Peter J. Rogers

Effects of anorexie drugs on food intake, food selection and preferences and hunger motivation and subjective experiences

Two anorexic drugs with contrasting neurochemical profiles—amphetamine and fenfluramine—have been incorporated into the experimental technique of double dissociation to explore relationships between certain features of feeding behaviour and subjective experiences surrounding eating. In general, the drugs brought about opposing effects on behaviour and subjective states. A test meal revealed that amphetamine gave rise to a marked suppression of protein consumption whereas fenfluramine exerted a greater suppression on carbohydrate intake. Similar directional effects were observed on a food preference questionnaire, and it is noted that subjective preferences were a more sensitive indicator of the presence of an active drug than actual behaviour. The drug treatments exerted quite different effects on ratings of hunger, various subjective feelings and bodily sensations. Amphetamine brought about prominent effects on the appetitive phase of eating, whereas fenfluramine appeared to enhance or prolong the post-prandial state. The strength of the correlation between hunger and eating varied markedly between the conditions. Taken together these findings are in good agreement with the results of research on animals and they suggest that certain drugs may be useful as tools to explore processes influencing feeding activities.

Breakdown of dietary restraint following mere exposure to food stimuli: interrelationships between restraint, hunger, salivation and food intake

It was hypothesised that the hunger-enhancing effects of exposure to the sight and smell of palatable food would disinhibit eating in restrained eaters (self-reported dieters). In two experiments exposure to palatable food stimuli led to increases in motivational (hunger) ratings and salivation, and was followed by overeating in restrained subjects compared with the control condition (no food during exposure) and a condition in which nonpreferred food was presented during the exposure phase. The food intake of unrestrained subjects, on the other hand, was reduced following exposure to palatable food in the first experiment. This shows that breakdown of dietary restraint can be induced by food stimuli even when the food does not constitute a preload. Mere exposure to the sight and smell of palatable food is sufficient to precipitate loss of dieting motivation. The effects of exposure on hunger and salivation were, in general, unrelated to food intake or degree of dietary restraint. Therefore, changes in hunger do not appear to directly mediate increased food intake in dieters. Instead, it is tentatively suggested that anxiety resulting from exposure to liked food may play a role both in disinhibiting eating and suppressing salivation in restrained subjects.

Uncoupling sweet taste and calories: Comparison of the effects of glucose and three intense sweeteners on hunger and food intake

This study was carried out to disclose effects generated by the uncoupling of the sensory and energetic components of sweet solutions. A comparison was made between equi-sweet preloads of three intense sweeteners (saccharin, aspartame and acesulfame-K), a bulk sweetener (glucose) and a nonsweet water control. Measures were made of subjective ratings of motivation to eat, food preferences and energy intake in a test meal. The glucose load produced a consistent pattern of changes on all measures. The intense sweeteners tended to facilitate motivational ratings and food preference checklist responses, but marginally lowered intake in the test meal. The facilitative action is probably due to the stimulation of sensory receptors for sweetness by the high-intensity agents, while the effects on intake are most likely due to a ceiling effect imposed by methodological limitations of this particular design. The results of this study must be interpreted with reference to the prevailing experimental conditions, but they suggest that intense sweeteners can produce significant changes in appetite. Of the intense sweeteners, aspartame gave rise to the most pronounced effects.

Separating the actions of sweetness and calories: Effects of saccharin and carbohydrates on hunger and food intake in human subjects

A comparison was made of the effects on hunger and food intake of consuming preloads varying in sweetness and energy content. The preloads were a plain (unsweetened) yogurt, and the same yogurt sweetened to equal intensity with saccharin or glucose, or supplemented with starch. This balanced design made it possible to assess the consequences of adding sweetness to food as well as the consequences of substituting a nonnutritive sweetener for a caloric sweetener. Subjects (N = 24, repeated measures design) ate the preload at midday and returned one hour later for a sandwich lunch. Food intake in this meal was measured directly, and intake during the remaining part of the day was monitored using home recording in diaries. Hunger was assessed using subjective ratings of motivation to eat. Food intake at lunchtime was significantly greater following the saccharin compared with the plain preload, and parallel effects were revealed by the motivational ratings. Saccharin also stimulated further increases in intake after lunch. Food intake was lowest following the high-energy preloads, with the starch supplemented yogurt producing somewhat the largest suppression of intake. The results confirm and extend previous findings showing that intense sweeteners do not possess the same satiating capacity as glucose and sucrose. The stimulation of appetite by saccharin may be due to its sweet taste and also to effects on postingestive mechanisms.

Effect of anorexic drugs on food intake and the micro-structure of eating in human subjects

Human volunteer subjects of normal weight received oral doses of (+)amphetamine (10 mg) or (±)fenfluramine (30 mg and 60 mg) together with a placebo control according to a within-subjects design. The effects of these treatments were monitored by measuring food intake in a test meal, subjective ratings of hunger motivation and the micro-structure of eating behaviour abstracted from videotaped recordings of the test meal. Various measures of the rate of feeding were computed from these recordings. Amphetamine and fenfluramine (60 mg) showed generally similar effects on food intake and on the subjective experience of hunger, but displayed differing actions on the fine structure of eating. Amphetamine increased latency to initiation of eating and increased the rate of food ingestion, whilst fenfluramine slowed the local rate of eating and eliminated the characteristic decline in the rate of feeding across the course of a meal. These findings display certain resemblance to the results of animal experiments involving similar pharmacological manipulations and emphasise the importance of measuring rate of feeding in animal and human studies. The results of this study suggest that the micro-analysis of feeding behaviour not only provides a tool for understanding systems involved in the modulation of food consumption but also reveals information which may be helpful for the use of drugs in the treatment of obesity.

Meal patterns and food selection during the development of obesity in rats fed a cafeteria diet.

Offering preferred foods in addition to laboratory chow led immediately to a marked increase in both mean meal size (MMS) and meal frequency (MF). As body weight increased over a 5 months period, MF declined to a low level but MMS remained high. Within a majority of meals there was substantial consumption of only one food item. Nonetheless, when mixed meals were eaten these were usually larger than exclusive meals. With more than one preferred food available there was a significant tendency to alternate consumption of food types from one meal to the next. This disappeared at inter-meal intervals longer than 90 minutes. With one preferred food available, only MMS (and not MF) was increased and the degree of hyperphagia and obesity were reduced. The findings suggest the following conclusions: Both palatability (preference value for a particular food) and variety (availability of different types of food) have incremental, but distinguishable, effects on food consumption and mean parameters. Palatability mainly influences meal size, whereas variety exerts an effect on meal size and inter-meal interval. However, the potential effect of variety on overall intake is probably somewhat reduced by the tendency to eat only one type of food in each meal. Obesity has an inhibitory influence on feeding, operating primarily through a reduction in mean frequency.

Behavioural structure and mechanisms of anorexia: Calibration of natural and abnormal inhibition of eating

The study of experimentally induced anorexia poses a problem for investigations of the processes controlling food intake. Inhibition of food consumption may arise from a specific intervention in a physiological system controlling nutritional requirements or from non-specific changes leading to the suppression or contamination of behaviour. The present experiment used the analysis of the structure of behaviour to distinguish between normal anorexia (natural development of satiation) and pathological anorexia brought about by intestinal discomfort (injection of lithium chloride) or adulteration of food (quinine added to diet). The treatments produced marked changes in parameters of feeding and in the frequencies of behaviours associated with eating. Both lithium chloride and quinine treatments gave rise to a slow rate of eating accompanied by a disordered temporal sequence of eating, grooming and resting. This behavioural calibration of anorexia can contribute to the behavioural pharmacology of feeding by helping to diagnose drugs which facilitate normal processes of satiation and those which act via a non-specific disruption of behaviour.

Umami and appetite: effects of monosodium glutamate on hunger and food intake in human subjects.

Subjects consumed soup (beef consomme) preloads of a fixed size containing different concentrations of monosodium L-glutamate (MSG). Effects on appetite following these preloads, and when no soup was consumed, were assessed in 3 studies. The soups supplemented with MSG were rated as more pleasant, more savoury and more satisfying than soup with no added MSG. Compared with the no preload condition, consumption of the soups initially reduced appetitive motivational ratings and increased fullness ratings, but did not alter food intake in a test meal begun either 2 or 30 minutes later. This immediate inhibition of subjective motivation to eat was unaffected by MSG concentration. The failure of the soups to reduce subsequent food intake is presumably due to their low energy content (less than 10 kcal) and indicates that sensory stimulation alone is insufficient to reduce appetite. Indeed, the most important finding concerning MSG showed that motivation to eat recovered more rapidly following a lunchtime meal in which MSG-supplemented soup was served as the first course (compared both with the effect of unsupplemented soup and no preload). It is suggested tentatively that MSG through its stimulation of orosensory receptors and/or by improving the palatability of the soup may have influenced the metabolic disposal of nutrients consumed in the previous meal.

Why a palatability construct is needed

Palatability is a hypothetical construct which is needed to account for the hedonic aspects of the taste, smell, flavour, texture, etc. of food. Palatability is influenced by innate factors, but can also be modified by learning. Powerful preferences and aversions can be conditioned by the aftereffects of food ingestion. Despite the common assertion that palatability is enhanced in a state of relative food deprivation, there is evidence to suggest that hunger and palatability act largely independently to determine intake. Accordingly a distinction should be made between the pleasantness of the taste of food (influenced by palatability) and the pleasantness of ingesting that food (influenced by hunger/satiety). Increased palatability appears to be at least part of the explanation of why certain diets promote hyperphagia and obesity. However, the postingestive effects which contribute to the greater palatability of such diets remain to be identified.

Aspartame ingested without tasting inhibits hunger and food intake

The effects on motivation to eat and food intake of administering small amounts of aspartame (234 to 470 mg: lower dose equivalent to the amount of aspartame contained in 1-2 cans of some soft drinks) in capsules to human volunteers were examined in two separate experiments (the second was a replication of the first). The results provided clear evidence of a prominent postingestive inhibitory action of aspartame on appetite: consumed in capsules, aspartame reduced subsequent food intake and, to a lesser extent, motivation to eat. The mechanism underlying this effect has yet to be elucidated. A possibility is that the release of cholecystokinin by phenylalanine, a constituent of aspartame, is involved. A further result was that drinking aspartame-sweetened water did not reliably reduce motivational ratings or food intake (in the first experiment aspartame ingested in capsules significantly reduced food intake compared with the same amount ingested as a sweet drink). One interpretation of these together with previous findings is that the response to consuming aspartame is determined by at least two interacting influences: an inhibitory postingestive effect and a stimulatory effect of its sweet taste. In turn, the relative potency of these influences may be modified by certain other features of the aspartame-sweetened food or drink (e.g., its nutrient content). Another implication of these results is that it cannot be assumed that intense sweeteners will all have equivalent effects on appetite.