The aim of the present article is to evaluate critically the various methods employed in studies designed to quantify precisely skeletal muscle substrate utilization during exercise. In general, the pattern of substrate utilization during exercise can be described well from O2 uptake measurements and the respiratory exchange ratio. However, if the aim is to quantify limb or muscle metabolism, invasive measurements have to be carried out, such as the determination of blood flow, arterio-venous (a-v) difference measurements for O2 and relevant substrates, and biopsies of the active muscle. As many substrates and metabolites may be both taken up and released by muscle at rest and during exercise, isotopes can be used to determine uptake and/or release and also fractional uptake can be accounted for. Furthermore, the use of isotopes opens up further possibilities for the estimation of oxidation rates of various substrates. There are several methodological concerns to be aware of when studying the metabolic response to exercise in human subjects. These concerns include: (1) the muscle mass involved in the exercise is largely unknown (bicycle or treadmill). Moreover, whether the muscle sample obtained from a limb muscle and the substrate and metabolite concentrations are representative can be a problem; (2) the placement of the venous catheter can be critical, and it should be secured so that the blood sample represents blood from the active muscle with a minimum of contamination from other muscles and tissues; (3) the use of net limb glycerol release to estimate lipolysis is probably not valid (triacylglycerol utilization by muscle), since glycerol can be metabolized in skeletal muscle; (4) the precision of blood-borne substrate concentrations during exercise measured by a-v difference is hampered since they become very small due to the high blood flow. Recommendations are given in order to obtain more quantitative and conclusive data in studies investigating the regulatory mechanisms for substrate choice by muscle.

Raised serum cholesterol does not adequately explain the increased risk of CHD within populations or the relationship between diet and CHD. Nevertheless, the principal transport vehicle of cholesterol in the circulation, LDL, must still be regarded as the most atherogenic lipoprotein species, but not because of its contribution to serum cholesterol. The atherogenic potential of LDL in the majority of individuals arises from an increase in the number of small dense LDL particles and not from its cholesterol content per se. There is now a wealth of evidence from cross-sectional and prospective studies to show that LDL particle size is significantly associated with CHD and predictive of increased coronary risk. Moreover, there are a number of credible mechanisms to link small dense LDL with the atherogenic process. The rate of influx of serum lipoproteins into the arterial wall is a function of particle size, and will thus be more rapid for small dense LDL. Components of the extracellular tissue matrix in the intima, most notably proteoglycans, selectively bind small dense LDL with high affinity, sequestering this lipoprotein in a pro-oxidative environment. The oxidation of LDL promotes the final deposition of cholesterol in the arterial wall, and numerous studies have shown small dense LDL to be more susceptible to oxidative modification than its larger and lighter counterparts. An increase in the number of small dense LDL particles may originate from a defect in the metabolism of triacylglycerol-rich lipoproteins. One mechanism may involve the overproduction and increased residence time of large triacylglycerol-rich VLDL in the postprandial phase, a situation thought to arise through pathways of insulin resistance.

Muscle tissue serves as a protein reservoir which is mobilized to meet the specific metabolic needs associated with various catabolic conditions in human subjects, such as trauma and critical illness. Glutathione is one of the most abundant short-chain peptides and a major source of non-protein thiol in the body, and tissue glutathione concentration is related to its oxidative capacity. Skeletal muscle is relatively unique with respect to a variety of metabolic properties, such as oxidative potential, patterns of amino acid utilization, and antioxidant enzyme activity. The glutathione concentration is not influenced by food intake, or by food deprivation. Moreover, there is no diurnal variation on muscle glutathione levels. Following elective surgery the muscle concentration of GSH (the reduced form) decreases by 40 % 24 h post-operatively, while the concentration of GSSG (the oxidized form) remains unaltered. During critical illness a similar decrease in the GSH concentration is seen, but in addition a change in the redox status indicative of an elevated GSSG level occurs. Furthermore, correlations between the concentrations of glutamine as well as glutamate and GSH exist in these patients. From available evidence accumulated it is clear that glutathione plays a pivotal role in the maintenance of the intracellular redox status, the antioxidant vitamin levels, and the antioxidant enzyme functions under various metabolic conditions. The effectiveness of glutathione protection in the individual tissue depends on the tissue concentration of glutathione as well as the capacity of the tissue to import GSH and to export GSSG. The mechanisms by which catabolism regulates tissue glutathione levels and the enzyme activities associated with the γ-glutamyl cycle are not completely understood and further studies need to be conducted.

Meat quality describes the attractiveness of meat to consumers. The present paper focuses on two major aspects of meat quality, tenderness and flavour. Both aspects of quality can be influenced by nutrition, principally through its effects on the amount and type of fat in meat. In several countries, high levels of intramuscular fat (marbling fat), i.e. above 30 g/kg muscle weight in longissimus, are deemed necessary for optimum tenderness, although poor relationships between fat content and tenderness have generally been found in European studies, where fat levels are often very low, e.g. below 10 g/kg in UK pigs. Muscle lipid may be a marker for red oxidative (type 1) muscle fibres which are found at higher concentrations in tender muscles and carcasses. Nutritional treatment can be used to manipulate the fatty acid content of muscle to improve nutritional balance, i.e. increase the polyunsaturated (PUFA) : saturated fatty acid value and reduce the n−6 : n−3 PUFA value. Increasing PUFA levels may also change flavour because of their greater susceptibility to oxidative breakdown and the generation of abnormal volatile compounds during cooking. This situation particularly applies to the n−3 PUFA which are the most unsaturated meat lipids. In pigs, a concentration of 3 mg α-linolenic acid (18 : 3)/100 mg in muscle and fat tissue fatty acids can easily be achieved by including whole linseed in the diet. This level has led to abnormal odours and flavours in some studies, but not in others. In cattle and sheep, feeding whole linseed raised 18 : 3 concentrations in muscle fatty acids from about 0.7 mg/100 mg to > 1 mg/100 mg. As with pigs, this diet also increased levels of long-chain n−3 PUFA formed from 18 : 3, including eicosapentaenoic acid (20 : 5). Although this increase led to greater oxidative breakdown of lipids during storage and the generation of large quantities of lipid-derived volatile compounds during cooking, there were no deleterious effects on odour or flavour. When 18 : 3 levels are raised in lamb and beef because of grass feeding, the intensity of the flavours increases in comparison with grain-fed animals which consume and deposit relatively more linoleic acid (18 : 2). In ruminants, very high levels of 18 : 2 produced by feeding protected oil supplements cause the cooked beef to be described as oily, bland or pork-like.

CHD is a multifactorial disease that is associated with non-modifiable risk factors, such as age, gender and genetic background, and with modifiable risk factors, including elevated total cholesterol and LDL-cholesterol levels. Lifestyle modification should be the primary treatment for lowering cholesterol values. The modifications recommended include dietary changes, regular aerobic exercise, and normalization of body weight. The recommended dietary changes include restriction in the amount of total fat, saturated fat and cholesterol together with an increase in the consumption of complex carbohydrate and dietary fibre, especially water-soluble fibre. However, nutrition scientists continue to question the value of these universal concepts and the public health benefits of low-fat diets, and an intense debate has been conducted in the literature on whether to focus on reduction of total fat or to aim efforts primarily towards reducing the consumption of saturated and trans fats. Moreover, it is well known that there is a striking variability between subjects in the response of serum cholesterol to diet. Multiple studies have examined the genediet interactions in the response of plasma lipid concentrations to changes in dietary fat and/or cholesterol. These studies have focused on candidate genes known to play key roles in lipoprotein metabolism. Among the gene loci examined, APOE has been the most studied, and the current evidence suggests that this locus might be responsible for some of the inter-individual variability in dietary response. Other loci, including APOA4, APOA1, APOB, APOC3, LPL and CETP have also been found to account for some of the variability in the fasting and fed states.

Diet and immunity have been known to be linked to each other for centuries. In the last 30 years systematic studies have confirmed that nutrient deficiencies impair immune response and lead to frequent severe infections resulting in increased mortality, especially in children. Protein-energy malnutrition results in reduced number and functions of T-cells, phagocytic cells and secretory immunoglobulin A antibody response. In addition, levels of many complement components are reduced. Similar findings have been reported for moderate deficiencies of individual nutrients such as trace minerals and vitamins, particularly Zn, Fe, Se, vitamins A, B6, C and E. For example, Zn deficiency is associated with profound impairment of cell-mediated immunity such as lymphocyte stimulation response, decreased CD4+:CD8+ cells, and decreased chemotaxis of phagocytes. In addition, the level of thymulin, which is a Zn-dependent hormone, is markedly decreased. The use of nutrient supplements, singly or in combination, stimulates immune response and may result in fewer infections, particularly in the elderly, low-birth-weight infants and malnourished critically-ill patients in hospitals. The interactions between nutrition and the immune system are of clinical, practical and public health importance.

Microdialysis has been used for 25 years to study brain function in vivo. Recently, it has been developed for investigations on peripheral tissues. A microdialysis catheter is an artificial blood vessel system which can be placed in the extracellular space of various tissues such as adipose tissue and skeletal muscle in order to examine these tissues in situ. Molecules are collected from the tissue by the device and their true interstitial concentration can be estimated. Metabolically-active molecules can be delivered to the interstitial space through the microdialysis probe and their action on the tissue can be investigated locally without producing generalized effects. It is also possible to study local tissue blood flow with microdialysis by adding a flow marker (usually ethanol) to the microdialysis solvent. The microdialysis technique is particularly useful for studies of small and water-soluble molecules. A number of important observations on the in vivo regulation of lipolysis, carbohydrate metabolism and blood flow in human skeletal muscle and adipose tissue have been made recently using microdialysis.

Developing countries have very diverse food consumption patterns and agricultural production systems. The proportion of meat in national diets varies from negligible in some countries in central Africa to 30–40% in some countries in Latin America and Mongolia. However, the demand for meat in developing countries is increasing rapidly (53%/year from 1982 to 1993), as the result of population growth and the trend for people to move to the cities. Growth rates in consumption are greatest in Asia, with China dominating the statistics, in view of the size of its population. Theoretically, livestock production can be increased to meet this demand, but the multiple roles of livestock in developing countries must be recognized if this is to be achieved in a sustainable manner. Resource-poor farmers who keep livestock may value more highly their contribution to livelihoods and to crop production, through provision of draught power and improvements in soil fertility through the recycling of manure, than the production of more meat. Recognition of the goals of the farmer and the wishes of the consumer regarding meat quality need to be reflected in the way in which opportunities for increasing meat production are identified and communicated to farmers. The impact of the global economy on cereal prices, for example, will also influence which interventions will be economically viable. Interpretation of information in an integral manner, using geographical information systems, mathematical models and/or simple spreadsheet models will be an important ingredient in turning scientific knowledge into increased meat production in developing countries.

Quantitative dietary guidelines for fats were first issued in 1977 in the USA and these guidelines have changed little since then. In the UK only 14 % of the population achieve the dietary goal for fat (33 % energy) and only 3 % achieve the target (10 % energy) from saturated fatty acids. Analysis of the Dietary and Nutritional Survey of British Adults reveals that across quartiles of decreasing total fat intake, the actual fatty acid composition of this fat does not alter; i.e. when total fat is lowered, all fatty acid categories are equally lowered. This arises because 85 % of total fat and of each of the categories of fatty acids are provided by just five foods (milk, meat, cereals, spreads and vegetables). When total fat in the UK is lowered, the intake of polyunsaturated fatty acids is lowered. The problem is that if the intake of polyunsaturated fatty acids falls below a threshold of about 5 % energy, the cholesterol-raising properties of certain saturated fatty acids, e.g. myristic acid, are greatly augmented. In order to alter the balance of dietary fatty acids, more data are needed on food choices of those in the population achieving particular targets. These targets cannot be based on existing dietary goals, since so few people attain them. A new set of‘interim attainable dietary guidelines’, based on prevailing dietary intakes, must be the basis for establishing sensible food-based dietary guidelines.

Techniques in human skeletal muscle research are by necessity predominantly 'descriptive'.Microdialysis has raised high expectations that it could meet the demand for a method that allows 'mechanistic' investigations to be performed in human skeletal muscle. In the present review, some views are given on how well the initial expectations on the use of the microdialysis technique in skeletal muscle have been fulfilled, and the areas in which additional work is needed in order to validate microdialysis as an important metabolic technique in this tissue. The microdialysis catheter has been equated to an artificial blood vessel, which is introduced into the tissue. By means of this 'vessel' the concentrations of compounds in the interstitial space can be monitored. The concentration of substances in the collected samples is dependent on the rate of perfusate flow. When perfusate flow is slow enough to allow complete equilibration between interstitial and perfusate fluids, the concentration in the perfusate is maximal and identical to the interstitial concentration. Microdialysis data may be influenced by changes in blood flow, especially in instances where the tissue diffusivity limits the recovery in vivo, i.e. when recovery in vitro is 100 %, whereas the recovery in vivo is less than 100 %. Microdialysis data indicate that a significant arterial-interstitial glucose concentration gradient exists in skeletal muscle but not in adipose tissue at rest. While the concentrations of glucose and lactate in the dialysate from skeletal muscle are close to the expected values, the glycerol values obtained for muscle are still puzzling. Ethanol added to the perfusate will be cleared by the tissue at a rate that is determined by the nutritive blood flow (the microdialysis ethanol technique). It is concluded that microdialysis of skeletal muscle has become an important technique for mechanistic studies in human metabolism and nutrition.

The projected stagnation in the catch from global fisheries and the continuing expansion of aquaculture is considered against the background that fishmeal and fish oil are major feed stocks for farmed salmon and trout, and also for marine fish. The dietary requirement of these farmed fish for high-quality protein, rich in essential amino acids, can be met by sources other than fishmeal. However, the highly-polyunsaturated fatty acids eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3) present in high concentrations in fish oil are essential dietary constituents for marine fish and highly-desirable dietary constituents for salmonids. Currently, there is no feasible alternative source to fish oil for these nutrients in fish feeds. Vegetable oils rich in linoleic acid (18:2n-6) can partially substitute for 20:5n-3 and 22:6n-3 in salmonid and marinefish feeds. However, this is nutritionally undesirable for human nutrition because the healthpromoting effects of fish-derived 20:5n-3 and 22:6n-3 reflect a very high intake of 18:2n-6 relative to linolenic acid (18:3n-3) in Western diets. If partial replacement of fish oils in fish feeds with vegetable oils becomes necessary in future, it is argued that 18:3n-3-rich oils, such as linseed oil, are the oils of choice because they are much more acceptable lrom a human nutritional perspective, especially given the innate ability of freshwater fish, including salmonids, to convert dietary 18:3n-3 to 20:5n-3 and 22:6n-3. In the meantime, a more judicious use of increasinglyexpensive fish oil in aquaculture is recommended. High priorities in the future development of aquaculture are considered to be genetic improvement of farmed fish stocks with enhanced abilities to convert C18 to C20 and C22n-3 polyunsaturated fatty acids, enhanced development of primary production of 20:5n-3 and 22:6n-3 by single-cell marine organisms, and continuing development of new species.

Immune function declines with age, leading to increased infection and cancer rates in aged individuals. In fact, recent progress in the study of immune ageing has introduced the idea that rather than a general decline in the functions of the immune system with age, immune ageing is mainly characterized by a progressive appearance of immune dysregulation throughout life. Changes appear earlier in life for cell-mediated immunity than for humoral immunity. Thus, agerelated modifications in cell-mediated immunity, i.e. changes in naive: memory T-cells, mature: immature T-cells, T-helper 1: T-helper 2 cells are more important in the elderly than changes in humoral immunity, i.e. CD5: CD5+ cells or length of antibody responses. Such evolution of the immune system has been linked to declining thymus function and to accumulative antigenic influence over the lifespan. In contrast, innate immunity (macrophage functions) is preserved or even increased during the ageing process. This finding shows that the ‘primitive’ immune system is less affected by the ageing process than the sophisticated specific immune system. The present review focuses on innate and cell-mediated immune changes with ageing. It provides evidence that primary changes (intrinsic modifications in the immune system) and secondary changes (resulting from environmental influences during the lifespan) exert different influences on the immune system. Primary changes, occurring in healthy individuals, seem less important nowadays than they were considered to be previously. For example, interleukin 2 secretion in some very healthy aged individuals is comparable with that in younger adults. Primary immune changes may not explain the increased incidence and severity of infections observed in the elderly population. Secondary immunological changes are far more frequent and are certainly responsible for most of the immune modifications observed in the elderly population. Environmental factors leading to secondary immune dysfunctions include not only antigenic influence, which is a reflection of diseases experienced over the lifespan, but also many other factors such as drug intake, physical activity and diet; factors for which important changes occur in the elderly population. Nutritional factors play a major role in the immune responses of aged individuals and the present review shows that nutritional influences on immune responses are of great consequence in aged individuals, even in the very healthy elderly.

A public-health approach considers the relevance of nutritional research in the prevention and management of obesity. Well-defined and internationally-agreed definitions based on BMI allow an assessment of the worldwide prevalence of overweight and obesity. There are about 250 million obese adults in the world, and many more overweight. Obesity is emerging in the Third World, first in urban middle-aged women. With economic developments, obesity then occurs in men and younger women. In the West childhood obesity is rapidly emerging, with concern that early-onset obesity is especially hazardous. In Asians the risks of excess visceral fat occur at lower body weights than in Caucasians. The propensity to visceral obesity in Asians may relate to malnourished mothers and low birth weight. The International Obesity Task Force is considering many issues, including the health economics of obesity. It has developed a strategy to define childhood obesity, which in children over 6 years is likely to predict long-term weight and health problems. While the search for genetic markers of obesity continues, with particular interest in the leptin gene, it is clear that societal change, with the decline in physical activity and the passive overconsumption of high-fat diets are major contributors to the global increase in obesity. The public-health aspects of obesity research are therefore challenging.

CHD is a major cause of morbidity and mortality in women. The incidence of CHD in premenopausal women is low but increases substantially after the menopause, and this difference suggests that endogenous oestrogens are cardioprotective. Observational prospective studies have consistently shown that exogenous oestrogens also lower CHD risk. The biological mechanisms by which endogenous and exogenous oestrogens exert their protective effect are multifactorial, affecting lipids, carbohydrate metabolism, body fat distribution and blood pressure. The prevention of CHD with oestrogen therapy is therefore aimed both at correction of the traditional risk factors and at direct control of vessel structure and function. The wide international variation in rates of CHD together with the lower mortality in sub-groups of the population suggests that a considerable proportion of CHD may be prevented by dietary modification. Since phyto-oestrogens are structually similar to oestrogen, they have the potential to mimic its effects in vivo. The hypocholesterolaemic effects of soyabean protein (rich in phyto-oestrogen precursors) are well established, but the underlying mechanism and atherogenic potential of these changes are unknown. One isoflavone, genistein, has been shown in vitro to exert effects which may slow the development of atherosclerotic disease. However, further studies are required to determine the dose-related changes induced by phyto-oestrogens on serum lipoproteins, haemostasis and vascular function.

The incidence of infectious diseases, particularly respiratory diseases, increases with age. Age-associated decline in immune function contributes to the increased susceptibility of the aged to infections. Vitamin E supplementation has been shown to improve some aspects of immune function in aged animals and human subjects. The protective effect of vitamin E against viral or bacterial infections in experimentally-challenged young animals has been reported. We investigated the effects of supplementation with vitamin E and other antioxidants on resistance to influenza infection in young and old animals. While vitamin E-supplemented young mice showed only a modest reduction in lung viral titre, vitamin E-supplemented old mice exhibited a highly significant (P< 0.05) reduction in viral lung titre. In subsequent studies, we focused on the mechanism of vitamin E-induced reduction of influenza viral titre. The results of these studies as well as those reported by other investigators on the relationship between vitamin E and infectious diseases will be reviewed.

In the study of the regulation of skeletal muscle blood flow during exercise it is useful to obtain information regarding the concentrations of vasoactive substances in the muscle interstitium, a site where the compounds act on the vascular and skeletal muscle cells. The microdialysis technique is a useful tool for measuring interstitial substances in the muscle at rest and during exercise in human subjects, and the technique can also be used to study the effect of both systemic and local interventions in a specific area of an exercising muscle. Probe recovery, which represents the relative amount of a substance that is diffusing to the dialysis membrane, changes from rest to exercise and can be determined by the internal-standard technique which allows for a relatively high time resolution (min). Furthermore, the use of electrodes at the microdialysis outlet makes it possible to perform continuous measurements of interstitial substances. The present review gives examples of how the microdialysis technique has been applied to study potentially important vasodilators such as adenosine, NO and K+ in human skeletal muscles and highlights areas for future research to establish the functional importance of these compounds.

There is a growing body of scientific evidence which demonstrates that plasma triacylglycerol (TAG) concentration, especially in the postprandial state, is an important risk factor in relation to the development of CHD. Postprandial hypertriacylglycerolaemia is associated with a number of adverse metabolic risk factors, including the preponderance of small dense LDL, low HDL-cholesterol concentrations and elevated factor VII activity. Traditionally, a low-fat high-carbohydrate diet was used to prevent CHD because it effectively reduces plasma cholesterol concentrations, but this dietary regimen increases plasma TAG concentrations and reduces HDL-cholesterol concentrations. There is substantial epidemiological evidence which demonstrates that high plasma TAG and low plasma HDL concentrations are associated with an increased risk of CHD. Thus, there is reason for concern that the adverse effects of low-fat high-carbohydrate diets on TAG and HDL may counteract or negate the beneficial effect of reducing LDL-cholesterol concentrations. Although there have been no prospective studies to investigate whether reduced fat intake has an adverse effect on CHD, there is strong epidemiological evidence that reducing total fat intake is not protective against CHD. On the other hand, high-fat diets predispose to obesity, and central obesity adversely affects TAG metabolism. There is substantial evidence that in free-living situations low-fat high-carbohydrate diets lead to weight loss, which in turn will correct insulin resistance and plasma TAG metabolism. Clearly there is a need for prospective studies to resolve the issue as to whether low-fat high-carbohydrate diets play an adverse or beneficial role in relation to the development of CHD.

The present article is a review written at a level suitable for students and new workers to the field of techniques in common current use for the measurement of static and dynamic features of metabolism, especially nutritional metabolism. It covers the nature of radioactive and stable-isotope tracers, the means of measuring them, and the advantages and disadvantages of their use. The greater part of the review deals with methods for the measurement of pool sizes and metabolic processes, with the emphasis being on protein metabolism, a field the author knows best. The examples given are from a variety of sources, including the work of the author, but the principles underlying the techniques are universally applicable to all metabolic investigations using tracers.

The association between viral disease and nutrition has long been thought to be due to effects on the host immune system. This theory suggests that when a host is malnourished, the immune system is compromised, and thus increased susceptibility to viral infection will occur. However, the virus itself may also be affected by the nutritional status of the host. We have demonstrated that a normally-benign strain of coxsackievirus B3 (CVB3/0) becomes virulent in either Se-deficient or vitamin E-deficient mice. Although the deficient animals are immunosuppressed, the virus itself is also altered. Six nucleotide changes were found in the virus that replicated in the deficient mice, and once these mutations occurred, even mice with normal nutrition became susceptible to disease. Thus, the nutritional status of the host was able to transform an avirulent virus into a virulent one due to genomic changes in the virus. We believe that a common mechanism of oxidative stress is the underlying cause of the genetic changes. Both vitamin E and Se act as antioxidants, and benign virus inoculated into GSH peroxidase (EC 1.11.1.9)-knockout mice will also convert to virulence due to genomic changes. Our work points to the importance of host nutrition during a viral disease, not only from the perspective of the host, but from the perspective of the viral pathogen as well.