This was always a confusing question for me. I thought that if I didn't eat enough, I would lose muscle. Bulk and cut, or something like that. Anyway, John Barban and Brad Pilon did a study about caloric intake, and it has been a tremendous help in setting me straight (How Many Calories? is the name of the study, and it's floating around out there for free in cyberspace).
Essentially, it is our height and Lean Body Mass that determines our overall caloric intake. Start by using averages....according to the study....assume a male is 5'10 and 5'6 for females. The median LBM is 148 (for most men who workout) and female is around 104 pounds. For men, add or subtract 7 pounds per inch to the LBM depending on height. For example, if you are 5'9....subtract 7 pounds from 148. For females, use the rule of 3's instead of 7's. Now, the Katch-Mcardle formula looks like this:
370 + (21.6 * LBM (kg) = BMR (or RMR, I see them used interchangeably). Convert the LBM in lbs. to kilograms using any website (this is important, DO NOT USE LBS). Then, solve to get your recommended caloric intake. Now, I used an LBM of 148 as a starting point and my daily caloric requirement came out to around 1800 calories, about 1,000 less than I had been consuming on some days. This explains why my weight loss started to plateau....I was so fat in the beginning, that consuming 2,800 calories was doing me good for the first few months (Jesus, I was eating a lot). To get to a healthier weight, I had to cut back even more. Now, before, I had recommended that you go to a website to do this calculation. I now recommend that you stick to this method instead of going to a website to do the Katch-Mcardle because most will ask for your body fat %. Also, forget the activity factor. A couple of things....you need to do a boat load of exercising and gain a boatload of muscle just to put yourself in a nice caloric defecit. Forget what your heart rate monitor is telling you or your treadmill at the gym. Also, think about this. Why would you factor back in the exercise and add more calories? Aren't you trying to burn those with exercise? I know, ground breaking, but I have really ramped up my progress again by shooting for the BMR. Don't try and shrug this off and tell me that you are burning enough calories with your exercise program. You are not. Seriously, ask yourself, are you really making progress with your countless hours at the gym? You spend a lot of time moving on the stair stepper, why are you not happy? I am still losing weight, and I have actually averaged more than 1800 calories, probably 1,900 -2,100....which is fine....my initial calculation was only an estimate. Find that range in which you are losing weight, and stick to it. Exercise is an added bonus to improve your health, but not to be used as a means to control calories. Also, trust the formula. In the beginning, I would have said you are crazy to suggest that I only need around 1800-2000 calories a day. I was wrong, about many things, but I'm learning....slowly. I am not miserable, hungry or tired, I am feeling great at the gym and am not killing myself with cardio.
On a side note, I had a "consultation" with a trainer at LA Fitness the other day. Essentially, they want you to pay a lot of money to spin your wheels. The trainer asked what my goals were. I replied " to get a waist of 31 inches, and shoulder width of 50 inches. I am shooting to and 5-7 lbs. in LBM over the next 90-120 days. I plan on consuming 1,900 - 2100 calories of whatever the hell I want to eat. I will mix in my fruits, veggies and lean meats. I will not take supplements, will not do pre and post workout drinks. I will do heavy resistance training 3 days a week ( I also outlined my training program). I have lost 52 pounds and am shooting to burn through another 10-15lbs of fat. I only eat one to two meals a day. What can you do for me?" His eyes became big and he shook my hand, thanking me for the info and he looks forward to my progress. End of story.