Old Man Liver

Regular IRONMAN readers are familiar with the concept of fast and slow proteins and their connection with the two primary milk proteins, casein and whey. Precise food-absorption studies have found that casein is a slowly absorbed protein, taking as long as seven hours to get completely into the blood. Natural substances in the casein that slow the digestive process account for that. The advantage of a slowly absorbed protein is that you get a time-release effect of amino acid entry into the blood, and the gradual trickle of aminos, in turn, results in a pronounced anticatabolic effect in muscle.

The other primary milk protein, whey, is a rapidly absorbed, or ‘fast,’ protein. Whey offers little or no anticatabolic effects. What it does provide is the rapid release of amino acids, which favors protein synthesis, so whey is the winner in the protein-synthesizing category.

While the combination of fast and slow protein digestion offers the best of both worlds for most people, some recent studies show that there are exceptions. In particular, protein absorption and uptake change with age, and what may be optimal in a 20-year-old may not work as well for a person over age 40, as two recently published studies illustrate.1,2

What the studies found is that in younger people, casein’likely because of its anticatabolic effect’offers superior results in overall protein gains. Rapidly absorbed proteins such as whey tend to also be rapidly oxidized in the liver and thus ‘disappear.’ As a rule, the faster amino acids enter the blood, the more quickly they’re oxidized in the liver. The longer-acting casein results in less oxidation and provides the extended amino acid release that spares body protein, leading to an anabolic effect in muscle.

In contrast, older people often show defects in muscle-protein synthesis, but those defects can be overcome by having high levels of amino acids in the blood. Since whey is a rapidly acting and absorbed protein, it’s ideal for older people because it enables them to synthesize muscle protein at a rate that compares to what happens in their younger peers. IM

1 Dangin, M., et al. (2002). Influence of the protein digestion rate on protein turnover in young and elderly subjects. J Nutr. 132:3228S-3233S.
2 Dangin, M., et al. (2003). The rate of protein digestion affects protein gain differently during aging in humans. J Physiol. 549:635-644.