Eating calories

You should be eating your 1200 PLUS your exercise calories. Any less and your body won't be getting the fuel that it needs.

I just leave the exercise off and aim to eat 1200 calories, can't imagine why they ( fitness pal) would want to add calories on - sort of defeats the point of exercise.

No it doesn't... When you set up your profile with your age, activity level, gender, etc., you tell MyFitnessPal how much weight you're looking to lose per week. Based on that number, it sets up a caloric intake to achieve that level of weight loss, AS IF YOU ARE NOT EXERCISING. Basically it says, if you don't do anything extra besides your normal activity level, you need to consume 1200 (for example) calories per day to lose 1 or 2 pounds (for example) per week. It does that by creating a calorie deficit. You burn much more than 1200 per day in your normal daily activities, but you would only eat 1200, which creates a safe deficit.

If you then exercise and burn 600 calories IN ADDITION to the deficit, you're now down to 600 net calories, which is not enough fuel to run your body. If you do this consistently, your metabolism may slow and it may become harder to lose weight. Lots of people throw around the phrase "starvation mode" around here with reckless abandon... Not sure how long you'd have to sustain that low calorie level to get to starvation mode, but it won't happen from one low day. But I would not recommend having a net calorie level under 1200 for extended periods of time...

Hope this helps.