Having excessively high or low body mass index (BMI) may raise a person's risk of premature death, findings of a new study have revealed.
Many doctors use BMI to determine if someone is obese because it is accurate and relatively simple to measure. It is measured by dividing a person's weight by the square of his or her height. A BMI score of 18.5 to 25 falls within the healthy range.
In a research published in The Lancet Diabetes and Endocrinology on Oct. 30, researchers from London School of Hygiene and Tropical Medicine analyzed the data of 3.6 million people and more than 357,000 deaths, and found that too high or too low BMI can slash years from a person's life.
They found that obesity, or BMI of 30 or more, was associated with higher prevalence of heart disease and cancer, two major causes of death.
Obesity reduces life expectancy by 4.2 years in men and 3.5 years in women. Researchers also said that it can contribute to other chronic conditions, which include liver disease, diabetes and respiratory disease.
The study also revealed that being underweight is also associated with what the researchers described as a "surprising wide range of deaths," which include cardiovascular disease, suicide and Alzheimer's disease.
The researchers nonetheless said that the links between low BMI and causes of death were more "observative" because it was less clear if there is a direct causal link between low weight and illness, or if low weight is just a marker of poor health in general.
"The raised risks of many outcomes at low BMI, coupled with the fact that mental health conditions showed the strongest inverse associations with low BMI, might indicate pervasive effects of mental health problems on a range of outcomes, through pathways that could include poorer self-care and less access to or use of health-care services, or both," the researchers wrote.
The study reiterated the importance of maintaining a healthy BMI. The results showed that the lowest risk of cardiovascular death was linked to a BMI of 25 kg/m2. Each additional 5 kg/m2 is linked to a 29 percent increased risk of morbidity.
The findings also showed that the lowest risk of cancer death was linked to a BMI of 21 kg/m2. Every additional 5 kg/m2 was linked to a 13 percent increased risk of death.
"We have filled this knowledge gap to help researchers, patients and doctors better understand how underweight and excess weight might be associated with diseases such as cancer, respiratory disease and liver disease," study researcher Krishnan Bhaskaran told CNN.