By now, I know most people are aware of what it takes to lose weight, and more specifically to lose fat. With all due respect to the quality of the calories you eat (I'm not covering that today), the generic and essentially correct thinking is calories in must be less than calories out to lose weight. Many people take this as a literal translation, meaning the bigger the deficit between calories I eat and calories I burn, the more weight I'll lose. This is not necessarily true. The body doesn't like having to few calories, and at a certain point, will begin to make adjustments to create balance between incoming and outgoing calories (caloric homeostasis). Let’s first think about how the human body uses energy. When we eat calories, they are converted to energy, when you eat enough to fuel your body, no problem, you're in balance like a see saw with the same amount of weight on both sides; but when the total amount of calories you burn in a day through just living, and exercising, and regular daily activity is more than what you're eating, your body has some choices to make about where to get enough energy to make it up. It'll first use the stored energy it has in the liver and at muscles, called glycogen. Unfortunately, you don't have a heck of a lot of glycogen, so that's only good for a little bit extra. Next the body will turn to body fat, and that's great, but your body burns body fat like an ice cube melts, by that I mean, you can only burn so much at a time. Next up is muscle mass, the body will pull energy from muscle as a last resort, cannibalizing the least used muscle first in most cases. If all of these extra calories STILL don’t equal what you are burning, then the body will take drastic measures and start slowing down the metabolism. See my prior blog on short and long term fasting for a more in depth description of this process.
So, because of what we just outlined, there's a sweet spot for the body. Roughly this is the calorie deficits that allows you to burn fat, but not start pulling energy from muscle, and definitely not start lowering the metabolic rate. How do you know how much? Well, you usually don't, not at first at least, and this number will change as body fat percentage changes. That being the case, a little experimentation is usually in order. The first thing you need to know is roughly how much energy you burn in a day; to know that you can use a few different methods. The easiest method is to just use one of the tons of maintenance calorie calculators on line. I prefer the one at webmd.com (http://www.webmd.com/diet/healthtool-metabolism-calculator). The number won't be perfect, but it's a good start. To be a little more accurate, you might consider something like a body bugg. Please note, a body bugg is different from a regular HRM(calorie counting heart rate monitor) as HRM's have a hard time calculating calorie expenditure at normal heart rates (HRM's are essentially ECG machines and because the impulse from your heart is low during rest periods, it can misread these beats often) Body bugg type devices are far more sensitive (thus the reason they are so expensive) which means they are more accurate during low heart rate periods (such as sleeping). Lastly you can go to a lab and have a metabolic rate test done. These are generally the most accurate, but if you are having a "bad" day, it could be off (and remember, stress over GOING to the lab could affect the outcome). However you do it, you need a baseline of your daily calories.
Once you know your calories, you can start thinking about a deficit. Remember, we're looking for the sweet spot for calorie deficit. That means you want enough of a deficit to lose fat, but not too much where you're burning lots of protein or lowering your metabolic rate. To do this you should analyze your fat. Obviously, if you know your body fat percent, that's the best way to make a decision. But sometimes it's not easy to receive an accurate body fat test. Home scales are wildly inaccurate for body fat (they use hydration for their calculations, which can vary greatly day to day, thus giving really wide margins for error). If you can't have a body fat test, the next best thing is to use BMI. I am not a fan of BMI in general, but in the absence of body fat, it's essentially all we can go on. Just be sure to remember that BMI does NOT take into account muscle mass, so if you are heavily muscled, the BMI can inaccurately report obesity. Also women who are or who recently were pregnant shouldn't put much stock in BMI. That out of the way, your BMI number will give you a general starting point for deficit, for most people if you are obese (BMI of 31 or greater), you can have a relatively large calorie deficit, somewhere north of 1000 calories per day, the higher the BMI, the bigger (within reason) the deficit. The lower your BMI, the smaller the deficit. I usually tell clients that if your BMI is over 30, go for 1000 and see where that leads you, 28 to 30 I usually give a deficit of about 750 calories, between 25 and 28 I say about 400 to 600 calories, and below 25, if you want any deficit, it should never be above 300. I realize that this may seem rather small to some, but that's the reality of HEALTHY weight loss. It's never going to be super fast. And it shouldn't be, because if you want this to stick, losing fast weight isn't the answer, you need to give your body time to adjust.
On to the subject of adjustment; after the initial calorie goal is set, you should monitor and tweak, if it's not working like you suspected, tweak. Remember this though, any tweaks you make to calories should be relatively small, 100 calories per day changes is my recommended maximum. And in order to evaluate that tweak, you should give yourself at least 1 month at the new level. The human body doesn't react that quickly to changes in eating habits, so expecting changes in days or weeks is not realistic.
Lastly let’s talk about maintaining. Once you reach your end point and have reached a body fat percentage you feel comfortable with, you need to SLOWLY raise your calories, usually over the course of 6 to 8 weeks, back to maintenance. If you've followed what I've written above, you've probably already adjusted your calorie deficit as you approached your goal weight. If you have, then your deficit should be relatively small, and your body shouldn't have a hard time adjusting to maintenance. One of the main reasons why so many people fail after dieting is because they fail to properly come out of the "diet mode" and into maintenance. If you suddenly go from a 500 calorie deficit to maintenance, the body will inevitably try to put some weight back on. Give your body the chance to adjust and it won't feel the need to pack on extra fat.
Please note that I'm not going into the concept of exercise calories. All calorie amounts I talk about here are NET calories. Which means daily calories PLUS exercise calories. So a deficit of 500 calories would mean your daily calories plus exercise calories minus what you eat should equal a 500 calorie deficit.