Inexpensive food may be a key factor in rising obesity, according to a recent study.

A new review, authored by Roland Sturm of RAND Corporation and Ruopeng An of the University of Illinois at Urbana-Champaign, suggests that Americans having the cheapest food available in history is a factor fueling the obesity epidemic in the United States.

Today, two in three Americans are overweight or obese, with rates climbing steadily over the past several decades.

"Americans are spending a smaller share of their income (or corresponding amount of effort) on food than any other society in history or anywhere else in the world, yet get more for it," researchers said in their study.

For example, in the 1930s, Americans spent one-quarter of their disposable income on food. By the 1950s, that figure had dropped to one-fifth. The most recent data show the share of disposable income spent on food is now under one-tenth.

Sturm and An said other factors, such as the rise of electronic entertainment, increased use of cars, a shift in jobs away from those with physical demands, and increased urbanization, also contribute to obesity.

However, they say the evidence for those associations is not as strong as inexpensive food.

"Examining time trends for which there are data, what jumps out are changes in food availability, in particular the increase in caloric sweeteners and carbohydrates," they said.

They also said that policy interventions that focus on "positive" approaches or messages, such as increasing fruit and vegetable consumption and increasing physical activity, may not be the optimal approach. They conclude a more promising tactic is an emphasis on reducing calorie consumption; particularly sugar sweetened beverages and salted snacks.

 "Although increasing fruit and vegetable consumption may be a laudable goal for other health reasons, it is unlikely to be an effective tool for obesity prevention," they said.

The findings were recently published in the journal CA: A Cancer Journal for Clinicians.