Effect of diet, sex and age on fatty acid metabolism in broiler chickens: SFA and MUFA

Abstract

The present study was conducted to evaluate the effect of different dietary lipid sources, age and sex on the SFA and MUFA metabolism in broiler chickens using a whole body fatty acid balance method. Four dietary lipid sources (palm fat, Palm; soyabean oil, Soya; linseed oil, Lin; and fish oil, Fish) were added at 3 % to a basal diet containing 5 % Palm. Diets were fed to female and male chickens from day 1 to either day 21 or day 42 of age. The accumulation (percentage of net intake and ex novo production) of SFA and MUFA was significantly lower in broilers fed on Palm than in broilers fed on the other diets (85·7 v. 97·4 %). Conversely, β-oxidation was significantly higher in Palm-fed birds than the average of the other dietary treatments (14·3 v. 2·6 %). On average, 33·1 % of total SFA and MUFA accumulated in the body were elongated, and 13·8 % were Δ-9 desaturated to longer chain or more unsaturated metabolites, with lower proportions being elongated and desaturated for the Palm and Fish diets than for the Soya and Lin diets. Total in vivo apparent elongase activity decreased exponentially in relation to the net intake of SFA and MUFA, while it increased with age. Total in vivo apparent Δ-9 desaturase activity was not significantly affected by dietary treatment or age. Total ex novo production and β-oxidation of SFA and MUFA showed a negative and positive curvilinear relationship with net intake of SFA and MUFA, respectively. Sex had no effect on SFA and MUFA metabolism.

Dietary SFA are associated with an increase in LDL and a decrease in HDL as a risk factor for CVD, obesity and associated disorders^{(1, 2)}. The current dietary recommendations for reducing the risk of CVD underline reducing SFA intake (mainly 16 : 0) by increasing MUFA (mainly 18 : 1n-9) in the diet⁽³⁾. The ratio of TAG:cholesterol in postprandial TAG-rich lipoprotein of healthy men is inversely correlated with the ratio of 18 : 1n-9 over 16 : 0 and with the MUFA:SFA ratio in dietary fat⁽³⁾. High MUFA content in the diet favours lipid oxidation and reduces risk of obesity development⁽⁴⁾. MUFA are also of particular interest for their protective role against SFA-induced insulin resistance and cell death⁽²⁾.

White meat such as chicken is commonly preferred to red meat, supposedly as a more healthy choice, due to its lower SFA content and consequently healthier unsaturated:saturated ratio⁽⁵⁾. However, poultry nutritionists are increasingly urged by the industry to provide even better fatty acid composition in chickens. In attempting to optimise fatty acid composition of chicken products, previous studies reported that different broiler chicken tissues reflect dietary fatty acid composition^{(6, 7)}. However, information on metabolism (deposition, elongation, Δ-9 desaturation and β-oxidation) of individual SFA and MUFA in broiler chickens is limited. An early ex vivo study in chickens using isotopic dilution technique to study the metabolism of 16 : 0, 18 : 0 and 18 : 1n-9 in liver is available⁽⁸⁾. However, one of the drawbacks of the ex vivo methods is that the knowledge deduced from this method is restricted to a distinct tissue⁽⁹⁾. Several in vivo methods are able to give an indication about the fatty acid metabolism at the whole body (WB) level⁽¹⁰⁾. The WB fatty acid balance method described by Turchini et al. ⁽⁹⁾ proved to be a reliable method to estimate SFA and MUFA metabolism in fish⁽¹¹⁾, and a capable method to describe the fate of each individual dietary fatty acid.

It is believed that dietary fatty acid composition may affect WB fat partitioning for storage (accumulation) or energy (β-oxidation) utilisation⁽¹²⁾. The objective of the current experiment was to examine the effect of dietary fat source together with slaughter age and sex on the SFA and MUFA metabolism in a commercial strain of broiler chicken using the WB fatty acid balance method.

Materials and methods

Animals and diets

The experiment was carried out according to the guidelines of the Ethics Committee of Ghent University (Belgium) for the humane care and use of animals in research. Details concerning the experimental design and analytical methods can be found in Poureslami et al. ⁽¹³⁾. Briefly, four hundred 1-d-old male and female Ross 308 chickens were subjected to one of the four dietary treatments: palm fat (Palm, SFA source); soyabean oil (Soya, 18 : 2n-6 source); linseed oil (Lin, 18 : 3n-3 source) or fish oil (Fish, n-3 long-chain PUFA source). The four test oils were added at 3 % to the same basal diet containing 5 % Palm. The fatty acid composition of the diets is given in Table 1. Mass ingested and excreted fatty acids were recorded in two subsequent periods: 7–21 d and 21–42 d of age, and the WB fatty acid content was quantified as the sum of seven anatomical compartments of the broiler chickens, which were analysed individually⁽⁷⁾.

Table 1 Fatty acid profile of the diets (mg/g feed)

Palm, palm fat diet; Soya, soyabean oil diet; Lin, linseed oil diet; Fish, fish oil diet; ΣPUFA, sum of PUFA.

* –, not detected.

Whole body fatty acid balance calculations

The WB fatty acid balance method and calculation model described by Turchini et al. ^{(9, 11, 14)} were employed to estimate the different fates (excretion, net intake, body accumulation, ex novo production, elongation, Δ-9 desaturation and β-oxidation) of individual dietary SFA and MUFA. Briefly, in the WB fatty acid balance method, upon a feeding experiment, initial and final WB fatty acid mass as well as accumulation was quantified. Then, ingested and excreted fatty acids (apparent digestibility) were determined, and the net fatty acid intake (fatty acid absorption) was computed. The difference between fatty acid accumulation in the WB and the net intake was considered as the total appearance or disappearance of an individual fatty acid. At this step, the balance of SFA and MUFA was calculated. Backward calculations were made along the known SFA and MUFA metabolic pathway (Fig. 1). It is worth noting that some of the fatty acids involved in the metabolic pathway (namely 22 : 1n-9, 24 : 1n-9, 20 : 1n-11 and 22 : 1n-11) were not available in a detectable quantity in chickens' WB. Hence, in the present study, they were not considered in the computations of the model. In the backward computations, the number of μmol of longer chain or more unsaturated fatty acids that appeared was subtracted from the numbers of μmol of the previous fatty acids in the elongation and Δ-9 desaturation pathways, and so forth, backward until 12 : 0 (the shortest fatty acid identified in the present study). In this way, a new fatty acid balance was obtained, in which any fatty acid appearance was ascribable to ex novo production and any fatty acid disappearance was ascribable to β-oxidation. Hence, total ex novo production was computed. Subsequently, fatty acid accumulation and β-oxidation were calculated and expressed as ‘% of ex novo production plus net intake’, while the proportion of fatty acids elongated and/or Δ-9 desaturated was expressed as percentage relative to total fatty acid accumulation. Finally, the bioconversion (i.e. elongation and/or Δ-9 desaturation) and β-oxidation of a specific fatty acid (μmol of fatty acid per g body weight per d) were quantified. The overall accretion of elongation and Δ-9 desaturation for a given fatty acid was expressed as an indicator of the overall apparent in vivo elongase and Δ-9 desaturase activities (μmol/g per d).

Fig. 1 Schematic representation of the SFA and MUFA elongation and desaturation pathways considered in the present study for the computation of the whole body fatty acid balance method.

Data analyses

Data were analysed using S-PLUS for Windows (version 6.1, Insightful, Seattle, WA, USA). A linear model was used to analyse the fixed effects of diet, age and sex and their interaction terms. In case of a significant diet effect, the mean values were compared with the Tukey's post hoc test (P < 0·05). Regression analysis was performed to examine the effect of SFA and MUFA net intake on the total in vivo apparent elongase and Δ-9 desaturase activities, ex novo production and β-oxidation of SFA and MUFA.

Results

Table 2 shows fatty acid intake and apparent digestibility in broiler chickens. A higher 12 : 0, 16 : 0, 18 : 0, 20 : 0, ΣSFA, 18 : 1n-9 and ΣMUFA intake was recorded in the birds fed with Palm diet than in the birds fed with the other diets (P < 0·001). Intake of 14 : 0, 16 : 1n-7, 18 : 1n-7 and 20 : 1n-9 was 3·6-, 13-, 1·6- and 2·2-fold higher, respectively, in birds fed on Fish diet than in birds fed on the other diets (P < 0·001). The effect of age and sex was significant for the intake of all the given fatty acids, with a higher fatty acid intake in 21–42-d-old chickens and in male broiler chickens always.

Table 2 SFA and MUFA intake (mg/d) and apparent digestibility (%) in broiler chickens

Palm, palm fat diet; Soya, soyabean oil diet; Lin, linseed oil diet; Fish, fish oil diet; RMSE, root mean squares error; D, diet; A, age; S, sex; ΣSFA, sum of SFA; ΣMUFA, sum of MUFA.

^a,b,c,d Mean values for diets within a row with unlike superscript letters were significantly different (P < 0·05).

* Significant interaction terms at P < 0·05.

Apparent digestibility of 12 : 0, 14 : 0, 16 : 0 and ΣSFA was not influenced by diet, age and sex (P>0·05). Digestibility of 18 : 0 was higher for the Soya diet than for the Fish diet (P < 0·05), with intermediary values for the Palm and Lin diets. Digestibility of 20 : 0 was higher for the Soya and Lin diets than for the Fish diet (P < 0·05), whereas the digestibility of 22 : 0 was higher for the Lin diet than for the three other diets (P < 0·05). Digestibility of 16 : 1n-7 and 20 : 1n-9 was larger for the Fish diet than for the other diets (P < 0·001). Digestibility of 18 : 1n-7 was higher for the Soya and Fish diets than for the Palm and Lin diets (P < 0·001), and digestibility of 18 : 1n-9 was higher for the Soya diet than for the Palm diet (P < 0·01). The ΣMUFA digestibility was higher in the Soya- and Fish-fed birds than in the Lin- and Palm-fed birds (P < 0·001). Digestibility of 22 : 0, 18 : 1n-9 and 20 : 1n-9 and ΣMUFA was higher in the birds slaughtered at day 42 than in those slaughtered at day 21 (P < 0·001). Sex had no effect on FA digestibility (P>0·05).

The SFA and MUFA content of WB broiler chickens and fatty acid appearance/disappearance are presented in Table 3. Diet had a significant effect on the content of most individual SFA and MUFA, except for the major SFA (16 : 0) and for ΣSFA. The 14 : 0 and 14 : 1n-5 content was higher in the Fish diet than in the three other diets (P < 0·001). The 18 : 0 content was higher in the Fish diet than in the Palm diet (P < 0·05). The 16 : 1n-7 was higher for the Fish diet than for the Soya and Lin diets. Birds fed on the Palm diet had a higher 18 : 1n-7, 18 : 1n-9 and 20 : 1n-9 content than the birds fed on Lin, Fish, and Soya diets, respectively (P < 0·001). The ΣMUFA content was higher in Palm-fed birds than in Fish-fed birds (P < 0·05), with intermediary values in Soya- and Lin-fed birds. The effect of age was significant on the SFA and MUFA content of broiler chickens (except for 22 : 0), with a higher fatty acid content at 21–42 d of age always. The effect of sex was significant for 16 : 0, 18 : 0, ΣSFA, 18 : 1n-9, 20 : 1n-9 and ΣMUFA, with a higher fatty acid content in the female birds than in the male birds.

Table 3 SFA and MUFA content (mg/g broiler chicken) and fatty acid appearance/disappearance (μmol/g per d) in whole body broiler chickens

Palm, palm fat diet; Soya, soyabean oil diet; Lin, linseed oil diet; Fish, fish oil diet; RMSE, root mean squares error; D, diet; A, age; S, sex; ΣSFA, sum of SFA; ΣMUFA, sum of MUFA.

^a,b,c Mean values for diets within a row with unlike superscript letters were significantly different (P < 0·05).

* Significant interaction terms at P < 0·05.

† Sum of SFA, MUFA and PUFA.

A net disappearance was observed in all dietary treatments for 12 : 0, 16 : 0, 20 : 0 and 22 : 0. For 14 : 0, a net disappearance was observed in birds fed on the Palm and Fish diets that differed from the net appearance that was observed in birds fed on the Soya and Lin diets (P < 0·001). Birds fed on the Palm diet had a lower 18 : 0 appearance but a higher 18 : 1n-7 and 20 : 1n-9 appearance in contrast with the birds fed on the other diets (P < 0·001). Appearance of 16 : 1n-7 was lower for the Fish diet than for the other diets (P < 0·001). Age significantly affected SFA and MUFA appearance/disappearance, with higher values at 7–21 d for all the given fatty acids, except for 18 : 0 and 18 : 1n-9. The effect of sex was only significant on 18 : 0 appearance, with higher values in female birds (P < 0·05).

The different fates of ΣSFA and MUFA are summarised in Table 4. Across diets, sex and age, on average 94·5 % of total SFA and MUFA from the net intake and ex novo production were accumulated in the WB, while the rest (5·5 %) were β-oxidised for energy production. Accumulation was lower (85·7 v. 97·4 %) and thus β-oxidation was higher (14·3 v. 2·6 %) for the Palm diet than for the three other diets (P < 0·001). Accumulated SFA and MUFA may have either of the three fates in the WB, i.e. deposition as such in the tissues, Δ-9 desaturation of SFA to MUFA and elongation to longer chain metabolites (Fig. 1). Across diets, sex and age, 33·1 % of accumulated ΣSFA and MUFA were elongated to longer chain products, whereas 13·8 % were Δ-9 desaturated. Elongation as a proportion of accumulation was higher for the Lin diet than for the Palm diet (P < 0·01), and desaturation was higher for the Lin diet than for the Palm and Fish diets (P < 0·01).

Table 4 Fate of sum of SFA and MUFA (% net intake+ex novo production), fate of accumulated sum of SFA and MUFA (% of accumulation), and accretion of elongated and desaturated fatty acids (μmol/g per d) in broiler chickens

Palm, palm fat diet; Soya, soyabean oil diet; Lin, linseed oil diet; Fish, fish oil diet; RMSE, root mean squares error; D, diet; A, age; S, sex.

^a,b,c Mean values for diets within a row with unlike superscript letters were significantly different at P < 0·05.

* Significant interaction terms at P < 0·05.

† Mean values for diets were significantly different (P < 0·1).

The accretion of elongated 12 : 0 and 14 : 0 (namely 14 : 0 and 16 : 0, respectively) and the accretion of total elongated fatty acids were lower in the Fish- and Palm-fed birds than in the Soya- and Lin-fed birds (P < 0·05). The apparent elongation of 18 : 1n-9 to 20 : 1n-9 was higher in birds fed on Palm diet than in those fed on the other diets (P < 0·001). The apparent elongation of 18 : 1n-9 to 20 : 1n-9 was slightly higher in birds at 7–21 d of age than in the older birds (P < 0·001), whereas elongation of all the given SFA was always higher in birds at 21–42 d of age (P < 0·05).

The accretion of Δ-9 desaturated 14 : 0 (net appearance of 14 : 1n-5) was higher for the Fish diet than for the other diets, whereas the opposite trend was observed for 16 : 0 desaturation (net appearance of 16 : 1n-7; P < 0·001). Total Δ-9 desaturation activity and Δ-9 desaturation of 18 : 0 were not significantly affected by dietary manipulation (P>0·05). The apparent activity of Δ-9 desaturase on 16 : 0 was higher in 7–21-d-old birds than in the older birds, while the opposite was observed for 18 : 0 (P < 0·05). Sex did not influence apparent elongase and Δ-9 desaturase activities of the given fatty acids (P>0·05).

The β-oxidation of SFA and MUFA is presented in Table 5. Total β-oxidation of ΣSFA and MUFA and β-oxidation of 12 : 0, 16 : 0 and 18 : 1n-9 were higher for birds fed on the Palm diet than for those fed on the other diets (P < 0·001; P < 0·05 for 18 : 1n-9). β-Oxidation of 14 : 0 was higher for the Fish diet than for the Soya and Lin diets. β-Oxidation of 20 : 0 was higher for the Palm and Lin diets than for the Soya and Fish diets (P < 0·001). Birds fed on the Soya diet had a higher 22 : 0 β-oxidation than those fed on the other diets (P < 0·001). Age influenced β-oxidation of the fatty acids (except for 18 : 1n-7), with greater values in 7–21-d-old birds always (P < 0·05). Sex did not influence β-oxidation of the fatty acids (P>0·05).

Table 5 β-Oxidation of SFA and MUFA in broiler chickens (μmol/g per d)

Palm, palm fat diet; Soya, soyabean oil diet; Lin, linseed oil diet; Fish, fish oil diet; RMSE, root mean squares error.

^a,b Mean values for diets within a row with unlike superscript letters were significantly different (P < 0·05).

* Significant interaction terms at P < 0·05.

The total ex novo production and β-oxidation of ΣSFA and MUFA were plotted against the net intake of these fatty acids expressed as μmol/g per d (Fig. 2). Ex novo production of SFA and MUFA showed a negative curvilinear relationship with the net intake of these fatty acids (R ² 0·56). A similar trend was observed for individual SFA and MUFA (data not shown). The β-oxidation of ΣSFA and MUFA indicated a strong positive curvilinear relationship with the net intake of these fatty acids (R ² 0·97). A sharp increase in β-oxidation was observed by elevating the net intake of SFA and MUFA from 23 to 30 μmol/g per d. Similar strong relationships were observed for individual SFA and MUFA (data not shown).

Fig. 2 Total ex novo production and β-oxidation of total SFA and MUFA in relation to the sum of SFA and MUFA net intake (μmol/g per d) across diets, age and sex (n 16). Broken line regression equation for ex novo production: Y = 0, if X>24·51, and Y = 24·51+0·4076 × (24·51 − X), if X ≤ 24·51; R ² 0·56; root mean square means (RMSE) = 1·53 (n 16). Broken line quadratic regression equation for β-oxidation: Y = 0·078, if X < 15·28, and Y = 0·078+0·0362 × (X − 15·28)², if X ≥ 15·28; R ² 0·97; RMSE = 0·403. ■, Ex novo; , β-oxidation.

The relationship between total apparent elongase activity and the net intake of ΣSFA and MUFA (μmol/g per d) was exponential (R ² 0·70; Fig. 3). It was observed that increasing the SFA and MUFA net intake is associated with decreasing total elongase activity on SFA and MUFA. A slight negative linear relationship between total apparent Δ-9 desaturase activity and SFA net intake was noticed (r − 0·48; n 16; data not shown).

Fig. 3 Total apparent elongase activity on total SFA and MUFA in relation to the sum of SFA and MUFA net intake (μmol/g per d) across diets, age and sex. Exponential regression equation: Y = 146 × e^− 0·17X; R ² 0·70; root mean square means 4·23 (n 16).

Significant interaction terms are given in the tables, but these are not further discussed here since they were negligible compared with the main effects. P-values for the interaction effects were considerably smaller than those for the main effects. In addition, the significant interaction effects were in most cases the result of scale differences, and only in a few cases, they were the result of opposite effects. The most marked diet × age interaction effect was for the digestibility of 16 : 1n-7 and 18 : 1n-7, which was higher in the young birds than in the older birds fed on the Lin diet, whereas no difference or the opposite was found in the birds fed on the other diets.

Discussion

The experiment underlying the present study was primarily designed to examine the effect of different dietary PUFA sources on PUFA metabolism⁽¹³⁾. Hence, diets differed more in the content and source of PUFA than in those of SFA and MUFA. Nevertheless, we assumed that the experimental data are appropriate for making inferences on SFA and MUFA metabolism also, which is the topic of the present manuscript.

Apparent digestibilities of 12 : 0, 14 : 0, 16 : 0, 18 : 0, ΣSFA and 18 : 1n-9 in this experiment were in the same range as that reported previously^(15–17). We found no data on apparent digestibilities of long-chain SFA and MUFA in the literature, e.g. 20 : 0 and 20 : 1n-9, to compare with the present results. It has to be mentioned that the digestibility of SFA may be underestimated due to endogenous biosynthesis of SFA and biohydrogenation of unsaturated fatty acids in the chicken hindgut, while the opposite should be considered for MUFA⁽¹⁸⁾. A relatively higher fatty acid digestibility in birds slaughtered at 42 d of age than in those slaughtered at 21 d of age might be due to a low bile salt concentration and lower formation of mixed micelles and lower lipid absorption in younger broiler chickens⁽¹⁷⁾. An interesting finding of the present study was that across dietary treatments, the ΣSFA content of broiler chickens was almost constant (approximately 30 mg/g), suggesting that to increase the MUFA:SFA ratio in the broiler chicken tissues or final products, the prime strategy is to enhance dietary MUFA than to decrease SFA.

Thanks to the WB fatty acid balance method implemented in the present study, it was possible to estimate and follow the fate of dietary fatty acids. In Palm-fed birds, intake and total β-oxidation of SFA and MUFA were highest, whereas accumulation (percentage of net intake and ex novo production) was lowest when compared with the other groups. In Lin-fed birds, intake of SFA and MUFA was minimal, while apparent elongation and Δ-9 desaturation activities were higher when compared with the other groups. It can thus be stated that the dietary intake of SFA and MUFA has a negative relationship with the accumulation of these fatty acids, but a positive relationship with β-oxidation.

The Δ-9 desaturase (stearoyl-CoA desaturase) enzyme is a rate-limiting lipogenic enzyme, and is considered to modulate de novo fatty acid biosynthesis^(19–21). An earlier study suggested that Δ-9 desaturase in chickens is up-regulated by low-fat high-carbohydrate diets, and is down-regulated by the addition of dietary PUFA⁽²²⁾. In general, it has been reported that Δ-9 desaturase activity could be influenced by different factors including nutrition⁽²³⁾. In the present study, the dietary fat level was constant, and Δ-9 desaturase activity was not influenced by the PUFA content of the diets. Across diets, sex and age, 18 : 0 followed by 16 : 0 were the most Δ-9 desaturated fatty acids in the present study. This corresponds with a previous finding that the preferred substrates for Δ-9 desaturase are palmitoyl-CoA and stearoyl-CoA⁽²²⁾.

The elongation process is involved in many aspects of endogenous fatty acid synthesis by inserting two carbon units at the carboxyl terminal of fatty acids⁽¹⁹⁾. In the current experiment, accretion of an elongated fatty acid was used as an indication of the total apparent in vivo elongase activity on this fatty acid. Comparatively, accretion of elongated fatty acids was numerically higher than that of Δ-9 desaturated fatty acids. This is in harmony with a recent report in rainbow trout using a similar WB fatty acid balance method⁽¹¹⁾. According to another study, elongation activity in bovine adipose tissue is 3-fold greater than desaturation activity⁽²⁴⁾. In bovine, the desaturation of 18 : 0 to form 18 : 1n-9 is proposed as the limiting process in the biosynthesis of 18 : 1n-9 from 16 : 0⁽²⁴⁾. Hence, the addition of a double bond appears to be the rate-limiting step in MUFA biosynthesis rather than the chain elongation across species.

Total apparent in vivo elongase activity was significantly reduced when SFA and MUFA were abundantly provided (Palm diet). This, together with the fact that total Δ-9 desaturase activity seemed to be unaffected by dietary fatty acid composition, suggests that a dietary surplus of SFA and MUFA may be an obstacle in the biosynthesis of some MUFA, especially 18 : 1n-9, by suppressing elongase activity. However, in the birds supplied with abundant SFA and MUFA, the actual accumulation of MUFA directly originating from the diet coped with the inhibited endogenous biosynthesis of these fatty acids and therefore recorded the highest accumulation in the WB.

Comparing Fish- and Lin-fed birds in the present study with rainbow trout fish given similar dietary treatments⁽¹¹⁾, it was found that accretion of elongated SFA (14 : 0 and 16 : 0) is higher in broiler chickens than in rainbow trout fish, whereas elongation of MUFA (18 : 1n-9) in chickens seems to be half of that in trout. Interestingly, accretion of Δ-9 desaturated 16 : 0 and 18 : 0 in Fish and Lin groups of chickens seems to be up to 7-fold higher than that in similarly fed rainbow trout.

It is known that fatty acids with a chain length ranging from eight to twenty carbon atoms, both SFA and MUFA, are good substrates for mitochondrial β-oxidation⁽²⁵⁾. The effect of diet on β-oxidation of SFA and MUFA in this experiment confirms isotopic studies in rats and human subjects arguing that the nature of the ingested fat affects β-oxidation of the fatty acids⁽²⁾. In the present study across diets, 16 : 0 showed a higher β-oxidation rate than the other fatty acids. It has been explained by Sanz et al. ⁽²⁶⁾ that fatty acids are not utilised on an equal basis for oxidation. Our finding regarding higher β-oxidation of 16 : 0 over 18 : 1n-9, 20 : 0 and 22 : 0 is in general harmony with a previous report that the β-oxidation rate drops sharply when the fatty acid chain length increases from eighteen to twenty carbon atoms⁽²⁵⁾.

In the present experiment, the β-oxidation of ΣSFA seemed to be higher than that of ΣMUFA in broiler chickens. This is in agreement with an earlier study indicating a preferential β-oxidation of SFA over MUFA in rat muscle, heart, and liver, and beef liver⁽¹²⁾. Across diets, age and sex, β-oxidation of both SFA and MUFA in the present study followed the same trend as fatty acid net intake, suggesting that despite the possible existence of a preferential order of oxidation for some fatty acids over others, the fatty acid oxidation is primarily directed towards those provided in surplus by the diet. On the other hand, it has been reported that diets containing high amounts of PUFA result in greater rates of β-oxidation of fatty acids^{(15, 26, 27)}. This was not apparent in the present study. Total PUFA intake was in the order Lin>Soya>Fish>Palm diets. The β-oxidation of ΣSFA and ΣMUFA (percentage of net intake and ex novo production) was similar for the Soya, Lin and Fish diets, and was lower for these diets than for the Palm diet. In the accompanying paper⁽¹³⁾ on the PUFA metabolism, the β-oxidation of 18 : 2n-6 and 18 : 3n-3 was also not related to their proportions in the diet. Hence, higher proportions of PUFA in the diet did not increase β-oxidation of fatty acids in the present study, in contrast with the above-mentioned studies. In these studies, high amounts of PUFA in the diet also reduced abdominal fat pad weights. This was also not apparent in the present study. However, it should be mentioned that in the present study, 3 % of a PUFA source was included in the diet, whereas this was 8–10 % in the above-mentioned studies^{(15, 26, 27)}.

Opposite to the ΣSFA and MUFA accumulation, elongation and Δ-9 desaturation, β-oxidation in 7–21-d-old birds was higher than that in the older birds. Accordingly, as reported previously by Kloareg et al. ⁽²⁸⁾, it is known that the stage of growth and development influences β-oxidation of the fatty acids. Young fast-growing chickens possess a higher rate of metabolism than chickens approaching slaughter age.

Similar to isotopic methods, the WB method applied in the present study was unable to estimate the multidirectional metabolism of some fatty acids. For instance, 18 : 1n-9 can be elongated to form 20 : 1n-9 or saturated to form 18 : 0. Conversely, 18 : 1n-9 can be formed from these fatty acids⁽²⁹⁾. The WB fatty acid balance method estimates fatty acid metabolism based on the fatty acid detection in the WB pool over a specific time frame. Besides the drawbacks, the employed WB fatty acid balance method has proven to be reliable to study the turnover of SFA and MUFA in the WB^{(11, 30)}.

In conclusion, the present study has shown that increasing the intake of SFA and MUFA is associated with a lower accumulation of SFA and MUFA relative to the net intake and ex novo production in WB broiler chickens, a higher rate of β-oxidation and a lower rate of apparent elongase activity. Apparent elongase and Δ-9 desaturase activities increase with age, but sex has no effect on this.