A Nutrition Society member-led meeting was held on 9 January 2020 at The University of Surrey, UK. Sixty people registered for the event, and all were invited to participate, either through chairing a session, presenting a ‘3 min lightning talk’ or by presenting a poster. The meeting consisted of an introduction to the topic by Dr Barbara Fielding, with presentations from eight invited speakers. There were also eight lightning talks and a poster session. The meeting aimed to highlight recent research that has used stable isotope tracer techniques to understand human metabolism. Such studies have irrefutably shaped our current understanding of metabolism and yet remain a mystery to many. The meeting aimed to de-mystify their use in nutrition research.

In a previous study, carnitine supplementation to piglets during the suckling period resulted in an increased total muscle fibre number at weaning in piglets of low birth weight. The objective of the present study was to investigate whether this effect is maintained until market age and whether this would attenuate the negative consequences of low birth weight on carcass and meat quality. Using a split-plot design with litter as block, sex as whole plot and treatment as subplot, the effects of early-postnatal l-carnitine supplementation on female and castrated male piglets of low birth weight were investigated on a total of 56 German Landrace piglets from 14 litters. From days 7 to 27 of age piglets were orally supplemented once daily with 400 mg of l-carnitine dissolved in 1 ml of water or received an equal volume of water without carnitine. From weaning (day 28) until slaughter (day 166 of age) all pigs were fed standard diets. At weaning, carnitine-supplemented piglets had a twofold increased concentration of free carnitine (P < 0.001) and a lower concentration of non-esterified fatty acids (P < 0.05) in blood plasma indicating that carnitine became bioavailable and increased fatty acid utilization during the period of supplementation. Growth performance was not influenced by treatment in any growth period. Dual-energy X-ray absorptiometry revealed no differences in body composition between groups in weeks 12, 16 and 20 of age. LW at slaughter, carcass weight, measures of meat yield and fat accretion, as well as body composition by chemical analyses and dissection of primal cuts did not differ between treatments. No differences between control and carnitine-treated pigs in total fibre number (P = 0.85) and fibre cross-sectional area (P = 0.68) in m. semitendinosus (ST) measured at slaughter could be observed. The carnitine group tended to exhibit a smaller proportion of slow-twitch oxidative fibres (P = 0.08), a greater proportion of fast-twitch glycolytic fibres (P = 0.11), and increased specific lactate dehydrogenase activity (P = 0.09) in ST indicating a more glycolytic muscle metabolism. Compared with the controls, a lower pH24 value was observed (P = 0.05) in ST muscle of carnitine-supplemented pigs, which – in castrates only – was associated with an increased drip loss (P < 0.01). Meat quality traits in m. longissimus were not influenced by treatment. In conclusion, our hypothesis that early-postnatal carnitine supplementation to piglets of low birth weight permanently increases myofibre number and improves later carcass and meat quality could not be confirmed by this experiment.

The present paper describes the main procedures used to slaughter fowl, pigs, calves and adult cattle, sheep, and farmed fish, starting on the farm and ending with the death of the animal at the abattoir. It reviews the currently known causes of stress, indicated by behavioural and physiological measurements on the animal level, and by post-mortem muscle metabolism. During the pre-slaughter period, psychological stress is due to changes of environment, social disturbances and handling, and physical stress is due to food deprivation, climatic conditions, fatigue, and sometimes pain. The exact causes of stress depend, however, on the characteristics of each species, including the rearing system. For fowl, bird catching and crating, duration and climatic conditions of transport and of lairage and shackling are the main known pre-slaughter stress factors. For pigs, stress is caused by fighting during mixing of pens, loading and unloading conditions, and introduction in the restrainer. Handling and novelty of the situation contribute to the stress reactions. For veal calves and adult cattle, disruption of the social group, handling, loading and sometimes unloading conditions, fatigue, novelty of the situation and for calves mixing with unfamiliar animals are known stress factors. Gathering and yarding of extensively reared lambs and sheep causes stress, particularly when shepherd dogs are used. Subsequent transport may induce fatigue, especially if sheep are commercialised through auctions or markets. In farmed fish, stress is predominantly related to environmental aspects such as temperature, oxygen, cleanliness of the water and, to a certain extent, stocking density and removal of the fish from the water. If transport and lairage conditions are good and their durations not too long, they may allow pigs, calves and adult cattle, sheep, and fish to rest. For certain species, it was shown that genetic origin and earlier experience influence reactions to the slaughter procedure. Stunning techniques used depend on the species. Pigs and fowl are mostly electrically or gas-stunned, while most adult cattle are stunned with a captive bolt pistol. Calves and sheep may be electrically stunned or with a captive bolt pistol. Various stunning methods exist for the different farmed fish species. Potential causes of stress associated with the different stunning procedures are discussed. The paper addresses further consequences for meat quality and possible itineraries for future research. For all species, and most urgently for fish, more knowledge is needed on stunning and killing techniques, including gas-stunning techniques, to protect welfare.

Natural-abundance 13CNMR spectroscopy is a non-invasive technique that enables in vivo assessments of muscle and/or liver glycogen concentrations. Over the last several years, 13C NMR has been developed and used to obtain information about human glycogen metabolism with diet and exercise. Since NMR is non-invasive, more data points are available over a specified time course, dramatically improving the time resolution. This improved time resolution has enabled the documentation of subtleties of muscle glycogen re-synthesis following severe glycogen depletion that were not previously observed. Muscle and liver glycogen concentrations have been tracked in several different human populations under conditions that include: (1) muscle glycogen recovery from intense localized exercise with normal insulin and with insulin suppressed; (2) muscle glycogen recovery in an insulin-resistant population; (3) muscle glycogen depletion during prolonged low-intensity exercise; (4) effect of a mixed meal on postprandial muscle and liver glycogen synthesis. The present review focuses on basic 13C NMR and gives results from selected studies.

The aim of the present review is to present techniques used for measuring blood flow in human subjects and advice as to when they may be applicable. Since blood flow is required to estimate substrate fluxes, energy turnover and metabolic rate of skeletal muscle, accurate measurements of blood flow are of extreme importance. Several techniques have therefore been developed to enable estimates to be made of the arterial inflow to, venous outflow from, or local blood flow within the muscle. Regional measurements have been performed using electromagnetic flow meters, plethysmography, indicator methods (e.g. thermodilution and indo-cyanine green dye infusion), ultrasound Doppler, and magnetic resonance velocity imaging. Local estimates have been made using 133Xe clearance, microdialysis, near i.r. spectroscopy, positron emission tomography and laser Doppler. In principle, the aim of the study, the type of interventions and the limitations of each technique determine which method may be most appropriate. Ultrasound Doppler and continuous indo-cyanine green dye infusion gives the most accurate limb blood flow measurements at rest. Moreover, the ultrasound Doppler is unique, as it does not demand a steady-state, and because its high temporal resolution allows detection of normal physiological variations as well as continuous measurements during transitional states such as at onset of and in recovery from exercise. During steady-state exercise thermodilution can be used in addition to indo-cyanine green dye infusion and ultrasound Doppler, where the latter is restricted to exercise modes with a fixed vessel position. Magnetic resonance velocity imaging may in addition be used to determine blood flow within deep single vessels. Positron emission tomography seems to be the most promising tool for local skeletal muscle blood-flow measurements in relation to metabolic activity, although the mode and intensity of exercise will be restricted by the apparatus design.

The role of intramyocellular triacylglycerol (IMTG) as a substrate source during exercise has recently regained much attention as a result of the proposed functional relationship between IMTG accumulation and the development of insulin resistance. It has been speculated that elevated NEFA delivery and/or impaired fatty acid (FA) oxidation result in intramyocellular accumulation of triacylglycerol and FA metabolites, which are likely to induce defects in the insulin signalling cascade, causing insulin resistance. The progressive accumulation of IMTG in sedentary patients and patients who are obese and/or have type 2 diabetes should therefore form a major therapeutic target, and efforts should be made to develop interventions that prevent excess IMTG accretion by stimulating their rate of oxidation. Although regular exercise is likely to represent such an effective means, there is much controversy about the actual contribution of the IMTG pool as a substrate source during exercise. The apparent discrepancy in the published literature might be explained by differences in the applied research protocol and the selected subject population, but most of all by the techniques that have been employed to estimate IMTG use during exercise. Data obtained in trained-endurance athletes indicate that athletes can substantially reduce their IMTG pool following a single exercise session. With the growing awareness that skeletal muscle has a tremendous potential to oxidise IMTG during prolonged moderate-intensity exercise, more research is warranted to develop combined exercise, nutritional and/or pharmacological interventions that can stimulate IMTG oxidation in sedentary patients and patients who are obese and/or have type 2 diabetes.

The aim of the present study was to investigate the effect of soya-bean protein on growth and muscle metabolism in fish. Cod, Gadus morhua, were fed on a fish-feed formula with the high-quality fish-meal protein being replaced by 100, 200 or 300 g soya-bean protein/kg fish-meal protein. The feeding experiment lasted for 43 d at a water temperature of 7–8º and a sea water salinity of 3.5%. At the 200 g/kg level of soya-bean protein, food intake and growth rate were similar to those of the controls. At the 300 g/kg level of soya-bean protein, food intake was diminished by 6 % and growth by 67 % relative to control levels. In muscle, sarcoplasmic protein (/g wet weight) was significantly decreased by 14%. Myofibrillar protein (/g wet weight) was unchanged. Levels of RNA in the myofibrillar fraction decreased at all three levels of soya-bean protein, and that of the sarcoplasmic fraction decreased at the highest level of legume-protein. With increased levels of soya-bean protein, RNA: DNA declined by 18% from 1.88 to 1.54. The contractile protein myosin heavy chain (/mg protein and /g wet weight) and myosin heavy chain-specific mRNA (/mg RNA) were not significantly affected by dietary conditions. Expressed per g wet weight, the decline by 21 % of the specific mRNA depended on the total RNA content which decreased with the increase in soya-bean protein. Acid proteinase activity was lowest at the 200 g/kg level, showing a decrease of 23%. Glycogen content fell by 46% at both the 200 and 300 g/kg level of dietary soya-bean protein. The results show that muscle metabolic functions responded to the dietary plant protein before there were any measurable changes in growth rate. From the results it is concluded that 200 g/kg or less of the high-quality fish-meal protein may be replaced by soya-bean protein in a fish-feed formula.