We love protein in America. We love it so much that we want to add it to everything. So which protein should we add?
There are only a few major proteins widely available on the market. Whey and soy dominate, with a few other proteins like casein making some inroads.
Whey protein is the major protein of bodybuilders, as it is rapidly absorbed for muscle replacement from breakdown (study here). These are small studies on young people, and we don’t have the large scale studies that we should on whether whey would be superior to avoid muscle breakdown caused by aging (study reviews here). On studies of malnourished children, adding whey or casein helped with growth, but only because of the increased calories, not because the protein was superior to less costly grains (study review here). In a study of post-menopausal women, whey did not help add muscle produced by resistance training (study here). Rats who were not exercising experienced significant liver inflammation from whey intake (study here).
As a person ages, the need for muscle bulking wanes against an need to prevent things like bone loss. While exercise clearly helps, the data on whether adding protein or calcium to an exercise routine will really help are not available (review here). Soy isoflavones have shown benefit for bone health in animal studies that so far have not been confirmed by long term human studies (review here). Soy has received a great deal of attention for menopausal symptoms and the possible ill-effects it may have. But when all the literature is reviewed, the overall effect of soy is mildly positive (review abstract here) and the concerns about thyroid interaction appear to be based on faulty research (review here).
Would an alternative protein source be preferable given the issues with whey and soy? Yes, and as soon as it became popular someone would find fault with it. So at this point young exercisers would prefer whey, while older people, particularly women, would prefer soy.