کاهش کیفیت گوشت بره ناشی از تنش گرمایی: تغییرات در ترکیب اسیدهای چرب و ارزش تغذیه‌ای

Summary
Introduction
Lamb meat is a nutritionally valuable food source, prized for its high-quality protein and essential fatty acids, such as linoleic (C18:2n6c) and α-linolenic (C18:3n3) acids. The health implications of meat consumption are significantly influenced by its fatty acid profile, particularly the ratios of saturated (SFA), monounsaturated (MUFA), and polyunsaturated fatty acids (PUFA). Higher PUFA intake has been associated with improved cardiovascular and metabolic health. However, this nutritional profile is highly susceptible to various factors, including environmental stressors. In the context of global climate change, livestock in arid and semi-arid regions are increasingly subjected to prolonged chronic heat stress. This stressor disrupts metabolic homeostasis, including lipid biosynthesis and oxidative stability, potentially compromising the quality and health value of meat products. While dietary influences on meat quality are well-studied, the direct impact of natural, chronic heat exposure on the fatty acid composition of lamb muscle is not well characterized. This study aimed to provide a detailed examination of how chronic heat stress under real-world conditions alters the fatty acid composition and nutritional attributes of lamb meat from the Longissimus dorsi muscle.

Materials and Methods
The study was conducted in accordance with ethical guidelines approved by the Institutional Animal Ethics Committee of Yasouj University (Approval ID: 4024443009). Thirty male Turki-Qashqaei lambs (5-6 months old; mean body weight 26.2 ± 1.4 kg) from SabzBavarn-e-NouAndish Co. farm were subjected to a 90-day fattening period under controlled conditions. To simulate contrasting environmental conditions, fifteen lambs were reared under chronic heat stress in the semi-arid Afzar region (Fars Province), where the Temperature-Humidity Index (THI) consistently exceeded 80. The control group of fifteen lambs was raised in the thermoneutral highland region of Kamane (Isfahan Province), where THI remained within the comfort zone for sheep (<68). Both groups were housed in semi-open barns with natural shade and ventilation and had ad libitum access to water. They were fed the same Total Mixed Ration (TMR), formulated to meet NRC (2007) standards, consisting of alfalfa hay and a concentrate mix.
After the 90-day period, all lambs were humanely slaughtered. Samples from the Longissimus dorsi muscle were collected within 45 minutes postmortem, snap-frozen in liquid nitrogen and stored at −80 °C until analysis. Lipids were extracted from freeze-dried muscle tissue using a modified Folch method. Fatty acids were transmethylated to form Fatty Acid Methyl Esters (FAMEs), which were then analyzed using gas chromatography (Agilent 7890A) equipped with a flame ionization detector and a DB-23 capillary column. Fatty acids were identified by comparing retention times to known standards. Results were expressed as a weight percentage of total FAMEs, and nutritional indices (PUFA/SFA, n-6/n-3) were calculated. Data were statistically analyzed using SAS software, with differences between groups evaluated using independent t-tests and considered significant at P < 0.05.

Results
The results demonstrated that chronic heat stress induced significant and comprehensive alterations in the muscle lipid profile. A pronounced increase was observed in short-chain fatty acids (SCFAs). Butyric (C4:0), caproic (C6:0), caprylic (C8:0), capric (C10:0), and lauric (C12:0) acids increased by 19.4% to 35.7% (P < 0.01 for all).
The composition of long-chain fatty acids was also markedly affected. Key saturated fatty acids (SFAs) increased significantly: palmitic acid (C16:0) rose by 12.4% and stearic acid (C18:0) by 13.1% (P = 0.003 and P = 0.004, respectively). Conversely, major unsaturated fatty acids decreased substantially. The monounsaturated oleic acid (C18:1n9c) declined by 20.3% (P = 0.009). Critical polyunsaturated fatty acids were also reduced: linoleic acid (C18:2n6c) decreased by 17.4% (P = 0.001) and arachidonic acid (C20:4n6) dropped by 31.9% (P = 0.003).
The health-beneficial conjugated linoleic acid (CLA) isomers were severely impacted. The cis-9, trans-11 CLA isomer decreased by 24.1% and the trans-10, cis-12 isomer by 31.2% (P = 0.015 and P = 0.021, respectively).
At the class level, these changes culminated in a significant shift in the overall lipid profile. Total SFA increased from 44.7% to 49.2% (P = 0.007), while MUFA decreased from 34.2% to 30.1% (P = 0.011) and PUFA from 21.1% to 18.3% (P = 0.013). Analysis of PUFA subclasses revealed a significant decrease in n-6 PUFA (14.6% to 12.2%, P = 0.017) and a smaller but significant decrease in n-3 PUFA (6.5% to 6.1%, P = 0.049). Consequently, the n-6/n-3 PUFA ratio improved slightly from 2.25 to 2.00 (P = 0.021).

Discussion and Conclusion
The findings of this study provide compelling evidence that chronic environmental heat stress triggers a profound deterioration in the nutritional quality of lamb meat. The observed lipidomic shifts increased SCFAs and SFAs coupled with decreased MUFAs, PUFAs, and CLA isomers paint a consistent picture of metabolic derangement. The elevation in SCFAs may indicate a stress-induced activation of mitochondrial β-oxidation or a compensatory lipid remodeling mechanism for energy production. The significant rise in palmitic and stearic acids, alongside the sharp decline in oleic acid, strongly suggests a suppression of Δ9-desaturase enzyme activity, a known response to thermal and oxidative stress.
The most nutritionally concerning outcome is the substantial loss of polyunsaturated fats, including the essential fatty acids linoleic and arachidonic acid, and the bioactive CLA isomers. This decline is likely driven by accelerated lipid peroxidation, as PUFAs are highly susceptible to oxidative damage from reactive oxygen species generated under heat stress. The reduction in CLA also points toward potential heat-induced alterations in the rumen microbiome, which is responsible for the biohydrogenation processes that create these isomers. While the n-6/n-3 ratio showed a slight improvement, this occurred within a context of an overall depletion of the total PUFA pool. This means the relative change is misleading and does not represent a true nutritional enhancement; instead, it signals a loss of health-promoting lipids vital for anti-inflammatory, immune, and cardiovascular functions.
These changes have direct implications for animal physiology, meat quality, and human nutrition. A higher SFA and lower MUFA/PUFA content negatively impacts sensory traits like flavor and juiciness, reduces the meat's shelf life due to increased oxidative instability, and diminishes its value as a functional food. The loss of CLA further erodes its nutraceutical properties.
In conclusion, this study underscores the significant vulnerability of lamb meat quality to climate-related heat stress. The results highlight an urgent need for integrated mitigation strategies to preserve nutritional integrity in warming environments. Potential interventions include dietary supplementation with antioxidants (e.g., vitamin E, selenium) to combat oxidative stress, the use of rumen-protected PUFA sources to replenish lost reserves, and the implementation of cooling management practices. In the long term, genetic selection for thermotolerant breeds with more resilient lipid metabolism may offer a sustainable solution. Future research should adopt a more integrative approach, combining tissue biomarker analysis, transcriptomic data, and rumen microbiome profiling to fully elucidate the mechanisms behind these changes and develop effective countermeasures for the livestock industry.