There is no question that a reduction of calorie intake coupled with high-level energy expenditure results in weight loss (all other factors being equal). But what guarantees the weight loss to be fat only? Consider these facts:
It is a long-standing fact that a pound of fat equals 3,500 calories.
A pound of muscle renders around 600 calories.
People with a higher percent of body fat will lose more fat and retain more muscle with a significant calorie deficit.
People with a lower percent of body fat will lose more muscle and retain more fat with a significant calorie deficit.
Let’s do some maths. A 500 calorie deficit per day over one week could result in two outcomes: a loss of one pound of fat (3,500/3,500 calories = 1) or a loss of nearly six pounds of muscle (3,500/600 calories = 5.8).
Obviously, the loss of muscle is not desirable. This is why attention must be paid to the correct calorie deficit based on your existing percentage of fat and your activity level. If you possess a significant amount of body fat, you can probably get away with a larger deficit in the early stages. As you become leaner – or if you are relatively lean to begin with – the calorie deficit needs to be adjusted to preserve lean tissue. This also explains why it is so difficult to shed those elusive few pounds to get completely “ripped”.