Whey Became the Dextrose of the Protein World
Dextrose was recognized for quite some time as the quickest carb, and thus best for postworkout recovery. Naturally, a whey plus dextrose combination became the standard for postworkout nutrition due to the quickly absorbed nature of each substrate. But again, the postworkout nutrition objectives were all based upon unrealistic conditions in a nearly irrelevant population (fasted endurance trainees). People around the world spouted off familiar advice: "You shouldn't have casein postworkout because it will hinder the absorption of the whey and dextrose." And hence the whey-only era continued. Some folks have latched onto the speed of absorption idea and swear (without evidence) that whey hydrolysate is better than whey isolate or whey concentrate. Still others will take the concept even further and trade out their steak for a bowl of free-form aminos, which are more quickly absorbed. Naturally, milk was supposed to be avoided postworkout, because it formed viscous clumps of gel in your gut that scared off the whey, rendering it useless. Well, at least that's how a lot of people made it sound. Meanwhile, research began to trickle in and change the way we viewed the situation.
Is Faster Better?
About a decade ago, Boirie's team compared the absorption rate and leucine kinetics of 30 grams casein to 30 grams whey, and their results were very interesting. Casein elevated blood amino acid levels for up to 300 minutes post-ingestion [1]. Whey, on the other hand, caused higher amino acid levels at the 100 minute point, and returned levels to baseline at 300 minutes. Casein caused a smaller increase in protein synthesis, but unlike whey, casein also inhibited protein breakdown.
The striking aspect about this study was the casein group achieved a better leucine balance than the whey group, indicating better nitrogen retention, thus better protein utilization. Casein caused a greater net positive protein balance than whey. This was a big win for casein, because all we really care about is remaining in a trend of positive protein balance amidst continual protein turnover.
In a later similar study, it was found that aside from known factors (including quantity of calories, amino acid composition, and micronutrition), the speed of digestion is an independent regulator of protein balance [2]. In other words, slower actually was observed to be better. Now let's see what happens when the rubber meets the road in comparison trials measuring the relevant endpoints.
The True Test: Comparative Effects On Body Composition and Strength
Seven years ago, Demling and Desanti compared the effect of a casein-dominant meal replacement product with a whey supplement. Overweight healthy police officers underwent a structured exercise program that included resistance training, under slightly hypocaloric conditions [3]. The dosing of each treatment was twice daily, once postworkout, and again 8-10 hours separately. The results? Whey got whooped by the casein-based meal replacement for strength gain, lean body mass gain and fat loss. Can we blame the superiority of the casein blend's results all on the 20 grams of carbohydrate it contained? Overall carbohydrate intake was controlled between the groups, however carbohydrate timing was not controlled, so carb timing could have made a minor impact. About a year after that study, Wojcik and colleagues compared an all-carbohydrate drink with a milk-based carb/protein recovery drink [4]. Each drink was dosed immediately postworkout and again two hours later. The results? No significant difference in rate of glycogen resynthesis.
More recently, Kerksick's team compared the strength and body composition effects of three treatments on resistance-trained men. One group received 40 grams of whey plus eight grams of casein. Another received 40 grams of whey plus three grams of BCAA plus five grams of glutamine. The last group received 48 grams of carbohydrate [5]. The supplements were ingested postworkout and on the mornings of non-exercise days. The casein/whey group had the greatest increase in lean mass, the carbohydrate group came in second without any gain, and the whey/BCAA/glutamine group actually lost lean mass. Calories were poorly controlled in this trial, But still, fast-absorbing whey and BCAA didn't shine as the be all and end all of protein and amino acid supplements.
The latest dairy protein fight as of this writing was performed by Cribb's team, who saw high-dose whey beat high-dose casein for strength gain, lean mass gain, and fat loss [6]. But once again, calories weren't controlled as tightly as necessary to level the playing field and allow us to draw any firm conclusions.
So Where Does This Leave Us?
The research comparing casein and whey is split almost right down the middle. Where it's headed from this point is really anyone's guess. Undoubtedly, funding source will play a big role in the direction of the research results. As the evidence stands, it certainly wouldn't hurt, and might actually be optimal to have a blend of the two and reap the best of both worlds. A popular practice is to have casein pre-bed, and whey near the training bout. But the reality is, the training bout is the most acutely catabolic point in your day, even though it's a necessary trigger for anabolism. Think about it, sleep ain't got NOTHING on the immediate muscle damage caused by battling the iron. So, why not have both of the best tools for each job present in circulation at the most critical point? Let's look at things logically. By saying that casein might get in the way of whey, we might as well say that the carbohydrate might get in the way of the protein, so let's separate them? Wrong. Protein and carbs act synergistically to increase muscle protein synthesis and inhibit protein breakdown. The same synergy can be achieved by having a combination of whey and casein near the workout. Maybe Mother Nature was right all along.
References
1. Boirie Y, et al. Slow and fast dietary proteins differently modulate postprandial protein accretion. Proc Natl Acad Sci USA 1997;94(26):14930-5.
2. Dangin M, et al. The digestion rate of protein is an independent regulating factor of postprandial protein retention. Am J Physiol Endocrinol Metab. 2001 Feb;280(2):E340-8.
3. Demling RH, DeSanti L. Effect of a hypocaloric diet, increased protein intake and resistance training on lean mass gains and fat mass loss in overweight police officers. Ann Nutr Metab 2000;44(1):21-9.
4. Wojcik JR, et al. Comparison of carbohydrate and milk-based beverages on muscle damage and glycogen following exercise. Int J Sport Nutr Exerc Metab. 2001 Dec;11(4):406-19.
5. Kerksick, et al. The effects of protein and amino acid supplementation on performance and training adaptations during ten weeks of resistance training. J Strength Cond Res. 2006 Aug;20(3):643-53.
6. Cribb, et al. The effect of whey isolate and resistance training on strength, body composition, and plasma glutamine. Int J Sport Nutr Exerc Metab. 2006;16:494-509
but yeah, I know the view will be beautiful! You guys are so cute in the group shot, how hot that you all ended up having to have a Man Dogpile 
And you poor thing 
Well, of course you'll work through everything just fine I'm sure 
