What are nutrients?
Nutrients can be categorized as energy vs. non-energy
yielding. An energy nutrient is one which upon burning
in the laboratory or in our bodies yields heat (measured
energy nutrients are carbohydrates (4 calories per gram),
fats or lipids (9 calories per gram) and proteins (4 calories
per gram). Other nutrients are involved in helping the
body use energy, but none of them actually yields heat
or can be converted into a stored form of energy such
as fat or glycogen, a carbohydrate. Alcohol, not a nutrient,
yields seven calories per gram. These four compounds are
our only sources of calories.
the energy issue is important to a protein discussion.
The energy issue is important in determining human protein
requirements because the first demand of the body is for
energy. If calorie/energy requirements are not met, protein
will not be spared (or available) to do its jobs of maintenance
and growth, thus the term “protein-calorie malnutrition.”
However, assuming adequate energy/calories are provided,
and the appropriate other nutrients are available to facilitate
energy release and other biological mechanisms, then protein
can be reserved for its unique functions. Although the
daily allowance is expressed as protein, the biological
requirement is for amino acids.
Proteins are composed of building blocks called amino
acids all of which contain nitrogen. Proteins and other
nitrogen containing compounds are degraded and rebuilt
continuously in the body. Several times more protein is
turned over each day than is typically consumed, indicating
that the reutilization of amino acids is a major feature
of the economy of protein metabolism. The process of recycling
is not completely efficient and some amino acids are lost.
Metabolic products of amino acids—urea, creatinine,
uric acid and other nitrogen-containing compounds are
excreted in urine. Nitrogen also is lost in feces, sweat
and other body secretions and in sloughed skin, hair and
nails. All of these losses must be replaced by a continuous
supply of dietary amino acids which are usually provided
through dietary protein.
importance of the amino acids.
There are about 20 amino acids. However, nine amino acids—histidine,
isoleucine, leucine, lysine, methionine, phenylalanine,
threonine, tryptophan, and valine—cannot be synthesized
by mammals and therefore must be consumed in the diet.
They are called essential or indispensable amino acids.
The rest of the amino acids can be made by mammals if
nitrogen and other needed nutrients are available. Thus
the bottom line is that humans must eat protein for two
reasons: first, because food protein is the only source
of the indispensable amino acids and secondly, because
food protein is the only practical source of nitrogen
with which to build the dispensable amino acids.
why are some sources of protein considered better than
All plant and animal proteins have the same 20 or so amino
acids. However, the proportion of the indispensable amino
acids in a food compared to the indispensable amino acid
requirements of the 3-5 year old child is used by the
FAO/WHO and the FDA to determine the quality of a protein
source for human nutrition. This method is called the
Protein Digestibility Corrected Amino Acid Score (PDCAAS).
The best score for protein quality using PDCAAS is 1.0.
A score of 1.0 means the protein contains 100% of the
amino acids in the correct proportions per gram of protein
for the 3-5 year old child. If the requirements for amino
acids (measured per gram of protein) for growing children
are met, then the requirements for adults will be exceeded.
Of course, the absolute amount of protein needed will
be higher for an adult. The FDA requires food manufacturers
who make protein claims on their labels to take into account
protein quality when calculating the % Daily Value. Unfortunately,
most food manufacturers do not make protein claims and
do not voluntarily provide a % Daily Value for protein
(FDA does not require it). Since % Daily Value of protein
is not on most labels, it is very difficult for consumers
to become familiar with the protein quality of common
foods or to learn how a food contributes to their daily
are some high quality protein sources?
We’ve already mentioned that in order for a protein
to be high quality, it must contain the indispensable
amino acids in the proper proportion to meet the needs
of a growing child. Animal sources (meat, poultry, eggs,
milk, and fish) and the vegetable protein from soybeans
all meet 100% of the amino acid requirements of the young
growing child and these are considered the “gold
standards” for protein quality.
good sources of protein are peanuts, lentils and rice.
They score about 0.50 (out of a possible 1.0) when using
the Protein Digestibility Corrected Amino Acid Score,
the method recommended by FAO/WHO for evaluating protein
quality and the method used by FDA. By comparison wheat
protein scores 0.40. In typical diets, people consume
a combination of protein sources and can actually meet
their protein needs entirely from grains, beans, and vegetables.
Protein Quality Evaluation. Report of a Joint FAO/WHO
Expert Consultation. Rome, Italy. Food and Agriculture
Organization and World Health Organization. Food and Agriculture
Organizations of the United Nations; 1990.
Henley EC, Kuster JM. Protein quality evaluation by protein
digestibility corrected amino acid scoring. Food Technology.
Title 21, Code of Federal Regulation, Part 100-169 –
Food labeling, food standards, good manufacturing practices
for foods, low-acid foods, and acidified foods. 101.9(c)(7)p28-9.