Whey protein is the liquid portion of milk, which is separated out in the process of making cheese, dried, and sold in powder form, says Melissa Majumdar, MS, RD, LDN, CPT, a registered dietitian and Academy of Nutrition and Dietetics Spokesperson who specializes in sports nutrition. "[Whey] is a fast-acting protein source, easily digested, and usable in a quick manner," Majumdar says. You can also buy whey protein isolate, which is just a filtered and processed form of whey that doesn't contain lactose, and tends to have less fat content than plain ole whey, Majumdar says.

So, why is the fitness community so hooked on it? Whey protein contains all the essential amino acids, which "help stimulate muscle metabolism and protein synthesis," she says. It's also highly "bioavailable," meaning that almost all of it (99%) can be absorbed into your body — so it's really just straight-up protein that you're ingesting.

But building muscle involves way more fuel than you get chugging protein. "There is a misconception right now that we need more protein, especially for exercise and muscle gains, which is why whey protein has become popular," Majumdar says. "Most people think that they need protein, protein, protein, but that is not the case," she says. "Someone looking to gain weight or muscle might use whey protein powder, but they should keep in mind that extra calories and extra carbohydrates are important for muscle and weight gain."

Not to mention, drinking a protein shake is not the same thing as eating whole foods, Majumdar says. "Having protein within 15 to 30 minutes of resistance training is important for muscle repair, growth, and building," but you can get that from food, she says. Shakes do have their appeal (you can quickly prep one after a workout that was hard to fit into your schedule to begin with). But there are plenty of other post-workout snacks that are convenient and provide even more nutritional benefits, like eggs and fruit, Greek yogurt with cereal, a turkey sandwich, or an actual meal. And whey protein can be pretty expensive (about $60 a canister).

The bottom line: "Whey protein is a good source of protein, but most of us do not need to worry about eating more protein," Majumdar says. When taken in the recommended dose, whey protein is generally safe to consume, according to the Mayo Clinic. But there can be potential side effects, from abnormal heart rhythms, to stomach or intestine symptoms, so be careful if you're taking it for the first time. "If someone has a history of kidney stones, they may put themselves more at risk when consuming a diet high in animal protein, including whey," Majumdar says. And if you're someone who has a milk allergy, then you should avoid whey, because it is milk, after all.

"People want bigger muscles, so they drink whey protein shakes without knowing the science behind muscle gain," Madjumdar says. But if something claims to be a quick fix, that's usually a red flag that it's not the healthiest way to achieve your goals — and that's just the whey it is.