Food Lipids: Chemistry, Nutrition, and Biotechnology

to a glycerol molecule. The majority of dietary fats usually supply four to six different
fatty acids.
There is considerable evidence that both �6 and �3 polyunsaturated fatty acids
(PUFAs) can modulate a variety of immunological activities. There was considerable
interest in the clinical use of �6-enriched vegetable oils during the 1970s for patients
who had received kidney transplants, as well as those with multiple sclerosis and
autoimmune disorders. Development of better immunosuppressive agents and other
drugs for these conditions led investigators away from this area of research. However,
there remains considerable interest in the effects of dietary fats on immune response,
particularly in relation to development of cancer but also for cardiovascular disease
and general health. It is fair to state that beyond the generally accepted immunosuppressive
effects of polyunsaturated fats, there is considerable disagreement on
whether the specific fatty acid, ratios between different fatty acids, or the quantity
of dietary fat has the greatest impact on immunological responses. In general, when
the long chain �3 fatty acids increase in a cell membrane, there is a concomitant
decrease in arachidonic acid concentration.
Sources of �3 fatty acids used most commonly in studies of immune function
include fish oils, evening primrose oil, and flaxseed oil. Plant-derived oils generally
supply 18:3, while the fish oils have the longer chain fatty acids EPA and DHA.
There is substantial debate concerning the optimal range of the �6/�3 ratio in the
diet with many researchers favoring a range of 1.0–7.0 based on studies in rats. The
10th edition of the U.S. Recommended Dietary Allowances does not make specific
recommendations for any essential fatty acids in humans but indicates the desirability
of establishing these in the near future.
One study of the immune response in rats fed blends of different proportions
of sunflower oil (rich in linoleic acid) and flaxseed oil (rich in linolenic acid) found
that the higher the �3/�6 ratio of fatty acids in the plasma, the greater the reduction
of lymphocyte-dependent immune responses, including T-cell blastogenesis and natural
killer cell activity [11]. These authors concluded that �-linolenic acid was as
potent as fish oil for suppressing immune responses.
While most animal studies simply use feeding of a single source of fat, or a
blend, throughout an experiment, at least one study has tried to mimic the normal
human intake pattern of �3 PUFA [12]. Mice were fed a diet containing safflower
oil and switched to a sardine/olive oil mixture for periods of 1–7 days per week.
Peritoneal macrophages had phospholipid compositions that reflected the diet. Synthesis
of leukotrienes E 4 and C 4 decreased, while E 5 and C 5 levels increased with
more frequent consumption of fish ol. Prostaglandin F 1� also decreased with increasing
consumption of fish oil. The summary finding of this study—that fish oil must
be consumed at least twice a week to produce significant changes from the control
diet—touches on the current debate about how frequently a person has to consume
fish to derive a health benefit. As with most questions on diet and risk of chronic
diseases, there is no definitive answer, but some studies have suggested maximal
benefit with consumption of at least two servings of fatty fish weekly.
A fundamental study on this topic looked at the effects of feeding a low-fat
(26% of energy) diet that contained a high amount of fish in 22 subjects for 24 weeks
or a low-fat diet without fish [13]. Fish intake ranged from 4 to 6 ounces daily.
Therefore, this study used both type and amount of dietary fat as variables. Responses
on these diets were compared with those of subjects who followed a diet with 35%
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.