阅读短文，回答问题：Sometimes， people are not clear about what kind of food is healthy， and what kind of food may do harm to our health. The USDA has prepared a food guide to help people learn about which kind of food is the healthiest to eat. The food guide describes six main food groups: meat （like fish and chicken）， dairy （like milk and cheese）， grains （like bread and rice）， fruit， and vegetables. The last group is fats and sweets. The USDA also suggests how much of each food group is healthy to eat daily. Though this guide was prepared by the US government， it is very useful for people all over the world．As a result of years of study， we know that too much animal fat is bad for our health. For example， Americans eat a lot of meat and only a small amount of grains， fruit and vegetables.Because of the way they eat， they have a high rate of cancer and heart disease. In Japan， people eat large amounts of grains and very little meat. The Japanese also have a very low rate of cancerand heart disease. The Japanese live longer than anyone else in the worlD． However， when Japanese people move to the US， the rate of heart disease and cancer goes up as their eating habit changes. And as hamburgers， ice creams and other high-fat foods become popular in Japan， the rate of heart disease and cancer is going up there as well. People are also eating more meat and dairy food in other countries， in which the disease rate is going up along with the change of the way people eat. Doctors everywhere suggest people eat more grains， fruit and vegetables and less meat and dairy food．Eating healthily is important for children as well as their parents. When parents have poor eating habits， their children usually do， too. After all， children eat in the same way as theirparents. When parents eat healthy food， the children will learn to enjoy it， too. Then they will develop good eating habits. Doctors suggest parents give their children healthier food such as fruit， vegetables and juice.Everyone wants to live a long， healthy life. We know that the food we eat affects us in different ways. For example， doctors believe that fruit and vegetables can really protect people from many different diseases. On the other hand， animal fat can cause diseases. We can change our eating habits now and enjoy many years of healthy living. What is the USDA in the passage? 查看材料