Introduction to Human Nutrition

286 Introduction to Human Nutrition

prepared foods from raw ingredients are made. Thus,
the nutrient composition of a prepared or cooked
food is calculated from the analytical data of uncooked
food by applying suitable nutrient retention and yield
factors. To obtain the nutrient content per 100 g of
cooked food, the nutrient content per 100 g of raw
food is multiplied by the percentage retained after
cooking, and this is divided by the percentage retained
after cooking, divided by the percentage yield* of the
cooked product:

Nutrient content of cooked food per 100 g =

[(nutrient content of raw food × retention

factor)/yield of cooked food] × 100

The retention factor accounts for the loss of solids
from foods that occurs during preparation and
cooking. The resulting values quantify the nutrient
content retained in a food after nutrient losses due to
heating or other food preparations. This is called the
true retention method and is calculated as follows:

% True retention = [(nutrient content per g of
cooked food × g cooked food)/(nutrient content
per g of raw food/g of food before cooking)] × 100
The following example uses only the yield factor to
predict the nutrient content of the cooked food. The
yield factors for different foods are reported in the
USDA Agriculture Handbook 102 and for cooked
carrots it is 92%. Selected nutrient values in SR 21 for
100 g of raw carrots are 0.93 g of protein, 33 mg
of calcium and 5.9 mg of ascorbic acid. Using the
yield factor the composition of 100 g of cooked
carrots is calculated as 0.93 g/0.92 = 1.01 g protein,
33 mg/0.92= 36 mg calcium and 5.9 mg/0.92 = 6.4 mg
of ascorbic acid. This compares favorably to the deter-
mined values for carrots of 0.76 g of protein and 30 mg
of calcium, but less so for ascorbic acid at a value of
3.6 mg, probably because it is heat sensitive; therefore,
applying the nutrient retention factor for ascorbic
acid (70%) would have resulted in a more accurate
prediction (5.9 × 0.7/0.92 = 4.9) of 4.9 mg/100 g)
(http://www.ars.usda.gov/nutrientdata).

Missing values in food composition tables

In general, original analytical data provide informa-
tion of the highest quality for inclusion in a food

composition table or nutrient database. However, it is

seldom possible to construct a food composition table

with only such data. A plan of action should be devel-

oped by the compilers of the database to deal with

missing food items and values for particular nutrients.

Very often, values of a biologically similar food are

used. For composite or mixed dishes the composition

of the dish is estimated by calculation from a standard

recipe and applying appropriate nutrient retention

factors and, in some cases, adjusting for changes in

moisture content due to cooking loss or gain in the

different cooking procedures. If a food item forms an

important part of the population’s diet and analysis is

not possible, existing food composition databases

should be searched to see whether data on the same or

a similar food item could be borrowed. If a value for a

nutrient is missing a similar approach can be followed,

as it is more desirable to have a slightly incorrect esti-

mated value of lower quality than no value at all. A

value of “–” or “0” assigned to missing nutrient values

may lead to underestimation of nutrient intakes,

especially if those nutrients make a signifi cant

contribution to the diet.

Bioavailability and glycemic index

Nutrient composition information in food composi-

tion tables indicates the amount of nutrients as ana-

lyzed in that specifi c food sample and does not give an

indication of the absorption or bioavailability of the

nutrient from that food item. However, when dietary

reference intakes such as recommended dietary allow-

ances (RDAs) are drawn up, the recommendation

makes provision for the amount of ingested nutrient

that may not be absorbed. The concept of bioavail-

ability has developed from observations that measure-

ments of the amount of a nutrient consumed do not

necessarily provide a good index of the amount of a

nutrient that can be utilized by the body. The bioavail-

ability of a nutrient can be defi ned as the proportion

of that nutrient ingested from a particular food that

can be absorbed and is available for utilization by the

body for normal metabolic functions. This is not

simply the proportion of a nutrient absorbed, and

cannot be equated with solubility or diffusibility in

in vitro-simulated physiological systems. Bioavailability

is not a property of a food or of a diet per se, but is the

result of the interaction between the nutrient in ques-

tion, other components of the diet and the individual

consuming the diet. Owing to the many factors

*Yield of cooked food (%) = (weight of edible portion cooked food/
weight of raw food) × 100