Initial Posting: 11/4/2015    Last Update: 9/28/2019
Answer: As discussed in more detail in our Protein Powders and Drinks Review, each of these can be a good source of protein, but certain protein sources may be better for particular uses and in certain people.
Whey protein contains the highest percentage of branched-chain amino acids, which can become depleted during exercise and are needed for maintenance of muscle. However, some studies have found rice protein and pea protein equal to whey in increasing strength and muscle when taken after resistance exercise. Casein is absorbed more slowly than whey and, for this reason, some athletes take it before bed to help counter protein breakdown. Soy protein is a complete protein and is often promoted as a "heart healthy" option, although the evidence for this claim is inconsistent and has been called into question by the FDA. Like rice, pea and soy protein, hemp protein may be a good option for vegetarians and vegans, although there appears to be little published research on the use of hemp protein for muscle building or sports recovery, or comparing its effects with those of other sources of protein.
Heavy metals such as lead, cadmium, and arsenic have been found in protein powders. Typically, these have been at very low levels — below limits for safe use. When higher amounts have been found, it has been associated with added, non-protein ingredients such as cocoa powder (a source of cadmium) or bran (from rice). ConsumerLab has also found that a significant percentage of protein powders do not live up to their label claims regarding sodium and/or cholesterol.
Specific sources of protein should be avoided due to potential allergic reactions, food sensitivities, and medical conditions (e.g., soy protein should not be used by people with thyroid conditions). It is also important to understand differences in the forms of protein, such as concentrates, isolates, and hydrolysates.