More than 100 situps a night, one night doing 400. how quickly will she lose weight?

Not: I admire anyone who does 400 situps a day, but it probably won't make you (her?) lose any weight at all.

It depends...: Doing sit-ups burns 4-8 calories per minute, depending on the rate one does them (you burn more when you do them quicker). It takes 3500 calories burned to lose 1 pound. This calculates to needing to do vigorous sit-ups for 1 hour a day to lose 1 pound in a week! this isn't the most efficient weight loss strategy & 1 hour of vigorous sit-ups is impossible, but it will tonify the abdominal muscles.