Control of Eating Behaviours

Q: Investigating Ghrelin

__Cummings et al. (2004)__ investigated the role of ghrelin in controlling eating behaviour using human participants.

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Neural Mechanisms Controlling Eating Behaviour

The hypothalamus is located at the base of the brain and is a gland that is responsible for maintaining constant conditions in the body (homeostasis), including the regulation of thirst and hunger.

Ventromedial and lateral hypothalamus

The ventromedial hypothalamus (VMH) is the lower-middle part of the hypothalamus and is involved in the physiological process of feeling full that stops you eating, known as satiety.
The lateral hypothalamus (LH) is the outer sides of the hypothalamus and is involved in hunger.
The VMH and LH are said to have antagonistic roles in the regulation of eating behaviour.

Glucose

Glucose levels in the LH cause a person to feel hungry, encouraging a person to eat and restore equilibrium of glucose levels in blood.
Once a person has satisfied their hunger, increased glucose levels activate the VMH and inhibit the LH, causing the person to feel full.

Rat studies

Studies of rats have been used to show the differing roles of the VMH and the LH in the control of hunger.
LH damage in rats causes them to stop eating, while stimulation of the LH stimulates eating behaviour.
VMH damage in rats results in overeating, while stimulation causes them to stop eating. In humans, VMH malfunctions may lead to obesity.

Winn et al (1990)

Aim: To investigate the effects of lateral hypothalamus (LH) lesioning in rats.
Procedure: Laboratory experiment. Rats had the lateral hypothalamus in their brains lesioned with excitotoxin N-methyl-d-aspartate (NMDA).
Results: The rats with large lesions in the LH had much lower food and water intake levels than rats without lesions or smaller lesions.
Conclusion: Demonstrates the importance of the LH in regulating hunger and thirst.

Evaluation of Winn et al (1990)

Strengths
- Controlled, reliable experiment.
- Practical applications.
Limitations
- Cannot generalise from animals to humans.
- Ethics.

Hormonal Mechanisms Controlling Eating Behaviour

Hormones are involved in hunger and satiety. There are two crucial hormones involved in controlling eating behaviour: Ghrelin and Leptin.

Ghrelin

Ghrelin is released from the stomach and small intestine into the bloodstream.
If levels of Ghrelin increase to be greater than a norm, the lateral hypothalamus is stimulated and hunger is felt.
Ghrelin levels increase at a rate that is inversely proportional to a person’s bodily resources. So the longer the period of hunger, the higher the ghrelin levels.
Release of Ghrelin stops when food is eaten.

Leptin

Leptin is an appetite suppressing hormone. It tells the body when satiety is reached.
It is produced by fat cells when a certain amount of fat has built up in the body’s stores. Leptin travels in the blood where it influences the ventromedial hypothalamus.
There is a positive correlation between leptin levels and the number of fatty cells.

Lack of leptin

A lack of leptin cause obesity, but most studies involve mice and other animals and so can’t necessarily be extrapolated to make conclusions about humans.

Investigating Ghrelin

Cummings et al. (2004) investigated the role of ghrelin in controlling eating behaviour using human participants.

Method

The participants were six males.
The participants were given lunch, and then asked to record their level of hunger at 30 minute intervals.
The levels of ghrelin in the participants’ blood were measured at five minute intervals.
The participants were told to ask for their dinner when they felt hungry enough.

Results

Ghrelin levels decreased immediately after the participants ate lunch.
Levels were lowest around 70 minutes after eating.
Levels then rose until the participants ate again.
The hunger level reported by the participants positively correlated with the amount of ghrelin in the blood.

Conclusion

Hunger is related to ghrelin levels.
When ghrelin levels are high, we feel more hungry.

Evaluation

The findings are only correlational, and although the study shows that ghrelin levels and hunger are related, it does not provide any causal information about the influence of ghrelin on hunger.
There was only a small sample of male participants used for this study. This limits the validity of generalising the results to the wider population (low ecological validity).