Biology of Disease

docosahexaenoic acid (DHA). Omega-3 and V-6 fatty acids are EFAs because
they are not synthesized by the body but must be obtained in the diet. Seeds
and vegetable oils are excellent dietary sources of V-6 EFAs and the body is
able to convert linoleic acid to GLA and arachidonic acid. Many vegetable
oils contain only low amounts of ALA since this is normally present in the
chloroplasts of plants and may only be a significant dietary constituent if
green leafy vegetables are consumed. However, ALA can be used to synthesize
EPA and DHA by body tissues. Rich sources of the V-3 EFAs, EPA and DHA are
in the oils from the muscles and skins of a number of cold deepwater fishes,
such as herring, mackerel, salmon, sardines and tuna (Figure 10.7). Cod, which
live in similar environments, store fat in the liver: hence cod liver oil is also an
excellent source of EPA and DHA.

DIET AND NUTRITION

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Albacore tuna

Herring

Sardine

Mackerel Figure 10.7 A selection of fish rich in essential

fatty acids.

A dietary deficiency of EFAs leads to a dry scaly skin subject to erythema,
poor healing of wounds and hair loss and a failure to thrive in infants. The
first associations between V fatty acids and health came from studying
Greenland Inuit (Eskimo) and inhabitants of fishing villages in Japan and
people of Okinawa. These groups have much lower incidences of diseases,
such as coronary heart disease (CHD, Chapter 14), rheumatoid arthritis
(Chapters 5and 18 ) and diabetes mellitus (Chapter 7), than their European
or USA counterparts, even though their diets are high in fat from eating seal
and fish. However, these organisms are rich in V-3 fatty acids that have been
shown subsequently to provide significant health benefits. The metabolites
ofV-3 fatty acids decrease platelet function, reduce the risk from sudden
death caused by cardiac arrhythmias and slow the progress of atherosclerosis.
Modest decreases in blood pressure also occur with high intakes of V-3 fatty
acids. Some studies have indicated that V-3 fatty acids inhibit the synthesis
of very low density lipoprotein (VLDL) and triacylglycerols (TAGs) and so
decrease their concentration in plasma, particularly in patients suffering
from hypertriglyceridemia (Chapters 13 and 14 ). Furthermore, they have
been associated with reducing morning stiffness and the number of tender
joints in patients with rheumatoid arthritis. There is, however, no established
recommended daily intake for V-3s although it has been suggested that 1–2%
of the total daily energy intake should be in the form of EFAs. Human adults
are thought to require up to 10 g of linolenic acid daily.

The eicosanoids formed from V-6 fatty acids, although produced by many
of the same enzymes as those from V-3 acids, have different physiological
functions and effects. For example, the V-3 group is antithrombic and anti-
inflammatory, both processes promoted by V-6 acid derived metabolites.
Thus a balancing of the intakes of V-3 and V-6 fatty acids in the diet is
thought to be of importance. The diet typical of the developed world is
far richer in V-6 than V-3 fatty acids; it has been suggested that ratios of