No, the BMI is a only an approximate guide and it is not equally accurate for everyone. One reason for this is that people with the same weight can carry different amounts of fat compared to their lean body mass (bone and muscle). BMI can tend to overestimate in: athletes, particularly those who build a lot of muscle with weight or resistance training; and, some ethnic groups, such as Pacific Islanders, who tend to have higher levels of lean body mass compared to body fat. BMI can tend to underestimate in: older people, who tend to lose muscle and bone density as they age; and people of South Asian, Japanese and Chinese heritage, who tend to have higher levels of body fat compared to lean body mass. Also, BMI does not take into account where body fat is accumulated on the body. Not all body fat contributes equally to health problems. Fat underneath the skin, particularly on the hips and thighs, doesn't tend to cause as many problems as fat around your abdomen. This is because fat which builds up around the organs, inside the abdominal cavity, can cause inflammation within the body and contributes to many health problems associated with obesity. The greater the abdominal fat, the greater the risk of health problems such as diabetes and cardiovascular disease. People from some ethnic groups, including Aboriginal Australians, can tend to carry more fat around their abdomen. For this reason, waist circumference, which can be a good indicator of how much abdominal fat a person is carrying, is also often used to assess how much of a risk a person's extra weight may pose to their health.
