Protein powder isn’t just for cutoff-wearing, bicep-baring, iron-pumping meatheads anymore. The supplement is gaining popularity from teen athletes to moms trying to get back their pre-baby body to those simply filling in protein potholes in their diet. Studies have shown upping protein intake can help curb hunger, burn fat and build lean muscle without the bulk.

But which post-workout wonder powder should you choose? And what are whey and casein [kay-seen], anyway?

New research published in the American Journal of Clinical Nutrition found that whey packs more punch when it comes to protein supplements. One study from McMaster University in Canada found that whey is digested quicker and starts repairing muscle tissue faster. Whey simply outweighs casein among exercisers doing resistance training. Another study from the U.S. Army Research Institute of Environmental Medicine found whey also has a higher concentration of the amino acid leucine, which aids protein growth and repair.

Whey and casein both come from cow’s milk and are complete proteins, which means they have all nine amino acids — a must for growing muscles. But casein digests slower, making it a better choice before bedtime or between long stretches of time.

Recreational athletes should aim for half to three-quarters of a gram of protein daily for every pound of body weight. Competitors and athletes need up to nine-tenths of a gram for every pound of weight. Look for varieties that deliver 20 to 25 grams per serving, and be sure to pair your protein with a healthy dose of carbs and fat for better absorption and nutritional balance.

But avoid overconsumption, which can tax the liver and kidneys — besides, you don’t want to end up looking like the Hulk. Unless, of course, that’s what you’re going for.