> My guess is that there is little or no betacellulin in the fat part of the
> dairy--just a guess--but I suppose cream may contain more than just the pure
> fat, since it has a liquid component.

My understanding is that betacellulin is most heavily concentrated in dairy fat.  Here's a little bit of a blurb from Max Muscle website that I believe supports this: 

"Growth factors are contained within the fat globule membrane in whey. They can also be found in colostrum, the pre-milk breast fluid produced in the first 48 hours after giving birth. Because whey concentrates are higher in fat than whey isolates, growth factors are usually found at higher levels in concentrates."

Jim