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Should you use a fast or a slow protein?

If you want to gain muscle size, most people agree that you need more protein than the average joe. But not everyone agrees on the type of protein that works best for muscle growth.

One of the most contentious arguments about protein type centers on fast and slow proteins and which one is best for building muscle.

The classic comparison of fast and slow proteins involves comparing protein supplements containing whey with those containing casein (pronounced kay-seen). Both casein and whey are derived from whole milk. About 80 percent of milk protein is casein. The remaining 20 percent is comprised of whey.

Whey is considered a fast protein because it's rapidly absorbed by the body. Casein, on the other hand, is a slow protein that takes longer to break down and absorb. Casein seems to clot in the stomach, slowing gastric emptying. Casein also releases opioid peptides during digestion that slow down gastric emptying.

Think of fast and slow proteins a bit like the glycemic index, where foods differ in their impact on blood sugar levels over a 2-3 hour period. In much the same way, all sources of protein are not digested and absorbed at the same rate.

However, while fast and slow proteins do exist, the question is whether or not the differences between them matter to you.

In other words, will a diet comprising mainly fast rather than slow proteins (or vice versa) differ radically in its impact on muscle growth? Does it really matter if you use a fast protein after exercise and a slow one at night before bed? Should you use a supplement that contains a blend of fast and slow proteins to give your body the best of both worlds?

Whey and casein

The interest in fast and slow proteins started with a series of studies published by a group of French scientists, led Yves Boirie, in the 1990's [3]. Their research shows that whey and casein vary in their impact on the rate at which proteins are synthesised and broken down.

Your muscles are in a constant state of change. New proteins are being made, while old ones are broken down, or degraded. Skin goes through the same process. Old skin dies off, and your body synthesizes new skin to take its place.

To build muscle, the rate at which muscle proteins are being made has to be greater than the rate at which they're being broken down. Think of it in business terms. For a company to be in profit, for example, income must be greater than expenses. In much the same way, for muscles to grow, muscle protein synthesis must be greater than muscle protein breakdown .

In the first study, healthy subjects with...

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