Assessing protein quality: How is it done?

The question is: how do you determine protein quality? Experts use two criteria for rating and ranking proteins: bioavailability and amino acid profile.

  • Bioavailability

With bioavailability, protein quality is measured based on nitrogen measurements through Biological Value (BV), Nitrogen Balance (NB) and Net Protein Utilization (NPU).

These scales are based on the fact that 1) dietary protein is the only source of nitrogen for the human body and 2) all proteins that are not excreted are converted to bodily proteins.

Bioavailability is measured by understanding how much nitrogen is excreted from the body, calculating how much protein that number represents and compare that to the amount of protein that was ingested.

Although the Biological Value (BV) scale is still used today, the FDA’s official scale is now the Protein Digestibility Corrected Amino Acid Score (PDCAAS) because it can measure both the protein’s bioavailability and its amino acid profile.

  • Amino acid profile

As discussed, amino acids make up proteins and since essential amino acids (EAAs) are not naturally synthesized by the body, they have to be ingested as part of the diet to help with the body’s proper functions. Experts only categorize a protein as complete when it has enough of each essential amino acids.

Animal proteins contain more of these EAAs than plant proteins, which is why it’s important to plan a diet based on what each protein source can offer to the body.

But more often than not, those we think that offer the best sources of protein are actually not what they claim to be. Beef protein powder is a good example of this misconception because we are led to assume that it is made purely from animal meat, which is a great source of protein.

But it’s actually made from collagen that’s boiled from the animal’s bones, skin and other tissues, which means that while it is beneficial for skin and joint health, it doesn’t necessarily provide the protein quality that’s needed by the body.

Protein quality also considers two important aspects: the protein’s characteristics and the food matrix where it is consumed and the health demands of the individual who’s consuming the food.

For instance, athletes who have more physical activity than the average individual also require more protein in their diet. Protein quality and quantity requirements also vary depending on age, existing health conditions and overall physiologic status.

The future of protein quality assessment

The PDCAAS scale that was introduced by the Food and Agriculture Organization of the World Health Organization (FAO/WHO) in 1991 has been widely used ever since, even displacing the old Biological Value scale because of its ability to measure both bioavailability and amino acid profile.

But as nutrition and its technology evolves, experts from the Food and Agriculture Organization are now proposing to replace the PDCAAS with a newly developed scale, the Digestible Indispensable Amino Acid Score (DIAAS).

One of the biggest differences between these two scales is the manner in which the sample is taken from the subject. The old PDCAAS looks at an individual’s feces to determine bioavailability and amino acid profile while the newer DIAAS uses the contents of the ileum to determine the amount of protein that was absorbed after the food has passed the small intestine.

This method may be promising, but it is still far from perfect. In fact, there is still the challenge that all other scales have faced, which is the failure to determine real life measurements even after all factors are taken into consideration.

For instance, the new method allows a subject to eat only one kind of protein on an empty stomach to make sure that proper variables are calculated. But when is the stomach completely empty on a real-life setting?


Protein quality assessment is still an ongoing study for nutritionists and health experts. Different clinical trials are being conducted to make sure that all possible confusing factors are eliminated to get the best results out of the new DIAAS scale.

But more than anything, it’s best to consult the experts when it comes to knowing how much or what kind of protein should be ingested, especially considering the relation between protein quality & human performance.