Effects of dietary LnA v. long-chain n-3 derivatives

LnA is not equivalent in its biological effects to the long-chain n-3 FA found in
marine oils. The latter are more rapidly incorporated into plasma and membrane
lipids and produce more rapid effects, compared with LnA (Sinclair & Crawford,
1972; Sinclair, 1975; FAO/WHO, 1978; Dyerberg et al. 1980, Sanders & Younger,
1981; Sanders & Roshanai, 1983). Relatively large reserves of LA in body fat, as
are found in vegans, would tend to slow down the formation of long-chain "-3 FA
from LnA (Sanders & Naismith, 1980). Therefore, the role of LnA in human
nutrition should be regarded from the point of view of long-term dietary habits.
There are also some advantages in the consumption of LnA over n-3 FA from fish
oils; problems such as insufficient vitamin E intake, and excessive supply of
vitamins A and D, are not expected to arise from an increased intake of LnA from
plant sources.

The signi3cance of the dietary LA:LnA value

If LnA, and its n-3 derivatives, have their place in the human diet as modulators of
ArA metabolism and platelet function, the question arises as to the desireable level
of supply for optimal membrane conditions. Because of the wide range of species
differences, animal models do not provide an unambiguous answer to this
question. Since direct evidence is not available, some information might be
obtained from circumstantial evidence.

Because LA and LnA will compete for desaturation and incorporation, the
relative amounts of LnA and LA are important, rather than the absolute intake of
LnA. The ratio, LA:LnA, calculated from food balance sheets, is now roughly 10: I
in the UK (Taylor et al. 1979), and similar values are presumably found in other
western countries. Actual analysis seems to yield a higher ratio, e.g. 14:1 in a
French farming community (Renaud et al. 1980). The present preponderance of
n-6 FA over the "-3 FA may be traced to the steady increase in the production and
consumption of LA-rich vegetable oils since the turn of the century, due to
important technological innovations in the extraction and processing of vegetable
oils and agrotechnical advances in the selection and cultivation of oil-seed plants
(Budowski, 1984).

It is instructive to look at wild plants and animals which provided the food for
prehistoric man. Such foods are not only mostly low in fat, but their lipids are also
rich in PUFA (Crawford & Stevens, 1981). Moreover, the ratio, LA:LnA (or
n-6:n-3 FA) is, on the whole, fairly well balanced; LnA predominates in the
chloroplasts of green, leafy plants, while seeds are as a rule richer in LA (Hitchcock
& Nichols, 1971). Muscle phospholipids of wild animals exhibit n-6:n-3 FA values
ranging from 2: I to 4:1 (Crawford et al. 1969, 197oa,b; Crawford & Woodford,
1971). An analytical survey of thirty-two wild animal species in Africa yielded
average ratios of I : I in brain ethanolamine phosphoglycerides (EPG), the major
phospholipid receptacle for PUFA in brain grey matter, and I. ~: I
in liver (Crawford et af.1976). These findings show that, in spite of widely-varying
patterns of food selection, animals tend to incorporate both types of PUFA into
their membranes, in proportions ranging from I : I to 5 : I.

Human milk contains both n-6 and “-3 PUFA. A survey of over 2000 samples
from 250 nursing mothers from Hungary and Thailand over a 6-month period
(unpublished results) yielded a mean n-6:n-3 value of 4.8. Although the dietary
habits and the total fat content of the breast milk were very different in the two
countries, the above ratio was similar.

The evidence available from wild plants and animals which might have served as
food for evolving Man, as well as the FA composition of mothers’ milk, all seem to
point to an n-6:n-3 value of less than 5 as a ‘natural’ (and therefore desirable)
dietary PUFA ratio. This is well below what is found at present in western
industrialized countries.

Only 150 generations have passed since man emerged from the mesolithic era. It
is therefore reasonable to assume that, biologically, man is still a wild animal. At a
recent international conference on ‘Nutritional Adaptation in Man’ (Blaxter &
Waterlow, 1985), it was concluded that there was little evidence of significant
evolutionary adaptation to the contemporary extremes of either nutritional
deprivation or excess. The comparison of ‘wild’ and contemporary foods leaves
little doubt that there have been dramatic changes in what Man eats, and in
northern Europe and America the most striking changes are of recent origin.

It should be noted that the national and international recommendations on the
prevention of nutrition-related diseases are in agreement on the need to reduce the
intake of fat, particularly saturated fat. If these recommendations are implemented,
then a reduction in the level of total dietary fat must automatically redress to some
extent not only the ratio, EFA:non-essential fatty acids but also the dietary
n-6:n-3 balance, as the two PUFA families are fairly evenly represented in the
structural lipids of plants and animals. Increasing the consumption of fish at the
expense of meat would also contribute to a more balanced PUFA intake. Cell
membranes would then acquire a PUFA composition closer to what might be
considered as ‘natural’.

If LnA and its derived “-3 FA are required for the regulation of ArA, as well as for specialized cell structures, then the question of the dietary LA:LnA or n-6:n-3
values may be just as important, nutritionally, as the P: S ratio.

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α-Linolenic acid as a regulator of the metabolism of arachidonic acid