I'll indulge you a little further shags.. It's 6am, not much else to do.
I don't know anyone that refers to protein supps as proteins.
We had a similar discussion regarding powder vs powders. I've been involved in weight training for many years and I'm quite familar with the lingo used by gym rats. Never once heard anyone say proteins but given our lot are typically brain dead, you might expect it but no.
The value of marketing is in the marketing itself, it has nothing to do with the product or in this case, the domain name. I like pureprotein.com.au but only from a brand name perspective - there is no such thing as pure (as in clean or unmixed) when it comes to protein supps and unless you're selling to middle aged women on TVSN - which is what I guess is your thinking, they already have their own brand name.
I wouldn't call protein.com.au a category killer either as it's far too broad although still a good domain name. In any case, it sold for just over $2000. If that is all the category killer (in your opinion) fetched, it's not a stretch to suggest your domains are worth perhaps 5-10% of it's price (best case scenario).
Let's look at it this way, I own some supplement related domain names - bcaa.com.au (branched-chain amino acid), glutamine.com.au and casein.com.au. They don't come close to the popularity of supps such as protein though and I'm not overly excited about them even though I bought them each for under $50.
Don't get me started on organic food or people who don't like chemicals in their food because everything is a fucking chemical - perfect example of marketing.