In order to better understand the factors that may influence and regulate the intake of the macronutrients carbohydrate, fat and protein a 7 d diet diary technique was employed to study eating behaviour in the natural environment of free-living human subjects. In general, factors that promote energy intake tend to promote fat and protein intake to a greater extent than carbohydrate intake. This increased intake occurs as the result of: environmental factors such as social facilitation and the time of day, week or lunar phase; subjective factors such as hunger and elation; individual difference factors such as obesity, restraint and ageing. There are indications that the intake of macronutrients is regulated by negative feedback systems. In the short term, the amount of protein remaining in the stomach at the onset of a meal appears to have a restraining effect on intake, especially protein intake. Over several days, macronutrient intake appears to be affected by a nutrient-specific delayed negative feedback. Protein intake during 1 d is negatively associated with protein intake 2 and 3 d later, while carbohydrate intake is negatively related to later carbohydrate intake, and fat intake to later fat intake; both peaking after a 2 d delay. Studies of the intakes of twins suggested that many aspects of the control of macronutrient intake are influenced by inheritance; these factors include the overall amounts ingested, the before-meal stomach contents and the responsiveness of the subject to the negative impact of the stomach contents. The results indicate that macronutrient intakes are regulated by multiple persistent processes that are to a large extent inherited.

In the present paper we deal with the problems of explaining and predicting diet selection of animals under controlled conditions, i.e. conditions that can be described and in which any influences of the environment can be either controlled or at least monitored. Diet selection is considered within an integrative framework of feeding behaviour that views both food intake and diet selection as an outcome of the animal's internal state and knowledge of the feeding environment. Three questions that arise from the framework are considered: (1) how do animals learn about foods available to them as a choice? (2) what changes in internal state affect diet selection? (3) how much time is needed for a change in the animal's internal state to be detected and for it to react to this change through a modification of its diet selection? It is proposed that animals have developed behavioural mechanisms that allow them to recognize foods on the basis of their nutritional as well as other properties. The rate at which animals learn about foods depends largely on the extent of the animal's deficiency and on the extent of the post-ingestive consequences induced by the foods. There is little evidence that animals modify their diet selection in response to short-term systemic fluctuation of their internal environment. On the other hand, long-term changes in the internal state of the animal lead to consequent long-term changes in diet selection. The time needed for a change in diet selection to be observed depends on the deviation created in the animal's internal state, either as a result of a physiological change or as a consequence of feeding. Thus, a more appropriate question to consider is not 'what time period matters to the animal?' but 'how much change or deviation in the internal state is the animal prepared to accept?'

It is now widely accepted that obesity develops by way of genetic mechanisms conferring specific dispositions which interact with strong environmental pressures. It is also accepted that certain dispositions constitute metabolic risk factors for weight gain. It is less well accepted that certain patterns of behaviour (arising from biological demands or environmental influences) put individuals at risk of developing a positive energy balance and weight gain (behavioural risk factors). Relevant patterns of behaviour include long-lasting habits for selecting and eating particular types of foods. Such habits define two distinct groups characterized as high-fat (HF) and low-fat (LF) phenotypes. These habits are important because of the attention given to dietary macronutrients in body-weight gain and the worldwide epidemic of obesity. Considerable evidence indicates that the total amount of dietary fat consumed remains the most potent food-related risk factor for weight gain. However, although habitual intake of a high-fat diet is a behavioural risk factor for obesity, it does not constitute a biological inevitability. A habitual low-fat diet does seem to protect against the development of obesity, but a high-fat diet does not guarantee that an individual will be obese. Although obesity is much more prevalent among HF than LF, some HF are lean with BMI well within the normal range. The concept of 'different routes to obesity' through a variety of nutritional scenarios can be envisaged, with predisposed individuals varying in their susceptibility to different dietary inputs. In a particular subgroup of individuals (young adult males) HF and LF displayed quite different profiles of appetite control, response to nutrient challenges and physiological measures, including BMR, RQ, heart rate, plasma leptin levels and thermogenic responses to fat and carbohydrate meals. These striking differences suggest that HF and LF can be used as a conceptual tool to investigate the relationship between biology and the environment (diet) in the control of body weight.

We have introduced a framework that enables the identification of the important elements in complex nutritional systems, and the quantification of the interactions among them. These interactions include those among the multiple constituents of the ingesta, as well as between behavioural (ingestive) and physiological (post-ingestive) components of nutritional homeostasis. The resulting descriptions provide a powerful means to generate and test hypotheses concerning the mechanisms, ecology and evolution of nutritional systems. We provide an overview of the key concepts involved in our scheme, and then introduce four examples in which the framework is used to develop and test hypotheses. In the first example we use comparative methods based on a data set of 117 insect species to test a prediction about the relationship between evolving an association with bacterial endosymbionts and the composition of the optimal diet. Second, using two species of locusts (a grass specialist and a generalist), we consider the relationship between an animal's diet breadth and the decision rules employed when feeding on foods containing suboptimal protein: carbohydrate values. Third, we introduce a mathematical model that predicts the dose-response properties of gustatory systems in the context of nutritional homeostasis. Finally, we consider the interaction between tannic acid and macronutrient balance in the diet of locusts.

Effects of extreme environments on food intake in human subjects are analysed as behavioural and physiological adaptations to annual and circadian rhythms, temperature and altitude. Effects of the environment on food intake through food availability have direct consequences on energy balance and body weight. Different geographical regions show variations in the composition of dietary foods, i.e. the relative proportions of carbohydrate, protein and fat. In developing countries the annual cycle appears to affect body weight through dependence on food availability. In West-European countries this effect appears to depend on physical activity. Energy and macronutrient intakes appear to follow a circadian pattern, with breakfast being relatively high in carbohydrate and dinner being relatively high in fat. In cold conditions, maintaining an adequate food intake is important in sustaining normal physiological responses to cold. Evidence for a possible cold-induced increase in appetite is poor. A condition influencing level of intake is the palatability of the food. High altitude, i.e. hypobaric hypoxia, appears to reduce appetite, energy intake and body mass, irrespective of acute mountain sickness (AMS). Meal size is reduced and meal frequency increased. Under circumstances of AMS, dissociation between appetite and hunger occurs. Thus, spontaneous adaptation to extreme environments requiring increased energy intake occurs first by adaptation of body weight to a new energy balance. In general, prevention of a negative energy balance occurs by learning with respect to food intake. After return to normal, adjustment of energy intake to the original energy balance occurs with restoration of body weight.

Large herbivores play a major role in shaping vegetation community dynamics through selective consumption of particular plants and plant communities. An understanding of the factors influencing diet selection at the level of individual bites (‘bite scale’) is important for prediction of the impact of herbivores on vegetation at the habitat scale. Bite-scale diet selection represents an integration of the twin goals of maximizing nutrient intake and minimizing toxin intake. Recent research with ruminants in pen-fed situations has shown that animals are able to make choices between artificial foods that maximize growth and other production variables. The role of post-ingestive feedback as an important mechanism for allowing animals to assess the nutritional quality of particular foods, and so select optimal diets, has been recognized in a number of recent experiments. Our understanding of the role of toxin intake minimization in diet selection decisions is more rudimentary. An important advance in the last decade has been the acknowledgement of the role of post-ingestive feedback and learning as a mechanism for avoidance of dietary toxicity. Further research is required to assess the importance of these processes in relation to free-grazing animals. The extent to which an understanding of bite-scale diet selection can be used to predict habitat utilization is not well understood. At the habitat scale additional factors such as predator avoidance, social constraints, avoidance of parasitism and microclimatic effects have an important influence on foraging decisions. Future research needs to focus on developing a quantitative understanding of such decisions at the habitat scale.

Food choice is influenced by a large number of factors, including social and cultural factors. One method for trying to understand the impact of these factors is through the study of attitudes. Research is described which utilizes social psychological attitude models of attitude-behaviour relationships, in particular the Theory of Planned Behaviour. This approach has shown good prediction of behaviour, but there are a number of possible extensions to this basic model which might improve its utility. One such extension is the inclusion of measures of moral concern, which have been found to be important both for the choice of genetically-modified foods and also for foods to be eaten by others. It has been found to be difficult to effect dietary change, and there are a number of insights from social psychology which might address this difficulty. One is the phenomenon of optimistic bias, where individuals believe themselves to be at less risk from various hazards than the average person. This effect has been demonstrated for nutritional risks, and this might lead individuals to take less note of health education messages. Another concern is that individuals do not always have clear-cut attitudes, but rather can be ambivalent about food and about healthy eating. It is important, therefore, to have measures for this ambivalence, and an understanding of how it might impact on behaviour.

Conditioned food aversion is a powerful experimental tool to modify animal diets. We have also investigated it as a potential management tool to prevent livestock from grazing poisonous plants such as tall larkspur (Delphinium barbeyi), white locoweed (Oxytropis sericea) and ponderosa pine (Pinus ponderosa) on western US rangelands. The following principles pertain to increasing the strength and longevity of aversions: mature animals retain aversions better than young animals; novelty of the plant is important, although aversions can be created to familiar plants; LiCl is the most effective emetic, and the optimum dose for cattle is 200 mg/kg body weight; averted animals should be grazed separately from non-averted animals to avoid the influence of social facilitation which can rapidly extinguish aversions. Social facilitation is the most important factor preventing widespread application of aversive conditioning. When averted animals see other animals eat the target food they will sample it, and if there is no adverse reaction they will continue eating and extinguish the aversion. However, if averted animals can be grazed separately, aversions will persist. Aversive conditioning may provide an effective management tool to prevent animals from eating palatable poisonous plants that cause major economic loss.

Protein-energy undernutrition, or the possibility of its development, has been documented to occur frequently in surgical patients admitted to hospital. Nutritional status is known to deteriorate over the course of the hospital stay, with poor awareness by medical and nursing staff as to the deleterious effects of impaired nutritional status on clinical outcome and hospital costs. While there is no consensus on the best method for assessment of the nutritional status of surgical patients pre-operatively, there are a number of techniques available. These techniques can be divided into two types, those suitable for screening for nutrition risk on admission to hospital and those used to fully assess nutritional status. Both techniques have their limitations, but if used correctly, and their limitations recognized, should identify the appropriate degree of nutritional intervention for an individual patient in a timely and cost-effective manner. The techniques currently available for nutritional screening and nutritional assessment are reviewed, and their applicability to the Irish setting are discussed in the present paper.

Immunonutrition generally refers to the effect of the provision of specific nutrients on the immune system. These nutrients typically have immunoenhancing properties, and recent advances in nutrition support involve studies designed to exploit the desirable biological properties of these nutrients. The term immunonutrition strictly implies that we are focusing on the effect of certain nutrients on aspects of the immune system. However, in reality immunonutrition also refers to studies that not only examine the function of lymphocytes and leucocytes, but which also study the influence of key nutrients on the acute-phase response, the inflammatory response and on gastrointestinal structure and function. The interest, therefore, is on the impact of immunonutrition on all aspects of host defence mechanisms in response to a catabolic stress. Major surgery evokes an acute-phase response, a transient immunosuppression and alterations in gastrointestinal function. Normal function is usually restored after a few days; however, in a subgroup of patients homeostasis may be lost and development of the systemic inflammatory response syndrome (SIRS) ensues. Results of recent clinical trials suggest that provision of immunomodulatory nutrients, including glutamine, arginine, n-3 polyunsaturated fatty acids and dietary nucleotides, may promote restoration of normal tissue function post-operatively and prevent the occurrence of SIRS.

Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) are being increasingly used for investigations of human muscle physiology. While MRI reveals the morphology of muscles in great detail (e.g. for the determination of muscle volumes), MRS provides information on the chemical composition of the tissue. Depending on the observed nucleus, MRS allows the monitoring of high-energy phosphates (31P MRS), glycogen (13C MRS), or intramyocellular lipids (1H MRS), to give only a few examples. The observation of intramyocellular lipids (IMCL) by means of 1H MRS is non-invasive and, therefore, can be repeated many times and with a high temporal resolution. MRS has the potential to replace the biopsy for the monitoring of IMCL levels; however, the biopsy still has the advantage that other methods such as those used in molecular biology can be applied to the sample. The present study describes variations in the IMCL levels (expressed in mmol/kg wet weight and ml/100 ml) in three different muscles before and after (0, 1, 2, and 5 d) marathon runs for a well-trained individual who followed two different recovery protocols varying mainly in the diet. It was shown that the repletion of IMCL levels is strongly dependent on the diet post exercise. The monitoring of IMCL levels by means of 1H MRS is extremely promising, but several methodological limitations and pitfalls need to be considered, and these are addressed in the present review.

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.

31P magnetic resonance spectroscopy (MRS) offers a unique non-invasive window on energy metabolism in skeletal muscle, with possibilities for longitudinal studies and of obtaining important bioenergetic data continuously and with sufficient time resolution during muscle exercise. The present paper provides an introductory overview of the current status of in vivo31P MRS of skeletal muscle, focusing on human applications, but with some illustrative examples from studies on transgenic mice. Topics which are described in the present paper are the information content of the 31P magnetic resonance spectrum of skeletal muscle, some practical issues in the performance of this MRS methodology, related muscle biochemistry and the validity of interpreting results in terms of biochemical processes, the possibility of investigating reaction kinetics in vivo and some indications for fibre-type heterogeneity as seen in spectra obtained during exercise.

The arterio-venous difference technique is now well established in the study of organ and tissue metabolism. This technique requires samples to be obtained of the arterial blood supplying and the venous drainage from a tissue, together with a measurement of the blood flow through the tissue. The technique is most appropriate when the arterial concentration and tissue metabolism of a substance are constant, and when the blood flow is stable. If these criteria are not satisfied, care is needed in the interpretation of the results obtained. It should be recognized that the arterio-venous difference technique only measures the net exchange of a substance with the tissue, and that tracers are needed if unidirectional flux needs to be estimated. The other factors which must be borne in mind when intending to use this technique are the transit times of blood and the substance of interest through a tissue, the volume of distribution of the substance in the tissue, and the possibility that the venous samples obtained are derived from a mixture of different tissues.

The metabolism of white adipose tissue is regulated by many factors, including hormones and substrates delivered in the blood, the activity of the autonomic nervous system and the rate of flow of blood through the tissue. An integrated view of adipose tissue metabolism can only be gained, therefore, from studies in vivo. Of the various techniques available for studying adipose tissue metabolism in vivo, the measurement of arterio-venous differences offers some unique possibilities. In human subjects this technique has been performed mostly by catheterization of the venous drainage of the subcutaneous abdominal depot. Studies using this technique indicate that adipose tissue has an active pattern of metabolism, responding rapidly to meal ingestion by suppressing the release of non-esterified fatty acids, or to exercise with an increase in fat mobilization. Adipose tissue blood flow may also change rapidly in these situations; for instance, it increases markedly after a meal, potentially increasing the delivery of triacylglycerol to the enzyme lipoprotein lipase (EC 3.1.1.34) for hydrolysis. During exercise, there is evidence that adipose tissue blood flow does not increase sufficiently to allow delivery of all the fatty acids released into the systemic circulation. The various adipose tissue depots have their own characteristic metabolic properties, although in human subjects these are difficult to study with the arterio-venous difference technique. A combination of tracer infusion with selective catheterization allows measurements of leg, splanchnic and non-splanchnic upper-body fat mobilization and triacylglycerol clearance. Development of such techniques may open up new possibilities in the future for obtaining an integrated picture of adipose tissue function and its depot-specific variations.

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.