Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

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Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations

Transcript of Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

Page 1: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

Biological explanations to eating behaviour

Role of neural mechanisms

Evolutionary explanations

Page 2: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

Role of neural mechanisms in eating behaviour

• Eating behaviour = ingestive behaviour• Homeostasis

– if we are hot – we sweat– if we are dehydrated – thirsts– If we need food – hunger

• Homeostasis is maintained via a negative feedback loop: this assumes that all body variables have a set point (or range)

• The digestive tract and the hypothalamus play a

significant role in eating behaviour.

The process by which the body maintains a constant

internal environment

Page 3: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

Body weight• Each individual has a set point and their

weight is regulated around that set point• Body regulates hunger based possibly on– Fat stores (lipostatic hypothesis)– Glucose levels (glucostatic hypothesis)– Cellular energy – adenosine trisophate (ATP) is a

product of the breakdown of macronutrients (fats, carbohydrates and proteins) and considered important for the regulation and maintenance of homeostasis

Page 4: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

The hypothalamus

• Patient with tumours in the hypothalamus tend to become obese

• Methods used to research hypothalamus– Lesioning in animals– Investigation of feeding patterns after brain damage– Effects of neurotransmittersintroduced into parts of brain– Impact of drugs on eating– Use of fMRI (FunctionalMagnetic Resonance Imaging)

Page 5: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

Dual-Centre Theory of feeding behaviour

• 2 areas of the hypothalamus involved in eating behaviour– Ventro Medial Hypothalamus (VMH) as a "satiety

center" – Lateral Hypothalamus (LH) as a "Hunger center".

• NOTE: VMH and LH are also called VMN and LN (N stands for NUCLEUS!!!)

Page 6: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

Hunger -> eating starts

Increase in blood glucose,

decrease in ghrelin release

VMH satiety centre

activated

Satiety – feeding stops...

...Feelings of declining nutrient

levels

LH feeding centre

activated

Dual-Centre Theory of feeding behaviour

Page 7: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

The lateral nucleus of the hypothalamus

• Contains the feeding centre

• This initiates eating. • It responds to

decreased blood glucose and increase in ghrelin a hormone released from the stomach when it is empty.

Page 8: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

Ventro medial nucleus (VMN)of the

hypothalamus • Contains the satiety centre. • This inhibits eating

behaviour when we are full.

• Responds to – an increase in blood

glucose, – a decrease in ghrelin and to

CCK, a hormone released when food is detected in the duodenum

– leptin a long term satiety signal released by fat cells.

Page 9: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

Role of hypothalamus - evidence

• Aphagia (failure to eat when hungry) can be caused by damage to the LH

• Rats whose VMH had been lesioned developed overeating and obesity

• However, Gold (1973) found that lesions restricted to the VMH alone did NOT result in hyperphagia and only produced overeating when they included other areas such as the parvoventricular nucleus!

• However, subsequent research has failed to replicate Gold’s findings...

Page 10: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

Role of ghrelin - evidence• Cummings et al (2004) monitored PPs’ ghrelin levels every 5 minutes • PPs had to assess their level of hunger every 30 mins. • In 5 of the 6 participants there was a significant correlation between

ghrelin levels, emptiness of the stomach and hunger. • The results support the theory of the role of ghrelin in eating

behaviour• AO3: The study used a small sample limiting how far the findings can

be generalised and it is likely that the participant’s subjective judgements of hunger were influenced by expectations of food based on meal times (cultural factors rather than biological factors)

• Ghrelin injections result in increased food intake in animals • Gastric bands used in treating obesity reduce ghrelin secretion.

Page 11: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

Role of neurotransmitters

• 3 main neurotransmitters are found to influence appetite:– Catecholamines (dopamine, norepinephrine, and

epinephrine): sympathomimetic "fight-or-flight" hormones that are released by the adrenal glands in response to stress

– Serotonin: found extensively in the gastrointestinal tract, it activates the muscles used for feeding. Is also associated to aggression and “happiness”

– Peptides: short polymers formed from the linking, in a defined order, of α-amino acids

Page 12: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

Neurotransmitters that increase food intake

• Norepinephrine (NE) – injections of NE in hypothalamus– can stimulate feeding if injected into the paraventricular nucleus – Can reduce feeding if injected into the perfornical area

• Neuropeptide Y (33 amino acid peptide)– high concentrations in the paraventricular hypothalamus and perfornical hypothalamus.– Rats injected with neuropeptide Y continue eating large amounts of

food even when full– It also seems to cause a preference for carbohydrates

• Galanin (29 amino acid peptide)- particularly found in the paraventricular hypothalamus.– Injections of galanin into rats cause an increase of food intake and a

preference for fats rather than carbohydrates

Page 13: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

Marie et al (2005) role of neuropeptide Y

• Genetically manipulated mice so that they did not produce neuropeptide Y

• Found no subsequent decrease in their feeding behaviour!!

• However, injections of NPY cause hunger...• ...May be due to an experimental artefact,

because of the unexpected intake of NPY in experimental conditions (through injection)

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Neurotransmitters and modulatorsthat decrease food intake

• Cholecystokinin (CCK) – endogenous 33 amino acid peptide that is released into bloodstream during meals– Causes reduction in appetite and satiation– Suppresses weight gain

• Bombesin – peptide found to reduce food intake in rats• Corticotropin-Releasing Hormone (CRH) – 41 amino acid

peptide produced in the paraventricular hypothalamus and other regions of brain that reduces food intake

• Serotonin (5HT) – neurotransmitter which decreases food intake.

Page 15: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

KEY WORDS – matching exercise

• Homeostasis• Negative feedback loop• Set point• Lipostatic hypothesis• Glucostatic hypothesis• adenosine trisophate

(ATP)

• Lateral Nucleus• Ventro Medial Nucleus• Ghrelin• CCK• Leptin• Neuropeptide Y (NPY)

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Impact of drugs on eating behaviour

• Nicotine – decreases food intake.– Smokers generally weigh 3 kg less than non-smokers (US

Department of Health and Human Services, 1990)– Ogden (1994) some dieters use smoking as a weight-loss

strategy and those who stop smoking increase their calorie intake, especially from sweet foods

• Amphetamines - have a dramatic suppressant impact on both subjective hunger and food intake

• Marijuana – increases hunger and food intake

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Impact of drugs on eating behaviour

• Alcohol – influences food intake in contradictory ways– Some studies indicate it can have a weak inhibitory effect– Others show it can stimulate hunger

• Anti-psychotic drugs – both lithium and chloropromazine cause considerable weight gain

• Tricyclic anti-depressants – cause cravings for sweet food and weight gain

• Selective Serotonin Reuptake Inhibitors (SRRIs) – used for patients with depression, might promote weight loss...

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Impact of drugs on eating behaviour

• Analgesics – used for pain relief– Naloxone can trigger deceased food intake– Morphine has also a weak depressant effect on

appetite• Appetite-suppressant drugs – have a consistent depressant effect on hunger and

reduce food intake – Have been removed from the market because

linked to heart problems

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Does it increase or decrease hunger?

• Morphine• Anti-psychotic drugs • Tricyclic anti-depressants • Selective Serotonin Reuptake

Inhibitors (SRRIs)• Naloxone• Alcohol• Appetite-suppressant drugs

BOTH!

INCREASE

INCREASE

DECREASE

DECREASE

DECREASE

DECREASE

Page 20: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

• Ethical issues with non-human animalsissues

• Is the biological approach reductionist?• Free will vs determinism (can biological drives be

overridden?)debates

• Compare and contrast explanations to eating behaviour• Biological, • Learning (Behaviourist)• Cognitive• Social

approaches

IDAs?

Page 21: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

Evaluation of neural mechanisms and eating behaviour

+Reflection on people’s experience of hunger\satiety+Insight into brain chemicals – could be used to develop medical

interventions to help change what we eat+Provides explanation of some differences in eating behaviour+Studies such as those involving lesions to the LH and VMH in rats have

supported the role of the hypothalamus in regulating eating behaviour

+Studies involving electrical stimulation of these centres have confirmed their role in feeding and satiety.

+Such studies provide sound scientific evidence but there is the issue of extrapolating finding from rats to humans.

+Research evidence • Cumming et al, 2004 - ghrelin

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Cont.-Reductionism: focus exclusively on biological aspects of hunger

and satiety-Biological determinism: focus exclusively on the role of nature

and no space left to choice and cultural and social influences-there is substantial and convincing evidence that social, cultural

and psychological factors affect our eating behaviours as is evident from psychological explanations of eating disorders

-Use of animals in research implies lack of generalisability-Highly controlled lab experiments may lack ecological validity-Physiological drives can be overriden (eg desire to loose

weight; dislike of certain foods; fear of losing control; social cues to continue eating; food availability)

Page 23: Biological explanations to eating behaviour Role of neural mechanisms Evolutionary explanations.

Outline and evaluate the role of neural mechanisms involved in controlled eating and

satiation(25 marks)• AO1:

– Intro: homeostasis; negative feedback loop; set point – Lipostatic and glucostatic hypotheses – describe dual centre model (diagram will be also credited); – Role of LH and VMH; – neurotransmitters and hormones involved in eating regulation (ghrelin; CKK; leptin;

norepinephrine; Neuropeptide Y...) • AO2:

– research evidence for effects of hormones + contrasting evidence (eg Cummings et al, 2004 ghrelin AND Marie et, 2005 NEY al OR Gold, 1973 VMH lesioning) (4)

– Impact of drugs (eg alcohol, anti-depressants...) on appetite confirms biological basis of eating behaviour

– I: ethics with non-himan PPs; reductionism – D: free will vs determinism; nature vs nurture – A: biological vs social, behaviourist, psychodynamic

• AO3– Lack of ecological validity – Lack of generalisability – Sampling issues (eg Cummings et al, 2004)