Back in the history when Whey protein was just introduced in the market, the contained as low as 30-40% protein and contained high amounts of lactose, fats and un-denatured protein. This type of protein is known as Whey protein concentrate (WPC). Nowadays WPC contains around 70-80% of protein with reduced amount of lactose and fats. Because of the marketing of WPIs people are under false impression that WPC is inferior to a WPI.
I think this is very wrong. Though Whey protein concentrate has less percentage protein than WPI, it contains many compounds which are absent in the whey protein isolate.Good quality WPC contains higher levels of growth factors such as IGF-1,TGF-Beta 1 and TGF-Beta 2. WPC contains higher levels of various phoso lipids and various bio-active lipids such as conjugated immunoglobulins and lactoferin. Though there is no significant data available to say that these compounds are helpful for muscle mass building and performance but studies do show that these compounds can improve immunity, intestinal health and have many other effects that both body builders and normal people.
The only drawback in using WPC is that it contains less protein and high level of Fats and Lactose than a WPI if contains are considered in percentage. But this does not mean that WPC is always inferior to WPI The choice between the WPC and WPI totally depend on the goals of the individual. In my opinion WPI is a better choice for the individual who is tolerant to lactose.