Whey is absorbed rapidly, with plasma levels peaking about 90 minutes after you take it in. It potently stimulates increased muscle protein synthesis, which depends on its essential amino acids. Casein is released into the bloodstream and absorbed by tissues more slowly. Studies show that casein curdles in the stomach, gradually releasing amino acids over a seven-hour period.
So while the rapid uptake of whey favors muscle protein synthesis, the extended amino acid release characteristic of casein helps minimize muscle protein breakdown and blunt the effects of catabolic hormones, such as cortisol.
Children today are generally taller than children of the past, and many attribute that increase in height to their drinking milk. It’s not just the high-quality protein in milk that’s responsible for the added height. Since milk is the first food given to humans and other mammals, it’s a specialized growth formula. Its nutrient balance has evolved to help maximize the rate of early growth.
There is something else special about milk. It contains factors that stimulate the release of anabolic hormones, which are conducive to growth and muscle building. For example, small peptides that are active in the body may be involved in the hormone stimulation that milk provides.
A recent study of eight-year-old boys compared milk to meat intake, finding that milk aided the release of two anabolic hormones: insulin and insulinlike growth factor 1. Meat, although high in quality protein, did not have a hormone effect. The increased IGF-1 that milk provides acts as a growth factor and could explain the overall increased height observed in recent generations of children.
Hormone-induced growth implies that milk contains something that may not have been present in the past. Some suspect that the addition of IGF-1 to milk may account for the added growth, but that’s pure speculation. A peptide hormone, IGF-1 degrades into its constituent amino acids in the digestive process.
In a follow-up study of the eight-year-old boys, the same researchers sought to determine whether the primary protein fractions in milk had different hormone effects. For one week the study’s 57 subjects were given differently compounded milk drinks, as follows:
1) Whey with low mineral content (calcium and phosphate)
2) Whey with high mineral content
3) Casein with low mineral content
4) Casein with high mineral content
The whey increased fasting insulin levels significantly more than casein and increased insulin resistance to a greater degree. Conversely, the casein milk provided a greater increase of IGF-1 and its protein binder in blood. While the increased insulin could be explained by the milk sugar lactose, the fact that the casein drinks contained the same amount of lactose precludes that explanation.
Other studies show that while milk has a moderate glycemic index number, it has a far higher insulinemic index number—a measure of the potency of insulin released by milk intake. The increased insulin release is not related to milk’s carbohydrate content. The minerals added to the milk drinks had no effect on the hormone release.
Those findings can explain why children tend to be taller these days as well as why a combination of milk proteins appears to be best for adding muscle. The whey-induced insulin release favors increased amino acid uptake into muscle, providing an anabolic action. The casein-induced IGF-1 activity may extend the anabolic effect and support an anticatabolic effect. The two major milk proteins appear to act synergistically in terms of growth stimulation.
Hoppe, C., et al. (2009). Differential effects of casein versus whey on fasting plasma levels of insulin, IGF-1, and IGF-1/IGFBP-3: Results from a randomized 7-day supplementation study in prepubertal boys. Eur J Nutr. 63(9):1076-83.