Human Limits

My recent focus on physical activity and obesity leads to an important question.  How much of the big increase in obesity rates seen in the U.S. over the last 30 or so years is due to what we eat?  Like some of my earlier posts on physical activity and obesity this one is going to be data intensive.

 

We are eating more!

The U.S. Department of Agriculture (USDA) reports interesting statistics on what people are eating in the U.S.  The figure below shows trends in calories per person.  The top line is total food supply.  The bottom line is an estimate of what people might actually be eating.  As the title of slide indicates there has been about a 19% increase sine the early 1980s.  This is equal to about 400 calories per day!

Clearly these are average statistics and there is all sorts of individual variability related to what people eat.  However, the story for society as a whole is pretty alarming.  If these changes in consumption are not offset by at least some increased energy expenditure this would work out to a pound of weight gain every 9 days or about 40 pounds (18kg) per year based on the idea that a pound of fat is 3500 calories of excess energy consumption.

 

 

The USDA statistics about increased calorie consumption have been challenged and better estimates are shown below for all adults (ALL), normal weight (NW), overweight (OW), and obese (OB) adults in the U.S.  These data compare estimates made in the early 1970s to data from 2005-2006.

 

 

 

So, if we use these data, about 200 calories per day, then we would expect people to gain about 20 lbs (9kg) per year.  Also remember from my earlier posts only about 5% of people are meeting the guidelines for physical activity and there has been an average loss of about 100-150 calories a day of physical activity at work for many people.

When you see statistics like this the question isn’t why is the population getting fatter, the question is why isn’t the population getting even fatter!

 

We are less fat than predicted!

The figure below shows the average adult is “only” about 10kg heavier now then in the early 70s when caloric consumption starts to rise.  This figure comes from an excellent paper with a lot mathematical modeling on how and why our energy expenditure goes up when we eat too much and why it goes down when we diet.

 

1975                                                                                                          2010

 

Here is a link to a simulator you can use to project changes in your body weight with dieting using the computer models that generated the simulated changes in body weight noted above.

So the good news is that it is actually pretty hard to gain weight and that  the combination of a lot of extra calories and a lot less activity is required to pack the pounds on.  The bad news is that these same principles apply in reverse and make it hard to lose weight.   The figure below from the modeling paper is an example or what happens to most people when they try to lose weight.

The bottom panel shows a baseline energy consumption of 10 Mega Joules or about 2500 calories per day.  Dieting starts and people reduce their caloric consumption by 30-35% but unless they really start to exercise the energy consumption also goes down.  After a couple of months their eating begins to pick back up, but energy expenditure is still down and weight begins to creep back up over the next couple of year.   This pattern of weight gain is shown in the top panel.  The other interesting thing is just how much inter individual variation there is both total weight loss, energy expenditure and how they interact over time.

 

 

The concepts outlined in this figure explains why the National Weight Control Registry data on successful long term “losers” shows that a combination of a low calorie diet and high levels of physical activity are required to take and keep weight off over time.  In coming posts I will try to figure out where the extra calories are coming from over the last 30-40 years and why there is not a linear relationship between eating less (or becoming more active) and losing weight.

 

This entry was posted on Monday, September 3rd, 2012 at 6:10 am and is filed under Current Events, Research and Health. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.