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Snacking and super sizing are two of the dieter's worst enemies, research suggests.

The average daily calorie intake in the US has increased by almost a third in 30 years, reaching 2,374 kilocalories.

The influence of bigger portion sizes and excessive snacking outweighs the shift towards high-calorie foods, say experts.

Focusing on reducing how much and how often people eat could help tackle obesity, they report in .

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 categorised 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.

"These findings suggest a new focus for efforts to reduce energy imbalances in US adults," write Kiyah Duffey and Barry Popkin of the University of North Carolina in the journal PloS Medicine.

Commenting on the paper, Dr Áine O'Connor, a scientist at the British Nutrition Foundation said: "Many factors influence total energy intake that can lead to [being] overweight and obesity but it is possible that having more eating occasions through the day, for example by frequent snacking, would increase calorie consumption and so lead to weight gain.

"This study also looked at portion size and studies have shown that having larger portions of food leads to an increased intake.

"The researchers were based in the US, but many of the factors causing the obesity epidemic there are mirrored in the UK and, for those trying to control their weight, it is important to manage both how much and how often they eat."