Protein supplements and meal-replacement products containing proteins have four commonly used scientific measure of protein quality. 

  1. Protein Efficiency Ratio (PER) which is a measure of growth of animals consuming a fixed amount of dietary protein of a single type. Often considered less applicable to humans. 
  2. Biological Value of a protein is a measure of the amount of protein that is retained from total absorbed quantity for body composition. 
  3. Net Protein Utilization measure the amount of amino acids available by any one given protein source which are then synthasized in the body as a new protein. 
  4. Chemical Score is a measure of the concentration of the nine essential amino acids from a single protein source. 

Whey

Whey protein is one of two protein types found in milk often during cheese making, the other being casein. The two type soft proteins are separated from each other and whey was often considered useless. It was discovered that it was a high quality protein. 

It is a complete protein and contains all 9 essential amino acids and rates well on all 4 measures for protein measures. 

Whey Isolate: Purest form (90-95% by weight) of whey that is great for those who are lactose intolerant as it has had all the lactose removed. 
Whey Concentrate: Most common form of whey protein (60-70% by weight). It is inexpensive and can easily be added to different types of products. 
Whey Hydrolysate: The protein has water added to the polymers breaking them down into it's individual amino acids. This is to help with the body's digestion and absorption. This is a more expensive form to produce. 

Casein

As I mentioned above, casein is distilled through the production of cheese. One for the most important differences in Casein from Whey is that it is an excellent source of glutamine. 

It has lower concentrations of BCAA's than whey and because of a higher lactose content can be problematic for those who don't digest the sugar well.

For many strength and physique athletes the slower digestion aides for longer periods of time when there will not be any consumption of food like while sleeping.

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Soy

Rich in BCAA but contains only a small amount of methionine. Most forms of soy are highly digestible and absorbed quickly but for many it is considered a slightly inferior source of protein. 

Soy isolate is fortified with methionine which now makes it fairly comparable to whey and casein. One of the disadvantages are the isoflavones (anti-nutrients) which have been shown to have an estrogenic effect in the body.

Other Commonly Found Proteins

Rice Protein

Gluten free, neutral tasting and very budget friendly. May be derived from genetically modified rice. 

Egg Protein

Fat free, contains a full spectrum of essential amino acids. May cause upset stomachs. 

Milk Protein

Often seen as Calcium casein ate. High in BCAA's, contains lactose which may or may not cause upset stomachs. 

Hemp Protein

Great source of Omega 3 fatty acids. Provides fiber as well. 100% plant based protein. 

Which Protein should you take?

Well that would vary depending on your goals and dietary preferences. But first and foremost is making sure that you're getting the majority of your protein from whole foods and less processed animal and plant based sources. They're called dietary supplements and shouldn't be considered a main staple of your daily intake, rather in addition to.