To produce foie gras, male mule ducks were force-fed for 10 days under two different conditions of indoor air exchange (A and C, corresponding to good and poor conditions respectively). Force-feeding led to a dramatic increase in liver weight (+8-fold, p < 0.0001) in both groups. However, the steatotic livers from both groups had a very similar macroscopic composition (dry matter = 67%, total lipids = 60%, protein = 3.5%) and showed no signs of fibrosis, as indicated by their similar hydroxyproline contents. Throughout the force-feeding period, levels of HIF (hypoxia-inducible factor) 1 alpha and 2 alpha increased (+30% and +38%, p < 0.0001 respectively) in the livers of both groups, but at the end of this period, HIF1 alpha levels were significantly higher in group C (+16%, p < 0.05) than in group A. The malondialdehyde (MDA) content in the livers was also significantly higher in group C (+21%, p < 0.05) than in group A, indicating that lipid oxidation was influenced by the different air renewing conditions. Force-feeding also led to an increase in the transcription levels of several genes (IL-8, IL-9, IL-18, TNF-alpha, NF-kappa B2) associated with inflammatory responses, with this increase being particularly marked for IL-9 in the steatotic livers of group C (+204%, p < 0.0001). Thus, different air renewing conditions during the period of force-feeding influence the development of hepatic steatosis and modify the oxidation of accumulated lipids.

IntroductionThis study investigated the physiological, biochemical, and molecular shifts in male Mule ducks during an assisted-feeding period and the subsequent recovery phase following the cessation of overfeeding. While assisted-feeding is known to induce significant hepatic changes, the timeline and extent of the liver's capacity to return to a basal state remain critical areas of inquiry.MethodsMale Mule ducks were subjected to a period of assisted-feeding followed by a recovery phase where they returned to an ad libitum diet. We monitored body weight and liver mass, analyzed plasma metabolic markers (including lipids and liver enzymes), and evaluated hepatic composition. Molecular analysis was conducted to assess gene expression related to lipogenesis, inflammation, and apoptosis, while antioxidant enzyme activities and hypoxia markers were measured to determine cellular stress levels.ResultsAssisted-feeding significantly increased body weight, liver mass, and hepatic steatosis, accompanied by a sharp rise in plasma markers such as triglycerides, cholesterol, and liver enzymes (ALAT, LDH, ALP). At the molecular level, there was a marked upregulation of genes involved in lipogenesis (scd1, dgat2), inflammation (TNFα, IL8), and apoptosis. Furthermore, increased activities of antioxidant enzymes (SOD, Cat, GPX1) and markers of hypoxia (HIF-1α, HIF-2α) indicated significant metabolic load and cellular stress. Following the cessation of overfeeding, most physiological and biochemical parameters, including liver mass and enzyme levels, returned to control values within 20 to 29 days. Notably, while hepatic alterations were fully abolished, abdominal fat remained significantly higher in ex-force-fed ducks compared to the control group.ConclusionThe study demonstrates that hepatic steatosis induced by assisted-feeding in Mule ducks is a highly reversible process. Despite triggering significant oxidative stress, hypoxia, and inflammatory responses, the avian liver exhibits a remarkable regenerative capacity, returning to its basal physiological and molecular state within 29 days of returning to a standard diet.

Metabolic dysfunction-associated steatotic liver disease (MASLD), and its more advanced stage metabolic dysfunction-associated steatohepatitis, is the most common chronic liver disease, constituting a major public health issue. Relevant preclinical models are needed to define molecular mechanisms underlying MASLD pathogenesis and evaluate therapeutic approaches. The majority of the lipids accumulating in the liver upon MASLD originate from adipose tissue and appropriate models to study the liver-adipose tissue dialog are also needed. Here, we demonstrated that, compared to standard temperature housing, thermoneutral housing aggravated western diet (WD)-induced obesity, diabetes, and steatosis in male mice, which was associated with increased hepatic expression of inflammation- and fibrosis-related genes. Accordingly, thermoneutral-housed WD-fed mice developed more severe hepatic inflammation and fibrosis compared to standard-housed mice. We next used thermoneutral-housed WD-fed mice to question the effect of MASLD during β3-adrenergic stimulation. We found that diet-induced MASLD is associated with defective inter-organ metabolic cross-talk which leads to impaired activation of brown adipose tissue.

Metabolic dysfunction-associated steatotic liver disease (MASLD), and its more advanced stage metabolic dysfunction-associated steatohepatitis, is the most common chronic liver disease, constituting a major public health issue. No medication is approved for MASLD treatment, and relevant preclinical models are needed to define molecular mechanisms underlying MASLD pathogenesis, and evaluate therapeutic approaches. Here we demonstrated that compared to standard temperature housing, thermoneutral housing aggravated western diet (WD)-induced obesity, diabetes, and steatosis in male mice, which was associated with increased hepatic expression of inflammation- and fibrosis-related genes. Accordingly, compared to standard-housed mice, thermoneutral-housed WD-fed mice developed more severe hepatic inflammation and fibrosis. The liver is the central metabolic organ in whole-body metabolic homeostasis. We used thermoneutrally housed mice with WD-induced MASLD to examine the effect of MASLD during β3-adrenergic stimulation, and found that diet-induced MASLD was associated with defective inter-organ metabolic cross-talk, leading to impaired brown adipose tissue activation. ### Competing Interest Statement The authors have declared no competing interest.

Male and female mule ducks were subjected to a force-feeding diet to induce liver steatosis as it is generally done only with male ducks for the production of foie gras. The different biochemical measurements indicated that the course of hepatic steatosis development was present in both sexes and associated with a huge increase in liver weight mainly due to the synthesis and accumulation of lipids in hepatocytes. In livers of male and female ducks, this lipid accumulation was associated with oxidative stress and hypoxia. However, certain specific modifications (kinetics of lipid droplet development and hepatic inflammation) indicate that female ducks may tolerate force-feeding less well, at least at the hepatic level. This is in contradiction with what is generally reported concerning hepatic steatosis induced by dietary disturbances in mammals but could be explained by the very specific conditions imposed by force-feeding. Despite this, force-feeding female ducks seems entirely feasible, provided that the final quality of the product is as good as that of the male ducks, which will remain to be demonstrated in future studies.

Mule ducks have been force-fed to develop a hepatic steatosis, also called “foie gras”, which is similar to the non-alcoholic fatty liver disease (NAFLD) described in humans and mammals. However, in hepatic steatosis resulting from force-feeding of ducks, very little is known about the fine biochemical events that occur due to the enormous and very rapid increase in total lipids that mainly accumulate in hepatocytes. To begin to reduce this lack of knowledge associated with the development of this specific hepatic steatosis, liver samples were taken at different times to follow the overall biochemical transformation of the liver as well as different markers of oxidative stress, hypoxia and apoptosis. The results indicate that the lipid content increases rapidly in the liver throughout the force-feeding period while the protein content decreases. The amount of hydroxyproline remains constant indicating that no liver fibrosis develops during the force-feeding period. On the contrary, all the tested biomarkers of cellular oxidative stress increase rapidly but without any visible disorder in the coordination of paired activities. At the same time, hypoxia-inducible factors also increase indicating that a hypoxia situation is gradually occurring in hepatocytes. This leads, in addition to the lipotoxicity induced by the accumulation of lipids, to an increased number of liver cells to enter into apoptosis. A relative variability in the level of these cellular responses was also observed indicating that, probably, certain animals support the development of this steatosis differently. This leads us to imagine that the physiological status of these birds may differ widely for reasons that remain to be clarified.

SCOPE:Non-alcoholic fatty liver disease (NAFLD) is a sexually dimorphic disease influenced by dietary factors. Here, the metabolic and hepatic effects of dietary amino acid (AA) source is assessed in Western diet (WD)-induced NAFLD in male and female mice. METHODS AND RESULTS:The AA source is either casein or a free AA mixture mimicking the composition of casein. As expected, males fed a casein-based WD display glucose intolerance, fasting hyperglycemia, and insulin-resistance and develop NAFLD associated with changes in hepatic gene expression and microbiota dysbiosis. In contrast, males fed the AA-based WD show no steatosis, a similar gene expression profile as males fed a control diet, and a distinct microbiota composition compared to males fed a casein-based WD. Females are protected against WD-induced liver damage, hepatic gene expression, and gut microbiota changes regardless of the AA source. CONCLUSIONS:Free dietary AA intake prevents the unhealthy metabolic outcomes of a WD preferentially in male mice.

Understanding the evolution of fatty liver metabolism of ducks is a recurrent issue for researchers and industry. Indeed, the increase in weight during the overfeeding period leads to an important change in the liver metabolism. However, liver weight is highly variable at the end of overfeeding within a batch of animals reared, force-fed and slaughtered in the same way. For this study, we performed a proton nuclear magnetic resonance (1H-NMR) analysis on two classes of fatty liver samples, called low-weight liver (weights between 550 and 599 g) and high-weight liver (weights above 700 g). The aim of this study was to identify the differences in metabolism between two classes of liver weight (low and high). Firstly, the results suggested that increased liver weight is associated with higher glucose uptake leading to greater lipid synthesis. Secondly, this increase is probably also due to a decline in the level of export of triglycerides from the liver by maintaining them at high hepatic concentration levels, but also of hepatic cholesterol. Finally, the increase in liver weight could lead to a significant decrease in the efficiency of aerobic energy metabolism associated with a significant increase in the level of oxidative stress. However, all these hypotheses will have to be confirmed in the future, by studies on plasma levels and specific assays to validate these results.

The liver is a vital organ that sustains multiple functions beneficial for the whole organism. It is sexually dimorphic, presenting sex-biased gene expression with implications for the phenotypic differences between males and females. Estrogens are involved in this sex dimorphism and their actions in the liver of several reptiles, fishes, amphibians, and birds are discussed. The liver participates in reproduction by producing vitellogenins (yolk proteins) and eggshell proteins under the control of estrogens that act via two types of receptors active either mainly in the cell nucleus (ESR) or the cell membrane (GPER1). Estrogens also control hepatic lipid and lipoprotein metabolisms, with a triglyceride carrier role for VLDL from the liver to the ovaries during oogenesis. Moreover, the activation of the vitellogenin genes is used as a robust biomarker for exposure to xenoestrogens. In the context of liver diseases, high plasma estrogen levels are observed in fatty liver hemorrhagic syndrome (FLHS) in chicken implicating estrogens in the disease progression. Fishes are also used to investigate liver diseases, including models generated by mutation and transgenesis. In conclusion, studies on the roles of estrogens in the non-mammalian oviparous vertebrate liver have contributed enormously to unveil hormone-dependent physiological and physiopathological processes.

The weight of the liver is one of the important selection criteria in the quality of "foie gras". This factor is highly variable despite the fact that individuals are reared, overfed and slaughtered in the same way. In this study, we performed an analysis of the proteome profile of two weight classes of light (between 550 and 599 g) and heavy (more than 700 g) livers. For the analysis of the proteic extracts, a liquid chromatographic analysis coupled with mass spectrometry was carried out. In low-weight livers, aerobic energy metabolism, protein metabolism and lipid metabolism oriented toward export and beta-oxidation were overexpressed. On the contrary, high weight livers were characterized by anaerobic energy metabolism and a more active protein catabolism associated with cell apoptosis and reorganization of the cell structure.

The "Foie gras" or fatty liver is the result of hepatic steatosis from nutritional origin and induced by the force-feeding of palmipeds. Despite identical rearing and force-feeding conditions of ducks from the same breed, different liver weights, within a range of 500 to more than 700 g, are generally observed at the time of evisceration. To better understand the determinism of this large variability in fatty liver weights, the activity of various metabolic pathways has been explored in 4 groups of steatotic livers differing by their weights. Different analyses were performed using biochemical assays on metabolites as well as ELISA tests or enzyme activity assays. The result showed that an increase in the final liver weight is always associated with a hypoxic response and even a severe hypoxia observed in livers with the highest weights (more than 650 g). This is also combined with a rise in the cellular oxidative stress level. In addition, for the heaviest livers (more than 700 g), signs of cell death by apoptosis were also observed, while others programmed cell death pathways, such as ferroptosis or necroptosis, seemed to be nonactive.

In foie gras production the technological yield after the cooking process is one of the main issues of processors as it is closely linked to the cooking melting rate. This rate is subjected to strict laws and regulations since it directly affects the organoleptic and technological qualities of this gourmet product. The objective of the study was to better understand the liver fattening and the technological yield decrease during the overfeeding kinetics. A flock of 210 mule ducks was reared and then overfed during 12 D with 2 overfeeding programs; in the test group the amounts of corn in the first meals were higher than in the control group (+430 g during the whole period). Ducks were slaughtered at the end of the rearing period (D0, n = 15) and every other day (D2 to D12, n = 15 by group). Duck performances, anatomical dissections and physical and biochemical liver characteristics were registered. The performances were equivalent in the groups (P > 0.1). The evolution of the liver weight was then analyzed in detail in relation with the evolution of its biochemical composition. A two-step evolution occurred in the liver metabolism, first a main glycogen storage and then a strong lipid storage. A model to predict the liver weight was established with only BWs and feed intakes (R² = 0.83). The technological yield was determined on foie gras weighing more than 300 g (D6 to D12). The melting process was high during the last 2 D. The technological yield reached 72% at D12, for 758 g foie gras, and a strong negative correlation was observed with liver weight (-0.83; P < 0.001). A model to predict the technological yield was established with the liver weight and the liver color parameters (R² = 0.71). This study highlights the compromise between foie gras weight and its quality.

Mule ducks were force-fed for 12 d to determine whether or not signs of apoptosis could occur during the development of the hepatic steatosis induced by the huge quantities of corn ingested twice daily by the birds. Presence of apoptosis in hepatocytes was assessed through the measurements of increased activities of capsase-3 +-7, -8, and -9. From d 0 of the force-feeding period until d 8, activities of the different caspases remained at a low level. On the contrary, at d 10 and d 12, activities of all measured caspases dramatically increased, indicating that apoptosis occurred at this stage, which corresponds to the time of accumulation of large quantities of lipids in the hepatic cells. The melting level of the liver issued from force-feeding (“foie gras”) during cooking is a point of interest for processors because it could degrade the quality of this delicate dish. In this study, we used the levels of caspases activities to improve the predictability of foie gras cooking, in addition to other parameters usually used, such as its weight or lipid content. From this improvement, we suggest that part of the variability of melting during cooking of fatty livers could reside in more or less intense activity of hepatic proteases.

"Achieving sustainable production of poultry meat, volume 3, health and welfare." British Poultry Science, 59(3), p. 356

The purposes of the study were to investigate the effects of Wooden Breast (WB) myodegeneration on poultry meat quality and to give a contribution in typing lesions morphology. At a poultry meat cutting facility, 474 carcasses of a high-breast-yield hybrid chickens were inspected for WB condition, and 30 normal (N) and 30 affected (WB) breast fillets (Pectoralis major) were randomly selected. The WB condition represented 53.2% of the examined carcasses. Weight, cross sectional area (CSA), pH, L*, a*, b* colour values, water-holding capacity, and Warner-Bratzler shear force were determined. Samples were also visually and histologically evaluated. Affected samples were heavier, thicker, paler (P < 0.001), and characterized by palpatory hardness and lower water holding capacity (P < 0.05). Macroscopically, abnormalities were primarily localized in the cranial portion of the fillet and defined by the presence of bulges, petechiae, fluid and clear exudate, and higher pH. Microscopically, the WB condition was characterized by muscle fibres with greater CSA (P < 0.001) and higher giant fibres prevalence (P < 0.01). Data suggest a relationship between breast weight and WB condition.

"Achieving sustainable production of poultry meat, Volume 1: safety, quality and sustainability." British Poultry Science, 58(4), p. 469

Steatosis, the accumulation of triglycerides in hepatic cells after a rich in carbohydrate force-feeding, is a non pathological condition in palmipeds. The steatotic liver presents an enhanced metabolism. Protease activity in duck liver cells with steatosis was electrophoretically characterized. From exemplars of lean and fatty duck liver, three different homogenates were prepared: a plain homogenate, a metalloprotease extract and a cathepsin extract. The purpose of the study was to introduce a one-dimensional zymography (1DZ) and two-dimensional zymography (2DZ) approach in order to characterize the proteolytic profile present in hepatic cells under steatosis, with the ultimate goal to demonstrate the feasibility of this technique in determining proteolytic activity present in fatty liver in a semiquantitative way. Protein concentration was 30% higher in fatty liver extracts. Employing casein as substrate, proteolytic activity measured at 280 nm was determined to be 25% higher in fatty liver extracts. Zymographic techniques allowed semi-quantitative results, successfully detecting cathepsin- and metalloproteaseactivity using polyacrylamide gels copolymerized with gelatin and quantified by densitometry. The use of inhibitors confirmed the identity of the proteins studied. A metalloprotease activity was determined to have a relative molecular mass (Mr) ~90 ± 5 kDa, with an isoelectric point (pI) of 5.55 ± 0.01, and a cathepsin activity of 38 ± 2 kDa with a pI of 4.40 ± 0.01. The results confirm an increase in these protease activities present in extracts from steatotic livers. The analysis of liver proteases activities in forcefed ducks may elucidate the mechanisms behind steatosis development.

Protease activity present in liver cells with steatosis can be electrophoretically characterized. Zymographic techniques allow semi-quantitative results, successfully detecting cathepsin and metalloprotease activity using polyacrylamide gels copolymerized with gelatin and quantified by densitometry. By using specific inhibitors, the identity of the proteases can be confirmed. 2D zymography allows the determination of both M r. and pI of the metalloprotease and cathepsin activity present in the homogenates. The analysis of liver proteases activities in force fed ducks may elucidate the mechanisms behind steatosis development.

The aim of the study was to evaluate some meat physical quality and muscle fibre properties of rabbit meat when considering 2 sire breeds (SB: Vienna Blue [VB]; Burgundy Fawn [BF]; both coloured and slow-growing breeds), several parity orders (P: 1, 2, ≥3), gender (G), and 2 slaughter seasons (SS: spring, summer) in an organic production system. The effect of storage time (ST) at frozen state (2 mo at –20°C) of Longissimus lumborum (LL) meat was also evaluated. Animals were slaughtered when they reached 2.8 kg of live weight. Then, pH and L*a*b* colour values of Biceps femoris (BF) and LL muscles, water loss and Warner-Bratzler shear force of LL and hind leg (HL) meat, and the fibre typing and enzymatic activity of LL muscle were analysed. LL meat from females showed higher b* values than males (0.04 vs. –1.25; P<0.05). Significant (P<0.05) SB×P, SB×G and P×G interactions were observed for the b* value of LL: VB and BF crossbreds presented a higher b* value when born as P≥3 and P2 respectively, VB females showed higher b* value than VB males, and P2 and P≥3 produced males with a significantly lower b* value. HL thawing losses were significantly (P<0.05) higher in rabbits slaughtered in summer than in those slaughtered in spring, whereas the opposite result was obtained for LL meat (P<0.01). Cooking loss of LL meat was significantly lower in P2 group than P≥3 group (P<0.05). The lactate dehydrogenase activity in LL muscle was higher in VB than in BF crossbreds (930 vs. 830 IU; P<0.05), albeit not supported by differences in fibre type distribution. The ST significantly (P<0.01) reduced pH, a* and b* colour values, and increased lightness of LL meat. It was concluded that the crossbreeds derived from VB and BF genotypes and farmed organically did not show remarkable sexual dimorphism, considering their elder slaughter age than rabbits reared under intensive conditions. Physical quality of meat was mainly affected by slaughter season, indicating that in the organic rearing system, specific attention needs to be paid to the farming environmental conditions.