A GLOBAL survey has found that Hong Kong has one of the lowest child mortality rates in the world, but local doctors have warned that changing lifestyles are leading to increasing nutritional problems and obesity among young children. The Unicef (United Nations Children's Fund) report, released yesterday, shows that Hong Kong has the fifth lowest infant mortality rate out of 145 countries for children under five. The report, which compares the survival chances of children throughout the world, shows that only seven of every 1,000 children in Hong Kong will die before they reach the age of five. Ireland has the lowest mortality rate with only six deaths per 1,000, while Niger has the highest with 320 deaths per 1,000. The territory also compares well on other indicators, with a life expectancy of 78 years - beaten only by Japan where the average is 79 years. At the other end of the scale, figures for some African nations tell a tragic story, with many having an infant mortality rate of well over 100 per 1,000 and a life expectancy in the 40s. Hong Kong's infant mortality rate has remained stable since 1990 when it dropped to seven per 1,000 live births from 15 in 1980 and 64 in 1960. Paediatricians agree that the main reasons for this are the effective control of many diseases through immunisation and a general improvement in lifestyle and health care. The head of the Paediatrics Department at Queen Mary Hospital, Professor Yeung Chap-yung, said: ''In the post-war years we have developed a very good maternal and childhood health care system which has helped to make parents aware of the importance of immunisation. ''As a result, diseases which once killed a number of children, such as diphtheria and polio, have been wiped out in Hong Kong, while others have been brought under control. ''Improvements in sanitation and other areas of general health care have also helped to lower the infant mortality rate and we should be very proud of all these achievements.'' But doctors warned that as Hong Kong became increasingly affluent, new medical problems were emerging which could lead to a high mortality rate among adults in the future. Senior lecturer in paediatrics at the Chinese University of Hong Kong, Dr Sophie Leung Suk-fong, said: ''A change in lifestyle and diet in Hong Kong has led to nutritional problems such as obesity in childhood, which increases the risk of heart disease and diabetes in later life. ''We have put so much effort into primary health care but we must now do something to prevent a high mortality and morbidity rate in the future by directing our health care services accordingly.'' Although Hong Kong's infant mortality rate is still far lower than most Asian countries and beats leading industrial nations like the United States and Britain, the territory's children now have the second highest blood cholesterol level in the world. The amount of fat consumed by children here today is double that of one or two generations ago, with the Western concept of fast food playing an increasing part in the local diet. Professor Yeung said: ''Western eating habits might promote growth, producing heavier and taller children, but bigger is not necessarily better. ''The eating habits we have adopted may not be such a good thing, while the indiscriminate intake of protein is equally bad because it cannot all be digested and so it is deposited as fat.''