Body-mass index (BMI) above the ideal range leads to increased mortality rates
ST LOUIS (MD Consult) - A study of around 900,000 adults has revealed a body-mass index (BMI) above the ideal range of 22.5-25 kg/m2 leads to increased mortality rates, according to a report published online in The Lancet.
Above 25 kg/m2, each extra 5 kg/m2 results in an increased overall mortality of around one third. Moderate obesity (BMI 30-35) is quite common but has only one-third the effect on mortality of smoking. Severe obesity (BMI 40-50) has a similar effect to smoking, but is still relatively rare. These are the conclusions of Professor Sir Richard Peto and Dr Gary Whitlock, Clinical Trial Service Unit, University of Oxford, UK, and colleagues from the Prospective Studies Collaboration.
Generally, BMI is a reasonably good measure of how overweight a person is. It is an established risk factor for various causes of death, and many populations have seen their average BMI rising by a few percent per decade, fuelling concerns about the impact of rising obesity on health.
The authors analyzed the relationship between BMI and the risk of dying from particular diseases. They used data from 57 prospective studies involving 894,576 participants, mostly in western Europe and North America. The mean age was 46 years and the mean BMI 25 kg/m2, and 61% of participants were men.
The authors found that in both sexes, the mortality was lowest in the BMI range 22.5-25 kg/m2. This means that if a person were 1.70m (5 feet 7 inches) tall, for example, that person’s optimum weight would be about 70 kg (154 pounds or 11 stone). Each additional 5 kg/m2 translated into the following increases in mortality: overall 30%; heart disease, stroke, and other vascular disease 40%; diabetes/liver disease/kidney disease 60-120%; cancer 10%; lung disease 20%.
Moderate obesity (BMI 30-35 kg/m2), which is now common, reduced survival by between 2 and 4 years. However severe obesity (40-45kg/m2), which remains relatively rare, reduced survival by eight to 10 years, comparable to the effects of smoking. There was also a higher death rate among those who had a BMI well below the optimum range. This was due mainly to smoking related diseases, however further study is required to fully understand the reasons behind this relationship.
“Excess weight shortens human lifespan,” Dr Whitlock said. “In countries like Britain and America, weighing a third more than the optimum shortens lifespan by about 3 years. For most people, a third more than the optimum means carrying 20 to 30 kilograms (50 to 60 pounds, or 4 stone) of excess weight. If you are becoming overweight or obese, avoiding further weight gain could well add years to your life.”
“This study has shown that continuing to smoke is as dangerous as doubling your body weight, and three times as dangerous as moderate obesity,” Peto said. “Changing your diet but keeping on smoking is not the way to increase lifespan. For smokers the key thing is that stopping smoking works.”
The authors conclude with some advice echoing the adage “prevention is better than cure.”
“In adult life, it may be easier to avoid substantial weight gain than to lose that weight once it has been gained,” the authors said. “By avoiding a further increase from 28 kg/m2 to 32 kg/m2, a typical person in early middle age would gain about 2 years of life expectancy. Alternatively, by avoiding an increase from 24 kg/m2to 32 kg/m2 (ie, to a third above the apparent optimum), a young adult would on average gain about 3 extra years of life.”
