In your battle to bulk up, two contenders likely stand out for your support: whey and casein protein.
Picking a protein powder is almost a rite of passage for bodybuilders; in fact, protein powders are currently the biggest-selling supplement in the industry. 
But with so many options, it can be hard to know what protein benefits you most.
Learning which protein your body needs increases your likelihood of success.
Whey Vs. Casein Protein: What’s the Difference?
Whey and casein protein are the most popular protein forms available.
Made from separated milk, whey protein comes from whey, and casein protein comes from curds. Both are abundant sources of essential amino acids.
The biggest difference between the protein sources, however, is absorption time.
Whey protein is fast acting, delivering a large amount of amino acids at rapid speed. Casein protein works slowly, decreasing the rate of protein breakdown.
The Argument for Whey Protein
Whey protein promotes an explosive increase in protein synthesis, the mechanism by which muscles are formed. It also supplies high amounts of leucine, which stimulates protein synthesis.
According to experts, leucine is the key to whey’s power: “Those amino acids… act as a signal to elevate muscle protein synthesis — crucial for ongoing growth, repair, and maintenance of muscles.” 
In one clinical study, participants given whey protein experienced significantly greater increase in strength gains and muscle fiber strength. 
The Argument for Casein Protein
Casein is more abundant in milk than whey; it makes up about 80% of milk protein.
A proper balance of protein synthesis and protein breakdown is needed to maintain muscle growth.
As casein protein slowly enters the bloodstream, it significantly inhibits protein breakdown. This makes casein protein especially beneficial for muscle repair and recovery. 
Which One Should You Choose?
While only you can make the final decision about which protein meets your needs, the most popular winner in the whey vs. casein protein battle is surprising.
Many experts conclude there is no winner; for the best results, you’ll need to utilize both.
Whey protein builds muscle but doesn’t affect protein breakdown. Casein protein does little for protein synthesis but aids in repair and recovery. When taking both, you’ll get the best of both worlds.
However, timing is everything. Below are a few guidelines on how to utilize whey and casein protein together.
• Use whey protein immediately before and after exercise. This boosts muscle growth during workouts.
• Take casein protein right before bed and in-between meals. Casein gives a feeling of fullness and repairs muscle as you sleep.
• Resist the urge to mix the two together. Casein slows whey protein absorption. It’s best to wait at least 3 hours after taking whey protein before using casein.
A Word of Caution
As with any dietary supplement, use caution when supplementing with protein.
Side effects of whey and casein protein include nausea, cramps, and headache. In more severe cases, overdosing on protein leads to protein poisoning.
Make sure to follow all directions from manufactures and consult a doctor if you’ve any questions before starting a protein regimen.
 “Top Protein Powders on the Market.” Muscle and Fitness. Available from: http://www.muscleandfitness.com/supplements/build-muscle/top-protein-powders-market
 Whey Protein May Beat Casein After Workouts. WebMD. Available from: http://www.webmd.com/fitness-exercise/news/20110819/whey-protein-may-beat-casein-after-workouts
 Cribb P, Williams A, Stathis C, Carey M, Hayes A. Effects of whey isolate, creatine, and resistance training on muscle hypertrophy. Medicine & Science in Sports & Exercise. 2007;39(2):298–307. Web. http://www.ncbi.nlm.nih.gov/pubmed/17277594
 “The Case for Casein Protein.” Muscle and Fitness. Available from: http://www.muscleandfitness.com/supplements/build-muscle/case-casein