Obesity is joining and even surpassing malnutrition as a dietary concern in some of the farthest reaches of the planet, experts warned Saturday.

Weight problems have long been recognized as a health hazard in the United States, Europe and other industrialized places, but in recent years the same worries have begun to emerge in many less well-off places.

At the annual meeting of the American Association for the Advancement of Science on Saturday, biological anthropologists documented this trend, both in people who migrate to wealthy countries and in those who stay put.

"Obesity has penetrated the remotest places on Earth,'' Stanley Ulijaszek of the University of Oxford said, adding that too little food, however, is still a more important concern than too much.

A recent Vatican conference concluded that about 800 million people worldwide are underfed, while the International Obesity Taskforce estimates that 300 million are obese.

Nevertheless, experts say obesity is becoming an issue in hard-to-reach areas where it was unknown just a few years ago. In many parts of the world, malnutrition and obesity now exist together, one a problem of the very poor, the other of a growing middle class.

"The recognition that this is a worldwide problem is very recent,'' said Marquise Lavelle of the University of Rhode Island.

Ulijaszek said obesity has begun to appear in the Purari delta of rural Papua New Guinea, where there was none at all in 1980. In the latest survey, conducted five years ago, 1 percent of men and 5 percent of women were found to be obese. This is defined as a body-mass index -- a widely used measure of fatness -- of over 30. People with a BMI of over 25 are considered overweight, while those with a BMI over 30 are obese.

In parts of the Pacific islands, obesity has been known for at least 50 years, but it has substantially increased in recent times to levels that Ulijaszek calls "astonishingly high,'' and there is no hint that weights there have leveled off.

For instance, in Rarotonga, capital of the Cook Islands, 14 percent of men and 44 percent of women were obese in 1966. Now, 52 percent of men and 57 percent of women there are obese.

Lavelle surveyed weight in South Africa and rural Australia three years ago and found more signs of an emerging weight problem.

In Cape Town, 12 percent of girls and 16 percent of boys were considered overweight. In much poorer rural Klein Karoo 300 kilometers to the west, just 1 percent of boys and 2 percent of girls weighed this much.

In a similar survey among nomadic people in the central desert of Australia, she found that about 4 percent of children and 15 percent of adults are obese.

The obesity is blamed on the growing worldwide availability of high-calorie foods and less physically demanding jobs. In some ways, fatness is a sign of better health, since children who avoid chronic infections grow up to be larger.

But even though people might be better off obese than malnourished, the trend toward fatness worries many health experts. "We are concerned about this because of the higher disease rates that go with obesity,'' said Lavelle.

Obesity increases the risk of a variety of health woes, especially diabetes, which is rising rapidly in many parts of the world.

Some of the most extreme weight gains are seen among people who move from poor countries to places like the United States, where clean water prevents many childhood diseases and high-fat food is plentiful.

Dr. Barry Bogin of the University of Michigan-Dearborn surveyed Mayan children moving from Guatemala to Los Angeles and rural central Florida. He found that nearly half are overweight and 42 percent are obese.

By comparison 14 percent of white and black children in the United States are overweight or obese.