We all know about fast and slow carbohydrates and the use of the glycemic index or glycemic load to determine which carbs are burnt more fast or slow in the body. What many trainees don't know is that there are also fast and slow proteins. Body builders have spoken about it for a long time but the average guy at the gym doesn't rally know what it means.
If we take a look at whey protein and also casein protein, we see a fast protein (whey) and a slow protein (casein). The whey protein is digested much more quickly and spikes amino acid levels in the body quickly, but for only a short amount of time. The casein is digested more slowly and has a more steady level of amino acid release. This is why I would often read to have this before bed in the old bodybuilding magazines I had as a kid.
Whey protein increased protein synthesis by 68% in one study, whereas casein only increased it by 31%. Where casein was better though was by inhibiting protein breakdown. Casein reduced protein breakdown by 34%.
By looking at these results, you get the idea that you should combine both whey and casein post workout for greatest results. They whey will increase protein synthesis and the casein can help prevent protein breakdown.
The only problem I see with this with our clients is that many people don't tolerate casein very well and I am finding more and more people are struggling with whey. If you tolerate these two proteins well, then try and combine them for greater gains in muscle mass.