In last blog I made the following observations:
1. The
world’s population is becoming more obese, and the United States is leading the
pack.
2. The
most common measure of obesity is “body mass index” or BMI.
3. Obesity
has many negative health implications.
4. But
there have been reports that obesity confers a health advantage (the “obesity
paradox”).
5. The
most recent thinking is that the obesity paradox is an artifact of the BMI
classification; when measures other than BMI have been used to calculate fatness,
the obesity paradox did not hold up.
6. Recent
data indicates that about 40% of obese people are actually “fit” and
metabolically normal; they do not have increased cancer, cardiovascular disease,
or morbidity.
7. As
of 2008, obesity cost the US $147 billion/year, or $1,429/year for each obese
person.
In this blog I want to focus on the two primary explanations
for obesity. But first, there is a
“law of nature” and a definition to cover.
The Law of Conservation of Energy (also known as the First
Law of Thermodynamics) is usually forgotten when it comes to discussion of
diets and dieting. It tells us
that energy can not be created or
destroyed. This natural law was
popularized by Einstein’s famous equation E=mc2, which states that
mass and energy are interchangeable.
That means you can’t gain weight (increase your mass) unless
you get energy from somewhere. E=mc2
tells us that food, having mass, also has energy. And since you, as a human, can’t get
energy directly from the sun through photosynthesis the way plants do, you get it
from food instead. By eating.
You can conclude both from everyday experience and the First
Law that as long as you are alive, you are expending energy, and if you don’t
eat, you will get the energy you need to continue living from energy reserves within
your own body. Energy is stored in
your body in several different forms (fat, protein, creatine, etc.), but for purposes
of an obesity discussion, the only one that is important is fat.
If you don’t eat, you will burn fat (and then muscle). WWII supplied ample evidence of the
effects of not eating—through photographs of starving prisoners in
concentration camps. There were no
fat people there. And if you’ve
ever read about the exploits of adventurers who ran out of food in such hostile
places as Antarctica or the Sahara, you know that the descriptions of their
gradual emaciation are quite memorable.
Anyone who stumbles 5-10 miles a day at -20F or +120F on limited or no
rations will end up losing weight (fat).
The Law of Conservation of Energy, being a law, applies wherever you are.
It even holds up at your kitchen table. If you don’t eat, i.e., replace the
energy your body expends, you will consume your body fat until there is no more
fat to consume. Your body has to
get energy from somewhere.
At this point we should define how energy is measured
relative to food. As I’m sure
everyone knows, the unit of measurement is a “calorie,” defined as the amount of energy it takes
to raise one gram of water one degree Celsius. (Okay, this is actually the definition of a “small calorie.”
The calorie we hear about relative
to food and nutrition is a “large calorie,” equal to 1,000 small calories. I don’t have a clue why there are two
different definitions, except that using large calories keeps us from having to
deal with a bunch of zeroes when we’re counting them.)
In any event, the energy in food is measured in calories per
unit of weight. If you read the
nutrition information on a candy bar wrapper, for example, you will see that the
candy inside has a certain number of calories per gram. Or per ounce. Sometimes this is referred to as a food’s “energy density.”
In the course of the next three or four (!) blogs, I want to
look at several diet-related explanations for obesity. There are two “main” theories: too many calories or not enough
exercise. Then there are
what I think of as the “minor” explanations, all of which can be lumped
together as the “depends on WHAT you eat, not just calories” theories.
Too many calories. We take in more energy (consume more calories) than we
expend through exercise. We are simply
the victims of our own success in being able to feed ourselves, and the reason
for the obesity epidemic is because we’re taking in more calories than we used
to.
It is no coincidence that those countries with the highest
rates of obesity also consume the most calories. In fact, the average person in the 10 countries with the
highest caloric intake (United States, Austria, Greece, Belgium, Luxemburg,
Italy, Malta, Portugal, France, Israel) consumes 3,653 calories each day and has
a BMI of 25.1. On the other hand, the
average person in the 10 countries with the lowest caloric intake (Eritrea,
Democratic Republic of the Congo, Burundi, Haiti, Comoros, Zambia, Ethiopia,
Angola, Central African Republic, Republic of Tanzania) consumes only 1,830
calories each day and has a BMI of 21.36.
Here is a chart about world energy
consumption that pretty much says it all: we just keep eating more and more.
The worldwide consumption of
calories has been steadily increasing since 1960, and obesity has been
increasing at the same time. Here
in the United States, caloric consumption started increasing about 1980, as
shown in the graph below.
Interestingly, that is the same time that obesity in the United States
started increasing too (see graph in
previous blog).
http://www.usda.gov/factbook/chapter2.pdf
So, we have a worldwide correlation between increasing
caloric intake and obesity, as well as a similar correlation in the United
States.
Kind of makes you think that obesity has increased as a
result of overeating, no?
Not enough exercise. As you will recall, the other primary explanation for the
obesity epidemic is that we’re not getting as much exercise as we used to. But has energy expenditure really decreased? That is, have we become more sedentary?
This is important because what
makes us gain weight is NET calories, i.e., those that are consumed but not
used up by activity.
After all, in order for weight
to remain stable, we must adhere to the following equation: calories in = calories out.
1988–2008 No Leisure-Time Physical
Activity Trend Chart
http://www.cdc.gov/nccdphp/dnpa/physical/stats/leisure_time.htm
This graph reflects the percentage of U.S. citizens who
engage in no physical activity during their leisure time. Note that during the 20-year period from
1988 to 2008, the percentage of people who do engage in leisure-time physical
activity actually went UP (the sedentary percentage dropped from 30% to 25%).
Additionally, the Centers for Disease Control has developed
a map of the U.S. that correlates particular counties with three related
factors: obesity, no leisure time
physical activity, and diabetes.
Again, the data seem to make a pretty convincing argument
that obesity is caused by too many calories—those parts of the U.S. that are the
most obese are also the least active, and by inference have the least calorie
expenditure. And conversely, the
places with the least obesity expend the most calories.
However, a recent (2011) study reported counterintuitive
results when the energy expenditure of developed countries was compared with
that of developing countries. This
was a meta-analysis of 98 studies in which the energy expenditure of 4,972 subjects
(both male and female) was measured using the “doubly labeled water” technique,
considered the gold standard. In
this procedure, the test subject drinks water tagged with heavy isotopes of
hydrogen and oxygen, and then several days or even weeks later, the CO2 in
the subject’s respiration is analyzed to determine how much heavy oxygen it
contains. Since CO2 leaves
the body only as a result of metabolism, the heavy oxygen that remains in subject’s
CO2 at the end of the study provides a direct measure of total energy
expenditure during the study. One
of the beauties of this technique is that it integrates energy expended in all of
a person’s activities, from sleeping to working. (Interestingly, this same procedure has been used to measure
the activity levels of more than 200 wild animals.)
So, what were the results? Essentially, they found no difference in energy expenditure
between developing and developed countries. Individuals from developed countries were fatter (as
measured by BMI and weight), but their energy expenditure was the same.
The authors conclude that increased obesity is primarily the
result of eating too much rather than not exercising enough, and they cite many
other studies that have reached the same conclusion.
In the same vein, a widely-cited 2012 study compared the
energy expenditure of Tanzanian hunter-gatherers belonging to the Hadza “tribe”
with the energy expenditure of U.S. and European males and females
(“Westerners”). Here too, the
researchers found no difference between the two populations—their levels of
activity were essentially the same even though at the time of the study, the Hadza
were actively engaged in both hunting and foraging. But the Westerners were fatter (more body fat as well as higher
BMI’s).
These two studies strongly suggest that there is no
relationship between increased “fatness” and decreased energy expenditure. And in fact, the study authors state
this pretty unequivocally.
Is it what we eat? When looking at changes in obesity over time, it is
important to note that the TYPE of foods we consume has also changed. Thus our attempts to identify the causes
of the obesity epidemic are confounded by the fact that increased caloric
intake has been accompanied by a change in diet.
And that brings us to another explanation for our obesity: it’s related to WHAT we eat rather than
just calories in and calories out. Consider a chunk of firewood, for example. It has lots of calories—but good luck
deriving any energy from it.
Unless you are a termite.
So let’s look at our diet here in the United States. You can see from the following chart that it changed quite a
bit from 1909 to 2000. In
particular, consumption of sugars and sweeteners increased 7%, and fats and
oils increased 10%. But grains
actually decreased.
http://www.cnpp.usda.gov/publications/foodsupply/FoodSupply1909-2000.pdf
Now take a look at the following graph. Note that consumption of carbohydrates,
as a class, remained fairly stable—if anything, our carb consumption in 2005
was a little lower than it was in 1909.
http://www.ers.usda.gov/data-products/food-availability-(per
capita)-
data-system/summary-findings.aspx#.Uknv3WTXhUh
However, carbohydrates are a very big class that includes
grains, bread, potatoes, and other starches, as well as sugar. In fact, what’s happened is that
although carbohydrates from grains have decreased, carbohydrates from sugars
have increased. More about sugar later.
It is also interesting to look
at the KINDS of fats that we are consuming—and to wonder whether some types of
fats might also be correlated with obesity.
Saturated,
monounsaturated, and polyunsaturated fat in the U.S. food supply, per capita
per day, 1909-20000
http://www.cnpp.usda.gov/publications/foodsupply/FoodSupply1909-2000.pdf
This graph
shows that there was no increase in consumption of saturated fats in the United
States from 1909 to 2000, so we can’t blame the obesity epidemic on that. What HAS increased, however, is the
consumption of both monounsaturated and polyunsaturated fats. I’ve already discussed fats in an
earlier blog and I don’t want to bore you by repeating myself, but let me say it
again anyway: saturated fats have been unfairly maligned
over the years. Go ahead and enjoy
that juicy steak.
So, the conclusions reached here are as follows:
Obesity is correlated with increased caloric intake but
probably NOT with decreased activity.
There has also been a substantial increase in consumption of sugars as
well as monounsaturated and polyunsaturated fats.
We’ll try to sort out these correlated factors next
time. But, it seems to me that
anyone who is interested in losing weight need only read the previous paragraph—and
behave accordingly.
References
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0040503
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