Food Lipids: Chemistry, Nutrition, and Biotechnology

of energy from fat. Feeding the low-fat, low-fish diet (which was enriched in plant
PUFAs) increased the response of mononuclear leukocytes to the T-cell mitogen
concanavalin A (con A), interleukin 1� levels, and tumor necrosis factor. No effects
were seen on PGE 2 production, interleukin 6 levels, or delayed-type skin hypersensitivity.
In contrast, the low-fat, fish-enriched diet resulted in a significant decrease
in CD4 � and a concurrent decrease in CD8 � cells. There were significant reductions
in the lymphocyte mitogenic response to con A, delayed-type hypersenitivity, interleukin
1�, interleukin 6, and tumor necrosis factor production by leukocytes. The
practical implications of these immune alterations are not clear cut. It was conjectured
that these decreases in immune responses would be favorable for atherosclerosis
and inflammatory diseases but harmful for host defense against microorganisms.
The issue of immune surveillance against cancer cells was not discussed but is a
valid concern.
In another important study in this area, Purasiri et al. [14] found that �3 fat
supplement of 4.8 g daily decreased the blood concentrations of a number of cytokines
significantly (by 60–80%) in individuals with colon cancer; these included
several forms of interleukins, tumor necrosis factor �, and interferon-�. Cytokines
returned to baseline levels 3 months after the cessation of the supplements. This
study clearly demonstrated a rapid and profound decline in a number of cytokines
in response to an easily consumed amount of �3 fatty acids. The implications of
this study are that regular consumption of �3 fatty acids might reduce the ability of
the immune system to respond to infections or tumors.
Some studies have measured prostaglandin metabolites in urine as an index of
whole-body metabolism. Supplementation of �3 fatty acids to a low-fat diet reduced
thromboxane A 2 production, while a low-fat diet, with or without fish oil, resulted
in reduced prostacyclin production in healthy men. Accompanying these dietary manipulations,
however, were increases in thromboxane A 3 and prostacyclin A 3 [15].
There have been many studies in which fish oil supplements were administered
to people with presumptive autoimmune inflammatory conditions such as rheumatoid
arthritis. Most studies find that fish oil supplements are effective in reducing symptoms
of these conditions. However, there does not seem to be any benefit of �3 fatty
acid supplements over conventional NSAID therapy.
C. Total Dietary Fat
The early work on total dietary fat in the 1960s showed that animals fed high-fat
diets were more susceptible to a wide range of spontaneous infections. These observations
led to the experimental examination of specific parts of the immune response
in animals fed different levels of dietary fat.
Differences in immune responses as a function of changes in total dietary fat
have been studied in many animal models and in a few human trials. In general, the
more fat in the diet, the lower the immune response. Many animal studies have
shown high-fat diets are immunosuppressive in comparison to low-fat diets. The
basal level in most animal studies have been about 10% of energy from fat, and the
high levels have ranged from 40 to 60% of energy from fat, usually of a single
source. Most animal studies have fed a polyunsaturated vegetable oil, so it is difficult
to distinguish between the effects of total fat and the effect of increased linoleic acid
or other individual fatty acids.
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