There are many options available for post workout protein beverages. Some are primarily protein while others have varying amounts of carbohydrates or fat. Few studies have directly compared the effects of different combinations of protein and carbohydrate after exercise.
Researchers from the Netherlands addressed the impact of co-ingestion of various amounts of carbohydrates combined with protein on the rate of protein synthesis in healthy young men. On three separate occasions subjects performed a 60-minute resistance exercise workout followed by one of three supplements and further measurements of protein synthesis for 6 hours during recovery. Supplementation over the 6 hour recovery period consisted of 0.3 g/kg of a casein hydrolysate per hour combined with either no carbohydrate, low carbohydrate (0.15 g/kg) or high carbohydrate (0.6 g/kg). For a 180 pound man this equates to about 25g protein per hour, and about 12g and 49g carbohydrate per hour for the low and high carbohydrate trials.
As expected, there was a significant increase in muscle protein synthesis during recovery in the protein-only trial. The co-ingestion of carbohydrate with adequate protein did not further augment protein synthesis. These findings show protein as the most important nutrient to consume after resistance exercise, and further indicate that adding carbohydrates offers little benefit. This is good news if you are trying to lose body fat because it means you can get an optimal anabolic effect from protein alone without adding additional calories from carbohydrate and the accompanying insulin response that will inhibit fat breakdown.
Reference: Am J Physiol Endocrinol Metab. 2007 Sep;293:E833-E842.