Bulking agents are intended to support muscle gain. They combine various protein sources with carbohydrate sources.
The most common source of protein in bulking agents is whey concentrate, and the carbohydrate source is some form of rapidly metabolisable sugar. This combination is perfect for consumption immediately after exercise. Because whey protein concentrate is relatively fast absorbing, it provides your body with a nutrient for muscles that have been catabolized during exercise.
In addition to damaging muscles during exercise, the body uses sugar (stored as intracellular glycogen) as energy. This means that glycogen stores may be completely depleted by the end of exercise. If we were to consume only protein immediately after exercise, the body will produce sugar from the protein (through a process called gluconeogenesis), replenishing glycogen stores, but there will be no protein left for muscle building. When protein is combined with sugar, glycogen stores can be replenished and new muscle protein production can begin. Bulking formulas contain the right proportions of protein and sugar to further support the muscle building process.
The absorption of protein can be accelerated even more if whey concentrate is replaced by isolate or hydrolysate.
Bulking agents are also available in dairy-free, lactose-free and vegetarian forms, in which case they contain different protein sources.
There are also bulking agents that contain slow-absorbing carbohydrates, which prolong the absorption of carbohydrates and are ideal if you cannot eat for a long time after exercise.
Many bulking agents also contain digestive enzymes to aid the absorption of nutrients.