Food Lipids: Chemistry, Nutrition, and Biotechnology

CLA is also present in small amounts in other food products. Turkey meat has
the highest CLA content of 2.5 mg/g fat for nonruminant species (18). Chicken
contained CLA (0.9 mg/g fat) as did pork (0.6 mg/g fat) with c9,t11 being the major
isomer (84% and 82%, respectively) (1). The amount of CLA in chicken egg yolk
lipids ranges from 0 to 0.6 mg/g of fat (1,33–36). CLA was found in plant oils
(0.1–0.7 mg/g fat) and selected seafood (0.3–0.6 mg/g fat) in small quantities (1).
Unlike in ruminant derived food where c9,t11 is the chief isomer of CLA, this isomer
accounts for only 38–47% of the total CLA in plant oils and rather interestingly
appears to be absent in seafood lipids. Banni et al. (37) carried out a series of analyses
to characterize the fatty acids with conjugated dienes in partially hydrogenated oil
(mixture of partially hydrogenated soybean oil and palm oil) and confirmed the
presence of CLA isomers in these oils. Moreover, Mossoba and coinvestigators (26)
reported that conjugated cis,trans and trans,trans 18:2 isomers were present in hydrogenated
soybean oil and margarine.
CLA has been identified in various human tissues, such as adipose, serum,
breast milk, and in bile and duodenal secretions (38). Fogerty et al. (39) reported
5.8 mg/g fat of CLA in human milk for subjects consuming a normal Australian diet.
Precht and Molkentin (40) reported a value of 3.8 mg/g fat (range 2.2–6.0 mg/g fat)
in human milk obtained from 40 German women. McGuire et al. (16) analyzed 14
human milk samples from subjects in the Pacific Northwest and reported that CLA
values ranged from 2.23 to 5.43 mg/g fat (mean 3.81 mg/g fat) or from 0.02 to 0.30
mg/g on a milk weight basis. In this last study, all milk samples contained 83–100%
of the c9,t11 isomer, and in 8 of the 14 samples the c9,t11 isomer was the only form
observed. Jensen et al. (41) reported a lower level of CLA in human milk that ranged
from 1.4 to 2.8 mg/g fat with an average of 1.8 mg/g fat. Based on these data, CLA
in human milk for Western societies ranged from 1.4 to 5.8 mg/g fat.
The concentration of CLA found in human plasma and serum appears to be
linked to dietary fat type and food consumption patterns. Herbel et al. (38) reported
that plasma CLA concentration ranged from 6.4 to 7.3 �mol/L in human subjects
that were given a high level of safflower oil for 6 weeks. The high LA intake from
safflower oil did not increase the plasma level of CLA, indicating that dietary LA is
not converted to CLA in these subjects. In another study, the effect of trans fatty
acid intake on serum CLA was examined (42). Eighty human subjects were put on
a diet high in saturated fat mainly from dairy for 5 weeks. Then they were separated
into two dietary groups: 40 subjects were assigned to a diet high in trans fatty acids
from partially hydrogenated vegetable oil, and the other 40 given a similar diet high
in stearic acid. At the termination of the study, serum samples from the trans diet
group (CLA was 0.43% of total fatty acids) contained 30% more CLA than samples
obtained while on the dairy fat diet (0.32% CLA). Samples from those given the
stearic acid diet had only half of the amount of CLA compared to those given the
dairy fat diet. These data indicate that a possible relationship exists between trans
fatty acid intake and serum CLA concentration. In another dietary intervention study
of nine healthy men, Cheddar cheese was added to their diet at a level of 112 g/day
for 4 weeks. Plasma CLA concentration increased from 7.1 to 9.6 �mol/L at the end
of the cheese supplement period and was maintained at 7.8 �mol/L after another 4
weeks following intervention (43).
The CLA concentration and isomeric distribution have recently been characterized
in human adipose tissue. Fritsche et al. (44) reported that human subcutaneous
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.