One of the biggest challenges is that the production of useful protein - based delivery systems is in its infancy. (pnnl.gov)
It should also be noted that both whey and casein are highly useful protein sources (both derived from milk) that have slightly different uses. (blog.nkdnutrition.com)
The best sources of protein are lean meats and sea foods, but there are many plants, which contain useful proteins, as well. (abmachinesguide.com)