Oxford University wrote:Background The increasing global prevalence of overweight and obesity has serious implications for the environment, as well as for health. We estimate the impact on greenhouse gas emissions of increases in the population distribution of body mass index (BMI).
Methods We estimated the food energy required to maintain basal metabolic rate in two hypothetical adult populations using the Schofield equations for males and females. Additional greenhouse gas emissions due to higher fuel energy use for transporting a heavier population were estimated.
Results Compared with a normal population distribution of BMI, a population with 40% obese requires 19% more food energy for its total energy expenditure. Greenhouse gas emissions from food production and car travel due to increases in adiposity in a population of 1 billion are estimated to be between 0.4 Giga tonnes (GT) and 1.0 GT of carbon dioxide equivalents per year.
Conclusions The maintenance of a healthy BMI has important environmental benefits in terms of lower greenhouse gas emissions.
And it sounds like they didn't even consider flatulence!