Tråd: Bv

Biological value (BV)

Biological value (BV) is probably one of the most commonly used measures of a protein's quality. The BV of a protein is given as the amount of nitrogen retained in the body divided by the amount of nitrogen absorbed from that protein. Therefore, digestibility of that protein is taken into account. Thus:

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.

To measure BV, subjects are typically fed a zero protein diet so that baseline losses of nitrogen can be measured (i.e. the amount of nitrogen that is lost normally). Then the test protein is fed at varying levels (generally 0.6, 0.5, 0.4 and 0.3 g/kg are fed) and a nitrogen balance study is done (9). Some studies use longer periods of starvation and this is an important consideration in evaluating the data.

For example, the study often cited by advertisers to demonstrate the 'superiority' of whey protein hydrosylate measured nitrogen balance in rats after three days of starvation, which corresponds to a longer period in humans (10). In this study, whey protein hydrosylate led to better nitrogen retention and growth than the other proteins studied. What is not mentioned is that starvation affects how well the body will store incoming protein, leading to falsely elevated BV measures. This study has little bearing to an individual with a habitual high-protein intake. A full discussion of the effects of low protein eating (i.e. protein cycling) will appear in Part 3 of this article series.

Although nitrogen balance methodology has it's problems (see Part 1 of this article series), it is a rough indicator of how well or poorly a given protein supports the body's needs. If a given amount of protein (more accurately, a given amount of nitrogen) places an individual in nitrogen balance (or positive nitrogen balance) it can be assumed that the protein in question is of sufficient quality to support maintenance of body protein stores.

The biggest drawback of the nitrogen balance method is that it gives no information regarding specific amino acid metabolism (and deficiencies) or the specific tissues which are being affected (e.g. muscle vs. liver), only an indication of what is occurring on the whole body level (9). Depending on the individual amino acid requirements of a given tissue, it is possible that a protein might optimally support protein synthesis in one organ, such as the liver, while not optimally supporting synthesis in another tissue, such as muscle. The issue of whole body versus specific tissue protein metabolism will be discussed in Part 3 of this series.




Despite what is sometimes claimed, it is impossible to have a BV greater than 100. Additionally, there is no indication that the percentage sign was ever dropped from the BV measure. For example, it's been suggested that whey protein has a BV of 157, but this would imply that 1.57 grams of nitrogen were stored for every 1 gram of nitrogen consumed. Since it is thermodynamically impossible for the body to store more nitrogen than was ingested, a BV of 157 is equally impossible. Protein advertisements claiming BV higher than 100 should be looked upon with suspect.



One aspect of measuring BV that can cause problems in interpretation of results is that the BV of a protein is affected by a number of factors. The first of these is caloric intake. A very high caloric intake will improve nitrogen balance at any given protein intake and vice versa. This means that an individual consuming a lot of calories (e.g. a bodybuilder on a mass-gaining diet) will show improved nitrogen retention and 'apparent' BV will go up (i.e. more nitrogen will be retained compared to the amount eaten). By the same token, if calories are decreased (e.g. during a diet), BV will go down. A secondary factor which affects BV is activity. Exercise, especially weight training, increases nitrogen retention which will give a protein a higher apparent BV.

A third factor, and one that is typically ignored in popular literature is that the BV of a protein is related to the amount of protein given (9). BV is measured at levels below the maintenance level. 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 70 or so (9).

To quote from Pellett and Young, "....protein is utilized more effectively at suboptimal levels than at levels in the near-maintenance range of intake. Accordingly biological measures of protein quality conducted at suboptimal levels in either experimental animals or human subjects may overestimate protein value at maintenance levels." (9) Therefore, while BV may be important for rating proteins where intake is below requirements, BV has little bearing on individuals with protein intakes far above requirements. Table 2 presents the BV of some common proteins.

Table 2: BV of some common proteins
Protein BV
whey 100?
egg 100
milk 93
rice 86
casein, fish and beef 75
corn 72
peanut flour 56
wheat gluten 44

Considering the high protein intakes of most strength athletes (2.0 g/kg or higher) it is hard to see how BV will play a meaningful role in rating proteins in this population. In all likelihood, any decent quality protein will be as good as any other at these types of protein intakes. Additionally, even if proteins such as whey have slightly higher BV ratings than protein sources like casein (milk) or egg, such a small difference is unlikely to affect mass gains in the long run.

PDCAAS


In 1993, the US Food and Drug Administration adopted the Protein Digestibility-Corrected Amino Acid Scoring (PDCAAS) as the standard by which to evaluate protein quality. It is now the most accepted and widely used method in the scientific community.

PDCAAS is based on several factors: the profile of essential amino acids, the digestibility of the protein, and the ability of the protein to supply the essential amino acids in the amounts needed to meet the requirements of growing human beings. The protein quality rankings are determined by comparing the amino acid profile of the specific food protein against a standard amino acid profile.

The highest possible score under these guidelines is 1.0. The following are the PDCAAS scores for the main sources of protein:

1.0 meat, fish, eggs, dairy, WPC, WPI, soy
0.7 nuts
0.6 pulses and legumes
0.4 wheat and grains

When shopping or selecting foods to eat, PDCAAS is easy to use. Simply multiply the PDCAAS by the protein content as displayed on the nutritional information (or food facts) panel. For example, a popular breakfast cereal contains 6.6g protein per serve (before milk). Multiply 6.6 by 0.4 to get the total useable protein: 2.6g.