Is it bad to lose 6 lbs in one day when sick?

Yes, you should contact your doctor. It can depend on what you are sick with, but whatever it is, you should consult your primary health care professional. It can be very bad for some people and not good for others. It likely does represent a loss of a lot of water weight because fat does not burn away that quickly. You are probably either dehydrated or at risk of serious dehydration from that. Loss of fluids in that amount from your body could come from profuse sweating with fever, vomiting, diarrhea or overuse of diuretics.

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