As you may know, we are in the midst of what many experts call an obesity epidemic in which two-thirds of adults are above a healthy body weight and half of those are classified as obese.

More than one-third of children ages 2 to 18 are above a healthy body weight, and 19 percent of school-aged children and 10 percent of infants, toddlers and preschoolers are considered obese.

It wasn’t always this way. As recently as 1980, the prevalence of obesity in adults was less than 15 percent and about 5 percent in kids. This means that obesity has more than doubled in the past 30 years!

The big question is what caused this increase in obesity in our population?

Unfortunately, there are no easy answers. Weight gain is the result of an imbalance between the calories people eat and the calories they burn through activity. Even a small difference each day can add up over time.

So, what is it? Are we eating more? Or are we less active? Probably both, but lower levels of activity may play a larger role.

One way to find answers is to look back at what life was like 100 years ago. A century ago most people did physical labor and were active for most of the day, even though participation in exercise was rare. They didn’t worry so much about what they ate, but obesity was uncommon.

Fast forward to today when being overweight is the norm. Most people are inactive for much of the day since most jobs require little to no movement. Even jobs that involve physical activity require less now than they used to due to mechanization and other labor-saving devices.

Food is more readily available and portion sizes are bigger than ever before. At the same time, though, the consumption of low-fat and reduced-calorie foods has increased.

In fact, the average calories per day in the American diet isn’t much different now than it was 100 years ago.

Unfortunately, there is no good data about how active people were 100 years ago. But a researcher at the University of Tennessee, Dr. David Bassett, came up with a way to measure how active people likely were back then. I call it his physical activity time machine.

He visited an Old Order Amish community and observed physical activity, diet, and body weight. No surprise, the prevalence of obesity among the Amish was low – 4 percent.

A look at their diet reveals that it is not much different from the typical American diet. They ate less processed food, but the balance of carbohydrates, fat and protein was similar to what most of us eat.

What was different was physical activity. The Amish spent much of their day engaged in active work. Amish men and women spent almost 40 hours per week engaged in moderate activity.

Compare that to the fact that less than 50 percent of Americans get 2.5 hours of moderate activity per week now.

On average, Amish adults took more than 16,000 steps per day; the typical American gets about 5,000!

There is no question that what we eat now is different than what we ate decades ago and contributes to the obesity problem. But a trip in Dr. Bassett’s time machine shows that low levels of physical activity are at least as important as changes in the diet.

Achieving the high levels of activity common among the Amish is unlikely, but you can make an effort to include more activity in your day. Spending less time sitting, more time moving around, and dedicating time to be active everyday are three easy ways to increase your level of physical activity.

Brian Parr, Ph.D., is an associate professor in the Department of Exercise and Sports Science at USC Aiken where he teaches courses in exercise physiology, nutrition and health behavior.