James Bentham, a co-author of the research from Imperial College, London, says the global trend is likely to be due primarily to improvements in nutrition and healthcare.

The doubt expressed by some bone doctors and nutrition experts doesn't surprise Dr. John A. Baron, a co-author of the study and professor of medicine at Dartmouth Medical School.

When children are involved in meal preparation, "they come to at least try the food," said Isobel Contento, professor of nutrition education at Teachers College and a co-author of the study.