Calorie deficit for dummies (a little long)

I wanted to break it down a bit for newbies and maybe a few long timers that have questions regarding calorie deficits. I see a lot of people on here just setting a 2 lb goal and not thinking about it again, I wanted to explain a few things.


INTRO:
OK so you're a newbie (or maybe not so new), you go through the goals wizard, and of course, you set your weight loss goal to the maximum. Everyone does it, but is it right? Maybe, maybe not. Moving to a healthy lifestyle with diet is trickier than just lowering your calorie deficit, there are things to consider, and work to be done. You need to make some hard choices along the way, and do some self-analysis to see where you really fit in this plan. Everything we do should be tailored to our individual needs. While there are general concepts that will work for everyone, results WILL vary. With that in mind, here's what I've come up with, boiled down to the important stuff that everyone should be able to digest. For a more detailed explaination of our metabolic process, please read this post:
http://www.myfitnesspal.com/topics/show/10589-for-those-confused-or-questioning-eating-your-exercise-calo

How your body thinks on calories:
Our body uses calories as energy. There are 3 main types of food out there called macro nutrients; they are carbohydrates, proteins, and fats. ALL macro nutrients are vital to good health, and while the body may subsist in the absence of one (namely carbohydrates) for most of us, living a carb free life is difficult at best, a serious commitment for sure, and untenable for most.
When the metabolism is in balance between energy taken in and energy expended (Total Daily Energy Expenditure or TDEE) we maintain our weight. When we create a REASONABLE deficit, we lose weight, when we create a calorie surplus, our body converts any unusable extra energy to one of 3 types of fat (none of which are good generally speaking). While we need a certain percentage of body fat to be healthy, over that amount is unnecessary, and when that fat percentage begins to become high (around 18% for men, and around 28% for women), it begins putting stress on other parts of the body (heart, lungs, joints, muscles ...etc)

What happens when the deficit becomes larger than the body likes:
The human body has evolved from a state where food sources were either abundant or scarce. Evolution has built mechanisms into our body to deal with this. The process commonly referred to as "starvation mode" is a response to less calories than the body needs to sustain full efficiency over an extended period (usually begins after 3 to 5 days of lower than required energy). Without going into the details of hormone responses, basically what happens during starvation mode is that the body recognizes a lack of fuel, and in order of importance it begins to shut down, or slow down organ function and brain activity. Because of the time involved, and that it takes extremes for organs to completely shut down, we may or may not immediately notice the changes that occur.

Why does this concern you? Because the TDEE goes down in starvation mode. This means that what you THOUGHT was your maintenance calories, and what any calorie calculator will say your TDEE is will be wrong, it will be lower. That means less of a deficit than expected, and sometimes none at all (surplus is even possible).

What else happens: as a secondary effect of starvation mode, our body begins fat conservation and muscle deterioration. These processes are symapthetic. When the body sees a calorie deficit, it will begin the process in earnest of breaking down existing muscle tissue to convert the amino acids into glycogen for fuel, it will also store any calories deemed non-vital as body fat. Muscles not being used are broken down, muscles in use will still be scavanged, but at a lower rate.


Caveats:
The above is all true for people with normal levels of fat. By that I mean someone who has a slightly low percentage of body fat, to someone who has a slightly high percentage of body fat. People who have abused their body and are considered very obese or morbidly obese (starting somewhere around 35% body fat) have a different set of rules. Starvation mode generally doesn't apply, or applies less to these people because the extreme availability of extra fuel resources blunts the body's need for extra energy, and while people who are morbidly obese can still have a stunted metabolic rate, they can still generally sustain a large calorie deficit (over 1000 calories below TDEE) without further lowering their metabolism.

What this means:

For all you guys out there that are relatively active, and/or don't have extreme amounts of weight to lose, setting that goal of 2 lbs per week is usually a mistake. For all the reasons above, you need to temper your attempt at weight loss, give it time, and be consistent.
Regarding exercise. Generally exercise is considered an add-on on the MFP site. That means that MFP allready gives you a calorie deficit. Exercising after should create a zero sum number after that deficit. That means that since you're already at a calorie deficit, any exercise you do should be balanced by eating that amount of calories to KEEP you at that pre-existing calorie deficit.

Figuring out what you SHOULD be eating every day to maintain (TDEE) and deciding how big your calorie deficit should be can be daunting, don't be afraid to query others on this site, there are many knowledgeable and helpful people on here. If you are still confused, seek out a registered dietitian and use their services. And by all means, check the general section of the forums for the sticky posts, which have lots of resources available to help you better understand.

hope this helps!

-Banks