Jean-François Hocquette

Intramuscular fat content in meat-producing animals: development, genetic and nutritional control, and identification of putative markers.

Intramuscular fat (IMF) content plays a key role in various quality traits of meat. IMF content varies between species, between breeds and between muscle types in the same breed. Other factors are involved in the variation of IMF content in animals, including gender, age and feeding. Variability in IMF content is mainly linked to the number and size of intramuscular adipocytes. The accretion rate of IMF depends on the muscle growth rate. For instance, animals having a high muscularity with a high glycolytic activity display a reduced development of IMF. This suggests that muscle cells and adipocytes interplay during growth. In addition, early events that influence adipogenesis inside the muscle (i.e proliferation and differentiation of adipose cells, the connective structure embedding adipocytes) might be involved in interindividual differences in IMF content. Increasing muscularity will also dilute the final fat content of muscle. At the metabolic level, IMF content results from the balance between uptake, synthesis and degradation of triacylglycerols, which involve many metabolic pathways in both adipocytes and myofibres. Various experiments revealed an association between IMF level and the muscle content in adipocyte-type fatty acid-binding protein, the activities of oxidative enzymes, or the delta-6-desaturase level; however, other studies failed to confirm such relationships. This might be due to the importance of fatty acid fluxes that is likely to be responsible for variability in IMF content during the postnatal period rather than the control of one single pathway. This is evident in the muscle of most fish species in which triacylglycerol synthesis is almost zero. Genetic approaches for increasing IMF have been focused on live animal ultrasound to derive estimated breeding values. More recently, efforts have concentrated on discovering DNA markers that change the distribution of fat in the body (i.e. towards IMF at the expense of the carcass fatness). Thanks to the exhaustive nature of genomics (transcriptomics and proteomics), our knowledge on fat accumulation in muscles is now being underpinned. Metabolic specificities of intramuscular adipocytes have also been demonstrated, as compared to other depots. Nutritional manipulation of IMF independently from body fat depots has proved to be more difficult to achieve than genetic strategies to have lipid deposition dependent of adipose tissue location. In addition, the biological mechanisms that explain the variability of IMF content differ between genetic and nutritional factors. The nutritional regulation of IMF also differs between ruminants, monogastrics and fish due to their digestive and nutritional particularities.

Nutritional and hormonal regulation of energy metabolism in skeletal muscles of meat-producing animals

In meat-producing animals, skeletal muscle is a tissue of major economic importance. Energy metabolism of skeletal muscle should be assessed in relation to its main physiological functions which are growth as well as posture, physical activity and thermogenesis. These functions have specific energy requirements which may entail competition in the utilization of energy-yielding nutrients, and modify the efficiency of ATP production and use. In this review, the energy requirements of muscles will be presented as well as the main metabolic pathways involved in the utilization of carbohydrate and lipids for energy supply and deposition. Regulation of metabolism will also be tackled in relation to the major physiological functions of skeletal muscle in various domestic species. Special focus will be given to the energy metabolism of growing muscle and its interactions with meat quality. The question will be asked whether the optimization of the efficiency of ATP production and utilization in muscle for nitrogen retention is compatible with an improvement of meat quality (as partly determined by glycogen and lipid deposition)? For this purpose, the interactions between muscle carbohydrate, lipid and protein metabolism will be reviewed.

Relationship between collagen characteristics, lipid content and raw and cooked texture of meat from young bulls of fifteen European breeds.

Variations in texture were determined for 10 day aged raw and cooked Longissimus thoracis (LT) muscle from 436 bulls of 15 European cattle breeds slaughtered at an age of 13-16 months. Variations in texture were related to differences in pH 24 h post-mortem, sarcomere length, collagen characteristics and lipid content. The shear force of cooked meat samples varied from 43.8 to 67.4 N/cm². Simmental, Highland and Marchigiana cattle had the highest shear force values and Avileña-Negra Ibérica, Charolais, Casina and Pirenaica cattle had the lowest values. Cooked meat toughness showed a weak negative correlation to lipid content (P<0.001) but no correlation to collagen characteristics. Raw meat texture measured by compression correlated positively (P<0.001) with total and insoluble collagen. In conclusion, collagen characteristics showed correlation to raw meat texture but not to cooked meat toughness of LT muscle in European young bulls.

Lipoprotein lipase activity and mRNA levels in bovine tissues

Lipoprotein lipase (LPL) in cattle has been extensively studied in adipose tissue, milk and mammary gland, but only to a limited extent in muscles. Therefore, we have adapted our in vitro LPL assay method for the measurement of LPL activity and describe, for the first time, sensitive procedures to quantify LPL activity and mRNA levels in bovine muscles. In vitro activation of bovine LPL activity is approximately 5-fold greater with rat than with bovine sera for heart and muscles, but not for adipose tissues. Values of LPL activity are in the upper range of those previously reported for rat or bovine tissues. With rat serum as activator, LPL activity in the heart of seven calves (662-832 mU g-1) is at least 3-fold lower than in the rat heart (2150-2950 mU g-1, P < 0.05). LPL activity is higher in bovine heart and oxidative muscles (412-972 mU g-1), except the diaphragm, than in mixed or glycolytic muscles (33-154 mU g-1, P < 0.05). The levels of LPL transcripts are positively related to LPL activity in bovine tissues, including muscles and adipose tissues.

Enhancing the nutritional and health value of beef lipids and their relationship with meat quality

This paper focuses on dietary approaches to control intramuscular fat deposition to increase beneficial omega-3 polyunsaturated fatty acids (PUFA) and conjugated linoleic acid content and reduce saturated fatty acids in beef. Beef lipid trans-fatty acids are considered, along with relationships between lipids in beef and colour shelf-life and sensory attributes. Ruminal lipolysis and biohydrogenation limit the ability to improve beef lipids. Feeding omega-3 rich forage increases linolenic acid and long-chain PUFA in beef lipids, an effect increased by ruminally-protecting lipids, but consequently may alter flavour characteristics and shelf-life. Antioxidants, particularly α-tocopherol, stabilise high concentrations of muscle PUFA. Currently, the concentration of long-chain omega-3 PUFA in beef from cattle fed non-ruminally-protected lipids falls below the limit considered by some authorities to be labelled a source of omega-3 PUFA. The mechanisms regulating fatty acid isomer distribution in bovine tissues remain unclear. Further enhancement of beef lipids requires greater understanding of ruminal biohydrogenation.

Recent advances in cattle functional genomics and their application to beef quality

The advent of high-throughput DNA sequencing techniques, array technology and protein analysis has increased the efficiency of research in bovine muscle physiology, with the ultimate objective of improving beef quality either by breeding or rearing factors. For genetic purposes, polymorphisms in some key genes have been reported for their association with beef quality traits. The sequencing of the bovine genome has dramatically increased the number of available gene polymorphisms. The association of these new polymorphisms with the variability in beef quality (e.g. tenderness, marbling) for different breeds in different rearing systems will be a very important issue. For rearing purposes, global gene expression profiling at the mRNA or protein level has already shown that previously unsuspected genes may be associated either with muscle development or growth, and may lead to the development of new molecular indicators of tenderness or marbling. Some of these genes are specifically regulated by genetic and nutritional factors or differ between different beef cuts. In recognition of the potential economic benefits of genomics, public institutions in association with the beef industry are developing livestock genomics projects around the world. From the scientific, technical and economical points of view, genomics is thus reshaping research on beef quality.

Indicators of milk and beef quality

All the review papers and the short communications presented in this session are in this EAAP publication entitled “Indicators of milk and beef quality”. Additional papers from invited authors were written in 2004 and then added to complete the publication although not all topics related to quality indicators are covered in the present publication. Review papers aim to describe the state of the art in different disciplines including genetics, physiology, nutrition, biochemistry as well as social and economic aspects. Short communications are recent and new results given as examples of the research done in these different areas. All the papers were peer-reviewed by international experts.

Increased peripheral lipid clearance in an animal model of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is the most common adult motor neuron disease, causing motor neuron degeneration, muscle atrophy, paralysis, and death. Despite this degenerative process, a stable hypermetabolic state has been observed in a large subset of patients. Mice expressing a mutant form of Cu/Zn-superoxide dismutase (mSOD1 mice) constitute an animal model of ALS that, like patients, exhibits unexpectedly increased energy expenditure. Counterbalancing for this increase with a high-fat diet extends lifespan and prevents motor neuron loss. Here, we investigated whether lipid metabolism is defective in this animal model. Hepatic lipid metabolism was roughly normal, whereas gastrointestinal absorption of lipids as well as peripheral clearance of triglyceride-rich lipoproteins were markedly increased, leading to decreased postprandial lipidemia. This defect was corrected by the high-fat regimen that typically induces neuroprotection in these animals. Together, our findings show that energy metabolism in mSOD1 mice shifts toward an increase in the peripheral use of lipids. This metabolic shift probably accounts for the protective effect of dietary lipids in this model.

Comparison of composition and quality traits of meat from young finishing bulls from Belgian Blue, Limousin and Aberdeen Angus breeds

Thirty-six young finishing bulls from three breeds (Belgian Blue, Limousin and Aberdeen Angus) were fattened over five months with finishing diets based either on sugar-beet pulp or on cereals. Nutritional quality traits of meat - fat content and fatty acid composition with emphasis on the n-6 and n-3 polyunsaturated fatty acids - along with some organoleptic quality traits were measured. The Belgian Blue bulls had the lowest intramuscular fat content associated with lower saturated and monounsaturated fatty acid contents. The polyunsaturated fatty acid content did not differ to a large extent between the breeds, the Aberdeen Angus bulls showing slightly higher values. Relative to energy intake, the overall contribution of meat to the n-3 fatty acid recommended intake was small, whatever the breed. By contrast, the contribution of meat to daily fat intake was of greater importance, especially for the Aberdeen Angus bulls. The quality traits of meat varied also according to the breed: compared to the Aberdeen Angus, the Belgian Blue bull meat had the stablest colour, the highest drip and the lowest cooking losses. The meat of Limousin bulls had intermediate characteristics for all the parameters.

Opportunities for predicting and manipulating beef quality.

Meat quality is a complex concept and can be defined as the characteristics of meat which satisfy consumers and citizens. The quality concept can be divided into intrinsic quality traits (which are the characteristics of the product itself) and extrinsic quality traits (which are more or less associated to the product for instance the price, a major determinant of purchase, or any brand or quality label). Quality can also be generic for the mass market or specific for niche markets. The relative importance of the different quality traits varies with human culture and time with a general trend of an increasing contribution of healthiness, safety and extrinsic quality traits. This review underlines the need for the development of methods to interpret and aggregate measures under specific rules to be defined in order to produce an overall assessment of beef quality. Such methods can be inferred for example from genomic results or data related to muscle biochemistry to better predict tenderness or flavor. A more global assurance quality scheme (the Meat Standards Australia System) based on the aggregation of sensory quality traits has been developed in Australia to ensure palatability to consumers. We speculated that the combination of indices related to sensory and nutritional quality, social and environmental considerations (carbon footprint, animal welfare, biodiversity of pasture, rural development, etc.) and economic efficiency (incomes of farmers and of others players along the supply chain, etc.) will provide objective assessment of the overall quality of beef (i.e. incorporating an all encompassing approach) not only for the mass market but also to support official quality labels of niche markets which are so far mainly associated with the geographical origins of the products.