.
Read More: Can Exercise Cause Upper Abdominal Pain?
Athletes in action may lose 1 to 2 percent of body weight through fluid loss. Unless you've done 500 of them in an Amazonian rain forest, sit-ups by themselves are unlikely to cause this degree of fluid loss. But if you're well into a long and strenuous workout, it's important be mindful of hydration. Thirst, however, is not necessarily a good indicator of dehydration. You can lose as much as 1.5 liters of body fluid without feeling thirsty. The optimal amount of fluids to drink depends on many factors: your size and weight, the level of intensity in your workout, heat and humidity, and diet. It's a good idea to drink extra water before, during and after heavy workouts or exposure to warm temperatures. The goal is to drink before you get thirsty. For workout sessions longer than 90 minutes, sports drinks with electrolytes are best.
Read More: Importance of Drinking Water During Exercise
During exercise the body produces 15 to 20 times more heat than when it is at rest. If for some reason the body isn’t adequately dissipating heat, body temperature rises. High humidity, dehydration and anything else that prevents sweat from evaporating can cause your body to overheat. That can cause nausea, muscle spasms, vomiting and other symptoms. Once again, it's unlikely that sit-ups alone would cause over-heating unless they're done at the end of a long and strenuous workout, or after time spent in sauna or steam room.
During exercise, blood flow to gut and liver is reduced by almost 80 percent. Physical activity also prevents gastric emptying. That can cause gastrointestinal complaints. And because sit-ups stress the very core of your digestive tract, it's possible that this could enhance any tendency toward nausea. So that alone is a good indication that you don’t want to carry a heavy digestive load into your workout. Exercising immediately after eating has been shown to make nausea more likely. A study in the journal Appetite found that athletes reported experiencing more nausea when they engaged in exercise immediately after eating, and that nausea increased with the intensity of the exercise.
Do you find that exercise in general brings on symptoms of heartburn? You might be experiencing gastric reflux, more commonly known as acid reflux, which can trigger nausea. This occurs when acidic stomach contents back up into the esophagus because of a weakness in the muscular band that normally seals tightly to prevent this from happening. Gastric reflux is triggered by impact and gravity, so the rocking motion you experience during sit-ups—especially if doing them on a reclining board—could be causing reflux that triggers nausea.
If you’re really working out hard, it’s important to watch out for signs of fatigue. According to the American College of Sports Medicine, cautions against doing reps to the point of complete muscle failure. It can lead to unconscious breath holding, which can cause nausea and dizziness.
Read More: I've Been Throwing Up After a Workout