Simultaneous Measurement Of Energy Expenditure And Protein Turnover In Pigs

Energy and protein metabolism are closely linked and substantially influence each other. Therefore, the combined measurement of protein and energy metabolism is a logical approach. There are several methods available for measurement of energy expenditure and of protein turnover that usually can be applied simultaneously without affecting either measurement. For energy expenditure, indirect calorimetry is the most accurate and most commonly applied method, but other methods like the doubly labelled water technique or the bicarbonate recovery method have also been applied in conjunction with measurement of protein turnover. For determination of protein turnover, isotope dilution methods are generally used. The precursor method relies on the measurement of the rate of appearance of an amino acid to be metabolized, while the end product method measures the amount of tracer appearing in urea and/or ammonia. The flooding dose technique depends on the ratio of the isotope enrichment in tissues to that in plasma. Each of these methods has distinct advantages and disadvantages that include: the accuracy and precision of results, complexity and necessary skills for application, cost, and applicability in laboratory or field. Thus, the choice of method for energy and protein metabolic measurements depends firstly on the research questions, and secondly on the equipment, expertise and funds available. The preferred method for measurement of energy expenditure is indirect calorimetry, which should be combined with the end product method for determination of protein turnover. The best tracer for measurement of protein turnover is carboxyl-C labelled leucine, unless for the indicator amino acid oxidation technique, for which carboxyl-C labelled phenylalanine is preferred. If indirect calorimetry cannot be performed, the doubly labelled water technique is recommended. If determination of CO2 production is not possible, the precursor method with N-15 labelled amino acids, usually glycine, is a viable alternative. However, different methods to determine energy expenditure or different tracers should be considered for research questions that make the recommended approach impractical or inappropriate. The combined measurement of protein turnover and energy expenditure is especially useful for studies in humans and large mammals where the recruitment of subjects may be difficult. The main applications have been determination of energy cost of protein synthesis under a variety of conditions. The research objectives were mainly the study of the effects of health and disease on protein and energy metabolism, assessment of intervention strategies and maximisation of production efficiency in animals. Because of the close linkage of protein and energy metabolism, their simultaneous study can provide additional insights either method alone cannot offer.