Experts have said that obesity can be prevented if people focused on snacking less and reducing the portion of food each time they ate.
A recent study revealed that increases in the number of eating occasions and portion size are responsible for tipping the scale for many Americans.
Obesity levels have risen sharply in many Western countries since the 1970s. In the US, where the study was carried out, a third of all adults - more than 72 million people - are now categorized as obese.
A team from the University of North Carolina analysed data from food surveys carried out in the seventies, eighties, nineties and the last decade.
The surveys record all food and drink a person consumes over a 24-hour period. The average daily energy intake of a US citizen increased from 1,803 kcal in 1977-78 to 2,374 kcal in 2003-06. In the last decade of the study alone, the average daily calorie intake went up by 229 kcal.
Several factors are involved in energy intake - the number of calories (energy) in a specific amount of food (energy density), portion size and how many meals and snacks a day eaten.
The researchers say that while all of these have gone up, increases in the number of eating occasions and portion size seem to account for most of the change.
They suggest efforts to prevent obesity should focus on reducing the number of snacks and meals a day as well as portion size.
The study has been published in PLoS Medicine.