Not all proteins are made equal. The source of your protein powder and how it was processed will determine whether you’re getting a performance boost or simply making an expensive chocolate-flavored drink.
Higher protein intake has been shown to improve performance and increase lean body mass. With an abundance of protein products that all seem to say the same things, how can you be sure what protein is best for you?
Dairy-derived proteins are by far the most common. Between whey and casein products, the rates of digestion and the purity of the protein you’re getting vary widely. We’ll break down the most common variations here. [1-3]
Whey protein concentrate:
Milk is typically composed of 87.7% water, 4.9% carbohydrates (lactose), 3.4% fat, 3.3% protein, and 0.7% minerals. The protein content of milk is largely broken into 80% casein proteins and 20% whey proteins.
During the process of cheese production, liquid whey is left as a byproduct. That liquid whey goes through processes of ultrafiltration and concentration to become the first tier of whey protein powder. Interestingly, whey protein concentrate (WPC) has a range of acceptable protein content. 
In order to be classified as WPC, the protein content only needs to reach 35% of the total solid product. Instead of protein, low end whey protein concentrate products are composed of different solid particles like fats and carbohydrates. Manufacturers are not required to disclose the percentage of whey protein in the package as long as it is within the 35-80% window.
If you are buying whey protein concentrate, be wary of companies that do not disclose the percentage of whey protein you are getting (i.e. this is typically shown as WPC80, for 80% whey protein content).
WPC is considered a fast digesting protein source with all 9 essential amino acids. Consuming WPC after a workout can give you an easily digestible, rapid source of protein to support muscle protein synthesis and anabolic states. Because of the lower protein concentration and associated lower cost of manufacturing, whey protein concentrates will generally be the less expensive whey option. 
Whey protein isolate:
Unlike whey protein concentrate, whey protein isolate (WPI) typically contains over 90% whey protein. Compared to WPC, you will find far fewer non-protein solids in the final product.
WPI is the purest form of whey protein on the market and is one of the fastest (if not the fastest) digested protein options available. Because of the additional processing steps, WPI is also typically more expensive than WPC alternatives.
Similar to WPC, WPI has all of the 9 essential amino acids. In general, whey proteins are high in a group of essential amino acids called branched-chain amino acids (BCAAs). One of those BCAAs, leucine, is especially active in the rebuilding phase post-training.
As mentioned above, casein is the major protein component of milk. Because of the underlying structure of casein proteins, they are digested much slower than their whey counterparts.
Casein supports a sustained release of the 9 essential amino acids, often for several hours after consumption. There are numerous ways athletes can tailor their protein supplementation and diet to take advantage of the longer release dynamics of casein.
How do the digestion rates of whey and casein products affect performance and recovery?
In studies between casein and whey, findings generally revolve around rates of digestion. One such study evaluated the release of amino acids after protein consumption, which would suggest greater availability for muscle protein synthesis and anabolic states.
In this study, one group was administered whey protein and one group was administered the amino-acid-equivalent dose of casein protein. The whey consumers displayed a quick release of amino acids at 100 minutes after consumption, but by 300 minutes the amino acid levels had returned to baseline. Alternatively, the casein group displayed a slower release of amino acid content, but their amino acid concentrations remained elevated at 100 minutes and 300 minutes. Overall, whey protein increased protein synthesis by a whopping 68% while casein increased protein synthesis by 31%. Over the course of the experiment, casein increased leucine levels across the body more than whey protein.
Using some creativity, an athlete might be able to strategically apply these findings to day-to-day fueling. If you are finishing a workout, a quick bolus of available amino acids is highly desirable. In this case, whey protein is a suitable option. To maintain anabolic states, muscle protein synthesis must continue past the 100 minute mark. In order to support the sustained states of muscle-building, athletes may find casein as an effective protein stack to whey isolates or high quality whey concentrates. [1-3]
- Hoffman JR, Falvo MJ. Protein - Which is Best?. J Sports Sci Med. 2004;3(3):118-130. Published 2004 Sep 1.
- Wilson, J., Wilson, G.J. Contemporary Issues in Protein Requirements and Consumption for Resistance Trained Athletes. J Int Soc Sports Nutr 3, 7 (2006). https://doi.org/10.1186/1550-2783-3-1-7
- Boirie et al. Slow and fast dietary proteins differently modulate postprandial protein accretion. Proceedings of the National Academy of Sciences. 1997.
- Adamson, N. Chapter 15 Whey Processing. The Dairy Processing Handbook. 2015.