One of the most common things we hear among our female clients is the fear of gaining weight. Because it’s common see so many of their male counterparts chugging protein shakes and gaining muscle and size, the assumption tends to be that all that protein must be leading to bulk.

This is actually part true: When men consume protein and pair it with intense resistance training, the end result is a stronger, bigger muscle. And women who still want to fit a certain dress size might see that and steer clear of protein.

However, protein gets quite the bad rap in this situation, because for all its benefits as a muscle-builder — and there are many — the substance truly responsible for a man jumping from a size L to an XL, for his legs and back getting broader and thicker, for the rounded, bulky shoulders, is not protein at all, but testosterone. And unless women everywhere develop a new-found love of hormone therapy, they just don’t produce enough of it to create the same effects as men do.

In fact, protein’s reputation as a muscle-builder actually dilutes its appeal as a fat-loss agent. Women who are afraid of gaining weight would be better off drinking three protein shakes a day instead of, for example, having a few bowls of fruit or a small bagel with jam. While the second examples sound like light fare, it is instead carbs and calories, not protein, that should be the focus if body mass is a concern. Simply put, protein shakes provide the most protein with the fewest calories.

We’re going to spend the week on this blog post breaking down protein, its mythical status as a female weight-gainer and why all those misconceptions are wrong. But if you’re waiting for the bottom line, it’s here already: Protein is a fat-burning, muscle-building must-have for any diet.

“I Don’t Want to Get Huge”

By far, this is the most common reason people — women specifically — do not supplement with protein powder. As mentioned before, however, it’s a myth. Increasing body mass has to do with mostly four things: genetics, calories, training and hormones. The first is a pretty good indicator of whether or not you even can get huge — what does your family look like? What is your body’s natural frame type? If you’ve found that you have trouble keeping weight on, or if all the women in your family are tiny (if you’re female), chances are you don’t have the genetic coding to suddenly become the She- Hulk with a few 120-calorie protein shakes a day.

That leads us to calories. Remember, they are simply units of energy; there’s nothing mystical to them. The more you consume, the more energy you have — and depending on how much of it you use, more of it gets stored, usually as body fat. Fat loss occurs in calorie deficit, when you are burning more than you are consuming, so if your training is intense and regular and your diet isn’t loaded with high-calorie foods and too-frequent meals, there’s no good reason you shouldn’t stay lean. And for all its reputation as a bulk-builder, protein’s calories provide feelings of satiety and fullness, meaning you don’t need to eat a lot of them to be satisfied. You can’t really say that about fruit salad, can you?

Lastly, there’s the case of hormones — and it’s probably the most important distinction between men and women when it comes to training and muscle size. Simply put, women lack the testosterone to build muscle at the rate men can. Yes, there surely some genetic exceptions (they’re usually called female bodybuilders) who can actually put on significant skeletal muscle mass, but most of the time, what you’re likely to see with increased protein intake and resistance training is tighter, more toned muscles and a leaner overall body. If the sexes were cars, a heavy-training, protein-eating man with great genes is a big, bulky Hummer — his female equivalent is probably something closer to a ’72 Stingray: sleek, strong and curvy.