Fatty Liver (cont.)
In this Article
How common are NAFLD and NASH and who are at risk?
The NAFLD spectrum is
probably the most common disease of the liver in the United States. Although
precise information on the number of cases of NAFLD and NASH is limited,
estimates have been made. Moreover, information is available on which
individuals are at risk to develop NAFLD and NASH.
Why is information on the number of cases limited?
To date, no data are
available on the incidence (number of new cases per year) of NAFLD or NASH in
the United States, or anywhere else in the world for that matter. To determine
the incidence of this disease, long-term studies in populations at risk will be
needed. Such data collection has only just begun in several medical centers.
We do have data, however, on the prevalence (number of cases observed at one
time) of NAFLD and NASH in cross-sections of populations at risk. Therefore,
knowing which individuals are at risk is important. In this regard, a large body
of evidence now supports the concept that NAFLD is associated with a condition
called insulin resistance, which is described in more detail below. Suffice it
to say at this point that diabetes mellitus type 2 (DM2) and, in addition,
especially the overweight condition and obesity are the most recognizable
features of insulin resistance.
What is the BMI and how does it relate to NAFLD and NASH?
As just indicated,
obesity is linked closely to NAFLD. Therefore, to get a grasp on the frequency
of NAFLD and NASH and the impact of obesity on NAFLD in the population, one has
to understand how obesity is defined. A calculation of the
body mass index (BMI)
is a method that can be used to determine degrees of obesity.
The BMI is calculated by dividing a person's weight in
kilograms by his or her height in meters squared (kg/m2). In the non-metric
system, BMI =
(lbs/inches2) x 703. In adults, normal weight is defined as a BMI between 20 and
25, overweight from 25 to 30, obesity from 30 to 35, significant obesity from 35
to 40, morbid obesity from 40 to 45, super obesity from 45 to 50, and
super-morbid obesity greater than 50.
In addition to the BMI method, obesity can be described according to the
distribution of body fat. Fat can be distributed predominantly either to the
hips (gynoid or pear-shaped) or to the abdomen (central, android, or
apple-shaped). An abdominal predominance of fat is the most commonly observed
type of obesity in insulin resistance. Abdominal obesity is defined in men by a
waist-to-hip circumference ratio greater than 1.0 or a waist circumference
greater than 40 inches (102 centimeters). In women, abdominal obesity is defined
by a waist-to-hip ratio greater than 0.8 or a waist circumference greater than
35 inches (88 centimeters). In fact, simple waist measurements alone seem to be
the best predictor of the type of body fat distribution that is most closely
associated with insulin resistance.
The normal limits that have been established for the BMI and waist
circumference are not based on considerations of appearance. Rather, they are
based on the significant risk of developing complications due to being
overweight (BMI-related morbidity), such as coronary artery heart disease and
diabetes. In children, however, normal ranges of the BMI vary according to age
and gender. Furthermore, up until the age of 16, the upper limits for the normal
BMI are lower than 25. Therefore, in children, the degree of overweight is more
often expressed as a percentage above ideal body weight.
Next: What are the estimated numbers of cases of NAFLD and NASH? »
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