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© 2004 The American Society for Nutritional Sciences J. Nutr. 134:2407-2414, September 2004

Nutritional Models

Tissue Fatty Acid Profiles Can Be Used to Quantify Endogenous Rumenic Acid Synthesis in Lambs1,2

D. L. Palmquist3, N. St-Pierre and K. E. McClure4

Department of Animal Sciences, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster, OH 44691

3To whom correspondence should be addressed. E-mail: palmquist.1{at}osu.edu.


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 
Proportions of vaccenic (trans-11 18:1) and rumenic (cis-9, trans-11 18:2) acids in mesenteric adipose, subcutaneous adipose, and longissimus muscle tissue lipids from lambs fed varying proportions of forages and concentrates were used to develop a mathematical model to predict exogenous and endogenous contributions to rumenic acid (RA) in the several tissues. From the model, we were able to estimate the proportion of absorbed RA, the proportion of vaccenic acid (VA) desaturated, the original proportion of VA in the tissue (before desaturation), and finally the proportion of RA synthesized endogenously. Estimates of endogenous RA were in the range of published data estimated by independent procedures. An independent data set of VA and RA in milk fat was used to challenge the model. Predictions were concordant with observations, although estimates of endogenous RA synthesis were lower than previous reports. Changing the amount of exogenous RA through manipulation of the diet influenced desaturation of VA inversely, so that endogenous RA synthesis was decreased when exogenous supply was increased (r = –0.80). The model should be challenged with data from human and nonruminant, as well as ruminant studies.

KEY WORDS: • CLA • rumenic acid • vaccenic acid • endogenous synthesis • tissue fatty acids

Increased public awareness of the nutritional and health aspects of food composition and quality has caused research to focus on the fatty acid composition of animal products; it is of particular interest to increase the content of conjugated linoleic acid (CLA)5 and other desirable fatty acids, including (n-3) fatty acids (1–8). CLA is a term commonly applied to any one of a series of octadecadienoic acids with conjugated bonds in the cis or trans configuration. The isomer found in greatest abundance in ruminant fat, cis-9, trans-11 18:2, is commonly known as rumenic acid (RA). Studies have shown that animals grazing pasture have higher tissue contents of CLA (3) and of (n-3) fatty acids (1,3) than stall-fed animals, and that supplementing diets with vegetable oils increases the proportion of CLA in milk fat (9), whereas the effect in tissues is smaller (4,7).

The relatively high RA content in ruminant fats originates from ruminal biohydrogenation of unsaturated fatty acids. The predominant unsaturated fatty acids of ruminant diets are linoleic and linolenic acids. Linoleic acid is isomerized to cis-9, trans-11 18:2, the first product in the classical biohydrogenation pathway (10); in a subsequent step, RA is reduced to VA. The biohydrogenation pathway for linolenic acid, the major fatty acid of forages, does not include RA as an intermediate, whereas VA is a product (11). Although early investigators assumed that RA in ruminant fats arose directly from intestinal absorption of RA, desaturation of trans octadecenoic acids was known to occur in rat liver microsomes (12,13), and subsequent research showed that the majority of RA arises from endogenous synthesis by {Delta}-9 desaturation of VA, via stearoyl-CoA desaturase (EC 1.14.99.5). Griinari et al. (14) used sterculic oil to inhibit stearoyl-CoA desaturase activity and estimated that 64% of CLA in bovine milk fat was of endogenous origin. This observation led to the conclusion that efforts to increase milk fat CLA should focus on increasing ruminal synthesis of VA (14). By a slaughter technique, we found that 50% of VA in tissues was converted to RA in mice (15), and more recently by a slope-ratio technique, we estimated that 19% of dietary VA was desaturated to RA by liver in humans (16). Because greater amounts of VA than RA are found in ruminant fats (11), the contribution of VA desaturation to total RA supplies of the human body can be substantial. Stearoyl-CoA desaturase mRNA expression is highly correlated with the amount of oleic acid found in ovine subcutaneous, but not abdominal adipose tissues (17). Thus, it is likely that the extent of VA desaturation to RA differs among tissues.

Kelsey et al. (18) calculated a desaturation index as follows: the product of {Delta}-9 desaturase/[product of {Delta}-9 desaturase + substrate of {Delta}-9 desaturase]. They applied this ratio to 4 product/substrate pairs: 14:1/14:0; 16:1/16:0; 18:1/18:0 and cis-9, trans-11 CLA/trans-11 18:1. Although a useful approach, enzyme/substrate kinetics for these different fatty acids are not likely to be similar (19); differences in substrate concentrations or affinities of the desaturase for fatty acids of different chain length could yield erroneous conclusions concerning the contribution of desaturation to rumenic acid deposition in tissues. In the present study, we developed an independent approach to estimate the contributions of ruminal and endogenous synthesis to the total RA found in tissues. To accomplish this, we fed lambs finishing diets designed to produce a range of concentrations of VA and RA in the tissues.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 LITERATURE CITED
 
    Animals and diets. Weaned lambs (n = 32) were allotted to pens (2 lambs/pen) by gender and body weight (light or heavy) and to 4 feeding regimens. Diets (Table 1) were high- or low-forage, as follows (proportions, dry matter basis): corn silage (HCS):high-oil corn (HOC), 2:1; alfalfa silage (HAS):HOC, 2:1; alfalfa silage (LAS):HOC, 1:2; LAS:whole shelled corn (WSC), 1:2. A soybean meal/vitamin/mineral mix was supplemented as needed to balance crude protein intake; energy intake was not equalized across treatments. Feed intake was measured by pens and recorded daily and animals were weighed once each week; all other measures were on individual lambs. Animal care was according to practices approved by the Animal Care and Use Committee of The Ohio State University.


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  		TABLE 1 Ingredient and chemical composition of high- and low-forage diets with differing oil contents fed to finishing lambs

 
    Diet analysis. Feeds were sampled 1 time/mo and composited for analysis. Silages were stored frozen until analysis. Crude protein was determined by the method of Kjeldahl, and neutral detergent fiber was measured according to Van Soest et al. (20). The fatty acid content of the diets was determined according to Sukhija and Palmquist (21); total fatty acid content ranged from 3.08 to 5.28%, and linoleic acid comprised 1.50–2.41% of dry matter, respectively (Table 1). The fatty acid content and composition of diet ingredients are in Table 2.


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  		TABLE 2 Fatty acid content and composition of diet components

 
    Carcass. Lambs were slaughtered in a commercial abattoir at a target live body weight of 46–48 kg; fat samples (subcutaneous and mesenteric) and loin muscle were obtained and frozen immediately. Adipose tissue (500 mg) and muscle (1000 mg) were homogenized with 15 mL hexane:isopropanol (3:2), filtered, and washed with distilled water. Two milliliters of the hexane layer was removed and combined with 2 mL of methyl-t-butyl ether to facilitate solution of the lipid in the sodium methoxide reagent used for transmethylation. Methyl esters were obtained by alkaline transmethylation and separated by temperature-programmed GLC as described by Santora et al. (15). Samples were rerun isothermally at 155°C to separate trans-10 and trans-11 18:1 isomers; proportions of these were then applied to the total trans-18:1 peak from the first run to obtain proportions of individual isomers in the total sample. Peaks were identified by comparison with authentic standards (Nu-Chek-Prep), including trans-9 and trans-11 18:1, cis-9, trans-11 CLA, and trans-10, cis-12 CLA, and by comparison with published profiles of octadecenoic acid isomers separated on highly polar GLC columns (22). No internal standard was used.

    Statistical methods. Data were analyzed as a completely randomized experimental design using Proc GLM of SAS, version 8.1 (23). The treatment design was a factorial arrangement consisting of 4 diets, 2 genders (female or neutered), and 2 weight classes. Interactions of diet x gender, diet x weight class, and gender x weight class were included in the model; only diet effects are reported, because few significant (P < 0.05) interactions were observed. Pens were the experimental unit for lamb performance data, whereas individual lamb data were used for fatty acid evaluations. Means were separated by Fisher’s protected LSD (24). Regressions were computed by Proc REG of SAS (23).

    Endogenous synthesis of rumenic acid: model development. To explore further the relations among diets and tissues that affect the observed amounts of RA in tissues we developed a model, using fatty acid concentrations within individual animals, to estimate endogenous synthesis of RA, as follows (in our usage, the terms "total" and "amount" refer to the fatty acids as g/100 g of the total fatty acids measured in the tissue). The model assumes the following: 1) within a diet and tissue, the amount of RA of ruminal origin is the same across animals; 2) the total amount of RA in a given tissue is the sum of RA of ruminal origin transferred to the tissue plus the amount from desaturation of VA within the same tissue; and 3) all of the endogenous synthesis of RA is from desaturation of VA.

Let i be the ith diet, j be the jth tissue, and n be the nth animal. Additionally, we define:

RATotijn as the total RA of the jth tissue of the nth animal on the ith diet,

RARumij as tissue RA from ruminal origin,

RAEndoijn as tissue RA from desaturation of VA,

VATotijn as total VA available for RAEndoijn synthesis,

VADesatijn as the amount of VA desaturated to RAEndoijn,

VATisijn as the amount of VA measured in tissue j, and

Mijn as the mass of the jth tissue of the nth animal on the ith diet.

Assuming that total RA measured in a tissue is the sum of RA from ruminal origin and from desaturation of VA within the tissue:

(1)

Also,

(2)


(3)

We now define kij as the proportion of total VA that is converted to RA in the jth tissue and the ith diet:

(4)

Thus, from Eqs. 4 and 2:

(5)

and

(6)

Rearranging Eqs. 5 and 6 and equating:

(7)

Thus,

(8)

Statistically, Eq. 8 has the form of a simple regression:

(9)

where {epsilon}ijn is an error term, assumed {approx} N(0, {varsigma}e2), Bij0 is an intercept specific to each tissue and diet, and Bij1 is a slope also specific to each tissue and diet. From this regression, the total amount of VA available for RA synthesis is easily calculated by rearranging Eq. 6:

(10)

where kij is calculated from Bij1 as follows:

(11)

First, we determined that the proportion of RA in tissues varied among diets, but also significantly by the interaction of diet, tissue and the proportion of VA within each tissue and diet. A plot of observed RA vs. predicted (Eq. 9) RA revealed 1 clear outlier residual. Deleting the corresponding observation (Diet LAS/HOC, subcutaneous fat) yielded unbiased predictions of RA, as shown by a plot of residuals vs. predicted RA (data not shown). The least-squares means from Eq. 9 at VA = 0 represents exogenous RA (RARumij); subsequently, computing Eqs. 10 and 11 yields the total VA available for desaturation and the proportion of VA desaturated, respectively.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
LITERATURE CITED
 
    Diet fatty acids. Corn silage contained 70% more fatty acids than did alfalfa silage, and high-oil corn contained 85% more fatty acids than conventional shelled corn. All corn products had similar fatty acid profiles, whereas alfalfa silage contained lower proportions of oleic and linoleic acids, and 6–25 times higher proportions of linolenic acid. Interestingly, trans monoenes were found in the silages, likely a result of the fermentation process. Only VA was found in corn silage, whereas only trans-10 18:1 occurred in the alfalfa silage. The difference may be the result of higher amounts of rapidly fermentable carbohydrates found in fresh hay crop forages than in corn at ensiling (25–27).

    Diet utilization. Only a few significant (P < 0.05) effects of gender and initial body weight on growth were observed, and none was unexpected (1); therefore, only diet effects are reported. One large ewe lamb was removed from the HAS/HOC after 41 d due to lack of weight gain. Other ewe lambs, 1 fed HCS/HOC and 1 fed HAS/HOC, gained poorly but remained in the trial (final weights, 41.3 and 39.5 kg, respectively). All data for these 2 lambs were used; however, if not used, treatment mean final body weights would have been 0.86 and 0.64 kg higher for diets HCS/HOC and HAS/HOC, respectively.

Initial body weights (Table 3) were not different among treatments; lambs fed low-forage diets consumed more dry matter and energy daily and consequently had higher daily gain, gain/feed, and fewer days of feeding (all P < 0.01). Lambs fed whole shelled corn were heavier at slaughter than those fed the same proportion of high-oil corn (P < 0.01). Lambs fed corn silage consumed less dry matter, but gained more per unit dry matter intake than those fed the high level of alfalfa silage (HCS/HOC vs. HAS/HOC). Lower dry matter intake by lambs fed the diet containing corn silage may have been caused by greater energy density compared with alfalfa silage diets or by slower ruminal degradation of corn silage neutral detergent fiber.


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  		TABLE 3 Body weight, dry matter, and fatty acid intakes and utilization of feed for gain of lambs1

 
    Tissue fatty acid composition. The mean proportions of pertinent fatty acids in mesenteric, subcutaneous and loin tissues are given in Tables 4, 5, 6, respectively. The proportions of myristic, myristoleic, vaccenic, and linoleic acids were not affected by any diet in any tissue. Although the diets fed in our study did not cause significant changes in mean VA concentrations in tissues, inspection of data revealed that VA varied by 2- to 7-fold within tissues among lambs. Considering differences in diets, tissue fatty acid profiles responded to diet changes similarly to those reported by Daniel et al. (28). Palmitic acid was increased in mesenteric fat of lambs fed whole shelled corn, whereas palmitoleic acid was lowest in loin muscle of lambs fed corn silage. Low-forage diets decreased stearic acid in subcutaneous fat and loin muscle (P < 0.01), but not in mesenteric fat (P = 0.30). Diet LAS/WSC increased the proportion of oleic acid (cis-9 18:1) in subcutaneous fat (P < 0.05). Diet LAS/HOC increased proportions of trans-10 18:1 in all tissues (P < 0.01), RA in subcutaneous fat (P < 0.02), and trans-10, cis-12 18:2 in mesenteric fat (P < 0.01). The percentage of linolenic acid was altered in all tissues by diet; it was lowest when corn silage was fed and highest with high-alfalfa silage feeding (P < 0.0001). The proportion of linoleic acid in tissues was less variable among dietary treatments than was the proportion of linolenic acid, reflecting more consistent ruminal biohydrogenation of linoleic acid. Independence of linoleic and linolenic acids in tissues and milk with respect to diet type was reported by others (1,29). The proportion of trans-10 18:1 in tissues was also more variable among diets than was VA or RA. The proportion of trans-10 18:1 in tissue fatty acids was 2- to 3-fold as high (P < 0.01) when the LAS/HOC diet was fed. Effects of high dietary linoleic acid and low-forage were confounded in this study, but dependency of these in combination to achieve high trans-10 18:1 in ruminant milk fat was shown clearly by Griinari et al. (30).


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  		TABLE 4 Fatty acid composition of mesenteric fat from lambs fed high- or low-forage diets with differing oil contents1

 

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  		TABLE 5 Fatty acid composition of subcutaneous fat from lambs fed high- or low-forage diets with differing oil contents1

 

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  		TABLE 6 Fatty acid composition of loin muscle from lambs fed high- or low-forage diets with differing oil contents1

 
The ratio of oleic/(stearic + oleic acids) was consistent across diets within tissues, but differed among tissues (P < 0.0001). Among tissues, the ratio was lowest in mesenteric fat and highest in loin muscle. Such ratios are interpreted to reflect the activity of {Delta}-9 desaturase (18); thus, desaturase activity was lowest in the internal adipose tissue and highest in muscle. Within tissues, the ratio tended to be lower when corn silage was fed and higher when low-forage diets were fed. The ratio of RA/(VA +RA) varied among tissues (P < 0.0001) similarly to oleic acid/(stearic + oleic acids); however, it was relatively higher in loin muscle. In addition, the RA/(VA + RA) ratio varied little among diets within tissues. Tissue contents of 14:1 were too low to compute 14:1/14:0 ratios reliably; trends among diets for ratios of 16:1/16:0 (not shown) in adipose tissues were comparable to RA/VA ratios, but in muscle, they were more similar to the 18:1/18:0 ratios. There were no significant diet x tissue interactions (P > 0.10).

The proportion of linolenic acid in tissues reflected the intake of this fatty acid (Table 7), suggesting that significant ruminal by-pass of linolenic acid in forage occurred. The slope of the regression of tissue concentration on dietary intake for linolenic acid was 2-fold higher than for linoleic acid in the 2 adipose tissue sites, whereas it was 3-fold higher in muscle, indicating that tissue content of linolenic acid was more dependent on diet content than was the case for linoleic acid. The greater difference in muscle reflects a greater contribution of membrane lipids to total fatty acid content of this tissue. Of all of the regressions tested for relations among diet and tissue fatty acids, slopes for trans-10 18:1 regressed on diet linoleic acid were highest, indicating that its synthesis is highly dependent on ruminal availability of linoleic acid. Also, the slope for incorporation of trans-10 18:1 into muscle was one half that for incorporation into adipose tissues.


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  		TABLE 7 Parameters of regressions for dietary fatty acid effects on fatty acid composition of tissues from lambs fed high- or low-forage diets with differing oil contents1

 
    Model evaluations. Information derived from the computations of endogenous synthesis of RA appears in Table 8. Although differences in VA concentration across diets were less than anticipated, differences within diets were up to 7-fold, allowing differences in VA metabolism to be suggested by the model. Total VA estimated to be available for desaturation varied over a narrow range among diets and tissues. Ruminal supply of RA was less, and desaturation of VA was about twice as high for high-forage diets compared with low-forage diets. Consequently, the proportion of RA synthesized in tissues was greater for high-forage diets. Among tissues, the proportion of VA desaturated was greatest for muscle, and tended to be lowest for mesenteric fat, consistent with observed oleic acid/(stearic + oleic acids) and RA/(VA +RA) ratios (Tables 5, 6, 7). For some comparisons in Table 8, total RA does not equal the sum of exogenous and endogenous RA. This is because the latter were computed independently by our model and reflect the uncertainty associated with their estimation. In a sense, the inequality of their sum to measured total RA serves as an independent assessment of the model’s precision in estimating these 2 pools.


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  		TABLE 8 Parameters derived from the model for rumenic acid synthesis in lambs by diet, tissue, and diet x tissue

 
An independent data set using milk fatty acid profiles was used to challenge our model. There were 383 individual observations from 114 cows with 9 dietary treatments from published studies (31–35). Ranges of data were 0.05–3.51 and 0.05–1.29% of total milk fatty acids for VA and RA, respectively. Both dietary treatments and proportions of VA influenced (P < 0.01) the proportion of RA in milk fat. Because of the high variability of CLA in milk fat among cows (18), cow within treatment was used as a covariate in the computation (Eq. 9) for each dietary treatment. Again, the plot of residuals of RA on predicted RA within treatment indicated no bias in the predictions (not shown). For VA, 10–25% was desaturated, and desaturation of VA in treatment 9 was lower than for other treatments (Table 9). However, the amount of VA available for desaturation was not lower. Endogenous RA synthesis tended to be lower than for lamb tissues, and lower than the 64–98% reported previously for milk fat (14,36–38). The proportion of RA synthesized endogenously was inversely proportional to the amount of trans-10 18:1 in the milk fat (r2 = 0.42), whereas the proportion of VA desaturated was not related to trans-10 18:1 (r2 = 0.006). The sum of predicted exogenous and endogenous RA was more consistent with measured values than occurred for the lamb data set, possibly because the challenge data set was much larger (n = 383 vs. 90).


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  		TABLE 9 Parameters derived from the model for rumenic acid synthesis in bovine milk fat from the challenge data set

 
Diet LAS/HOC significantly increased the proportion of trans-10 18:1 in all lamb tissues, and of trans-10, cis-12 18:2 in mesenteric fat. The latter CLA isomer was barely detectable in loin muscle. Although trans-10, cis-12 18:2 was shown to inhibit {Delta}-9 desaturase gene expression and activity (39), we cannot conclude from either our lamb study or the challenge data set that desaturase inhibition occurred, even though desaturation of VA was lower when the low-forage diets were fed. The proportion of VA desaturated was inversely related (r = –0.80 for the lamb data set) to the amount of exogenous RA, similar to our earlier observation (40). Although the explanation remains to be determined, the solution likely involves direct or indirect regulation of the interaction of {Delta}-9 desaturase with its substrate (VA), possibly by global effects on melting point of the total lipid in the tissue (41).

A model was presented to use fatty acid profiles to partition exogenous and endogenous contributions to RA content of tissues and to estimate the proportion of VA desaturated by tissues. Endogenous synthesis of RA was higher in muscle than in adipose tissue, and twice as great when high-forage diets were fed as when low-forage diets were fed to finishing lambs. Linolenic acid provided in high-forage diets had a greater effect on its content in lamb tissues than dietary manipulation of linoleic had on linoleic acid content. High-linoleic, low-forage diets significantly increased trans-10 18:1, but not VA in all tissues; the same diets increased RA only in subcutaneous fat. Mesenteric and subcutaneous fats, and loin muscle differed in the degree of unsaturation and apparent activity of {Delta}-9 desaturase, as estimated from our RA synthesis model and by ratios of oleic acid/(stearic + oleic acids) and RA/(VA + RA). The model provides more information than is obtained from product/substrate ratios and should be tested as an alternate and independent approach to partition dietary and endogenously synthesized RA in animal research and also in human studies by sampling fatty acid profiles in serum VLDL triglycerides for short-term effects (16) or adipose tissue (42) for long-term effects.


   ACKNOWLEDGMENTS
 
Appreciation is extended to Doug Clevenger for animal care and management, to Gary Lowe for carcass evaluation and data collection and analysis, and to Maria Sol Morales for assistance with analysis of statistical models. Thanks also to W. P. Weiss, R. R. Grummer, and S. G. Onetti for permission to use their milk fatty acid data sets.


   FOOTNOTES
 
1 Presented in part at Experimental Biology 03, April, 2003, San Diego, CA [Palmquist, D. L., & St-Pierre, N. (2003) Estimating endogenous synthesis of CLA from tissue fatty acid profiles. FASEB J. 17: A808 (abs.)]. Back

2 Support provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Center. Back

4 Deceased 1 July 2001. Back

5 Abbreviations used: CLA, conjugated linoleic acid; HAS, high-alfalfa silage; HCS, high-corn silage; HOC, high-oil corn; RA, rumenic acid; VA, vaccenic acid; WSC, whole shelled corn. Back

Manuscript received 12 December 2003. Initial review completed 26 January 2004. Revision accepted 17 June 2004.


   LITERATURE CITED
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 LITERATURE CITED
 

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