CT scans reveal whether abdominal fat is stored under the skin (top -
subcutaneous fat) or has accumulated around the internal organs (bottom
- visceral fat). Kevin Davy is determining how visceral fat is associated
with high blood pressure.
Kevin
Davy uses high-resolution ultrasonographic imaging of the carotid artery
to quantify arterial stiffness. Photo by Michael Kiernan.
Computed tomography (CT) imaging reveals abdominal fat.
Individuals with
an "apple," or abdominal, fat distribution pattern are at a
substantially higher risk of developing cardiovascular and metabolic diseases
compared with those with a "pear," or lower body, fat distribution
pattern.
BMI — The math of obesity
Are you overweight or obese? Your mirror may not lie, but it can be subjective,
so scientists use numbers based on the body mass index, or BMI.
It’s pretty straightforward. If you are over 18 years old, you divide your
weight by your height squared. If the result is between 18.5 and 25, your
weight is healthy. If the number is over 30, you are obese.
Didn’t work? That formula works with weight in kilograms and height in meters.
Using pounds and inches, do this:
Multiply your height in inches by that same number. If you are 5 feet, you multiply
60 times 60, which is 3,600. Divide that number into your weight. If you
weigh 120 pounds, you end up with .0333. Now, to return to the metric
world, you must multiply that number by 703 (the Northern Virginia area
code). Our 5-foot, 120-pound arithmetic word-problem model has a BMI of
23.
Back
to the mirror. If your BMI is high and you wear your fat around your stomach,
your risk of developing cardiovascular and metabolic diseases is also
high.
Over the years, Virginia Tech's College of Agriculture and Life Sciences
has been known for its research in apple and pear production. These days,
the college is taking a more "human" approach to the subject
of apples and pears. Kevin Davy, associate professor of human nutrition,
foods, and exercise, and his colleagues are studying people with apple-
and pear-shaped body types to learn more about how different types of
obesity influence cardiovascular physiology and overall health.
Obesity has reached epidemic proportions, says Davy, citing some very
telling statistics. "Approximately 65 percent of the population is
considered to be overweight and 31 percent is considered to be clinically
obese." Individuals are considered to be clinically obese if they
have a body mass index (BMI) of greater than 30. BMI is a measurement
of body fat content found by dividing an individual's weight in kilograms
by their height in meters squared. Overweight and obese individuals are
at increased risk of developing heart disease, high blood pressure, stroke,
diabetes, and many forms of cancer. However, there is considerable variability
in the relationship between obesity and chronic disease. Individuals with
an "apple," or abdominal, fat distribution pattern are at a
substantially higher risk of developing cardiovascular and metabolic diseases
compared with those with a "pear," or lower body, fat distribution
pattern. Big hips are better than a big stomach.
Davy initially became interested in obesity while conducting postdoctoral
research at the University of Colorado in the area of aging and cardiovascular
physiology. "We always excluded people who were overweight because
it was our impression that they would somehow confound our understanding
of how aging influenced the cardiovascular system. I became intrigued
with understanding how the people we were excluding differed in their
physiology."
Since 1999, Davy's research has been focused on determining the role
of body fat distribution patterns on the sympathetic nervous system (SNS),
that part of the nervous system that speeds the heart, contracts blood
vessels, and initiates other physiological reactions to mobilize the body
for action in response to stress. The sympathetic nervous system plays
a critical role in metabolism and cardiovascular physiology.
"The sympathetic nervous system has been implicated as both a cause
and consequence of obesity. Low levels of SNS activity may contribute
to weight gain and obesity development while high levels of SNS activity
may increase the risk of developing hypertension and other cardiovascular
diseases. There was no consensus in the literature and you could identify
literature to support your favorite hypothesis. Unfortunately, these previous
studies relied on measurements of SNS activity that were difficult to
interpret because of confounding factors. To overcome these pitfalls,
we have used direct recordings of the SNS from peripheral nerves using
a neuro-physiological technique." This approach, termed "microneurography,"
requires the placement of very small needle-like electrodes through the
skin and into sympathetic nerves in the arm or leg of willing subjects.
Through his research, Davy has found that some, but not all, obese individuals
have increased SNS activity. "As it turns out, SNS activity is increased
in those with increased levels of intra-abdominal fat," says Davy.
This may explain, at least in part, the increased risk of hypertension
observed in overweight and obese individuals. "People with abdominal
obesity are at the greatest risk for developing virtually all of the diseases
that we know are associated with obesity," Davy points out.
The apple body fat distribution pattern is typically characterized by
an excess accumulation of intra-abdominal fat (or fat located around the
internal organs), whereas the pear body fat distribution pattern is characterized
by subcutaneous (under the skin) fat storage in the buttocks and thigh
regions.
According to Davy, abdominal obesity (also called visceral obesity) is
associated with a number of cardiovascular and metabolic disease risk
factors that cluster together in an individual. The clustering of these
risk factors is referred to as the metabolic syndrome. These risk factors
synergistically increase the risk for cardiovascular disease. "If
clinical disease is not already present, then these individuals are frequently
pre-diabetic and pre-hypertensive. They are on the fast track to the clinical
expression of these diseases if drug and/or lifestyle intervention is
not initiated," says Davy.
Davy is continuing his efforts to understand the consequences of visceral
obesity on cardiovascular health and physiology. He has been particularly
interested in understanding how visceral fat links obesity with high blood
pressure. "In addition to elevated SNS activity, individuals with
excess visceral fat may exhibit reduced compliance, or elasticity, of
the major arteries. Arterial compliance is defined as the ability of an
artery to expand and recoil during the contraction and relaxation of the
heart. We know that reduced arterial compliance is related to a number
of adverse outcomes, particularly high blood pressure," says Davy.
"All these risk factors that comprise the metabolic syndrome have
negative influences on blood-vessel health. Arterial stiffening is considered
to be a very early event in the atherosclerotic process. All of these
risk factors converge to reduce the compliance of an artery."
Arterial stiffening has its own consequences, Davy points out. The heart
has to work much harder to pump blood through a stiff artery than through
a compliant one. What does the heart do to be able to accommodate this
elevated workload? It gets bigger. That's not a good thing.
There are other consequences of arterial stiffening that impact the body's
ability to regulate blood pressure. For example, when an individual moves
from lying down to standing up, the heart rate speeds up and blood vessels
constrict to help keep blood pressure from going too low. If blood pressure
goes too low, a person might faint.
"The ability to increase the heart rate in response to falling blood
pressure, for example, appears to be blunted in individuals that have
less compliant arteries. This may be related, in part, to the fact that
some of the receptors that signal the brain to increase heart rate when
blood pressure falls are located in the aorta and carotid arteries,"
explains Davy. "We think that visceral obesity leads to arterial
stiffening as a result of the risk factors clustering in the metabolic
syndrome, but arterial stiffening also has direct effects on the structure
and function of the cardiovascular system. Over time, reductions in arterial
stiffness can lead to high blood pressure."
Many researchers are trying to understand why these risk factors cluster
together in the same individuals and how this clustering of risk factors
alters disease risk. Davy and his colleagues are taking a slightly different
approach by examining whether the change in the amount of visceral fat
is associated with predictable changes in the structure and function of
the cardiovascular system.
With funding from the National Institutes of Heath, Davy and his multidisciplinary
team of researchers are having overweight and obese people lose weight.
"Our plan is to compare those individuals with relatively large and
small reductions in visceral fat with weight loss. We expect that subjects
with the largest visceral fat loss will demonstrate the greatest improvement
in the structure and function of their cardiovascular system and vice
versa," says Davy. "We are also interested in determining whether
any observed improvement in arterial stiffness can be maintained with
sustained lower weight."
According to Davy, the available literature suggests that there is a
linear relation between the amount of visceral fat and the degree of arterial
stiffness among individuals with varying degrees of total body and visceral
fat. However, the relation between visceral fat and arterial stiffness
may not be cause-and-effect, explains Davy. "It could mean that the
person with elevated visceral fat has a particular genetic make-up or
constitution which is also associated with stiffer arteries. We need intervention
studies," he says.
Davy is hoping that he can verify his hypothesis that weight loss, particularly
from the abdominal visceral region, will reduce arterial stiffness. He
anticipates that reductions in arterial stiffness will also have a favorable
impact on other aspects of cardiovascular structure and function.
Davy's research subjects go through a battery of testing to determine
their eligibility for participation. The individuals are obese and may
have risk factors for developing disease, but they cannot be so far down
the road that it would be unsafe for them to participate. Subjects must
be free of any overt disease and cannot currently have diabetes or coronary
heart disease. Davy works closely with Jose Rivero, a cardiologist from
Heart Specialists of Southwest Virginia in Christiansburg, to determine
each individual's eligibility.
Body composition and abdominal fat distribution measurements are critical
measurements for the study. Body composition of each participant is measured
using a low level X-ray device in Davy's laboratory called a dual energy
X-ray absorptiometer (DEXA). Subjects are also sent to Montgomery Regional
Hospital, where computed tomography (CT) scans of their abdomen are performed.
The CT scan allows Davy to precisely measure the amount of visceral fat
for each subject.
Davy also measures the stiffness of the arteries and heart size and function
using high-resolution cardiovascular ultrasound. He measures the distance
between the walls of the artery (major artery in the neck) when the heart
is contracting and when it is relaxing. An artery's compliance can be
estimated by relating the change in diameter of the artery to the change
in blood pressure during the contraction and relaxation of the heart.
Once baseline testing is complete, subjects are randomly assigned to
a test group or a control group. The test group undergoes a weight loss
program by reducing calories over three to four months. The subjects lose
approximately 10 percent of their body weight during that time period.
After maintaining this weight for one month, the battery of testing is
repeated. The individuals assigned to the control group are instructed
not to change their diet or physical activity over the same period of
time.
Arterial stiffness is a surrogate measure of cardiovascular disease,
according to Davy. "It is an early form of atherosclerosis. Coronary
artery disease is already well developed by the time someone may need
to visit the emergency room for chest pain. We can detect arterial stiffening
way before a person actually has the symptoms of cardiovascular disease,"
he says. "We can most effectively prevent disease by intervening
early."
The best thing a person can do is to avoid obesity, states Davy. He suggests
a low-fat, high-fiber diet along with plenty of physical activity. "The
key is to avoid becoming obese in the first place," he says. "Preventing
further weight gain should be the number one priority for individuals
who are already overweight or obese."
Most people can lose weight but maintaining weight loss over time is
much more difficult. Davy suggests setting reasonable and attainable weight
loss goals. "Becoming thin is not a reasonable goal for most obese
individuals." Weight-loss should be slow, about one to two pounds
per week. Even modest weight loss of 5 to 10 percent (e.g., 10 to 20 pounds
for a 200-pound person) can dramatically reduce the risks associated with
obesity. Including physical activity as part of the weight management
program should improve a person's likelihood of success.
