Biological value

For accurate determination of BV:[4]

1. the test organism must only consume the protein or mixture of proteins of interest (the test diet).
2. the test diet must contain no non-protein sources of nitrogen.
3. the test diet must be of suitable content and quantity to avoid use of the protein primarily as an energy source.

These conditions mean the tests are typically carried out over the course of over one week with strict diet control. Fasting prior to testing helps produce consistency between subjects (it removes recent diet as a variable).

There are two scales on which BV is measured; percentage utilization and relative utilization. By convention percentage BV has a percent sign (%) suffix and relative BV has no unit.

The determination of BV is carefully designed to accurately measure some aspects of protein usage whilst eliminating variation from other aspects. When using the test (or considering BV values) care must be taken to ensure the variable of interest is quantified by BV. Factors which affect BV can be grouped into properties of the protein source and properties of the species or individual consuming the protein.

BV provides a good measure of the usability of proteins in a diet and also plays a valuable role in detection of some metabolic diseases. BV is, however, a scientific variable determined under very strict and unnatural conditions. It is not a test designed to evaluate the usability of proteins whilst an organism is in everyday life — indeed the BV of a diet will vary greatly depending on age, weight, health, sex, recent diet, current metabolism, etc. of the organism. In addition BV of the same food varies significantly species to species. Given these limitations BV is still relevant to everyday diet to some extent. No matter the individual or their conditions a protein source with high BV, such as egg, will always be more easily used than a protein source with low BV.

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.[10]

Common foodstuffs and their values: (Note: this scale uses 100 as 100% of the nitrogen incorporated.)

• Whey Protein: 96 [11]
• Whole Soy Bean: 96 [12]
• Human milk: 95[13]
• Chicken egg: 94[13]
• Soybean milk: 91[12]
• Buckwheat: 90+[14]
• Cow milk: 90[13]
• Cheese: 84[15]
• Quinoa: 83[16]
• Rice: 83[15]
• Defatted soy flour: 81[12]
• Fish: 76[17]
• Beef: 92[17]
• Immature bean: 65[12]
• Full-fat soy flour: 64[12]
• Soybean curd (tofu): 64[12]
• Whole wheat: 64[17]
• White flour: 41[12]

Common foodstuffs and their values:[18] (Note: These values use "whole egg" as a value of 100, so foodstuffs that provide even more nitrogen than whole eggs, can have a value of more than 100. 100, does not mean that 100% of the nitrogen in the food is incorporated into the body, and not excreted, as in other charts.)

• Whey protein concentrate: 104
• Whole egg: 100
• Cow milk: 91
• Beef: 80
• Casein: 77
• Soy: 74[19]
• Wheat gluten: 64

By combining different foods it is possible to maximize the score, because the different components favor each other:

• 85 % rice and 15 % yeast: 118[20]
• 55 % soy and 45 % rice: 111[20]
• 55 % potatoes and 45 % soy: 103[20]
• 52 % beans and 48 % corn: 101[20]

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.[21] 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, a process where there is no change in overall protein balance, is created.[22] 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 the 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 protein's maximal potential quality and not its estimate at requirement levels.[23] Also, the study by Poullain et al., which is often cited to demonstrate the superiority of whey protein hydrolysate by marketers, measured nitrogen balance in rats after three days of starvation, which corresponds to a longer period in humans.[24] The study found that whey protein hydrolysate 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.[25]

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 to around 70.[25] Pellet et al., concluded that "biological measures of protein quality conducted at suboptimal levels in either experimental animals or human subjects may overestimate protein value at maintenance levels." As a result, while BV may be important for rating proteins where intake is below requirements, it has little bearing on individuals with protein intakes far above requirements.

This flaw is supported by the FAO/WHO/UNU, who state that BV and NPU are measured when the protein content of the diet is clearly below that of requirement, deliberately done to maximize existing differences in quality as inadequate energy intake lowers the efficiency of protein utilization and in most N balance studies, calorie adequacy is ensured. And because no population derives all of its protein exclusively from a single food, the determination of BV of a single protein is of limited use for application to human protein requirements.[26]

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 of the ability of organisms to conserve and recycle EAAs as an adaptation of inadequate intake of the amino acid.[27]

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 led 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 cysteine).

As a result, the analytical method that is universally recognized by the Food and Agriculture Organization (FAO), World Health Organization (WHO), the U.S. Food and Drug Administration (FDA), the United States Department of Agriculture (USDA), United Nations University (UNU) and the United States 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.[28][29]

• Edible protein per unit area of land
• List of foods by protein content