Body concentrations of Arachidonic Acid (AA, 20:4 n6) and Eicosapentaenoic Acid (EPA, 20:5 n 3) are influenced by diet.  Previously, we reported that the concentration range of AA and EPA might explain that %AA and %EPA are positively associated, and that variability of OA (18:1 c9) influences this association. We now investigate whether also the range of ALA (18:3 n3) might influence the association between %AA and %EPA, using data from a diet trial in chickens.  A broadening (narrowing) of ALA-variability made the %AA vs. %EPA scatterplot improve (be poorer), as observed both when calculating percentages of all fatty acids, and when using ALA, AA, and EPA only in the denominator.
Thus, the positive association between relative amounts of AA and EPA in breast muscle lipids of chickens is influenced by ALA variability. We raise the question of whether differences in concentration ranges between the many types of fatty acids (possibly acting via skewness) might serve as an evolutionary mechanism to ensure that percentages of fatty acids will be positively or negatively associated: a Distribution Dependent Regulation.

Fatty acids in blood and tissues are important in health and disease, and body amounts are influenced     by diet [1, 2]. For example, ALA cannot be synthesized by mammals, and adequate dietary intake is essential for human health.  Increased ALA intake may decrease proinflammatory cytokines [3].
Furthermore, it is well known that EPA (20:5 n3) and AA are metabolic antagonists [1, 4]. Eicosanoids derived from EPA may decrease inflammatory diseases [5, 6], improve coronary heart diseases [7, 8], and cancer [9].  However, in a systematic Cochrane Review of selected studies the beneficial effects of long-chain n3 fatty acids on all-cause and cardiovascular mortality was questioned [10].
AA is formed in the body from linoleic acid (LA 8:2 n6), a major constituent in many plant oils. This fatty acid is converted by cyclooxygenase and lipoxygenase into various eicosanoids, i.e. prostacyclines, thromboxanes and leukotrienes [1].  In contrast to the eicosanoids derived from EPA, those derived from AA, i.e. thromboxane A2 (TXA2) and leukotriene B4 (LTB4), have strong proinflammatory and prothrombotic properties [1, 11].  Furthermore, AA derived endocannabinoids may have a role in adiposity and inflammation [12]. 
We previously suggested that ALA, and possibly other fatty acids, might be involved in the regulation of AA metabolism [13].  Thus, the inverse relationship between relative amounts of AA and oleic acid (OA 18:1 c9) in muscle lipids of chickens [13-15] might possibly be explained by inhibition by ALA of the synthesis of AA, and stimulation of OA formation [13]. Additionally, the fact that ALA is precursor of endogenous synthesis of EPA [1] may probably explain some of the health effects of ALA.  Furthermore, it has been reported that a decreased level of the serum EPA/AA ratio may be a risk factor for cancer death [9]. Thus, when considering the beneficial health effects of foods rich in ALA, many of the positive effects would be anticipated if the fatty acid works to counteract effects of AA. It would appear, accordingly, that a coordinated regulation of the relative abundances of EPA and AA could be of physiological interest, so that an increase (decrease) in the percentage of one of these fatty acids would be accompanied by a concomitant increase (decrease) in percentage of the other. In accordance with these considerations, we reported that percentages of AA and EPA were indeed positively associated in breast muscle lipids of chickens [16-18].
Using random numbers in a computer experiment, we previously suggested [19] that, with 3 positive scale variables, two of which having low-number distribution, and low variability, as compared with the third variable, we might expect a positive association between percentages of the low-number variables, and a negative association between percentage of the high-number variable and percentage of each of the low-number variables. Furthermore, a decrease (increase) in the variability of either or both of the two low-number variables seemed to improve (make poorer) the association between their relative amounts. In contrast, a narrowing (broadening) of the distribution of the high-number variable seemed to make poorer (improve) the association between percentages of the low-number variables. These observations raise the question of whether the rules may apply for fatty acids. In support of this suggestion is our finding of a positive association between relative amounts of EPA and AA in breast muscle lipids of chickens [16-18], where these fatty acids are low-number variables with low variability (concentration range 0.13 – 0.24, and 0.25 – 0.42 g/kg for EPA and AA, respectively) relative to OA (range 1.04 – 8.56 g/kg), [20]. Alterations in the OA, EPA, and AA ranges strongly influenced the association between percentages of AA and EPA, in line with the general rules above [19].  For example, high OA variability improved the %EPA vs.

Chickens and Diet

We refer to our previous article [22] for details concerning the diet trial. In brief, from day 1 to 29 one-day-old Ross 308 broiler chickens from Samvirkekylling (Norway) were fed wheat-based diet containing 10 g fat per 100 g diet.  ALA (18:3 n3), a precursor of EPA, provided 15% of the fatty acids, and LA (18:2 n6), a precursor of AA, provided 21%. The n6/n3 ratio was 1.4. Energy content of the feed was about 19 MJ/ kg. ALA provided 2.5% of the energy, and LA 4%. Other components in the feed were: Histidine 0.1%, choline chloride 0.13%, mono-calcium phosphate 1.4%, ground limestone 1.3%, sodium chloride 0.25%, sodium bicarbonate 0.2%, vitamin A, E, D, K, B 0.18%, L-lysine 0.4%, DL-methionine 0.2%, and L-threonine 0.2%.

Calculations and Statistical Analysis

We first reanalysed our previously reported association between %EPA and

The present study is a spin-off study of a previously published diet trial, conceived and conducted by AH. ATH conceived and designed the present study, analyzed and interpreted the data, conceived the hypothesis of Distribution Dependent Correlations/-Regulation, and wrote the article. ATH emphasizes that the excellent diet trial of AH - and the nice correlations observed - were crucial for the hypothesis.  AH contributed substantially to the interpretation of data and revising the article critically for important intellectual content. Both authors read and approved the final manuscript.

The diet trial in chickens was performed in accordance with National and international guidelines concerning the use of animals in research (Norwegian Animal and Welfare Act, European Convention for the protection of Vertebrate Animals used for Experimental and other Scientific Purposes, CETS No.: 123 1986). The Regional Norwegian Ethics Committee approved the trial, and the experimental research followed internationally recognized guidelines. There are no competing interests.

Descriptive Data
Descriptive data for the fatty acids under investigation are shown in Table 1. ALA, AA, and EPA contributed with 5.2, 3.4, and 2.0% respectively of all fatty acids. There was a striking difference in variability between ALA and the other fatty acids; ALA showed a 20-fold increase from lowest to highest value (CV 60.4%). In contrast to this, the variabilities of AA and EPA were low, with CV 9.7 and 11.1, respectively. Total weight of all fatty acids in breast muscle lipids of the chickens was 8.86 + 2.62 g/kg wet weight (mean + SD, n = 163).

Table 1. Descriptive statistics for the fatty acids under investigation (n= 163); minimum and maximum values, mean values (g/kg), with SD, % of total weight, and CV= (SD/Mean)*100.
Percentages calculated from total amount of fatty acids
Using the measured (physiological) values of ALL fatty acids, including ALA

We first investigate the association of

Since this work was confined to studying the association between percentages AA, and EPA, as modified by ALA, we do not know to what extent the suggested phenomenon of Distribution dependent correlations/- regulation is valid for other fatty acids as well. Furthermore, the analyses was based upon the fatty acid pattern in breast muscle lipids of chickens and we do not know the generalizability of our results, as related to different organs, tissues or compartments, and to various species, including man. Thus, the ALA influence on the association between 

The present analyses show that the concentration range (distribution, variability) per se of ALA, EPA, and AA, including where on the scale they are placed, will determine (possibly via skewness of the %ALA distribution) whether percentages of the fatty acids will be correlated. High tissue ALA (likely to be diet related) might improve the positive

The study of which the present one is a spin-off, was funded by grant no 190399 from the Norwegian Research Council, and Animalia; the Norwegian Meat and Poultry Research Centre. We thank the collaborators at the Norwegian University of Life Sciences, and at the Animal Production Experimental Center, As, Norway, especially Nicole F. Nyquist.

Variability: the width or spread of a distribution, measured e.g. by the range and standard deviation.

Range: showing the largest and smallest values.

Distribution: graph showing the frequency distribution of a scale variable within a particular range. In this article, we also use distribution when referring to a particular range, a – b, on the scale.

Uniform distribution: every value within the range is equally likely. In this article, we may write “Distribution was from a to b”, or “Distributions of A, B, and C were a – b, c – d, and e - f, respectively”.

“Low–number variables” have low numbers relative to “high-number variables”.

ALA = Alpha Linolenic Acid (18:3 n3) OA = Oleic Acid (18:1 c9)

LA = Linoleic Acid (18:2 n6)

ALA = Alpha Linolenic Acid (18:3 n3) AA = Arachidonic Acid (20:4 n6)

EPA = Eicosapentaenoic Acid (20:5 n3)