When it comes to weight loss and achieving the body you want, it is very important to understand the difference between fat and muscle. Muscle and fat are very different metabolically, in energy storage, and of course in appearance.

The main principle to understand is that muscle is denser so it takes up less space than fat for the same weight. So 1 pound of fat is a much larger mass than 1 pound of muscle. Fat also contains a lot more calories per pound. Fat tissue contains 3,500 calories per pound, whereas muscle tissue only contains 600 calories per pound.

You may accurately think of fat as a large blob of concentrated calories and muscle as smooth tissue with much fewer calories per pound.

Here's a picture that explains what we mean:

Looking at that picture and the facts about muscle and fat, we can understand some important things when it comes to losing weight.

First, let's talk about appearance.

Your Appearance (Body Composition) and How the Scale Can Fool You

Considering what you have just learned about fat versus muscle tissue, you should also expect that:

A person who has more muscle, or who has gained more muscle, may weigh more than expected
Given two people who weigh the same, the person who has a higher fat percentage will look fatter

Again, let's take a look at this visually. You might not have believe it before, but the two women pictured below are probably very close to the same weight:

Of course most of us aren't aiming to have the muscle tone of a model. The point is that if all you are measuring is your weight as a measure of progress, you can be deceived and end up making bad decisions.

Remember this: If you are losing inches, you are losing fat ... regardless of what the scale says.

Incidentally, these facts are the reason why BMI measurements don't work for fit or athletic individuals. Calculations will tell them they are "overweight" when in reality they simply have more muscle mass than the average person of the same height and gender.

Fat Loss vs Muscle Loss

If you were to create a 3,500 calorie deficit over a week, your body would burn up 3,500 calories of energy stored in body tissues of some kind. These body tissues may be glycogen (carb) stores, fat stores, or muscle (lean body tissue). What typically happens is that you burn energy in each category (you lose glycogen, fat and some muscle).

It is considered normal for obese or overweight persons to lose up to 25% of their weight as muscle mass (lean body tissue). Why? Because when you gain weight, you also gained some muscle and supportive tissue to support and carry all that extra weight. In effect, it's just like how weight lifting for exercise increases muscle mass.

Fair Warning: It's important to realize that the more muscle you lose, the more weight you will lose. Look at the fat vs muscle scale picture above again. If your body burned all 3,500 calories as muscle tissue, you'd lose almost 6 pounds! -- compared to just 1 pound of fat. In reality, you will never lose all muscle, but you do want to minimize it. Just realize that fat loss is slower and that preventing muscle loss or even gaining some muscle will mean your weight won't go down as much (you will have lost fat instead).

It's the fat loss that counts. We'll say it again: If you are losing inches, you are losing fat ... regardless of what the scale says.

Some people don't really care about muscle loss -- weight is weight. But it's something to keep in mind if you are concerned about retaining lean muscle.

There are several ways to ensure you don't lose muscle while dieting:

Get adequate protein (see Protein)
If you don't get adequate protein, then eat adequate carbs (see Carb Smart Plan)
Do resistance exercises while dieting

Does Muscle Provide a Weight Loss Advantage?

Popular belief in the past said that muscle burned 15X more calories than fat -- however, that has since been proven wrong. There is still a metabolic advantage, just not as great as previously believed.

Here's the scoop: At rest, muscle burns roughly 6 calories per pound per day. Fat tissue burns 2.5 to 3 calories per pound per day. So muscle burns twice as many calories per pound as fat, which is great -- but to make a difference in your daily calorie burn you would have to gain a LOT of muscle. For example, if you built up 15 extra pounds of muscle beyond what you already have, you'd only burn 90 calories extra per day. Better than nothing for sure -- but it's not the game changing effect people once thought it was.

Many weight loss programs (especially those designed by bodybuilder gurus) still emphasize weight training over other exercise. In reality it's up to you and your preferences.

If you want to look better or get really lean, and/or have a slight metabolic advantage, among many general health benefits ... then you should do resistance exercise. Otherwise any exercise is great for weight loss purposes if calorie intake is accounted for. See Exercise and Weight Control and Diet Determines Exercise.