News from Cass Business School

The impact of obesity on life expectancy

Monday, 20 April, 2009

A new chart enables people to work out the impact of having a few extra inches around the waist on their life expectancy.  The cost of an unhealthy lifestyle can be calculated by entering height and waist measurements to find out the years a person is likely to lose due to obesity.

The chart shows that a heavily obese 30-year-old man could lose up to 20 years of his life when compared with a man with optimum measurements.  A woman of a similar waist-to-height (WTH) ratio is likely to lose 13 years.

The chart was developed as part of a new study by academics at Cass Business School, sponsored by the Actuarial Profession.  It uses actuarial calculations of WTH ratio as well as BMI (weight, in kilograms, divided by the square of the height, in metres) to measure the impact of excess fat on mortality.  The mortality data have implications for the insurance and pensions industries, health care and social policy.

The study suggests that WTH ratio is a stronger predictor of mortality risk than BMI, and is therefore a better method of measuring the impact of obesity on life expectancy. WTH ratio should therefore be adopted in preference to BMI when, for example, carrying out research which might influence government healthcare policy.

Other findings are:

 Obesity, at the top end, can have a huge impact on life expectancy, particularly in the case of males. A young, heavily obese man whose waist is over 74% of his height, or has a BMI of over 40, is likely to lose over 10 years of his life, and possibly as much as 20 years, when compared with a man with optimum measurements.

 A 30-year-old woman whose waist is over 80% of her height, or with a BMI of 45, can expect to lose 13 years (which equates to 23 per cent less in life expectancy compared with a female having optimum measurements). However, a man might expect to lose 20.8 years (which equates to a 42 per cent reduction in life expectancy).

 Even those who are moderately obese can expect to lose several years of their lives; a 30-year-old man whose waist measurement is two thirds of his height is expected to lose five years in life expectancy.  A woman of the same age and BMI is expected to face a loss of three and a half years.

 The data imply that it might be possible to turn back the clock: obese men and women could gain in life expectancy if they reduced their BMI or WTH.  For example, a 30-year-old man could increase his life expectancy by 3 years if he trimmed his waistline from 46 inches to 41 inches. This does assume that, purely by reducing the waistline, the individual gains the extra years of life.

 The charts show the Years of Life Lost (YLL) for male and female non-smokers at ages 30 for different measurements of waist and height compared with the optimum levels. Individuals of around age 30 can therefore look up where they stand with regard to years of life lost due to obesity.

Co-author Dr Ben Rickayzen, Head of the Faculty of Actuarial Science and Insurance at Cass and Fellow of the Institute of Actuaries, said:

This study covers a topic that is important to the planning of health care, social policy and insurance in the UK and validates recent government policy and the surrounding publicity over nutrition. According to a recent Department of Health report, approximately 24% of the population in England is obese. If current trends continue, it is likely that obesity will become an even larger influencing factor on mortality. Medical advances may mitigate this influence to some extent but at increased cost to society. Obesity is an avoidable condition and a relatively recent phenomenon.  Obesity is wasteful of scarce health resources because it increases the prevalence of diseases such as diabetes and heart disease.

To date, the majority of the research into the effects of obesity on mortality is based on US population samples. However, there is a dearth of research into obesity using UK-specific data. The aim of this paper is to redress some of this balance by considering the effect of obesity on life expectancy in the UK.

Research Methodology

This paper is an investigation into the effect of excess body fat on mortality within the UK. Health surveys from the UK were used to apply a Cox proportional hazards model to UK-specific data (Health and Lifestyle Survey, 1985) to provide an analysis, at various ages, of the effects of obesity on life expectancy. Researchers explored the issues by replicating and extending US research with UK data using both Body Mass Index (BMI) and Waist to Height (WTH) as obesity measures.

The researchers measure the impact of obesity in adults on life expectancy and find that mortality risk associated with obesity in the UK is similar to that found in US studies. However, importantly, the researchers also show WTH to be a better indicator of mortality risk than BMI. The results include the number of years of life lost for UK lives in various severity categories of obesity compared with lives of the same age at optimum levels of BMI or WTH.

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Dr Ben Rickayzen, Head of the Faculty of Actuarial Science

Les Mayhew, Professor of Statistics

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