Obesity - Genetic or Environmental - Nature
or Nurture?
As we approach the full sequence of the
human genome the question of 'nature versus nurture' has never been more
topical.
Obesity & Genes
Today we recognise a number of single gene defects, primarily in the leptin
and melanocortin systems, associated with profound obesity. Yet in the
overwhelming majority of cases of human obesity it has proved difficult
to isolate individual genes, which consistently segregate with the obese
phenotype across various populations. For the most part the genetics of
body weight represent the balance between genes causing susceptibility
to and protection from obesity.
Obesity & Social Systems
At the other extreme we must accept that some cases of obesity are strongly
determined by social and cultural systems. The fatness of Sumo wrestlers
is not a genetic accident, but the result of targeted eating and exercise
habits. In our own society the increase in weight or fatness in adult
life, which frequently accompanies declines in physical activity, provides
further evidence of a behavioural component to body weight.
Obesity & Children
However, few cases are clear-cut examples of 'nature or nurture' as a
cause of obesity. Evidence is beginning to emerge of children, disciplined
by over-controlling parents, who are unable to take responsibility for
their own food consumption and who appear particularly prone to excess
weight gain. It may be argued that this represents either a consequence
of familial eating behaviours or an underlying genetic factor transmitted
from parent to child.
Weight Gain - Genetic or Environmental?
This and many more examples provide strong evidence that most cases of
obesity result from interactions between genetic factors and environmental
influences. We are beginning to acknowledge that a strong hunger drive
or a preference for high fat foods may be innate rather than learnt phenomenon,
conveying an increased risk of hyperphagia and weight gain. However we
are perhaps still reluctant to consider that the willingness or capacity
to exercise may have a genetic basis. The recent increase in the prevalence
of obesity represents the impact of the 21st century environment on our
underlying susceptibility to weight gain, not a fundamental change in
the genome.
Obesity is not a single disease. It is
a heterogeneous group of diseases of diverse origin, each of which is
manifest as an obese phenotype. As we understand more fully the relative
contribution of 'nature and nurture' it will be possible to develop more
precisely targeted interventions to prevent and treat obesity.
Source: Susan Jebb, MRC Human Nutrition Research, Cambridge. (2000)
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