Understanding the Biological Value (BV)
The biological value (BV) is a metric that measures the efficiency with which the body can use a dietary protein for growth, maintenance, and repair of body tissues. It is essentially a measure of how closely a protein's amino acid composition matches the body's requirements for protein synthesis. The BV is primarily determined by the protein's essential amino acid (EAA) content and its overall profile. There are nine EAAs that the human body cannot produce on its own and must obtain from food. If a protein source lacks or is low in one or more of these EAAs, it will be less efficiently utilized and will have a lower BV.
The BV is traditionally expressed as a percentage, representing the proportion of absorbed nitrogen (a key component of amino acids) that is retained and used by the body. Whole egg protein has historically been considered the benchmark, with a BV of 100. However, modern measurements show that other protein sources, like whey protein, can have even higher relative BV scores, sometimes exceeding 100, because the egg protein's actual physiological utilization is slightly lower than 100%. This is why BV is sometimes expressed relative to egg protein.
The Role of Amino Acid Profile
The amino acid profile is the most critical factor influencing a protein's BV. The body's ability to build new proteins is limited by the availability of the scarcest EAA present in the diet, known as the "limiting amino acid". For example, a protein-rich food like rice is a good source of protein but is low in lysine, making it a lower BV protein if consumed alone. Combining it with another protein source, like beans, which is rich in lysine, can create a meal with a higher combined BV through a process known as protein complementation.
- Essential vs. Non-Essential Amino Acids: There are 20 total amino acids, but only nine are considered essential for humans. A protein source that contains all nine EAAs in sufficient quantities is called a "complete protein" and generally has a high BV. Animal-based proteins are typically complete, while most plant-based proteins are incomplete.
- The Limiting Amino Acid: The EAA that is in the lowest proportion relative to the body's needs acts as a bottleneck for protein synthesis. If this limiting amino acid is missing or present in low amounts, the body cannot effectively build new proteins, regardless of how much of the other amino acids are available. This directly lowers the BV.
How Biological Value is Measured
Measuring the BV of a protein is a complex process typically performed under controlled lab conditions. It involves a nitrogen balance study, where nitrogen intake is compared to nitrogen excretion.
- A test subject is placed on a controlled diet containing a known amount of the protein being tested.
- Urine and fecal samples are collected and analyzed to measure nitrogen excretion.
- The nitrogen absorbed is calculated by subtracting the fecal nitrogen from the dietary nitrogen.
- The amount of nitrogen incorporated into the body's proteins is estimated by subtracting the urinary nitrogen from the absorbed nitrogen.
- The final BV is calculated using the formula: $BV = (Nr / Na) \times 100$, where $Nr$ is the nitrogen retained and $Na$ is the nitrogen absorbed.
This method requires strict dietary controls and is typically performed on a low-protein diet to maximize the differences in protein quality.
BV vs. Other Protein Quality Metrics
While BV is a useful indicator, it is not the only metric for assessing protein quality. Newer and more comprehensive methods have emerged that address some of its limitations. The Protein Digestibility Corrected Amino Acid Score (PDCAAS) and the Digestible Indispensable Amino Acid Score (DIAAS) are two such examples.
| Feature | Biological Value (BV) | Protein Digestibility Corrected Amino Acid Score (PDCAAS) |
|---|---|---|
| Calculation Basis | Compares retained nitrogen to absorbed nitrogen, focusing on how well the body utilizes the absorbed protein. | Compares a protein's EAA profile to a human reference standard, correcting for true fecal digestibility. |
| Consideration of Digestibility | Does not directly account for how easily the protein is digested and absorbed. | Does correct for digestibility, making it a more complete measure of a protein's overall nutritional value. |
| Highest Score | Can exceed 100, especially in relative scoring systems where egg is the benchmark. | Is capped at 1.0, meaning scores above this are truncated. |
| Primary Use | Historically significant for comparing protein efficiency under controlled lab conditions. | The preferred method for human protein quality assessment by organizations like the FDA and FAO/WHO. |
Conclusion
The biological value of a protein is a key indicator of its nutritional quality, representing how efficiently the body can utilize it for essential functions like tissue repair and synthesis. A high BV is associated with a complete amino acid profile, meaning the protein source provides all nine essential amino acids necessary for human health. While egg protein is the traditional benchmark with a high BV, modern research recognizes that factors like protein source (animal vs. plant) and combination can significantly influence utilization. Despite its limitations in real-world application, understanding BV provides a foundational knowledge of protein quality. For practical dietary choices, especially for those on plant-based diets, combining different protein sources is essential to achieve a balanced amino acid intake and maximize overall BV. The BV, alongside other metrics, helps inform better nutritional strategies for athletes and the general public alike.
Sources: : Wikipedia - Biological Value : PMC - Protein – Which is Best? : Wikidoc - Biological Value : Thamesmead School - Protein Complementation : Wikipedia - Biological Value (scores) : nutri+ - Biological value of protein
Frequently Asked Questions
What is a high biological value protein?
A high biological value protein contains a balanced and sufficient amount of all nine essential amino acids, which the human body cannot produce on its own. These proteins are used most efficiently for tissue synthesis. Examples include whey, egg, and meat protein.
What is a limiting amino acid?
It is the essential amino acid present in a protein source in the lowest proportion relative to the body's needs. This amino acid effectively limits the body's ability to synthesize new proteins, even if other amino acids are plentiful.
Are plant-based proteins low in biological value?
Most plant-based proteins are considered incomplete, meaning they are deficient in one or more essential amino acids, which gives them a lower BV score when consumed alone. However, by combining different plant proteins, it is possible to achieve a complete amino acid profile.
Why can a protein's biological value be greater than 100?
This occurs in relative BV scoring systems where egg is set as the benchmark at 100. Since the true physiological utilization of egg protein is slightly below 100%, some highly efficient proteins like whey can score higher than 100 on this relative scale.
How does cooking affect a protein's biological value?
Cooking can affect the availability of amino acids in a food source. While some preparation methods can improve digestibility, overcooking or severe heat treatment can damage or destroy certain essential amino acids, reducing the protein's overall BV.
Is BV still a relevant measure of protein quality?
While BV is a foundational concept in nutrition science, newer methods like PDCAAS and DIAAS are now the preferred standard for measuring protein quality in human nutrition because they account for digestibility. Nonetheless, BV still offers valuable insight into a protein's efficiency.
Can you improve the biological value of a meal?
Yes, you can improve the BV of a meal through protein complementation. This involves combining two or more incomplete protein sources that have different limiting amino acids. For example, eating rice (low in lysine) and beans (low in methionine) together creates a complete protein meal with a higher overall biological value.
How does BV differ from protein digestibility?
Biological value specifically measures the proportion of absorbed protein that the body uses for synthesis, focusing on the quality of the amino acid profile. Protein digestibility, in contrast, measures how much of the ingested protein is actually absorbed, without considering how efficiently it will be used.
