Lecture 10.pdf

Conclusion • Food taboos occur in all cultures • They offer a very powerful means of controlling what we may and may not eat • A good (but not the only) explanation of the origin of food taboos is that they develop as a consequence of ecological forces operating within a society • We will return to...

Conclusion • Food taboos occur in all cultures • They offer a very powerful means of controlling what we may and may not eat • A good (but not the only) explanation of the origin of food taboos is that they develop as a consequence of ecological forces operating within a society • We will return to the social impacts of our dietary choices later in the course, as they have a major bearing upon the study of obesity… 33 33 Starting and stopping eating 1 Based upon Logue Ch.2 Appetite: The psychology of eating and drinking 34 34 Introduction • Why study what controls eating? – Encouraging healthy eating (i.e., consuming just enough) – Management of obesity • Assisting better approaches to dieting • Understanding what leads to overconsumption – Management of eating disorders • Increasing food intake – The food industry • Keen to understand what makes us consume more – The pharmaceutical industry • Keen to understand the neurobiology of food intake regulation so it can develop drugs to target obesity 35 35 Hunger • Three key concepts are hunger, satiety and satiation • Hunger has two scientific meanings – (1) The subjective desire to eat • This is primarily a psychological construct with some basis in physiology • It has varied meanings that differ within and between people – Stomach sensation of emptiness or related feelings – Bodily weakness, headache – in fact just about any sensation you care to name will be identified by someone as indicative of their hunger state – Desire to eat something ‘tasty’ (sometimes called hedonic hunger) – (2) The objective state of the body when nutrient depleted • There may be multiple signals for this as we will see 36 36 Satiation and Satiety • Satiation is our loss of desire for food that occurs during an eating bout – and it has two meanings too – It has a subjective meaning • Reduction in pleasure from eating • Increasing feeling of stomach fullness – And an objective meaning • This is reflected in multiple neural and hormonal signals that signal the intake of nutrients • Satiety refers to the state after a meal – This also has two meaning - the absence of a desire to eat (i.e., no hunger) and a physiological state reflecting the on-going digestion of nutrients 37 37 Measuring hunger and satiety • Asking people whether they are hungry can be a rather poor predictor of how much they are likely to eat – People may confuse arousal/anxiety/boredom for hunger – There are individual differences in sensitivity to hunger – While we believe that we eat when we are hungry, there is often only a weak correlation between eating and hunger • Consequently most experimental studies of humans rely upon more objective measures – Amount consumed – Eating rate – Food types selected • In studies involving animals we can also measure the animals subjective hunger as well as using more objective measures such as those above 38 38 What controls food intake? • The amount we eat could be hunger driven or satiety driven • If meals are hunger driven, then the longer you are without food, the more you should eat at the next meal • If meals are satiety driven, then the amount you ate at the last meal should predict what you will eat at the next • Time blinded humans are satiety driven • Caves study (no temporal cues at all) • Suggests reliance on the way we feel (bigger meal – longer interval till next) • When time cues are available we are hunger driven • Suggests a significant cognitive component, that is an awareness of time elapsed since the last meal and the time the next meal is due • Both contribute to intake control, but hunger driven eating may dominate 39 39 Short vs long-term intake • We also have to consider a further issue – Short-term energy needs (i.e., a meal) – Long-term energy needs (i.e., maintaining our body weight) • This is especially pertinent to small mammals – A mouse has around 10% body fat, enough to sustain it for around 2 days without food • If it stores more fat, then it becomes less agile and is easier to catch • If it stores too little fat, it becomes vulnerable to food shortages • Thus the mouse has to carefully manage short-term (immediate) and long-term needs – too fat it gets eaten, too thin it starves – The same is also true for humans and by and large we are fairly good at this - but not perfect… • We might expect that different mechanisms control long-term and short-term energy needs and as we will see they do 40 40 So what controls food intake? • The control of eating, as you will see, is very complex • Thirst, which only has one key stimulus - water - still utilises multiple mechanisms and food is a far more complex stimulus • Do not expect any grand theory - there is not one • However, there are three important and recurring themes, which I will return to later, but look out for them as we go on – Biological controls of appetite (machine-like; may be controllable?) – Psychological controls of appetite • No free will - environmental factors that promote intake unconsciously • Free will – the ability to exercise conscious control over food intake • To organise this material, I have broadly followed Logue… – Central controls (CNS factors) – more biological focus – Peripheral/environmental controls – more psychological focus 41 41 Peripheral factors – Contractions I • This is a good place to start both historically and because for many people a rumbling stomach is a sure sign that you are hungry • The basic rationale here is that an empty stomach produces contractions which then cause hunger (which prompts eating) • Filling the stomach stops the contractions and thus eliminates the desire to eat • So do stomach contractions cause hunger? – Washburn had a tube and balloon put in his stomach and then partly inflated to measure stomach contractions – His reports of hunger peaked at the height of a stomach contraction – Similar findings were obtained in other people – As there were no contractions when participants were not hungry and because the contractions started before people reported feeling hungry, they concluded that stomach contractions caused hunger 42 42 The equipment 43 43 Peripheral factors – Contractions II • How does Cannon & Washburn’s (1912) theory stand up today? • Well people without a stomach can and do feel hungry • People who have had gastric banding (for morbid obesity) and who also have a ‘tiny’ stomach also feel hungry • More sensitive and less invasive measurement techniques reveal only a weak relationship between stomach contractions and hunger – Some people seem able to detect stomach contractions (which do occur in an empty stomach) while others can not • Stomach contractions are a signal that the stomach is empty, similar to a dry mouth and thirst 44 44 Peripheral factors – Cues I • Environmental cues associated with food can trigger hunger • These can be sounds (e.g., cooking), smells (e.g., wafting from a bakery) or sights (e.g., pictures in a cook book, a logo) • Indeed we are almost constantly exposed to cues to food (e.g., adverts, food-related symbols, kiosks, cafes, vending machines etc etc) • If they do influence our behaviour then this may be very important in triggering (over) eating 45 45 Peripheral factors – Cues II • How might such cues work? – Probably via associative learning between the cue and prior episodes of ingestion (i.e., to pleasure and/or energy) – This is directly analogous to Pavlov’s famous classical conditioning experiments with dogs – A bell occurred prior to the delivery of food and this came to elicit an anticipatory response • Salivation (and hunger) • Insulin release, lower blood sugar (and hunger) • The real problem is in establishing at what level environmental food cues actually affect our behaviour – Are we aware that they can make us hungry? – Are we aware that this hunger then drives eating behaviour? 46 46 Peripheral factors – Portion size • The amount of food on your plate or in your serve, will significantly influence how much you eat • Portion size influences the amount you eat unconsciously and having a slightly smaller plate at home is a simple step to reducing intake • The same effect can also be observed with larger packets of food, which lead to larger serves • Portion sizes have relentlessly increased in recent years (see accompanying graph) 47 47 Peripheral factors - Variety • The greater the choice of foods available, the more one will typically eat • There are some obvious and not so obvious reasons for this – An important one that we encountered in a previous lecture and that we will come across again today is sensory specific satiety • It is easy to get sated on a single food, but hard with multiple foods – More choice makes it more likely that your favourite food will be represented – More choice is often associated with other factors that are known to promote intake (lots of people, excitement, alcohol) 48 48 Peripheral factors - Accessibility • Making food more accessible increases consumption – Within a room, the greater the distance between you and a plate of snack food, the less you will eat – Moving foods in a cafeteria, so that ice creams are equally visible, but more distant from the serving line, decreases their consumption – In salad bar displays, items at the edge are eaten more frequently than items in the middle or back – Fewer food items are eaten if they have to be picked up by tongs rather than with a spoon • In a nutshell we are lazy, and we will readily eat more if no extra effort is needed to obtain that food 49 49 Peripheral factors – Time I • Time as a cue to eating – People and animals can learn to expect food after a particular delay, so the passage of time can become a cue to the need to eat – Perhaps more importantly, we all have access to clocks, and we have learnt as children to expect meals/snacks to occur at rather specific times in the day – These ‘eating times’ are to some extent social constructs • Meal times have changed historically, driven by artificial light and work requirements • Hunter gatherers exhibit a range of eating behaviours from large single meals (on a given day with no further intake) to grazing (smalls amounts of food as it is found) • Rats (in the lab) can also readily adapt to all sorts of food presentation patterns – Clock regulated meals/snacks may have its pros and cons • CON: Eating when not hungry because it is a ‘meal time’ • PRO: May be hard to track food intake if eating is irregular 50 50 Peripheral factors – Time II • Evidence for time-related control of food intake – Multiple diary studies indicate that meal sizes increase and intermeal intervals (250 mins vs 150 mins) decrease across the course of a day in American participants – – – – Morning 400 Kcal/meal Afternoon 600 Kcal/meal Evening 700 Kcal/meal This corresponds to what all of us may recognise as a smallish breakfast, somewhat larger lunch, and a larger evening meal – Jet lag - hungry at inappropriate times, not-hungry at appropriate times (interaction of habit and circadian rhythms) – Night eating syndrome • Product of a phase delay in a persons circadian rhythm • Consume 25% of calories after evening meal, with night-time waking, hunger and eating • Very common in obese individuals and heritable too 51 51 Peripheral factors - Temperature • Ambient temperature and temperature regulation have been suggested to affect appetite • Body temperature drops, you become hungry, eat and dietary thermogenesis (i.e. metabolising the food) boosts body temperature reducing hunger • Evidence? – Food shifts more quickly from the stomach to the gut when you are cold – People in cold climates eat more calories (as do animals kept in cooled environments) – Hotter ambient temperatures reduce intake – Lower blood glucose (which induces hunger) also results in lower body temperature (and hunger) 52 52 Peripheral factors - Season • Do people eat more when winter approaches? • We might expect that either the drop in ambient temperature or shortening days might prompt increased ingestion as a way to prepare for winter • At least in the past winter would have been a time for high energy needs but low food availability • In long-term studies of food-intake in American’s the amount of food consumed consistently increases in the fall 53 53 Peripheral factors – People I • The most consistently powerful peripheral factors identified in promoting food intake is the number of people present when eating • Essentially the finding is that the more people there are the more food gets consumed per person 54 54 Peripheral factors – People II • Social facilitation effects are most pronounced for family members, then friends, but also work for strangers • They are no gender effects (overall) but… • When a man and a woman eat together, the woman will tend to eat more than she would alone, whilst the man eats a similar amount as normal • This effect may be moderated by romantic context • They occur for any type of meal or snack • They occur in many other organisms as well as people • In humans, they appear to be primarily the result of – – – – The greater amount of food available at social events Longer time spent at the table, so more nibbling Distraction, resulting in eating more, because of chatting Disinhibition (having a good time) 55 55 Peripheral factors - Distraction • Television viewing (TV) has several effects on food intake • Many people eat while watching TV – This can increase food intake via many of the same mechanisms as for social facilitation – The effects on food intake are most pronounced if the TV show is moderately distracting • Just sitting watching TV can trigger eating • This can occur via associative learning (i.e., from prior eating with TV), from adverts, or from show content (cookery shows etc) • Eating with TV can also have delayed effects – If you eat say a snack with TV, and later have lunch, you will eat more at lunch than if you had eaten that snack without TV – TV distracts you and makes it harder to remember what you have eaten 56 56 Peripheral factors – Mouth I • A further factor in peripheral control of eating is the mouth – The role of the mouth (i.e., sensory factors) in controlling intake has been extensively investigated in animals – The basic idea is to see how much the animal will eat if food intake is just controlled by the mouth – To test this a hole is made in the oesophagus so that food comes out at this point, a procedure called Sham Feeding • What happens when the animal is allowed to eat? – They eat far more than normal, but do eventually stop – How might the mouth affect eating? – The principal mechanism here is one we have already examined – sensory specific satiety 57 57 Peripheral factors – Mouth II • What about the taste of the food? – Sweet foods are consumed in greater quantities than non-sweet foods, even when matched for calories • Insulin mechanism (insulin secreted when sweet taste is sampled, lowers blood sugar and increases hunger) • Differential storage (body assumes sweet = higher calories, so more is eaten because the body can store the surplus calories for a ‘rainy day’) 58 58 Peripheral factors - Guts • What about the stomach and the intestine? • Their respective roles (especially in stopping eating) have also been explored using oesophagotmised animals, typically dogs or rats • They employ a technique called intragastric feeding, in which food is placed directly in the stomach or the gut • So what effect does this have on sham feeding (in other words how hungry or sated does this make the animal feel)? 59 59 Peripheral factors - Guts • Inflating a balloon in the stomach has no effect on sham feeding, except where the balloon is very very expanded • Putting food directly into the stomach does reduce sham feeding, even if there is a pyloric cuff • The nutrient density (and fibre content) of food placed directly in the stomach does hasten satiety • These effects are probably mediated by the vagus and splenic nerves, as these stomach signals are reduced by damage to these nerves 60 60 Peripheral factors - Digestion • What about the process of digestion? - When food is broken down in the stomach and gut, a range of chemicals are released into the bloodstream - These come from the food, the bodies response to the food and from the chemicals that the body utilises to digest it - These turn out to be some of the most promising candidates for controlling the termination of a meal and we will examine these in the next lecture 61 61