Obesity as a major public health problem Obesity is an increasingly important health problem in
the United States and also worldwide. The rates of obesity (defined as body mass
index (BMI) > 30.0) have nearly doubled in the U.S. population from
approximately 15 percent in 1980 to an estimated 27 percent in 1999 (1).
Although our society has made great strides in
reducing other threats to health such as smoking, we have been unable to stem
the rising tide of obesity in the population. This trend is particularly disturbing
because obesity is central to the development of many chronic diseases such as type 2 diabetes, heart disease,
hypertension and cancer. Clearly, the increase in rates of obesity is a matter of major public health
concern.
Obesity as a complex metabolic condition
Obesity is characterized by an
excessively high amount of body fat or adipose tissue. This condition is
common, but the condition varies from individual to individual. At one end of
the spectrum, a healthy weight can be attained by cutting down on certain foods,
taking smaller portions, and embarking on a regimen of regular exercise.
For individuals in this population, a moderate change in diet and exercise are
the proper interventions because they work.
At the other end of the spectrum, there is
another population of overweight people who do not respond as well to these
interventions. Everyone knows someone who tries diligently to lose weight
but who experiences limited success or loses weight only to gain it back over
time. In fact, more than 80% of those who lose weight will gradually
regain it unless a weight maintenance program is implemented over the long-term
(2). What is different about
these people compared to those who either stay thin or lose
weight readily and keep it off with relatively minor lifestyle changes? One difference may be genetic risk factors that affect energy metabolism and
result in an inborn susceptibility to gain weight.
Genetics as a risk factor for obesity
Obesity is a condition that results from an environment of caloric abundance
and relative physical inactivity that is modulated by a susceptible genotype.
Although rare obesity syndromes caused by mutations in single genes have been
described, by far the greatest proportion of obesity in humans is not due to
mutations in single genes. Genetic predisposition may not be health destiny, but
studies indicate that inherited genetic variation is an important risk factor
for obesity. Evidence from twin, adoption and family studies strongly
suggests that biological relatives exhibit similarities in maintenance of body
weight. Genetic factors also are beginning to be implicated in the degree
of effectiveness of diet and physical activity interventions for weight
reduction.
These genetic risk factors tend to be familial,
but are not inherited in a simple manner; they may reflect many genetic
variations, and each variation may contribute a small amount of risk and may interact with
environmental elements to produce the clinical condition of obesity. An
active area of research is determining associations between various
obesity-related phenotypes with variations in candidate genes or through linkage
studies with anonymous markers spread over the human genome. Learning how
genetic variations affect susceptibility to become or remain obese will lead us
to a greater understanding of how obesity occurs and, hopefully, how better to
prevent and treat this condition.
You can’t change your genes, but you can
change your behavior Does this mean that those with a
susceptible genotype are destined to a life of futile efforts to achieve a
healthy weight? This need not be the case. We can’t change our
genes, but we can change our behavior. Small victories in weight
loss—often as little as 10% of total body mass--can result in positive effects
on health and well-being, even if an ideal weight remains elusive. Also, the
positive effects of regular physical activity include lower blood pressure and
increased cardiorespiratory fitness' even in people who are significantly
overweight. In the longer term, understanding the genetic variations that
influence energy metabolism may help us to understand the underlying biological
factors that affect weight gain and energy expenditure and develop interventions
that capitalize on these insights. Finally, to recognize that obesity may be due
to a metabolic condition rather than a flaw in character is important both for
the people who are affected and for society as a whole.
The public health messages to prevent overweight
emphasize a nutritious diet and daily physical activity. Many who follow this
advice from the outset are able to maintain a healthy weight, even with a
genetic susceptibility to gain weight. However, these lifestyle
interventions have a range of uptake and effectiveness, especially if obesity is
already present. For people who are already overweight, the public health interventions
aimed at the general population are not a complete solution. Insights from
genetics and molecular biology in controlling appetite and activity may provide
more effective drug therapies for treatment of affected individuals. As we
continue to emphasize the importance of diet and exercise as major factors that
affect long-term health, public health needs to also seek new approaches—such
as considering genetic factors in risk factor assessment and intervention
design--to more thoroughly address this complex problem.