312 Introduction to Human Nutrition
animal models in human nutrition issues. There are
several types of diets offered to laboratory animals.
Commercially available diets made to internation-
ally accepted nutritional norms are often referred to as
chow diets or laboratory chow. For the vast majority
of laboratory animals in studies where nutrient intake
is not the central area of interest, such chow diets are
used. However, when nutrition is the area of research,
special diets will almost always have to be formulated.
The type of diet that needs to be formulated will
depend on the nature of the research question.
Terms such as semipurifi ed, purifi ed, and chemi-
cally defi ned diets are often used but frequently it is
diffi cult to know exactly which type of term fi ts dif-
ferent formulations. The least refi ned experimental
diet uses ingredients such as barley, soybean, and
wheat. An example is given in Table 13.5, taken from
a study of rapeseed glucosinolates on the iodine status
of piglets.
The purpose of the study was to assess the effects
of glucosinolate derived from ground rapeseed. A
direct comparison between the ground rapeseed and
the control is not possible because the ground rape-
seed contains twice as much fat as the controls. Thus,
the rapeseed oil diet is included because it contains
no glucosinolate, but the same amount of fat as the
control diet. The ingredients used in these diets, in
general, contain several nutrients. Thus, the main
ingredient, barley, contains protein, carbohydrate,
and fat as well as fi ber and micronutrients. That can
create problems when there is a need to examine the
effects of specifi c nutrients, such as fatty acids. The
fatty acids naturally present in barley cannot be
ignored. In the case of the rapeseed oil diet in Table
13.5, 40 g of the 56 g of fat per kilogram of diet comes
from the rapeseed oil, but 16 g (or 28.6%) comes
from barley lipid.
To deal with this, more refi ned diets are used. An
example of such a diet is given in Table 13.6. In this
instance, the authors were examining how different
dietary fats infl uence blood cholesterol in normal
mice and in transgenic mice not expressing the gene
for the cholesteryl ester transfer protein (CETP),
which is a key protein in lipid metabolism. In this
instance, the ingredients are almost all pure. Thus,
casein is pure protein and nothing else. Similarly,
sucrose is pure carbohydrate and cellulose is pure
fi ber. The diets differ only in the source of fat. The
high-fat diet obviously has more fat and thus more
energy per kilogram of diet. It is thus critically impor-
tant to note that as the energy density goes up, most
other things must also go up to ensure a common
concentration, not on a weight-for-weight basis but
on a weight-for-energy basis. A simple illustration is
the level of the mineral mix used: 2.5 g/100 g in the
control diet, 3.2 g/100 g in the low-fat diet and
4.2 g/100 g in the high-fat diet. But when considered
on a weight-for-energy basis, all fi ve diets contain
2.0 g/MJ. The only changes are in fat and in maize
starch, which always vary in opposite directions.
Variations in diet composition are often the key for
the design of nutrition experiments. In this context,
different feeding regimens can be applied to labora-
tory animals depending on scientifi c criteria. In ad
libitum feeding the animals have free access to food;
in controlled feeding animals are offered a limited
amount of food (restricted feeding) or receive as
much food as can be fed to them (forced feeding). A
specifi c form of restricted feeding is pair feeding,
which involves the measurement of food consumed
by some animals to match or equalize the intake of a
test group on the following day. There are many
reasons why pair feeding is critically important. An
experiment may seek to examine how a new protein
source, rich in some nutrient of interest, infl uences
some aspect of metabolism. Let us consider a com-
pound in the protein source that may reduce blood
LDL cholesterol. A control diet is constructed based
on casein. In the experimental diets, this casein isTable 13.5 An example of less refi ned experimental diets to test the
effects of rapeseed-derived glucosinolate
Control
(g/kg)Rapeseed oil
(g/kg)Ground rapeseed
(g/kg)Soybean meala 220 240 195
Rapeseed oil 5 40 –
Ground rapeseed – – 100
Barley 755 700 685
Mineral/vitamin 20 20 20
Total 1000 1000 1000
Energy (MJ/kg)b 12.6 13.3 13.0
Protein (g/kg) 183 177 182
Fat (g/kg) 29 56 58
Glucosinolate
(mmol/kg)
0 0 1.9aSolvent-extracted soybean meal.
bMetabolizable energy.
From Schone et al. (2001). Reproduced with permission.