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Guide to Protein Quality, Digestion and Absorption

Written by James Collier BSc (Hons), Registered Nutritionist, who devised the Huel formula. He has over 25 years of experience working in nutrition and dietetics, including 7 years as a Clinical Dietician in the NHS. Covering an array of clinical areas, he worked with people with a wide range of ailments and food intolerances. He also has an Honours Degree in Nutrition with Dietetics: read more about James here.

Protein Quality

There are numerous claims that different proteins vary in quality and that some are more beneficial than others. This is true, but only to a point…

There are a number of factors which may affect the protein quality from food, including:

  1. The amino acid profile of the protein
  2. The structure of the protein
  3. The amount of protein consumed in one meal
  4. Other nutrients and food constituents present in the meal
  5. How the foods been prepared
  6. Recent intake of protein
  7. The metabolic state of the individual, eg illness, exercise, sleep
  8. The individual's age, weight, sex and general health

Plus we also need to consider the reason why you’re wanting protein anyway; obviously, we all need adequate protein for good health, but bodybuilders will be looking at proteins for muscle growth, so the quality may be more important to them.

Protein Digestion & Absorption

Before we look at protein quality, let’s first look at the way proteins are digested and absorbed. Digestion of food begins in the mouth and continues until all nutrients have been absorbed in the intestines. A number of digestive enzymes are involved in the digestion process which break down, or hydrolyse, protein into short chain oligopeptides or amino acids. The simplest unit of proteins are amino acids of which there are 20-odd different types. Two amino acids linked together are called dipeptides, a few amino acids in a peptide chain are called oligopeptides and long chains of them are called polypeptides.

Amino acids are absorbed in their basic form by an active transport process, where they are pumped across the cell membranes and then into the blood. However, there is a second process which happens simultaneously to the active transport where oligopeptides can be taken up in their current form and, when inside the cells of the intestine, are then further broken down to free amino acids. The process of this is a cell enzyme-related system that relies on a chemical ion gradient.

As there are two independent protein absorption systems in operation, this allows for more protein absorption at times when it’s required as the second system only comes into play when there are oligopeptides present. In a high protein meal with more than one protein source (which is, in fact, the majority of meals), both systems come into play at similar levels.

Methods of Determining Protein Quality

There are different methods of analysis for assessing quality of proteins that look at the amino acid profile and how readily the protein is digested and absorbed. However, none are without flaws which we’ll discuss.

1) Amino Acid Scoring (AAS) aka Chemical Scoring (CS)
AAS is fast and cheap. It measures the essential amino acids (EAAs) present in a protein and compares the values with a reference protein. The rating of the protein being tested is based upon the most limiting EAAs. Obviously, this has real-world limitations.

2) Protein Efficiency Ratio (PER)
PER measures the ability of a protein to support the growth of a weanling rat. It represents the ratio of weight gain to the amount of protein consumed. The main limitation is obvious: it’s based on rats, but also that PER measures growth and not maintenance and exercise requirements..

3) Nitrogen Protein Utilization (NPU)
This is the ratio of the nitrogen used for tissue formation versus the amount of nitrogen digested.

4) Biological Value (BV)
BV is the most commonly used and well-known protein scoring system. It measures the amount of nitrogen retained in comparison to the amount of nitrogen absorbed (Chick & Roscoe, 1930). It looks at how similar the amino acid profile is to that of human requirements. Proteins are grouped into those of high BV (HBV), and low BV (LBV).

BV is significantly flawed, yet it’s still favoured as a reference guide when comparing protein, especially in sports nutrition. BV studies are in rats and rats have different digestive systems to humans and their protein requirements differ indicating that the reliability of BV in humans is questionable. Also BV does not take into consideration several key factors that influence the digestion and interaction of protein with other foods before absorption; it only measures a protein's maximal potential quality and not its estimate at requirement amounts. As more of a HBV protein is consumed, the actual amount of that protein retained decreases and BV doesn’t account for this.

Another limitation is that proteins which are missing one EAA can still have a BV of up to 40. This is because of our ability to conserve and recycle EAAs as an adaptation of inadequate intake of the amino acid in malnutrition.

Protein Digestibility-Corrected Amino Acid Scoring (PDCAAS)
Although more up-to-date and accurate, the PDCAAS isn’t without it shortfalls. PDCAAS takes into account the profile of EAAs of the protein in question, as well as its digestibility in humans; it is the AAS with an added digestibility component. Scores are from 0.1 to 1.0, with 1.0 being a high quality protein. The PDCAAS is the current accepted measure of protein quality, and is the method adopted by the World Health Organisation / Food and Agriculture Organisation (WHO/FAO) and the US Food and Drug Administration (FDA).

All proteins of a score greater than 1.0 are rounded down as scores above 1.0 are considered to indicate the protein contains EAAs in excess of the human requirements (WHO/FAO 1990). This obviously limits the possibility of comparisons between proteins.

Another flaw of the PDCAAS is that the scores are based on that of a 2 to 5 year old child (considered to be nutritionally the most demanding group). Adults have a proportionally larger maintenance:growth ratio and this is not considered when using the PDCAAS. Also the PDCAAS doesn’t account for certain factors influencing the digestion of the protein and is of limited use for application to human protein requirements because what is measured is maximal potential of quality and therefore it’s not a true estimate of the quality at requirement level.

However, the most important limitation of the PDCAAS is the fact that in human diets are mostly of varied protein sources, even in a single meal. This means the total amino acid profile of a meal is improved and there are also other food constituents that may affect protein hydrolysis, digestion and absorption. In order to assess the true PDCAAS of a meal, all individual amino acids would have to be taken into account, so the PDCAAS of each constituent is largely useless.

Let’s look at rice: the protein from rice PDCAAS is 0.4-0.5, limited only by the fact that it’s significantly low in the EAA lysine. However, it’s abundant in another EAA, methionine; a point that the PDCAAS doesn’t account for.

Now let’s look at pulses: the protein here ranges from 0.4-0.8, depending on the pulse, and the proteins are very low in methionine but abundant in lysine. So, when beans and rice are consumed together in a meal their combined constituent PDCAAS is 1.0: an ideal protein sources.

Bioavailability of Protein

Bioavailability is the amount of protein we actually absorb and both BV and the PDCAAS in part account for this. Bioavailability is affected by a number of factors including the total amount of types of protein consumed within one meal as well as other constituents of the meal.

Proteins are not simply a long chain of amino acids; the amino acid chain wraps around itself (known as the secondary structure of protein), and these double chains wrap around themselves again (tertiary structure) and then bonds form between amino acids as the chain’s folded over into a protein molecule forming its quaternary structure. The structure of proteins vary and this can have a bearing on bioavailability.

We’ve discussed the limitations of both BV and the PDCAAS above, yet BV is often cited as a reference for protein bioavailability. Indeed, plant proteins are often indicated to be inferior to animal proteins and, while this is partly true, it doesn’t account for the fact that meals often contain a combination of protein.

Whey protein - the number one protein choice of bodybuilders - has a very high bioavailability. Indeed, it’s considered to be so quickly digested and absorbed that a good amount of it goes to the liver after absorption where it’s converted to carbohydrate by gluconeogenesis for energy, indicating that the fate of whey is not as retained nitrogen but for energy, i.e. not the reason why the protein was consumed in the first place.

One study compared the effects of supplementation with rice protein isolate and whey protein isolate post resistance workout and found both to have an equally positive effect on body composition and exercise performance (Joy et al 2013); ie whey was no better than rice protein.

Protein in Huel

The protein in Huel is from all the main ingredients, but the two main contributors are pea and brown rice protein. As discussed above, the combination of proteins drastically improves the quality of protein in respect of amino acid profile and bioavailability. The pea protein used in Huel has a PDCAAS of 0.82 and the rice protein 0.47 and combined have a perfect score of 1.0: more than enough to ensure all amino acids are supplied and that the protein in Huel has high bioavailability.


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