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Biological Explanations for Obesity

Psychology: AQA A Level, topic 13.1.9

Introducing Obesity

Obesity is characterised by an individual being overweight with a body mass index (BMI) of 30 or over.

Body mass index

Body mass index

  • Individuals with a BMI of 30 or over are classified as obese. Those with a BMI of 40 or over are severely obese.
  • Healthy individuals usually have a BMI between 18.5 and 24.9.
Causes of obesity

Causes of obesity

  • When the amount of calories consumed are far greater than those burnt off, an individual can become obese.
  • There are certain medical conditions and medications that can also give people an increased risk of developing obesity.
Treatments for obesity

Treatments for obesity

  • Exercise and a strict diet are the most successful ways of treating obesity.
  • In the UK, the NHS leads a large initiative to reduce the levels of obesity in the country, and encourage exercise and healthy eating from a young age. The aim is to reduce the number of people becoming obese in the first place.
Explanations for obesity

Explanations for obesity

  • There are biological explanations for obesity including the role of genes, neural correlates and hormones.
  • There are also psychological explanations for obesity linked to a lack of cognitive control and self-restraint.

Genetic Explanations for Obesity

Genes may provide an explanation for an individual’s risk of obesity. Adoption studies have been used to show this.

Maes et al. (1997) - heritability of BMI

Maes et al. (1997) - heritability of BMI

  • In a meta-analysis of over 75,000 individuals, Maes et al. (1997) found that estimates of the heritability of BMI were far greater in identical twins (74%) than non-identical twins (32%).
  • BMI heritability was still higher in identical twins who were raised separately than in non-identical twins, so there is likely a genetic component beyond environmental factors.
  • Heritability rates have been shown to change over age ranges, which may be an environmental factor.
Stunkard et al. (1986) - adoption study

Stunkard et al. (1986) - adoption study

  • Stunkard et al (1986) carried out a Danish study of 540 adults who had been adopted.
  • The study showed strong links between the weight of adopted children and their biological parents, but no link between the weights of adoptive parents and adoptees.
  • This suggests that genes play a large role in determining obesity, and the effect of the environment is comparatively small.
  • But this is a very simple study and it is likely that there are other factors at play.
Leptin production

Leptin production

  • Some obese individuals have been shown to have a genetic mutation which limits the production of leptin.
  • Because leptin signals that an individual is full, if there is not enough leptin being produced, an individual will not stop eating and so may become obese.
  • Leptin injections have been shown to be successful in limiting the diet of obese individuals.

Neural Explanations for Obesity

As the hypothalamus associated neurotransmitters are involved in the control of eating behaviours, their abnormal functioning may be linked to obesity.

Hypothalamus

Hypothalamus

  • Over-activation of the arcuate nucleus (a bundle of neurons which signals when blood sugar is low and elicits a desire to eat) may cause obesity by triggering a desire to overeat.
Serotonin

Serotonin

  • Individuals with obesity often exhibit low levels of serotonin and its metabolite.
  • As serotonin triggers the activation of the ventromedial hypothalamus (VMH), low levels of serotonin lead to less VMH activation. This means individuals do not feel full.
  • If individuals are constantly hungry, they will overeat and gain even more weight.
Dopamine

Dopamine

  • Individuals with obesity often exhibit low dopamine levels and reduced levels of dopamine (D2) receptors.
  • These reduced levels are seen particularly in the striatum. So the pleasure centres usually activated by food consumption are not activated as much as in individuals with normal dopamine levels.
  • This may mean that individuals over-eat to try to increase activation of the pleasure centre.

Test yourself on Biological Explanations for Obesity

  • Which explanation of obesity involves the role of genes and hormones?

  • If a person had a BMI of 37, what would be their classification?

  • Were leptin injections successful in treating obesity?
    If so, why? If not, why?

  • According to Maes et al.'s (1997) study, the heritability of BMI is much greater in which group?

  • Which of the following factors is likely to have the greatest effect on the development of obesity?

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