What is a safe deficit?

Your body doesn't stop losing from eating too few calories. However, you do stand a chance of losing more muscle mass and developing other health issues from not getting adequate calories/nutrition. If you set your profile up to lose weight, then you should be eating the 1800 calories plus a portion of your exercise calories.

A safe net deficit is 0 - 1000kcals on average. That means, assuming you are overweight, you can safely eat up to 1000 calories less than you burn in a day on average. This is equivalent to losing up to 2lbs a week.

Now, it's hard to calculate your net with very high accuracy. Exercise calories are often overestimated by a bit. When cutting, I set MFP to "sedentary" and "lose 1lb per week." Then I typically shoot to eat somewhat less than the exercise calorie estimate. If you do that and track your food and exercise as accurately as possible, you'll lose somewhat more than 1lb per week.

dolphinsfan4ever wrote: »

I started MFP about a month ago. My calorie goal is about 1800. I tend to eat 1200-1400 and burn about 500 calories. I am not depriving myself or dieting. I am just tracking to be more aware of how much I am eating. Is there a limit on deficit where your body stops losing because its too low. I know its calories in vs calories out but I also know your body needs fuel to function well

Taking your numbers at face value, that would mean you are netting 7-900 calories, and that would be unsafe. However, unless you are logging precisely by gram weight and are an athlete, those numbers are probably not accurate.

A safe deficit is generally going to be 500-1000 calories a day, unless you are 250+ where you can stand to be a little more aggressive. MFP has already figured your deficit into your calorie allowance, so you should eat that full amount + a portion of exercise calories.

A good post on the subject:
http://community.myfitnesspal.com/en/discussion/10569458/why-eating-too-little-calories-is-a-bad-idea/p1