50 Part I: Principles
now well established that a high fat diet (specially of
SFA) not only increases the risk of heart disease but
also the risk of breast, colon, and prostate cancer.
Many health agencies, including the American Die-
tetic Association, the American Diabetes Associ-
ation, and the American Heart Association, recom-
mend that fat intake should be no more than 30% of
the total daily calories.
In research conducted by the United State De-
partment of Agriculture (USDA), scientists intro-
duced a recombinant bovine growth hormone
(rBGH) gene into pigs, with the purpose of under-
standing the relationship between rBGH expression
and the amount of fatty acids in the animal (Solomon
et al. 1994). Bovine growth hormone (BGH), also
known as bovine somatotropin, which is produced in
the pituitary gland, stimulates growth in immature
cattle and enhances milk production in lactating
cows (Leury et al. 2003). BGH is a protein hormone,
and as such, it is broken down during digestion in the
gastrointestinal tract, making it biologically inactive
in humans (Etherton 1991). In 1993, based on rigor-
ous scientific investigations, the U.S. Food and Drug
Administration (FDA) concluded that products from
transgenic-BGH and supplemented-BGH animals
are safe for human consumption.
Bovine growth hormone has been shown to
decrease fat content of transgenic pigs expressing an
rBGH gene (Pursel et al. 1989). The transgenic pigs
used in this study were created by pronuclear micro-
injection technique. The gene encoding rBGH was
introduced into the pig genome under the control of
the mouse metallothionein-I (MT) promoter. After
rBGH transgenic lines of pigs were established, suc-
cessive generations were produced by artificial in-
semination of nontransgenic females with sperm
collected from rBGH transgenic males. To deter-
mine the effect of rBGH in the pigs’ carcass compo-
sition, transgenic and nontransgenic (control) pigs
were raised under the same conditions and fed the
same type of diet. The animals were processed at
five different live weights: 14, 28, 48, 68, and 92 kg.
The entire left side of each carcass was ground, and
random samples of tissue were collected and ana-
lyzed for fatty acid and cholesterol content. The
researchers observed that as live body weight in-
creased, carcasses from transgenic pigs showed a
constant decline in the amount of total fat compared
to control pigs (Table 3.1). Although the results did
not demonstrate a difference in the cholesterol con-
tent of transgenic and control pigs, it was shown that
transgenic pigs expressing BGH had a significant
decrease in the levels of specific fatty acids com-
pared with nontransgenic pigs in the control group
(Fig. 3.11). These results indicate that consumers
might greatly benefit from a pork product with a low
fat content if regulation of BGH secretion levels can
be precisely controlled during the fast growth stage
of young pigs (Solomon et al. 1994).TRANSGENICPOULTRY: EGG ASBIOREACTORMammals and birds have been the focus of intense
research for their possible use as bioreactors. The
use of mammals as bioreactors became possible
with the creation of transgenic mice and the isola-
tion of tissue-specific promoters (Gordon et al.
1980, Swift et al. 1984). Clark et al. (1987) were the
first to propose the use of transgenic livestock mam-
mary glands for the production of biopharmaceuti-
cal proteins in milk. Although expression of foreign
protein in milk is high and milk production is large,
there are some problems associated with the use of
mammary glands as bioreactors, including the long
time required to establish a stable line of transgenic
founder animals and the high cost to purify foreign
protein from milk (Ivarie 2003). Researchers have
also long envisioned using chicken eggs for the
expression of exogenous proteins. There are many
advantages associated with the use of eggs as biore-
actors, including the fact that a single ovalbumin
gene controls most of the proteins in egg white
(Gilbert 1984). Also, egg white has a relatively high
protein content, is naturally sterile, and has a long
shelf life (Tranter and Board 1982, Harvey et al.Table 3.1.Comparison of Total Carcass Fat
(g/100g) between rBGH Transgenic and
Control Pigs, Measured at Different Live
WeightsTotal Fat, g/100g
Weight Group, kg Transgenic Control
14 6.19 10.04
28 7.62 12.32
48 8.16 16.58
68 5.97 26.78
92 4.49 29.07
Source:Adapted from Solomon et al. 1994.