Biological Value (BV)
Biological Value (BV) is a quantity used to evaluate certain foods in terms of the proportion of protein in the food that is both absorbed by the body and then retained for use in the body. Biological Value (BV) is included in some courses involving study of the human digestive system and is useful when considering diets and dieting, e.g. for weight loss or muscle gain.
This is a general introduction to the concept of BV. Obtaining accurate values of BV is not quick or easy and is not usually required for introductory-level courses in diet and nutrition and associated health science subjects.
What is Biological Value (BV) ?
Definition of Biological Value:
The Biological Value (BV) of a food is the percentage of absorbed protein from the food that is retained in the body and is therefore available for incorporation into the proteins within the body of the organism that consumed it - i.e. the person if this is discussed in the context of human biology.
Why is Biological Value a useful parameter ?
Proteins are a major source of diertary nitrogen, i.e. nitrogen in food. The Biological Value (BV) of a food summarizes how readily the broken down protein from the food can be used in protein synthesis in the cells of the organism. This matters because it is not sufficient to consume (ingest) protein; it is also important that such protein is in a form that can be readily accessed by the body via the digestive processes.
Equations for Biological Value (BV):
All of the following equations concern the Biological Value (BV) of a specific food.
The parameters therefore refer to nitrogen from that food that has been absorbed by or retained in the body, etc..
Simple equation for BV:
which can, alternatively, be written using symbols instead of words, as follows:
The shorter version shown in Eq (2) may be easier to remember - especially for people who tend to remember information as pictures and / or patterns rather than as words.
Another advantage of expressing the definition of biological value in symbols (as in Eq (2)) rather than in full words is that it is then possible to write longer definitions of exactly what each term, in this case Nr and Na, means. That is particularly useful when some of the terms in an equation are related to each other, such as Ni, Ne(f) and Ne(u) below.
More detailed equation for BV:
This more detailed equation (Eq (3), above) is useful because it includes more information about how the final figure is calculated. However, it is difficult to read when written out in full.
To include all the information in a shorter form, symbols are used to represent the quantities in the equation:
*sometimes Ne(f) and Ne(u) are even more tightly defined as 'while following the test diet only', see the section below.
Compare Eq (4) with Eq (2) and notice that:
Nr = Ni - Ne(f) - Ne(u)
i.e. nitrogen retained in the body
= ingested nitrogen
- nitrogen excreted in faeces
- nitrogen excreted in urine
Na = Ni - Ne(f)
i.e. nitrogen absorbed by the body
= ingested nitrogen
- nitrogen excreted in faeces
These symbols should be easy to understand because in all cases 'N' represents 'Nitrogen'.
- 'r' indicates that the nitrogen is retained (for use in the cells and tissues of the body).
- 'a' indicates that the nitrogen is absorbed (into the body by the digestive system).
- 'i' indicates that the nitrogen is that taken in to the body within the food whose BV is defined by the equation - so, not counting nitrogen that was already in the body or was received from some other (food) source.
- 'e' indicates that the nitrogen is excreted.
Nitrogen excreted form the body may be excreted in either faeces '(f)' or urine '(u)', hence 'e(f)' or 'e(u)'
Tip: An important part of stating a definition correctly by using an equation to express how the defined quantity is related to other quantities is the correct definition of all symbols used in the equation. It might be easier to remember the short form of an equation that uses symbols rather than words but in that case it is essential to also state correctly the meaning of every symbol used in the equation.
Provided that you can remember what the symbols mean, the short forms of the equations make it much easier to see the definition of BV "at a glance" and to appreciate the level of detail included in that particular definition.
Eq (4) is the most thorough of the above definitions of Biological Value (BV) because it takes into consideration:
- nitrogen leaving the body in the faeces, and
- nitrogen leaving the body in the urine, and
- if the nitrogen lost via faeces or urine actually came from ingested nitrogen (i.e. a source of nitrogen, e.g. protein that the organism had eaten) or "not from ingested nitrogen" (i.e. nitrogen that was already within the body).
Hence in order to obtain an accurate value of BV using Eq (4) it is necessary to measure or calculate the amount of urinary and faecal nitrogen excretion that did and did not come from ingested nitrogen.
One way to do this is to follow a protein-free diet for a period of time and consider the resulting urinary and faecal nitrogen excretion as a measure of urinary and faecal nitrogen not due to ingested nitrogen. However, the accuracy of that method is not universally accepted.
Question to check understanding:
Explain in a few words what the amounts of nitrogen in urine and nitrogen in faeces represent in the context of calculation BV
Answer : The amount of nitrogen in the urine indicates the amount of nitrogen absorbed and used by the body - but not retained in the body. The amount of nitrogen in the faeces indicates the amount of nitrogen that has been neither absorbed (used) nor retained by the body.
How is Biological Value evaluated ?
It is easy to put numbers into equations and do simple maths to calculate an answer. However, in biology and biology-related health science subjects it can be more difficult to obtain the correct values to put into equations.
It is difficult to evaluate the BV of a food because it depends on so many things, including not only the food itself but also:
- how the food is prepared, and
- how efficiently it is digested - i.e. the health and recent diet of the organism (e.g. person) eating it.
Practical issues in measuring the quantities needed to calculate BV
In order to calculate the BV of a specific protein (food) the 'test organism' (usually a person, e.g. bodybuilder) must only eat the protein or mixture of proteins being evaluated while participating the study or 'test' to evaluate the BV of that particular source of dietary protein. Therefore, during the study he or she must follow a strict 'test diet' and ensure that his or her intake of food and beverages during the study (i.e. while following the 'test diet') does not contain any non-protein sources of nitrogen. The test diet must also be designed and controlled in such a way as to avoid use of the protein primarily as an energy source.
These constraints tend to result in studies to calculate the BV of foods (sources of dietary protein) being carried out over a week, during which time the diets of the subjects or 'test organisms' are strictly controlled.
When several people are taking part in the same study or 'test' to evaluate the BV of a protein, fasting before testing increases the consistency of results across the 'test organisms' (different people participating in the study) by removing the variable 'recent diet'.