are overviewed and we recount some of the key findings from studies examining the unique benefits of casein on protein balance.
Casein is the most abundant protein in milk.
Milk actually contains a variety of proteins, and due to many unique properties is easily converted to a wide range of products such as cheese and yogurt. The amino acids in caseinare present in such a ratio to promote growth and development of the young. It has all the essential amino acids, which makes it score well on all methods of protein quality. The other main protein in milk is whey. Whey protein is more soluble in an acid environment, whereas individual casein molecules are relatively insoluble in the aqueous environment of milk. Because casein is insoluble it tends to form structures called micelles. Micelles are suspensions of spherical structures that increase solubility in water. The casein micelle also contains water and a salt (usually calcium or phosphorus) in the core. Because of these unique properties, casein has a much slower digestion rate. During normal treatment of milk, which usually involves heat or acid, the casein peptides and the micelle structure become disturbed or denatured. Simple put, the proteins are disrupted and broken apart to simpler structures. As a result a gelatinous material is formed - the curd - and this is the basis for formation of products such as cheese. Micellar casein is undenatured, containing more of the intact peptides in their natural form. In order to retain an abundance of micellar protein during processing, specialized treatment of milk has to be performed. Because the casein micelle is in suspension, it can be separated from the rest of milk by centrifugation at a very high speed. One method to preserve micellar casein is ultrafiltration or microfiltration. This avoids exposing the proteins to heat, acid or other chemicals thereby preserving micellar casein. Thus, more bioactive peptides with immuno- and growth-modulating effects are present in the protein.
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A first classic study documenting the effects of micellar casein on protein metabolism was done by French researchers and published in the prestigious Proceedings of the National Academy of Sciences (2). The researchers gave healthy subjects 30 g of either whey protein or casein protein and made several measures of the anabolic and catabolic effect for 7 hours after the meal. What they found was that whey protein resulted in a rapid increase in blood amino acids and protein synthesis, but it was short-lived. Casein on the other hand resulted in a prolonged increase in blood amino acids that resulted in a 34% reduction in protein breakdown. The net protein balance remained more positive for the casein protein over a 7 hour period. The authors attributed the superior long-lasting effect of casein to a delayed gastric emptying and slower absorption rate from the gastrointestinal tract. These findings were quite revolutionary. In fact, a commentary was written in the top science journal in the world - Nature, that went on to highlight the important aspects of how various proteins have different effects on protein turnover (3).
To gain further insight into these phenomena, researchers performed additional experiments to document the effects of digestion rate of proteins on protein turnover. In the first study (4), healthy young men were provided one of four meals:
- 1) a single meal composed of 30 g of casein (separated by microfiltration and ultrafiltration),
2) a single meal containing 30 g of individual amino acids equal to the casein meal,
3) a single meal composed of 30 g of whey protein, and
4) 30 g of whey protein provided in a sequence of 13 small meals given each 20 min.
These studies show unequivocally that a slow-digesting protein, in these cases microfiltered and ultrafiltered micellar casein, is an independent regulator of protein retention. Therefore micellar casein makes an ideal protein supplement to sustain long periods of an anabolic environment for muscle growth. This makes micellar casein particularly attractive as protein supplement for such periods as prior to bed or any time frequent meals cannot be consumed.
1. Dangin M, Boirie Y, Guillet C, Beaufrere B. Influence of the protein digestion rate on protein turnover in young and elderly subjects. J Nutr. 2002 Oct;132(10):3228S-33S.
2. Boirie Y, Dangin M, Gachon P, Vasson MP, Maubois JL, Beaufrere B. Slow and fast dietary proteins differently modulate postprandial protein accretion. Proc Natl Acad Sci U S A. 1997 Dec 23;94(26):14930-5.
3. Fruhbeck G. Protein metabolism. Slow and fast dietary proteins. Nature. 1998 Feb 26;391(6670):843, 845.
4. Dangin M, Boirie Y, Garcia-Rodenas C, Gachon P, Fauquant J, Callier P, Ballevre O, Beaufrere B.The digestion rate of protein is an independent regulating factor of postprandial protein retention. Am J Physiol Endocrinol Metab. 2001 Feb;280(2):E340-8.