Explanations of obesity

• Obesity= you are consuming more calories than you expend.
• Genetics theory:

  • Suggests obesity is something that is inherited; genes contribute to the development of obesity.
  • It could be that those genes indirectly cause the obesity, for example by producing more hunger hormones or fewer satiety hormones.
  • Twin studies have shown heritability factors ranging from 40%-75%, for example a meta-analysis that included 75,000 individuals concluded heritability estimates of BMI for MZs (same) was 74% compared to 32% for DZs (different).
• Genetic Theory:

  Stunkard et al (1990) found that MZs reared apart (so had dissimilar experiences/ environment), were more similar in terms of BMI than DZs reared together (so had similar experiences/ environment), further supporting a link between genes & BMI.
• Evaluation of genetics theory- strength:

  • Research support from Stunkard et al- gathered data about 540 adult adoptees as well as their adoptive & biological parents.
  • Results showed a strong relationship between the adoptee & their biological parent in terms of weight category (underweight, overweight, obese etc), but no significant correlation between adoptees & their adoptive parents in terms of their weight category.
  • This supports the idea that genes play a role in the development of obesity.
• Evaluation of genetics theory- weakness:

  • Genetic influence on obesity is not straightforward & varies depending on age.
  • Elks et al performed a meta-analysis & found that the influence is strongest during childhood & that this decreases with age.
  • This is probably due to the adults' ability to use knowledge & experience to make choices in terms of food intake & physical activity.
• Evaluation of genetics theory- weakness:

  • Genetics cannot explain the rapid increase in obesity in the last 30 years.
  • In 1993, government data tells us 13% of males in the UK were obese, but fast-forward to 2013 & that figure has risen to 26%.
  • The gene pool has remained fairly constant in that time, so genes cannot account for this.
• Hormonal- Leptin theory:

  • Satiety hormone released directly from fat cells that send messages of fullness & discourages further food consumption & promotes the use of energy, for example by blocking the effects of the appetite stimulating neurotransmitter neuropeptide Y.
• Hormonal- Leptin theory:

  • Bates & Myers (2003) produced research evidence to show that disrupting leptin signalling in the hypothalamus, resulted in obesity.
  • This evidence supports the key role leptin plays in the regulation of food intake & therefore the potential implications in causing obesity.
• Evaluation of leptin theory- strength:

  • Research ssupport- Montague et al looked at 2 severely obese Pakistani cousins living in the UK (case study).
  • Both were found to have very low levels of leptin despite having more fat to release the hormone.
  • Gibson et al reported on another child from the same region in Pakistan who was also severely obese.
  • For this child, after 4 years of leptin injections, the child has dramatically improved in terms of metabolism & weight- supporting the fact that leptin deficiency can be a contributory factor in obesity.
• Evolutionary theory- the 'thrifty gene':

  • In the environment of evolutionary adaptation (EEA), humans would have been reliant on catching & foraging for food, but food availability was sporadic & there would have been times of famine.
  • In order to survive, humans would've had to make the most of times of plenty in order to build enough fat reserves to survive times of famine.
• Evolutionary theory- the 'thrifty gene':

  • Eating lots when you could= evolutionary advantage & the survivors would breed & pass this trait on through genetics & offspring- this has been named the 'thrifty gene'.
  • Today this gene is more of a disadvantage, as all we have are times of plenty and hence lots of obesity & related diseases.
• Evaluation of evolutionary theory (thrifty gene)- weakness:

  • Are issues with the 'thrifty gene' theory- Speakman questions why aren't we all obese then. If majority of humans have thrifty gene, then the majority of us should be obese.
  • We are not in majority figures for obesity, and it has all happened fairly recently- in terms of human evolution (in the last 20-30 years), & it is only an issue in a few cultural groups where high fat & sugary foods are widespread, so we can't really blame a thrifty gene.