Fast vs. Slow Proteins



Protein supplements usually consist of one, or a combination of, whey, casein, egg, or soy protein. This review will focus on the two most popular sources of supplemental protein, casein and whey, whose names have become synonymous with “slow” and “fast” digestion, respectively.

Protein Science 101: General Digestion

For dietary protein to be utilized in the body, it must be absorbed by the small intestine and transported to the blood, where it will circulate to various tissues. Proteins can be thought of as long chains of linked amino acids, but these long chain peptides cannot be absorbed. It is generally thought that only single amino acids or chains of two or three amino acids (di- and tri-peptides) can be absorbed by intestinal cells and passed into circulation. In the stomach and small intestine, proteins are broken down into smaller peptides or single amino acids by gastric juices and a variety of proteolytic enzymes. However, not all proteins are created equal; some, depending on their source, may be harder to break down than others. Absorption becomes dependent not only how well your body can break down protein, but also on how easily the protein itself can be broken down.

What does this mean for protein supplement users? To tailor protein intake to your needs (say, for building muscle), understanding protein quality and absorption tendencies becomes an important step in reaping maximum nutritional benefits.

Related: Protein Supplements and Diet

Whey vs. Casein: The Basics

Whey and casein are both excellent sources of essential amino acids (those that cannot be produced by the body) and both share the highest Protein Digestibility Corrected Amino Acid Score (PDCAAS) of 1.0, a measure of protein quality based on amino acid utilization after digestion. The two dairy proteins, however, differ drastically in their rates of absorption.

A protein’s digestive properties are rooted in its chemistry. Whey protein’s solubility allows it to be emptied from the stomach and into the small intestine for absorption shortly after ingestion. This process results in a large and rapid spike in plasma amino acid levels, which may then be transported to various tissues to promote protein synthesis. Clinical research has supported whey protein’s ability to stimulate protein synthesis shortly after ingestion, but also indicates that the response is short-lived, lasting only a few hours post-ingestion.

Casein, unlike whey, is not very soluble in water and tends to coagulate with other casein molecules to form small, circular structures called micelles. This property makes it easy for casein to form a gel or clot in the stomach and effectively delay gastric emptying. This causes a much slower but steady rate of amino acid absorption into the blood, leading to a less pronounced effect on initial protein synthesis when compared to whey, but a positive long-term effect on preventing protein breakdown.

Does faster-digesting protein lead to a greater increase in muscle synthesis? This metric, net protein gain, is the result of the delicate balance between protein synthesis and the prevention of protein breakdown, and serves as the primary factor in determining muscle growth.

Net Protein Gain and Timing

In several clinical studies, casein has been shown to be absorbed at 6-7g/hour while whey is absorbed in the range of 8-10g/hour. Rapidly absorbed proteins, such as whey, have been shown to stimulate protein synthesis by up to 68% and moderately prevent protein breakdown. However, this effect only lasts a few hours, and peaks within an hour. Slowly absorbed proteins, on the other hand, only moderately increase protein synthesis but prevent protein breakdown by up to 30% for 7 hours post-ingestion. Overall, slow-digesting proteins, such as casein, promote a higher net protein gain than fast-digesting proteins such as whey.

To mitigate some of these effects, whey protein may be taken in smaller, but more frequent doses to maintain good protein balance. In several studies, 2.3 g of whey protein taken every 20 minutes showed a significantly larger net protein gain vs. one large dose.

It’s all in the timing. Optimal athletic benefit will likely be achieved if whey supplements are taken when muscles and tissues are in need of short-term repair, such as shortly after exercise or early in the day. Casein will be most effective when long periods of time without activity are expected, such as immediately before nighttime sleep. This will provide the body with a steady influx of nutrients, and limit the risk of muscle breakdown (catabolism) during a fasting period.

The principles of fast and slow digestion do not only pertain to whey and casein protein. Protein absorption rates also vary widely between supplemental and dietary protein sources. A better understanding of these tendencies will help increase the nutritional benefit of your supplements and shorten time-to-goal.

Related: Protein Quality: The 4 Most Important Metrics