How much of a calorie deficit is too much?

Starvation mode, insofar as people mean that your body will start hoarding fat somehow because it's scared, is a myth.

Eating too little will not stop your weight loss.

It can cause plenty of health problems though. However, one day of undereating is fine if it is not a habit. (As is one day of overeating if it is not a habit.)

Otherwise, you haven't given your deficit or your stats, so no one can advise you more specifically on what sort of deficit would be too big for you.

Welcome to the forums! Good luck with your calorie-counting!

18JessieS wrote: »

When does your body go into starvation mode?

Uh oh.

18JessieS wrote: »

Basically I have a huge deficit today and I want to know if it is too big? When does your body go into starvation mode?

Starvation mode, not a thing.

Of course we could answer questions about your deficit, but since you didn't give any numbers we can only speculate.

Generally, your MFP calorie allotment includes your deficit. Eat to that number.
If you log exercise and you're given more calories by MFP, eat about 1/2. Exercise burns are considered overestimated.

Having a large deficit on one given day is not a problem, but you do not want it to be a common thing, since you can be getting too few nutrients overall, and be losing muscle mass.