Four-fifths of Americans who decide to embark on a vegetarian diet will eventually return to eating meat, a study indicates, many of them within just months.

Eighty-four percent of vegetarians and vegans go back to eating meat or other animal products, half of them within a year's time and more than a third within three months, data released by the animal advocacy group Humane Research Council and Harris International shows.

The HRC set out 3 years ago to study the factors involved in someone's decision to become a vegetarian or a vegan.

Both both choose not to eat meat; vegetarians will consume dairy products and eggs while vegans avoid all animal products.

The new study provided "some potentially disappointing but illuminating conclusions," the HRC acknowledged.

Of the 11,000 Americans who responded to a survey, only 2 percent said they don't eat meat at all, while 88 percent said they have never tried being vegetarians or vegans.

And of the 12 percent who've tried a vegan or vegetarian lifestyle, five out of six go back to eating meat at some point.

"It's obviously a negative for animals," said HRC Executive Director Che Green.

Analyzing the factors that cause vegetarians or vegans to lapse, the researchers pointed to a lack of social support from partners or family and a dislike for being seen as "different" by their friends and social peers based on their dietary preferences.

Giving up meats completely and suddenly -- going cold turkey, so to speak -- puts vegetarians and vegans at risk of cravings that can be a real diet killer, says New York nutritionist Lisa Young.

"They say, 'I'm never going to eat that again,' " says Young, who warns against an all-or-nothing approach. "If you start by eating smaller portions of pork or chicken, then cutting out all meat and dairy for a month, you can get a better feel for it."

There is some encouraging news from the study, the researchers said; 37 percent of lapsed vegans and vegetarians spoke of a desire  to go back to a non-meat diet at some time in their future.

And the animal rights group's Green, although calling the study's finding "disappointing" overall, said he saw some positives in the study's data.

"The fact that so many people are trying vegetarianism is a higher number than I would have expected, so that's good," he said.