It depends what time the race is.
The only time I feel sick is when I haven't warmed up properly, and when I've eaten too much and/or too soon prior to very intense efforts.
We're not all the same, and I find that even a small meal two hours before very intense efforts is far too soon.
Also, it's my understanding (and experience) that the airways have to been warmed up along with the legs. Ever tried doing all-out track sprints with only 5 laps warm up? BLAH!! Takes me half an hour to get over it.
So, if your races are early (or earlyish) in the morning, I'd consider having a very, very light breakfast (or even skipping it, and have a larger and later than normal meal the night before.
Also, if your races start off very quick, right from the jump, try to get in a warmup with some intensity, perhaps by riding to the race.