4.3.6 - Biological explanations for obesity

Biological vulnerability and mechanisms

Biological accounts examine how inherited vulnerability and appetite or reward systems can contribute to obesity.

For AQA, organise the topic into two linked routes:

The strongest answers avoid two common mistakes.

First, do not write that a person "inherits obesity" as if it were a single trait. Most common obesity is polygenic, so many genetic variants each make small contributions, and their effects depend on environment.

Second, do not treat neural evidence as simple proof of cause. If a person has obesity, differences in dopamine, serotonin or hypothalamic signalling may help cause overeating, may help maintain it, or may partly result from long-term eating patterns. A good AO3 answer keeps these possibilities separate.

AO2 starting point: If a scenario says a student has several biological relatives with obesity, apply the genetic route. If it says someone feels hungry soon after eating or does not feel rewarded by ordinary meals, apply the neural route.

Genetic vulnerability

Genetic vulnerability changes the probability of an outcome; it does not specify an inevitable individual future.

Genetic explanations propose that differences in DNA affect appetite, satiety, food reward, fat storage, metabolic rate or sensitivity to food cues. This is a vulnerability model: genes shape the likelihood of obesity, but the environment influences whether that vulnerability is expressed.

Adoption evidence

Stunkard et al. (1986) studied 540 adult Danish adoptees. The researchers compared adoptees' weight class with the BMI of their biological parents and adoptive parents. The adoptees' weight class was strongly related to their biological parents' BMI, but not to their adoptive parents' BMI.

AO1 conclusion: because adoptees shared genes with biological parents but not the rearing environment, this supports a genetic contribution to adult body weight.

AO3 caution: adoption studies are valuable because they help separate genes from the family home. However, they are still natural studies, not controlled experiments. They may not fully control prenatal influences, selective placement, wider culture or the fact that obesity environments have changed since the original data were collected.

Twin evidence

Twin studies compare monozygotic twins, who share almost all their DNA, with dizygotic twins, who share about half their segregating DNA on average. If genes contribute to obesity, MZ twins should be more similar in BMI than DZ twins.

Maes, Neale and Eaves (1997) reviewed evidence on BMI and adiposity from more than 25,000 twin pairs and 50,000 biological and adoptive family members. They reported weighted mean correlations of about .74 for MZ twins and .32 for DZ twins, with adoption data also supporting genetic influence.

AO1 conclusion: higher similarity in MZ twins supports a genetic explanation. The comparison is consistent with genetic influence if weight-relevant environments are comparably similar for MZ and DZ twins. It does not by itself remove environmental explanations.

AO3 caution: MZ twins may be treated more similarly than DZ twins, and twin findings are about variation in populations, not destiny for one individual. MZ similarity below 1.00 is consistent with non-shared influences and measurement error; it does not identify a particular environmental cause.

Specific genes and modern genetics

Common obesity is not usually caused by one gene. One important early finding was Frayling et al. (2007), who found that a common variant in the FTO gene was associated with higher BMI. Adults with two copies of the risk allele weighed about 3 kg more on average and had increased odds of obesity compared with people with no risk allele.

Modern genome-wide evidence makes the polygenic picture clearer. Locke et al. (2015) analysed BMI data in up to 339,224 individuals and identified 97 BMI-associated loci. These loci accounted for only about 2.7% of BMI variation, while genome-wide estimates suggested common variation accounted for more than 20%.

AO3 precision: these findings support genetic involvement, but also weaken any simple "obesity gene" answer. The identified variants explain only a small proportion of BMI variation, so an exam answer should say that genes create vulnerability and interact with environment.

AO2 example: Sam was adopted as a baby. His biological parents both had obesity, while his adoptive parents do not. A genetic explanation would not say Sam must develop obesity. It would say he may have inherited a higher-risk combination of variants affecting appetite, satiety or energy storage, so the same food environment might have a different effect on him than on other family members.

Hypothalamus and satiety

Hypothalamic circuits integrate information about energy stores with signals that increase or reduce appetite.

The neural explanation for obesity begins with appetite regulation. Eating is controlled by signals from the body and brain. The hypothalamus is especially important because it helps coordinate hunger and satiety.

When energy stores are sufficient, the body should send satiety signals so eating slows down. If those signals are weak, blocked or not interpreted normally by the brain, a person may continue to feel hungry even when they have consumed enough energy.