314 Introduction to Human Nutrition
genome, permanent silencing of the target gene can
be obtained. Undoubtedly, new models of investiga-
tion will be developed, combining the different genetic
manipulation techniques to achieve the creation of
new models to understand the function and the regu-
lation of metabolism, nutritional, and disease-related
genes. Indeed, research concerning inhibiting/activat-
ing the expression of different genes (transgenic/
knockout animals), gene transfer, and RNAi applica-
tion is allowing us to specifi cally investigate functions
and metabolism of regulatory processes.
13.5 Human studies
In human nutrition, man is the ultimate court in
which hypotheses are both generated and tested.
Nutritional epidemiology, through its observational
studies, demonstrates possible links between diet,
physical activity, and disease (Willett, 1998). It is not
the only way in which such possible links are gener-
ated but it is a critically important one in modern
nutrition. Experimental human nutrition takes the
hypothesis and through several experiments tries to
understand the nature of the link between nutrients
and the metabolic basis of the disease. Once there is
a reasonable body of evidence that particular nutri-
tional conditions are related to the risk of disease,
experimental nutritional epidemiology examines
how population level intervention actually infl uences
the incidence of disease (see Section 13.6). In effect,
experimental human nutrition and experimental
nutrition epidemiology both involve hypothesis
testing. However, the former is more often intended
to understand mechanisms and generally involves
small numbers. The latter, in contrast, uses very large
numbers to examine the public health impact of a
nutrition intervention that, under the controlled con-
ditions of the laboratory, showed promise.
Human nutrition experimentation
The use of experimental animals for human nutrition
research offers many possible solutions to experimen-
tal problems. However, the defi nitive experiments,
where possible, should be carried out in humans.
Studies involving humans are more diffi cult to
conduct for two major reasons. First, humans vary
enormously compared with laboratory animals. They
vary genetically and they also vary greatly in their
lifestyle, background diet, health, physical activity,
literacy, and in many other ways. Second, it is far more
diffi cult to manipulate human diets since we do not
eat purifi ed or semi-purifi ed diets.Experimental diets in human nutrition
intervention studies
In the 1950s, an epidemiological study across seven
countries presented data to suggest that the main
determinant of plasma cholesterol was the balance of
saturated, monounsaturated, and polyunsaturated
fatty acids (MUFAs and PUFAs). To test this hypoth-
esis, a series of studies was carried out on human
volunteers using “formula diets.” Dried skimmed milk
powder, the test oil, and water were blended to form
a test milk with specifi c fatty acid compositions. The
volunteers lived almost exclusively on these formulae.
Although this type of study is simple to conduct, it
does not represent the true conditions under which
normal humans live. At the other end of the spectrum
of options for manipulating human diets is that of
issuing advice that the subjects verify by way of a food
record. It is diffi cult to prove that subjects actually ate
what they say they have eaten. Sometimes, adherence
to dietary advice can be ascertained using tissue
samples (blood, saliva, hair, fat) and biomarkers. For
example, adherence to advice to increase oily fi sh
intake can be monitored using platelet phospholipid
fatty acids.
In between these two extremes of formula feeds
and dietary advice lies an array of options in which
convenience is generally negatively correlated with
scientifi c exactitude. In the case of minerals and vita-
mins, it is possible simply to give out pills for the
volunteers to take and measure compliance by count-
ing unconsumed pills and perhaps using biomarkers.
When it comes to macronutrients this is not generally
possible. Whereas asking someone to take a mineral
supplement should not alter their eating habits, asking
someone to consume a liter of milk a day or a bowl
of rice bran per day will alter other aspects of the diets
of the volunteers. It will not then be possible to attri-
bute defi nitively an event to the intervention (1 l/day
of milk or 1 bowl/day of rice bran). The event could
have been caused by possible displacement of some
other foods by the intervention. The only option in
human intervention experiments is to prepare foods
for volunteers to eat, which differ only in the test
nutrient. If the objective is to examine the effect of
MUFAs relative to saturated fatty acids (SFAs) on
blood lipids, then fat-containing foods can be pre-
pared that are identical except for the source of fat.