Can someone explain calories in and calories out to me?

When you set up your profile, you told it you wanted to lose weight. The number of calories it gives you before exercise allows you to lose that weight. When you exercise, those are additional calories above what you need to lose weight. You should eat back about 50% of those calories to help fuel your workouts else you can lose lean body mass.

Your deficit is already included in the number you are given for calories - so if you exercise you need to eat those calories (or at least 1/2) or you are not consuming enough. For example. You are given 1,400 calories, which already includes a 500 calorie deficit. You then exercise 300 calories are added, if you don't eat them you now have an 800 calorie deficit, which over time can be harmful.

You burn so many calories every day just from living. Feel your flesh; it's warm. There's an industrious chemical engine in every cell in your body working all the time and giving off heat. Your brain also, all that blonde thinking, uses a lot of energy. So you start off your day needing 1,600 calories or so (depending on where you are starting from) to get you through the day. This is your daily Calories Out (CO).

If you eat a little less than the CO, your cells will take energy where they can; from stored glycogen in your liver, and then from your fat cells. All you have to do to lose weight is eat a little less than you need daily.

When you exercise, you put increased demands on your system (greater CO). MFP is compensating for this and raising your Calories In (CI) limit for the day. Please do eat your exercise calories back because you don't want to go TOO LOW on your CI daily. Unintended side effects like hair loss, brittle nails, dry skin, fainting...

If you are using MFP, you tell it how much you weigh, your age, height, and gender... and how much you want to lose. It calculates how many calories you need to eat less per day in order to achieve that goal.

When you exercise, you burn ADDITIONAL calories on top of the deficit that MFP has calculated, so if you don't eat those cals, you will be at a greater deficit than expected. This could either a) cause you to lose weight faster than you intended or b) if you are low cal, cause you to consume far too few calories.


Example... (making numbers up here)

MFP calculates how much I need to consume each day and in order to maintain (neither gain or lose weight) it figures I can eat 2000 calories a day.

I want to lose 1 pound a week, then it tells me I need to only eat 1500 cals a day (500 cals per day x 7 days = 3500 cals - the accepted # cals needed to lose 1 lb)

If I exercise and burn 500 cals on day one... I've already created that 500 calorie deficit - and therefore can eat the full 2000 that day.

Or if you look at it the other way... I'm allowed 1500 calories, but I've earned an extra 500 from exercise

Here's something else to read...
http://community.myfitnesspal.com/en/discussion/1318741/in-5-weeks-youll-lose-10lbs-why-is-it-not-working/p1

It makes perfect sense when you understand how the tool works.

First off, you have to understand that you "burn" 24/7, with most of your burn coming from merely existing on this planet...then add to that your normal daily activities. Without any deliberate exercise, we have a substantial burn already...calories out. So when you consume less calories in that what is going out, you lose weight.

I'll illustrate with my numbers. Lets say I tell MFP I want to lose 1 Lb per week based on an activity level without any exercise...MFP will give me 2,000 calories...this means that MFP is estimating my calories out to maintain weight to be 2,500 calories (500 calorie daily deficit). So in theory I should be able to eat 2,000 calories and lose about 1 Lb per week without any exercise.

Now lets say I go workout and burn 600 calories with that activity. This is unaccounted for as it is not included in my activity level...it makes perfect sense to account for that activity somewhere...so I log it and get 600 calories to "eat back". I will still lose the same 1 Lb per week given that my deficit is still 500 calories per day...I can now consume 2,600 calories and lose 1 Lb per week because my maintenance has also moved up by that same 600 calories...so 2,500 + 600 = 3,100...3,100 - 2,600 = 500 calorie deficit still.

This also makes perfect sense when you can start wrapping your head around fitness for the sake of fitness rather than weight loss or simply burning calories, etc...when you really start getting into fitness for the sake of fitness, you start to understand why you need to fuel that fitness ('cuz you aren't going to get better at anything without adequate fuel).

Beyond that, the more intensive your exercise, the more important it becomes to fuel it properly...failure to do so will ultimately result in recovery issues, injury, etc.