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(Comparative Anatomy and Physiology Brought Up to Date--continued, Part 7H)

Key Nutrients vis-a-vis Omnivorous
Adaptation and Vegetarianism (cont.)

Essential Fatty Acids (EFAs) (1 OF 2)


Fatty acids serve a number of important functions. As specified in Nettleton [1995]:

The primary fatty acids of interest here are, first, linoleic acid (an n-6 acid) and alpha-linolenic acid (an n-3 acid). Second, given the presence of suitable enzymes, linoleic and/or alpha-linolenic acid can be converted to arachidonic acid (AA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), eicosanoids, and other important metabolic compounds. DHA is very important in the central nervous system; Jumpsen and Clandinin [1995, p. 24] report:

Approximately one third of fatty acids with ethanolamine and serine phosphogylcerides in the cerebral cortex of humans, monkeys, and rats is docosahexaenoic acid (O'Brien and Sampson, 1965; Svennerholm, 1968; Neuringer and Connor, 1989).

Can synthesis of EPA and DHA from precursors maintain levels equal to obtaining them in the diet preformed? As we proceed, the central question that will be addressed here is how the conversion of precursors such as alpha-linolenic acid (from plant foods) to EPA and DHA compares to obtaining these crucial fatty acids preformed directly from animal foods. During evolution, humans would have been dependent on obtaining EPA and DHA primarily from animal sources [Eaton et al. 1998]. Given that DHA is particularly important in growth and development of the brain, which tripled in size during human evolution, the question of whether obtaining EPA/DHA primarily through synthesis from precursors is efficient enough to maintain optimum levels is of prime interest.

Understanding the terminology for EFAs

A number of systems and abbreviations are used to denote the various fatty acids of interest here, and these can at times be confusing. This section serves to introduce the relevant systems.

The two main groups or families of polyunsaturated fatty acids of interest here are the omega-3 and omega-6 families, which are written as n-3 or w-3, and n-6 or w-6. The different families of fatty acids have different chemical structures. They all consist of chains of carbon atoms with a methyl group (CH3) at one end of the chain, and an acid or carboxyl group at the other end (HO-C=O).

A standard system has been established for describing the various fatty acids. An example will illustrate the principles involved in the nomenclature.

18:2n-6 Linoleic Acid (LA): The first number specifies the number of carbon atoms in the chain; here it is 18. The second number, 2, tells us how many of the carbon atom bonds in the chain are double bonds. The last number, 6 in this case, specifies the position number of the first carbon double bond from the methyl end of the chain.

Putting the above together, letting "C" be a carbon atom, "-" a single bond, and "=" a double bond, we can crudely depict linoleic acid as:


Saturated vs. unsaturated fats. A fatty acid is saturated if it contains the maximum number of hydrogen atoms possible, i.e., it has no double bonds between adjacent carbon atoms. A fatty acid is unsaturated if it is not saturated, that is, if it has one or more double bonds between adjacent carbon atoms. Both linoleic and alpha-linolenic acids contain double carbon atom bonds, hence are unsaturated. (Note that the depiction of linoleic acid above is a summary and does not show all the hydrogen atoms.)

Abbreviations: an "alphabet soup" of terms. A sometimes-confusing array of numbers, abbreviations, and names are used for the various fatty acids and related terms. The following table is provided as an aid in understanding, and as a quick reference for fatty acids of interest here. The term "precursor" below refers to which fatty acid (FA) is a precursor in synthesizing another fatty acid. The precursors shown below are the ones of interest here, and are not necessarily the only precursors, and in some cases are actually intermediate precursors.


Numeric Designation

Fatty Acid Name


Can be Made from Synthesis Precursors


18:3n-3 Linolenic acid, also known as alpha-linolenic acid


[n-3 pathway]

20:5n-3 Eicosapentaenoic acid



22:6n-3 Docosahexaenoic acid




18:2n-6 Linoleic acid


[n-6 pathway]

20:4n-6 Arachidonic acid



22:5n-6 Docosapentaenoic acid



The table above is based on information from
Jumpsen and Clandinin [1995], and Nettleton [1995].

Note that EPA and DHA are synthesized in a pathway that starts with LNA (all three of which are n-3 FAs), while AA and DPA are synthesized in a pathway that starts with LA (all n-6 FAs). The primary focus here will be on the n-3 family and its longer-chain derivatives, EPA and DHA.

Additional terms of interest.

Requirements for EFAs

Linoleic acid: a minimum of 1 to 3+% of calories. Only small amounts of linoleic acid (an n-6 acid) are required. An RDA/RDI has not been formally adopted; however linoleic acid at 1-2% of calories will prevent deficiency. NRC [1989] recommends a minimum intake of linoleic acid, for adults, of 3-6 g/day. The comments of Jumpsen and Clandinin [1995, p. 29] are appropriate here:

...[B]ecause competition exists among the fatty acids for desaturating enzymes (Brenner, 1981), a level of at least 3% of energy should be met by n-6 fatty acids (FAO, 1977)...

Uauy et al. (1989) recently suggested that the recommendation of 3.0% of total energy is adequate to prevent clinical signs of deficiency but may be insufficient to ensure functional and biochemical normalcy.

FAO [1995], a joint publication with the United Nations World Health Organization (WHO), recommends linoleic acid consumption in the range of 4-10% of total energy. They specifically recommend consumption of linoleic acid at the 10% level when total intake of saturated fatty acids is high. Note that the FAO/WHO-recommended consumption level is higher than the suggested minimums.

Alpha-linolenic acid recommendation: 0.5% of calories. In reference to alpha-linolenic acid (an n-3 acid), Jumpsen and Clandinin [1995, p. 21] note:

...[A] dietary requirement for 18:3n-3 [alpha-linolenic] has not been clearly established, a growing body of evidence indicates that this series of fatty acids is also essential (Lamptey and Walker, 1976; Leprohon-Greenwood and Anderson, 1986; Bourre et al., 1990a). In Canada, nutrition recommendations have established the dietary requirement level for 18:3n-3 as 0.5% of energy (Nutrition Recommendations, Health and Welfare Canada, 1990).

Neuringer et al. [1984] has demonstrated n-3 fatty-acid deficiencies in animals, and Bourre et al. [1989b] showed that a diet deficient in n-3 oils affected enzyme activity and learning abilities in rats [Neuringer, Bourre, as cited in Jumpsen and Clandinin 1995]. The preceding lend credence to the hypothesis that alpha-linolenic acid is an essential nutrient.

Note that certain fruitarian extremists make the science-fiction claim that all, or nearly all, fats (i.e., fats above the levels found in sweet fruits) are "toxic." To date the extremists have not presented any credible evidence to support the idea that, say, avocados are "toxic" because of their fat/oil content.


(Bioavailability of EFAs: Plant vs. Animal Sources)

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GO TO PART 1 - Brief Overview: What is the Relevance of Comparative Anatomical and Physiological "Proofs"?

GO TO PART 2 - Looking at Ape Diets: Myths, Realities, and Rationalizations

GO TO PART 3 - The Fossil-Record Evidence about Human Diet

GO TO PART 4 - Intelligence, Evolution of the Human Brain, and Diet

GO TO PART 5 - Limitations on Comparative Dietary Proofs

GO TO PART 6 - What Comparative Anatomy Does and Doesn't Tell Us about Human Diet

GO TO PART 7 - Insights about Human Nutrition & Digestion from Comparative Physiology

GO TO PART 8 - Further Issues in the Debate over Omnivorous vs. Vegetarian Diets

GO TO PART 9 - Conclusions: The End, or The Beginning of a New Approach to Your Diet?

Back to Research-Based Appraisals of Alternative Diet Lore

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