Behind the Headlines

Wednesday April 22 2009

The study says a healthy BMI has important environmental benefits

"Fatties cause global warming," claims The Sun, which reports that scientists have warned the increase in ‘big eaters’ fuels the need for more food production. It also suggests the obese are more likely to drive and that both factors raise greenhouse gas emissions.

The research behind this story compared two theoretical populations: one with a ‘normal’ mix of body types, and the other an ‘overweight’ population where 40% of people were obese. The authors estimated that the overweight population would require 19% more food energy, and that the necessary increase in food production would raise carbon dioxide (CO2) emissions. Driving more often and carrying extra body weight would also use more fuel, further increasing emissions of the greenhouse gas.

The results of this study are based on mathematical modelling and involve making assumptions about weight distribution in the populations compared, plus estimating likely daily activities, food energy consumption and fuel use. As such, it may not accurately reflect what truly happens. Models such as these may be of use to policy makers to assess the potential non-health impact of the increasing prevalence of obesity in society.

Where did the story come from?

Phil Edwards and Ian Roberts of the Department of Epidemiology and Population Health at the London
School of Hygiene and Tropical Medicine carried out this research. No sources of funding were reported. The study was published in the peer-reviewed scientific journal, International Journal of Epidemiology.

What kind of scientific study was this?

This was a hypothetical modelling study estimating how increases in the population’s body mass index (BMI) might impact greenhouse gas emissions.

It is estimated that globally 1bn adults are overweight and a further 300m are obese. It is speculated that the population’s upward shift in BMI and food consumption habits may contribute to global warming, as food production accounts for roughly 20% of greenhouse gas emissions.

In this study, the authors aimed to compare two hypothetical populations, a ‘normal’ and an overweight one. This normal adult population comprised of 1bn people with a mean BMI of 24.5 kg/m2, with 3.5% of people being obese. The corresponding ‘overweight’ population had a mean BMI of 29.0 kg/m2 with 40% of people obese.

The authors say that their ‘normal’ population reflects the UK situation in the 1970’s and the overweight population BMI distribution reflects that predicted for the UK in 2010.

The authors carried out calculations to estimate daily energy expenditure and required calorie intake per person, and then used these to calculate yearly figures for both populations. They then calculated and compared the CO2 emissions from transport and food production in the overweight and normal populations.

What were the results of the study?

The authors based their estimates of CO2 emissions on three groups of calculations; energy requirements in relation to body mass, emissions due to increased food production and emissions due to increased vehicle use.

Energy requirements and relation to body mass

As a person gains weight they will have an accompanying increase in ‘metabolically active’ lean tissue expending energy. The rate at which an individual’s body expends energy is known as their basal metabolic rate (BMR), and a person’s increase in lean tissue mass will in turn increase their BMR. The greater energy cost of moving a heavier body also increases energy expenditure during any activity.

The authors expected that energy expenditure would be approximately balanced by energy intake, and therefore as BMI increases, total consumption of food energy would increase. The authors used standard BMR calculations to estimate the food energy required by the hypothetical adult populations.

The authors then assumed comparative patterns of daily activities divided into sleep, work, time at home and time walking, sitting and standing. For each activity they estimated the ratio of the metabolic rate in relation to being at rest, i.e. 1kcal per kg of body mass per hour of activity, referred to as 1 MET. Activity estimates were: sleeping 1 MET, office work 2 METs, light home activities 1.5 METs, sitting or standing 1.2 METs, driving 2METs and walking 3.5 METs).

Using a conversion of 1 kcal=4.184 kJ they estimated that the normal population would require an average of 6.49 megajoules (MJ) per person, per day to maintain BMR, with an additional 3.81MJ per person per day for normal daily activities. The overweight population would need an average of 7.05MJ per person, per day to maintain BMR, with an additional 5.25MJ per person, per day for daily activities. Compared with the normal population, this amounted to the overweight population requiring 19% more food energy for its total energy expenditure.

Food intake, production and emissions

Based on the 42 Giga tonnes (GT) of total global carbon dioxide emissions in the year 2000, which had a global population of roughly 6bn, means 1bn people would be expected to produce 7GT per year. With food production accounting for 20% of this amount, this amounts to approximately 1.4GT of the yearly emissions for one billion normal adults.

With a 19% increase in food energy requirements in an overweight population, this would amount to an extra 0.27GT produced per year, giving a total greenhouse gas emission of 1.67GT.

In addition to food energy requirement, the authors hypothesised that overweight people would use more fuel energy in transportation, with an additional amount of fuel needed to transport their heavier bodies. They estimated increase in fuel energy use as car weight plus half the mass of the person, divided by car weight.

Greenhouse gas emissions per car they based on the assumption that heavier people with BMI above 30kg/m2 would have a car with more internal space, so the authors calculated CO2 emissions produced by a shift to car travel for with higher BMI’s. Allowing for a shift to car travel amongst those in the upper end of the scale for BMI in the normal population, this would account for 0.005 GT of CO2 emissions per year in the overweight population where there would be a greater number of people with higher BMI switching to car travel. The total additional fuel energy used by the overweight population would therefore be expected to increase CO2 emissions by 0.17 GT per year.

What interpretations did the researchers draw from these results?

The researchers conclude that maintaining a healthy BMI has important environmental benefits in terms of reducing greenhouse gas emissions.

What does the NHS Knowledge Service make of this study?

This research estimated that an ‘overweight’ population (average BMI 29) with 40% prevalence of obesity would require 19% more food energy than a ‘normal’ population (average BMI 24.5). When added to the additional fuel energy used through increased transport, an ‘overweight’ population of 1bn would result in an increase of carbon dioxide emissions of between 0.4 and 1.0 Giga tonnes per year.

From this model, it can be estimated that increased prevalence of being overweight and obese within the population could be an environmental issue as well as a health issue (with the various chronic diseases that are associated with being overweight, e.g. cardiovascular disease and diabetes).

However, it should be noted that these results are based on mathematical models that simplify real life, and that the ‘normal’ and ‘overweight’ populations used are only an estimate of body size distribution within the population. As such, they may not be completely representative.

Additionally, the calculations of daily energy requirements, fuel consumption, likely daily activities (assumed to be the same for both normal and overweight populations), and annual carbon dioxide emissions within each population are only estimates and may not be truly representative of what actually occurs. As the authors acknowledge, if the overweight population’s daily physical activity was in fact lower than in their model, then     this group’s calculated energy expenditure would be an over-estimation.

Despite these limitations, models such as these may help policy makers to assess the potential non-health related impact of the increasing prevalence of overweight and obesity in society.

Analysis by Bazian

Edited by NHS Choices

Links to the headlines

Fatties cause global warming. The Sun, April 22 2009

1970s lifestyle 'protects planet'. BBC News, April 21 2009

Carbon emissions fuelled by high rates of obesity. The Guardian, April 21 2009

Obesity threat to individuals and planet. The Daily Telegraph, April 21 2009

Links to the science

Edwards P and Roberts I. Population adiposity and climate change. IJE 2009;  Advance Access published online on April 19

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