To any casual gym goer, they don't mean jack **** and they probably won't hold any advantage over normal whey.
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Going with current prices, 912kg of ON Whey costs £17, and I can make a mix just a good as that for less. 657g of WPI = £6.30, 105g of L-glut = £0.79, 150g BCAA = £3.03. Total is £10.12. Personally I wouldn't bother with WPI and would use concentrate, either way, it's almost half the price.
Been looking into this as an option of fine tuning my diet, how do you find the taste your custom formula? Also how do you work out how much of each ingredient to add in?