There's weight loss and then there's fat loss. When people say they want to lose weight, they mean they want to lose fat, because they certainly don't want to lose muscle.
Muscle is metabolically active tissue — it's the physical location in your body where fat is burned. What's more, muscle also creates the shape of your body. So when people talk about toning or shaping certain areas of their bodies, what they're really talking about is muscle, as you can't build a bigger or perkier anything without building muscle. And anyone who says otherwise either has no clue about basic human physiology, or is someone making false promises in order to separate you from your money.
When discussing fat loss, I want to make it clear that the relationship of how many calories you consume per day to the number you burn per day is the single most important factor when it comes to determining whether you lose fat.
The concept that you need to be in a caloric deficit in order to lose fat isn't personal opinion, nor is it up for debate by so-called diet gurus. This is the first law of thermodynamics, which states that energy — and fat is stored energy — can be neither created nor destroyed, only changed from one form to another.
In other words, fat loss is determined by burning more calories each day than you consume. Now, this isn't to discount that some calories are more nutrient dense than others. We've all heard the term "empty calories" before, but you can still be well-nourished and over-fed. So as important as it is to eat high-quality, nutrient-dense foods, you can still gain fat from eating "healthy" if you eat too many calories relative to what you're burning.
There are two ways to create a caloric deficit. You can either eat fewer calories or you can eat the same amount of calories and increase your activity level to burn more calories. Since we're looking for fat loss without muscle loss, that activity should focus primarily on strength training, not cardio training.
Although this may be contrary to popular belief, a 1999 study published in the Journal of the American College of Nutrition drives home the importance of strength training over cardio for fat loss without muscle loss while in a caloric deficit.
The study looked at two groups of obese subjects put on identical very low-calorie diets. One group was given an aerobic exercise only protocol (walking, biking, or jogging four times per week), and the other group was given resistance training only three times per week. After 12 weeks, both groups lost weight. The aerobic exercise group lost 37 pounds, 27 of which was fat and ten of which was muscle. However, the resistance-training group lost 32 pounds, and 32 pounds were fat; zero was muscle.
In other words, the resistance-training group lost significantly more fat and didn't lose any muscle, even at only 800 calories per day, which is far lower than anyone would want to go, but was done in this study in order to take any dietary variables completely out of the equation and compare the effects of the exercise regime on muscle maintenance and metabolism.
Additionally, when resting metabolic rate was calculated after the study, it was found that the aerobic (cardio) group was burning 210 fewer calories daily. In contrast, the resistance-training group had increased their metabolism by 63 calories per day.
So, the two takeaways for successfully losing fat without losing muscle: You need to be in a caloric deficit and you need to emphasize strength training over cardio when you're in such a deficit in order to maintain your muscle, which, as stated above is what gives you that sexy and athletic shape.
Last, it's important to note that just because you need to create a caloric deficit doesn't mean that you have to starve yourself or eat an incredibly low calorie diet as was used in the study discuss above, which involved obese individuals.
In fact, if you're not obese, maybe just a bit overweight, or you're already fairly lean and simply looking to lose that extra bit of fat, a large caloric deficit will generally make you lose some muscle even with strength training and adequate protein So, the goal in order to achieve fat loss without muscle loss, is to be in a caloric deficit without starving yourself, make sure that your diet delivers plenty of protein and focus your exercise efforts on performing regular strength training.