A new World Bank report warns that children who do not get enough good food in the first two years of life suffer lasting damage. They may be underdeveloped or underweight. They may suffer from poor health or limited intelligence. In addition, poorly nourished children are more likely to drop out of school and earn less money as adults. The report notes that too little food is not the only cause of poor nutrition. Many children who live in homes with plenty of food suffer for other reasons. For example, the study says that mothers often fail to give their newly born babies their first breast milk. This milk-like substance is called colostrum(初乳). It is full of nutrients that improve a baby's ability to fight infections and disease. The study also links malnutrition(营养不良)to economic growth in poor countries. A lack of nutrition in early childhood can cost developing nations up to three percent of their yearly earnings. Many of these same countries have economies that are growing at a rate of two to three percent yearly. The study suggests that poor countries could possibly double their economic growth if they improved nutrition. Africa and South Asia are affected the most by poor nutrition. The study says about half of all children in India do not get enough good food. The World Bank study also notes that rates of malnutrition in South Asia are almost double those in central and southern Africa. Other parts of the world are also severely affected, including Indonesia, Uzbekistan, Yemen, Guatemala and Peru. The study recommends that developing countries change their policies to deal with malnutrition. Instead of directly providing food, the study suggests educational programs in health and nutrition for mothers with young babies. It also recommends cleaner living conditions and improvements in health care. World Bank nutrition specialist Meera Shekar said the period of life between pregnancy and two years is extremely important. Governments with limited resources should take direct action to improve nutrition for children during this period. What's the main topic of the passage?