Neural mechanisms involved in controlling eating b
We have neural mechanisms which regulate our eating behaviour in terms of making us hungry to motivate us to eat, and of triggering a feeling of being ‘full’ or satiated so we do not eat too much.
When and how much we eat is determined by our metabolism, i.e the rate at which the body uses energy (calories). A number of physiological mechanisms try to keep the body in energy homeostasis (or balance) – neural mechanisms (brain mechanisms, neurones and hormones).
The rise in obesity over the past 25 years suggest for many people the balance is set too high, and it is clear social and environmental factors affect eating behaviour.
However, set point theory proposes we have a biologically determined set weight, focused on our ‘fat mass’. This is influenced by genetic factors, and this set point is where our weight is regulated through homeostasis.
This means if we eat too much or too little, homeostatic mechanisms alter our metabolism and appetite, to return us to our original weight. However, according to Passer et al (09) persistent over or under eating make it increasingly difficult to do this so over time people settle at a new set point.
At a basic level we can see the body has evolved two separate systems, one for turning eating ‘on’ and one for turning it ‘off’. These are both in the hypothalamus:
Ø The lateral hypothalamus produces feelings of hunger
Ø The ventromedial hypothalamus leads to feelings of satiation, reducing appetite
According to set point theory, food consumption and weight control are a result of a balance between these two parts of the hypothalamus.
The importance of the hypothalamus was shown by Lashley (1938) who was the first psychologist to suggest that hunger is not just a reflex response to an empty stomach.
Lashley argued that neural mechanisms are involved in making decisions about when and when not to eat. He found that after lesions (damage) to the lateral hypothalamus, animals stop eating spontaneously
and that the reverse occurred after lesions to the ventromedial hypothalamus- that is lesions in the ventromedial hypothalamus caused the rats to overeat to excess.
These findings have been confirmed by studies where the LH and VMH are stimulated by electrodes, leading to feeding behaviour when the LH is stimulated and an ignoring of food when the VMH is stimulated. (Stellar 54). This has led to a simple model of the neural control of eating behaviour:
GHRELIN - hormone
We now know that as well as social and environmental factors playing a large role in eating behaviour a number of hormones and neurotransmitters are also important (Hormones affecting the brain and neurotransmitters may be considered neural mechanisms!):
GHRELIN - an empty stomach sends signals to the brain to start eating. An important role is played by hormone ghrelin, which appears to act directly on the brain mechanisms of feeding behaviour, including the hypothalamus.
Ghrelin is a hormone is secreted from the stomach, and the amount released is directly proportional to the emptiness of the stomach, i.e. as the time from the last meal increases and we feel hungrier, so ghrelin secretion is increased. Injections of ghrelin increase food intake and body weight in animals and humans: interestingly, gastric bands used to treat obesity reduce ghrelin secretion from the stomach.
Cummings et al (2004)
Cummings et al. investigated changes in blood ghrelin levels over time between meals. 6 participants were allowed to eat lunch, then ghrelin levels were monitored from blood samples taken every five minutes (from a tube or catheter inserted into a vein) until the Ps requested their evening meal.
Ps assessed their degree of hunger every 30 minutes.
Findings were that ghrelin levels fell immediately after eating lunch, reaching their lowest level at about 70 minutes.
Then they slowly began to rise , peaking as Ps requested their evening meals. Importantly, in 5/6 Ps ghrelin levels were closely correlated with the degree of hunger reported by the Ps.
The authors concluded that ghrelin levels directly reflect stomach emptiness and are closely related to subjective feelings of hunger.
This supports a role for ghrelin as a key appetite signal in humans.
This research may seem useful in suggesting blocking ghrelin can help decrease appetite for people with obesity. However, ghrelin also has a role in stress - Lutter et al. (2008) in a study involving mice as Ps, found the body produces extra quantities in response to stress and that it reduces depressive and anxious behaviours. As it also boosts appetite, it may lead to increased comfort-eating, but blocking ghrelin may also increase depression
This means that even though blocking ghrelin may be helpful for an obese person, it's not appropriate to do so as it causes problems such as depression etc.
LEPTIN - neuropeptide, leptin, involved in satiety – a lack of leptin appears to lead to overeating.
Neuropeptides are very small proteins encoded by genes that act as chemical messengers.
Leptin is secreted from fat cells into the blood, signalling the hypothalamus that caloric storage is high, and therefore the individual may cease eating/not start eating.
Carlson (2007) investigated the role of leptin by studying of a strain of mice that are genetically obese – they are missing a gene that produces leptin.
This means they eat continuously and become obese. Injections of leptin into these mice stop them eating as much and their weight eventually returns to normal.
Although it may seem we could tackle human obesity with leptin, this only works in the few people who also lack this gene.
Generally, as fat cells secrete leptin, lean individuals secrete less as they have fewer fat cells, and overweight people actually secrete more leptin.
IDA - use of animals - generalisability
Use of animals when researching to explanation human eating behaviour
- Human eating behaviour differs from animal eating behaviour
- Human and animal hypothalamus' are different
- Animals may eat less/more than humans and humans may also eat more or less frequently
- Cannot generalise findings involving animals to humans as they are very different - reduced validity
- Hunger is due to neural mechanisms and the chemicals Ghrelin and Leptin - not taking other factors into account.
- Phsyiological redctionism - explaining the eating behaviour in terms of neurochemicals.
- Behaviour towards eating is due to factors our of our control.
- Determinstic view describes behaviour as not being under the control of the individual - behaviours determined by external/internal factors (in this case our biology) and also social/environment.
Eval points (from Bailey et al):
Neural mechanisms are still unclear: although the hypothalamic centres outlined are clearly very important in controlling hunger and satiety, they don’t explain the whole story. Exactly how ghrelin and leptin reach their targets in the brain is not wholly clear; both are large peptides that do not cross the blood-brain barrier readily.
The influence of biological rhythms: the various signals that send information to the hypothalamus are only part of the complex systems regulating when and how much we eat. Neural mechanisms are important in controlled eating and satiation but other factors play a part as well. Some of these relate to environmental factors. However, biological rhythms also affect our eating behaviour. For example, rats become most active and start to eat soon after darkness descends. This and similar rhythms are controlled by another area of the hypothalamus called the suprachiasmatic nucleus.
Eval. points (from Bailey et al):
Psychological Hunger: There is a difference between physical hunger (where the body requires energy to function) and psychological hunger (where an individual thinks they need food but it is not biologically required). There are various learned and cognitive components to eating. These will be considered in depth when we study the factors influencing attitudes to food and eating behaviour in the next pack, but these include:
Availability of rich foods: people tend to gain weight when rich foods are plentiful
Taste preferences: eating is also affected by taste preferences acquired through conditioning or observational learning
Smell: Some food smells are so attractive that people cannot resist feeling hungry even if they have just eaten a meal. Again, these preferences affect eating behaviour and are culturally learned preferences.
Psychological hunger cont.
Habits: People learn habits, such as when and how much they eat. Time of day can affect feelings of hunger beyond any physiological requirements. These habits also influence hunger and food intake. Schacter (1971) proposed the internal-external theory of hunger and eating of the obese. He devised and experiment where participants were measured by the amount of crackers eaten during a period when the real time was manipulated by a faster clock or a slower clock. It was hypothesized that if the obese person is more affected by the clock time than the real time, then he or she should eat more when the clock is close to dinner time. The results were consistent with the hypothesis. Schacter concluded that obese people respond to external cues of hunger, such as time, more than non-obese people, who tend to respond more to internal cues of hunger.
Psychological hunger cont.
Stress: The increased physiological arousal associated with stressful situations can stimulate hunger in some people, whereas in others stress can decrease hunger. Some people ‘comfort eat’ and use food as a way to suppress an emotional problem in order to avoid dealing with the actual problem High-fat, sweet foods are most often consumed.
Cultural altitudes: Different cultures have different expectations and ideals concerning food intake and body ideals. These factors can also affect controlled eating and satiation in any particular society.
All these external stimuli provide other ways to signal the hypothalamus to make us feel hungry even though the signals are not physiological (Hara, 1987).
Evolutionary explanations of food preferences
How evolutionary theory might explain food preferences:
- Most peoples favourite food is fatty food, unhealthy and high calories.
- Ancestors used a lot of energy (were hunters) so exerted a lot of energy - so we like the high energy food.
- As hunters when they found lots of food they'd stuff their face - they wanted to reserve food as fat (in case they got no food for a week or 2).
Evolutionary theory suggests that organisms should behave so as to maximize the survival of their genes. In this way, natural selection (survival and reproduction of the fittest individuals) occurs. Survival depends on any number of things; one involves remaining healthy by managing to obtain sufficient nutrients to meet the demands of the body.
In pre-agricultural societies, food supplies were almost certainly limited or erratic. There would not have been constant, regular and adequate food for the daily needs of all the hunter-gatherer population throughout the year. Given this perspective, it is clear that humans evolved in an environment that encouraged the maximisation of stored energy. In other words, binge eating would have been an adaptive behaviour.
Sweet, fatty or salty foods would have been particularly valued since they are vital requirements and were relatively rare in the ancestral environment
This in these situations it was advantageous to overeat in times of plenty, and those that did adopt this strategy were probably more likely to survive and pass on their genes to the next generation.
It was a good idea to retain as many calories as possible and expend as few as possible, as an insurance against future times of food scarcity.
In much of the developed world, such strategies are obsolete and yet people may find it hard to escape the evolutionary pressures on them for food preferences. Furthermore, although exercise could help alleviate these evolutionary led behaviours, the evolutionary hangover is also to conserve energy, and people do this through using labour saving devices.
Other reasons for taste preferences can be seen by considering the specialisation of taste receptors in humans:
Sweet: this would be used to identify foods rich in carbohydrates. These would provide the calories that we burn up in energy expenditure. Even 1-3 day old infants show a preference for sweet flavours (Desor et al 73) and Grill and Norgren (73) found lab rats immediately accept sweet foods, when they are reluctant to try other flavours. Together with the fact that we are biologically very sensitive to sugar (with, for eg, more nerve fibres sensitive to sugar being contained in the nerve that runs from our tongue to our brain, than for any other flavour), it appears this taste preference is genetic. As well as providing energy, sugary foods also supply our brain with the fuel necessary to make good and effective decisions quickly, clearly a survival advantage (Holt & Lewis)
Ø Sour: a sour taste is associated with food that has gone off (like milk) and may therefore contain harmful bacteria. Clearly to be avoided!
Ø Salt: This is critical to the normal functioning of cells in the body and it would therefore be important to identify foods providing this nutrient
Ø Bitter: A bitter taste is associated with plant chemicals that might be poisonous to humans
Ø Umami: this has been discovered relatively recently and represents a meaty or savoury quality. This would indicate a good source of protein
Food neophobia is another evolutionary aspect of food preferences. ‘Neophobia’ means ‘fear of the new’. Applied to food it means that animals have a powerful tendency to avoid foods they have not come across before. Although this can lead to a dull diet, it does mean that you always eat food you know is safe and avoid new foods that may be harmful.
Parents all know of the struggle to get children to eat certain foods. Birch (99) notes that neophobia is minimal when an infant is just eating solid foods, which is adaptive as they are completely dependent on adults for their food intake.
However, as they become increasingly independent and so theoretically able to find food themselves, neophobia forms a protective function.
An aspect of this neophobia is that we tend to show greater liking for foods as they become more familiar (Frost, 2006). Encouragingly, we do show a preference for variety in foods that we know are safe; children will eat more Smarties if they are multicoloured than if they are all the same colour!
As we develop, and require more energy, we learn to prefer flavours associated with high energy foods. This would have been adaptive at the time when most human evolution occurred – in the Environment of Evolutionary Adaptiveness, or EEA, when high energy foods were scarce.
Preference for meat
Humans are omnivores and the preference for meat doesn’t seem to be innate as it is usually introduced into children’s diets at a relatively late stage of development and many children are initially reluctant to eat it (unlike sweet tasting foods). In fact many cultures are vegetarian. [Gross & Rolls] Therefore, it must have evolved later than any preference for vegetables, and also has possible costs associated (eg food poisoning, Jacob-Cruetzfeld disease, longevity)
It is thought that human ancestors began to include meat in their diets 1 to million years ago. This was to compensate for a decline in the quality of their plant foods, caused by receding of forests. Evidence from fossils suggest the meat diet particularly consisted of organs such as livers, kidneys and brain which are extremely rich sources of energy (and relatively sweet tasting compared to other meat). Similarly, Stanford (99) found, through observation of chimpanzees in Tanzania, when a kill is made (which happens quite rarely and is usually of a monkey) they go straight for the fattiest parts (eg brain, liver).
Milton (08) proposes that meat eating led to the development of the human brain and spurred the increase in our intelligence. This is because he feels it unlikely the nutrition from vegetable matter alone would have been sufficient for humans to develop such a large brain. It is the size of our brain that leads to our high intelligence level.
Evidence for this comes from comparing our digestive system to that of (largely vegetarian) chimpanzees. We have a relatively long duodenum and small intestine, specialised for the digestion and absorption of protein, whereas chimpanzees have a relatively long large intestine, specialised for the digestion of plant material