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).


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


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.


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.

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

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.