Biological value

Biological Value or BV is a common method for measuring protein quality and biological utilization rates of protein for human and animal consumption. The method relies on nitrogen retention as an indicator of protein quality. Unlike carbohydrates and fats all protein contains nitrogen. Scientists trace nitrogen as it enters the body through protein intake and measure the amount that is retained. Since protein is used in the construction of bodily cells the more protein that is retained indicates a higher level of biological utilization of the particular protein. The more nitrogen that is excreted as urine and fecal matter the less utilizable the particular kind of protein.

The number is expressed as a score with 100 being the threshold. The number progressively lowers as more is consumed, while the protein's score is based on its consumption in a fasted state.

Humans

One method tested in humans for protein utilization is biological value of protein. Biological value is determined based this formula.

BV = (nitrogen retained / nitrogen absorbed) * 100

A BV of 100 would indicate complete utilization of a given dietary protein, in that 100% of the protein ingested was stored in the body with none lost. However, it is physically impossible to have a BV greater than 100. Having a BV of over 100 would suggest that more than 1 gram of nitrogen was stored for every 1 gram of nitrogen consumed. Since it is thermodynamically impossible for the body to store more nitrogen than is ingested, a BV of over 100 is equally impossible.

Egg protein scored the highest number of 100 to serve as a measuring stick. Whey protein has subsequently been found to have the highest known biological value of any protein in humans. Whey scored a 104 in its basic form relative to the original 100 egg scale.

Animals

The Biological Value method is also used for analysis in animals such as cattle, poultry, and various laboratory animals such as rats. It was used by the poultry industry to determine which mixtures of feed were utilized most efficiently by developing chicken. Although the process remains the same, the biological values of particular proteins in humans differs from their biological values in animals due to physiological variations.

Advantages

The BV and PDCAAS is more relevant than the Protein Efficiency Ratio (PER) for the measurement of protein utilization in humans.

The table below shows the Biological Value rating of various common proteins.

• Isolated Whey: 104
• Whole Egg: 100
• Cow’s Milk: 91
• Egg Whites: 88
• Fish: 83
• Casein: 80
• Beef: 80
• Chicken: 79
• Soy: 74
• Wheat Gluten: 54
• Kidney Beans: 49

Critics

Since the method measures only the amount that is retained in the body critics have pointed out what they perceive as a weakness of the biological value methodology. Critics have pointed to research that indicates that because whey protein isolate is digested so quickly it may in fact enter the bloodstream and be converted into carbohydrates through a process called gluconeogenesis much more rapidly than was previously thought possible, so while amino acid concentrations increased with whey it was discovered that oxidation rates also increased and a steady-state metabolism where there is no change in overall protein balance is created. They claim that when the human body consumes whey protein it is absorbed so rapidly that most of it is sent to the liver for oxidation. Hence they believe the reason so much is retained is that it is used for energy production not protein synthesis. This would bring into question whether the method defines which proteins are more biologically utilizable.

A further critique published in the Journal of Sports Science and Medicine, states that BV of a protein does not take into consideration several key factors that influence the digestion and interaction of protein with other foods before absorption, and that it only measures a proten's maximal potential quality and not its estimate at requirement levels. Also, the study by Poullain et al, which is often cited to demonstrate the superiority of whey protein hydrosylate by marketers, measured nitrogen balance in rats after three days of starvation, which corresponds to a longer period in humans . The study found that whey protein hydrosylate led to better nitrogen retention and growth than the other proteins studied. However the study's flaw is in the BV method used, as starvation affects how well the body will store incoming protein (as does a very high caloric intake), leading to falsely elevated BV measures.

So, the BV of a protein is related to the amount of protein given. BV is measured at levels below the maintenance level. This means that as protein intake goes up, the BV of that protein goes down. For example, milk protein shows a BV near 100 at intakes of 0.2 g/kg. As protein intake increases to roughly maintenance levels, 0.5 g/kg, BV drops only around 70.

Yet another limitation of the use of Biological Value as a measure of protein quality is that proteins which are completely devoid of one essential amino acid (EAA) can still have a BV of up to 40. This is because a the ability of ogranisms conserve and recycle EAA as an adaptation of inadequate intake of the amino acid.

Lastly, the use of rats for the determination of protein quality is not ideal. Rats differ from humans in requirements of essential amino acids. This has lead to a general criticism that experiments on rats lead to an over-estimation of the BV of high-quality proteins to man because human requirements of essential amino acids are much lower than those for rats (as rats grow at a much faster rate than humans). Also, because of their fur, rats are assumed to have relatively high requirements of sulphur-containing amino acids (methionine and cystine).

As a result, the analytical method that is universally recognized by the FAO/WHO as well as the FDA, USDA, United Nations University (UNU) and the National Academy of Sciences when judging the quality of protein in the human is not PER or BV but the Protein Digestibility Corrected Amino Acid Score (PDCAAS), as it is viewed as accurately measuring the correct relative nutritional value of animal and vegetable sources of protein in the diet. However, scientific studies demonstrate PDCAAS scale has limitations in predicting protein quality of those protein sources which may contain naturally occurring growth-depressing factors or antinutritional factors formed during alkaline and/or heat processing. In short, PDCAAS makes no distinction of their peformance relative to each other because after they pass a certain point they are all capped at 1.0 being the highest possible score and receive an identical rating. This is because in 1990 at a FAO/WHO meeting it was decided that proteins having values higher than 1.0 would be rounded or "leveled down" to 1.0 as scores above 1.0 are considered to indicate that the protein contains essential amino acids in excess of the human requirements. This approach implies injustice to high-quality proteins which can compensate for low-quality ones by virtue of their high content of essential amino acids (egg has an actual PDCAA score of 1.19 compared to 0.91 for soy, however when leveled down, they appear much closer).

Regardless, the scientific community has raised critical questions about the validity of PDCAAS.

On the other hand, BV makes a differentiation with respect to proteins with similar protein values such as egg whites at 88, whole eggs at 100, and whey at 104. Even though, as noted above it is thermodynamically impossible to achieve a BV of over 100.

Biological Value (BV), as demonstrated by research scientists - including early 20th century scientists K. Thomas, and H.H. Mitchell - is another method of choice as to estimating the nutritive value of proteins for muscle growth and synthesis. Some athletes and Dr. Michael Colgan also support BV as a reliable method for protein value.