Maximum Muscle Growth: Fast VS. Slow Protein
Muscular Development|December 2021
It’s well known that athletes engaged in intense training need to eat enough protein in their diets to repair tissue and promote growth.
By Richard B. Kreider

Research has also indicated that ingesting protein with carbohydrate prior to exercise can lessen the degree of muscle catabolism observed during exercise, while ingesting protein with carbohydrate following exercise can enhance glycogen and protein synthesis. However, not all protein is the same. Dietary proteins vary in amino acid content as well as the amount of fat and other nutrients they contain. These differences can affect the digestion rate of a protein and the manner in which amino acids are released into the blood. Research has shown that protein synthesis may be regulated in part by the manner in which amino acids are released into the blood. Theoretically, ingesting the right type of protein following exercise may help stimulate protein synthesis to a greater degree. This discussion addresses the way you can structure the timing and delivery of proteins in your diet in order to optimize protein synthesis during training.


Proteins vary depending on the amino acid, fat and micronutrient content. For example, proteins are comprised of various essential, conditionally essential, and nonessential amino acids (see Table 1). Proteins that contain all the essential amino acids are considered complete proteins, while those that don’t are considered incomplete.4,5 The quality of a protein can be classified using the Protein Efficiency Ratio (PER) and the Protein DigestibilityCorrected Amino Acid Score (PDCAAS). The PER is determined by assessing the weight gain of growing rats fed a particular protein in comparison to a standard protein (egg whites). The higher the PER value, the greater the quality of the protein. The PDCAAS method compares the amino acid profile of a protein to the essential amino acid requirements in humans established by the Food and Agriculture Organization.

A protein with a PDCAAS of 1.0 indicates that it exceeds the essential amino acid requirements of the body and therefore is an excellent source of protein.6 Table 2 describes the protein and fat content of common dietary proteins7 while Table 3 lists the PER and PDCAAS for various types of protein.4 As you can see, low-fat dairy products, skinless light chicken, and fish are generally considered among the best sources of low-fat dietary protein. Soy represents the highest quality plant protein, while egg, milk proteins (casein and whey) and bovine colostrum represent the highest quality animal proteins.


Research has indicated that several factors affect protein synthesis after consuming a meal, including the amount of calories consumed, the quantity and quality of protein ingested, the insulin response to the meal, and the digestibility of the food.3 The digestibility of food is affected by the fat, starch and fiber content of the food, as well as the acidity. Generally, the higher the fat, starch and fiber content, the slower a food is digested. The digestion rate influences the time course of amino acid release in the blood. In this regard, foods containing protein that are digested faster generally result in a greater release of amino acids in the blood over a shorter period of time. Conversely, foods that contain protein that are digested at a slower rate typically promote a smaller, but more prolonged, increase in amino acids.

Different types of protein also have different time courses of amino acid release. For example, since casein clumps when exposed to acid in the stomach, it’s digested at a relatively slow rate, which results in a modest but prolonged increase in amino acids in the blood.3 On the other hand, whey protein is comprised of a mixture of soluble proteins that are digested rather rapidly, resulting in a more pronounced but shorter increase in amino acids in the blood.

The time course of amino acid release following ingestion of a protein has been shown to affect protein metabolism and synthesis. For example, Boirie and colleagues8 compared the effects of ingesting 30 grams of whey (fast protein) and 43 grams of casein (slow protein) on protein utilization and synthesis. The amount of whey and casein evaluated was chosen to match the proteins for leucine content, since the researchers used labeled leucine methodology to assess protein use and synthesis rates.

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