New Study Suggests Excessively High or Low BMI Linked to Higher Risk Of Dying

A new study published in The Lancet Diabetes and Endocrinology purports a connection between both high and low Body Mass Index (BMI) numbers and increased mortality. BMI measurements that are above or below the “healthy” average were shown to increase the risk of dying from a range of diseases. The study was conducted by researchers at the London School of Hygiene and Tropical Medicine.

Credit: health.harvard.edu

Body mass index is one method to determine a person’s overall health. It is determined by dividing a person’s weight in kilograms by the square of their height in meters (KG/M^2). Optimal BMI range is between 21 to 25 kg/m^2 and has the lowest level of morbidity, according to the report. The study analyzed data from 3.6 million people and 367,512 deaths in the U.K.

“BMI is a key indicator of health,” said Krishnan Bhaskaran, lead author of the study and professor of statistical epidemiology. “We know that BMI is linked to the risk of dying overall, but surprisingly little research has been conducted on the links to deaths from specific causes…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.”

Body mass index measurements outside of the optimal range were shown to have a “J-shaped association” with virtually all causes of death; not just the most common diseases. This translates to increased mortality for BMIs both above and below this range.

BMI Above Optimal Range

A BMI number higher than 30 is considered to be obese. Researchers showed that obesity was linked to an increase of death from heart disease and cancer.

“BMI higher than 25, the upper end of healthy,” said Bhaskaran, “is linked to most cancers, most cardiovascular diseases, respiratory disease, and liver and kidney conditions.”

Credit: health.harvard.edu

Obesity can contribute to numerous other ailments such as diabetes, respiratory disease and liver disease. Obesity overall was shown to diminish life expectancy in men by 4.2 years and 3.5 years in women.

BMI Under Optimal Range

Having a low body mass index measurement was also shown to contribute to higher mortality rates. However, the connections weren’t as conclusive as the findings related to high BMI.

Being underweight was linked to a wider range of deaths, including dementia, cardiovascular disease and suicide. However, low body mass index levels and the associated causes of death were based more on observation. Essentially, it wasn’t altogether clear whether low weight was the cause of a specific illness or a marker of poor health generally.

The researchers acknowledge that the findings related to deaths and low BMI could be products of “reverse causation,” i.e. that low BMI was a result of the illness rather than the cause. For instance, a person with depression is likely to suffer from reduced appetite and poor health in general, which could lead to weight loss. The study said that further research is needed to “disentangle complex associations between these mental health and neurological conditions and body weight.”

Professor Bhaskaran noted other limitations of the study included lack of information regarding people’s diets and levels of exercise, which could have also impacted their morbidity. Nevertheless, the study’s findings reiterated the benefit of maintaining a BMI between 21 and 25.

The lowest risk of cardiovascular death was linked to BMI of 25. Every additional 5 kg/m^2 had an associated 29 percent increased risk of death. The lowest cancer risk of mortality was at a 21 BMI measurement, with a 13 percent increased risk of death for every additional 5 kg/m^2.