Results: There was a strong relationship between the weight of the adoptees and that of their biological parents. There was no relationship between the weight of the adoptees and their adoptive parents in any of the weight classes.
Conclusion: Genetic influences have an important role in determining adult weight, whereasenvironment seems to have little effect.
Evaluation: This finding is supported by other biological versus adoptive relative research and even by some twin studies. However, it's probably too reductionist to say that genetics alone are responsible for obesity. Also, the participants were all from Denmark, so the results can't be generalised to the whole population.
Montague et al (1997) - Leptin's role in obesity
Method: Two severely obese children (male and female cousins) were studied - a large proportion of their total body weight was made up of adipose (fatty) tissue.
Results: A mutation on the part of their DNA responsible for controlling their supply of leptinwas found - they didn't produce enough leptin. Leptin is a protein produced by adipose tissue to signal that fat reserves in the body are full.
Conclusion: Their leptin deficiency had caused the children's obesity. They did not have enough of this chemical to suppress appetite in the normal way.
Evaluation: A number of trials in which obese patients were given doses of leptin have had very little success. Research now suggests that most people with obesity in fact have high levels of leptin - they're just resistant to its effects. This was a case study of only two children, so although it revealed a lot about their particular situation, the findings weren't relevant to the majority of obese people.
Gene studies
Some forms of obesity are known to be monogenetic (caused by single gene), but this accounts for only 1% of obesity cases.
Prader-Willi syndrome is a rare genetic disorder characterised by hyperphagia (abnormality increased appetite) and obesity.
Nachman (1959) showed that rats could be selectively bred to have a greater or smaller preference for sweet foods, suggesting that a preference for these foods in obese individuals might be genetic.
Evaluation of genetic explanations:
Obesity problems are greatest in countries with constant and abundant food supplies, so obesity may be a consequence of environment (e.g. modern living).
- The very rapid increase in obesity rates in Western societies can't easily be explained by genetics and emphasises the importance of environmental factors.
In Fiji, obesity has significantly increased in the last 30 years, and US Pima Indians are on average 25kg heaver than those living in Mexico: the key factor in both cases is increased availability in the environment of high calorific food.
Most obesity is likely to be caused by a complex interaction of many genes, each contributing to susceptibility to the disorder.
Neural mechanisms
The VMH triggers the sense of feeling full, and damage to this causes overeating and obesity.
Increased levels of neuropeptide Y (NPY) in the PVN promote increased eating and weight.
Decline in leptin stimulates motivation centres in the brain, kindling interest in food.
Other biological factors
- Obesity is associated with a number of endocrine (hormone) disorders:
- Hypothalamus (affecting metabolic rate, thus predisposing sufferers to weight gain)
- Cushing's syndrome (excess corticosteroids causing rapid weight gain)
- Ovarian disease
- Pancreatic disease
- Changes in endocrine function tend to be secondary to obesity, as over-eating often causes endocrine problems which are then reversed by weight loss.
What You Need To Know:
- Evolutionary explanations, for one eating disorder: obesity
Evolutionary Explanations Of Obesity:
Food preferences
- Sugar would have been a much sought-after but scarce source of energy for our ancestors.
- Sugars would have been eaten sparingly.
- Nowadays sugary foods are abundant so we gorge rather than eat sparingly.
Survival advantage
- Storing lots of fat in times of plenty improves survival chances in times of famine.
- This was useful in the past where food supplies were variable, but not now where food is so plentiful.
- In a short space of time people have become exposed to constant food containing lots of fat, sugar and salt - there is now a mismatch between our bodies and modern living.
Environmental uncertainty
- The environment was much more unpredictable for our ancestors, leading to uncertainties about food supply.
- They would cycle through periods of famine and plenty.
- A useful strategy would be to eat as much as possible whenever they could so that if famine arrived then they would have stored fat to see them through.
- What results in obesity now might have provided an evolutionary advantage in the past.
Pima Indians - Chamala et al (2008):
- Modern Pima Indians show unusally high levels of obesity.
- Researchers found that their metabolic efficiency was being genetically influenced.
- They suggested that this provided an evolutionary advantage for their ancestors:
- The Pima would have lived in harsh barren desert-like conditions.
- Their genes provided them with the maximum advantage from any food they found