Diet and Obesity MiraCosta College


Uploaded by MeerdesIrrtums on 22.03.2012

Transcript:
Let’s start with the basics of body mass index or BMI—this is the metric used to
classify people as underweight, normal weight, overweight, or obese—and there are three
classes of obesity based on BMI values.
Your BMI is your weight in kilograms, divided by the square of your height measured in meters.
If you weigh 100 kilograms (roughly 220 lb) and you are two meters tall (just under six
feet seven inches), your BMI would be 100 (kilograms) divided by four (2 meters, squared)
, and 100 divided by four is twenty five. According to the BMI classification, a BMI
value of 25 would be on the borderline between normal weight and overweight. Let’s see.
Michael Jordan is 6’6” and his playing weight was 195. Larry Bird was 220 lb. but
he’s also taller at 6’9”. Neither of these NBA stars would have been classified
as overweight—can you see why?
Okay. Now it’s time for you to calculate your own BMI. Start by converting your weight
into kilograms—remember how to do this? It was in your first lab. I’m not going
to tell you. Okay so also calculate your height into meters. Be as precise as possible—don’t
guestimate. And be honest, because no one’s watching. Square your height in meters. Now
take your weight in kilograms and divide that by the squared value of your height in meters.
That’s your BMI.
Now look at the chart. Where do you fit?
As far as your health and well-being is concerned it’s generally better to be leaner. To put
things very bluntly, obesity dramatically increases the risk of death. Period. This
is demonstrated by this graph in rather stark terms. The height of the bars shows the relative
risk of death for different values of BMI for nonsmoking white men. For men with BMI
corresponding to slightly underweight to normal weight the risk value is approximately 1.0.
For obese men with BMI values between 35 and 39, the relative risk of death is about 2.0—twice
that of normal weight men. For even more severely obese men with BMI above 40, the risk is nearly
triple.
Roughly the same pattern exists for women though it’s not quite as severe.
Obesity kills, and it does so by increasing the risk of debilitating or lethal medical
conditions, including type 2 diabetes, which we discussed earlier, but also cardiovascular
disease and certain types of cancer like breast cancer, colorectal cancer, and non-Hodgkin’s
lymphoma. It also compromises a person’s quality of life through other impacts on one’s
physical and mental health.
Now obesity is also considered a preventable cause of death—preventable in the sense
that a person can avoid the added risk of death by not being obese. Smoking used to
be the leading preventable cause of death in the U.S., but with the decline in the prevalence
of cigarette smoking and an increasing prevalence of overnutrition, obesity has now overtaken
tobacco use as the leading cause of preventable death in our country. Cigarettes continue
to kill lots and lots of people, but junk food is killing even more Americans than are
cigs these days. It’s a pretty amazing statistic and one that has us scrambling.
At the level of a single person, the causes of obesity are pretty easy to identify—diet
(overconsumption of calories), a sedentary lifestyle, and to a certain extent genetics—but
there are other factors to consider as well, such as a person’s mental state and sociocultural
influences. We’ll focus on diet here. I think I’ve already told you once to get
off your butt and be more active.
The core of the problem seems so simple—people gain weight if they absorb more food-calories
than they burn. Remember the video on fat metabolism? All the excess of Acetyl CoAs
that result from your consumption of more food molecules than you can use for ATP synthesis,
these are stored as adipose tissue or fat. If Americans are getting larger, it’s because
they are eating more calories, while they may also be burning off fewer calories if
their lifestyles are becoming more sedentary.
And it’s not just a problem in the U.S. Obesity is increasing worldwide—and it’s
because people are eating more calories. Obviously. The harder question is why are people eating
more now than they were before? Understanding the causes of increasing obesity as a public
health concern is a more challenging question than “what causes a person to gain weight?”
And there are several models—i.e., explanations—that can be offered. We’ll look at three and
consider the evidence supporting each model.
The first model states that there is simply more food available for people to eat—more
now than ever before. According to the FAO (Food and Agricultural Organization of the
United Nations), in 2002 the amount of food that was available in the United States came
out to 3770 kcal worth of food every day for every man, woman, and child—a lot more than
what the average person needs. I’m not saying that all of these calories were consumed,
quite a bit of it was simply wasted. How much was actually consumed (as opposed to being
thrown out) is a much more difficult thing to keep track of than the amount of food that
was out there and available for people to eat. Compare this with 1961 during which the
estimated number of calories available per capita was under 3000 in the U.S. This overabundance
of food is most pronounced in developed nations. In sub-Saharan Africa the food availability
in 2002 was 2176 kilocalories per day (much less than the U.S.).
The second model relates increasing obesity of the U.S. population with changes in the
availability of the high-calorie/low-nutrition foods that we were discussing earlier. This
chart shows the increase in calories consumed over the second half of the 20th century—and
this is accounted for by increased consumption of carbohydrates—it looks like the consumption
of fats and proteins stayed about the same. Now check out this graph. Over that same period,
sweetened carbonated beverages—soda—is being consumed in higher and higher quantities
and it’s almost creepy the way the increase in soda consumption corresponds nearly perfectly
with increases in carbohydrate intake.
The basic argument of the third model is that the standards for portion size have changed.
To give an example, the original McDonald’s hamburger patty was 1.6 oz. of ground beef,
and the total calorie count for a cheeseburger was around 300 calories—in 1960, this would
have been the centerpiece of a standard fast-food meal. In contrast, a “modern” fast-food
single-patty cheeseburger, the Carl’s Jr. original six-dollar burger, has considerably
more meat and 991 calories. There’s a recursive dynamic that has gotten us to this point:
if you put more food in front of people, they will eat more. Over time, people become conditioned
to expect larger and larger portions. Food suppliers are pressured to accommodate this
demand by delivering more and more food, or else they will be outcompeted by other suppliers
who are willing to crank up their serving size.
So those are the three models I’m presenting here, and I hope that you’re thinking to
yourself that there’s not going to be a single answer to this complex pickle that
we’re in—it’s not a matter of “this model is the right one and the other two are
wrong.” Like a lot of phenomena that science attempts to explain, the causes are sometimes
numerous, and this is certainly the case here.