A small piece of fish each day may keep the heart doctor away. That's the finding of an extensive study of Dutch men in which deaths from heart disease were more than 50 percent lower among those who consumed  at least an ounce of salt water fish per day than those who never ate fish. The Dutch research is one of three human studies that give strong scientific backing to the long-held belief that eating fish can provide health benefits, particularly to the heart. Heart disease is the number-one killer in the United States , with more than 550, 000 deaths occurring from heart attacks each year . But researchers previously have noticed that the incidence  of heart disease is lower in cultures that consume more fish than Americans do. There are fewer heart disease deaths, for example, among the Eskimos of Greenland, who consume about 14 ounces of fish a day, and among the Japanese, whose daily fish consumption averages more than 3 ounces. During the next two decades, 78 of the men died from heart disease. The fewest deaths were among the group who regularly ate fish, even at levels far lower than those of the Japanese or Eskimos. This relationship was true regardless of other factors such as age , high blood pressure , or blood cholesterol   levels . The passage is mainly about   _  .
the high incidence of heart disease in some countries the changes in people's diet in the United States the daily fish consumption of people in different cultures the effect of fish eating regularly on people's health