|The US is home to 6% of the world's population, but 34% of its biomass|
We estimated global human biomass, its regional distribution and biomass attributable to
overweight and obesity. Our results underscore the need to take body mass into account when considering the ecological implications of population growth. UN world population
projections suggest that by 2050 there could be an additional 2.3 billion people.  The
ecological implications of rising population numbers will be exacerbated by increases in
average body mass.
Although the largest increase in population numbers is expected in Asia and sub-Saharan
Africa, our results suggest that population increases in the USA will carry more weight than would be implied by numbers alone. It is predicted that the US population will increase from 310 million in 2010 to 403 million by 2050 . Most of the increase will be due to migration and to the extent that migrants adopt the diet and lifestyles of the host population, we can reasonably expect that the body mass of migrants will rise. Our results show that this could have important implications for world energy requirements.
In Africa and Asia urban populations are increasing more rapidly than rural populations . This will also have implications for average body mass . Given the current trend of rising BMI, our scenario where all countries have a similar BMI distribution to the USA provides an insight into possible future challenges. If global biomass were to increase to a level where all countries had the age-sex BMI distributions of the USA, the biomass increase would be equivalent to an extra billion people of average body mass. Although, this is not the same as an extra billion people in terms of energy requirements, the increase corresponds to the energy requirements of about 473 million adults of current world average body mass.
Our findings should be viewed in the light of the following limitations. Firstly, in countries where data on average BMI, height and its standard deviation were unavailable, we used a regression model to estimate the missing parameters. Due to limited data availability, we assumed that height and BMI are independent variables, and that the mean and standard deviation of height are the same across the distribution of BMI. Furthermore, because of the lack of data describing the distribution of BMI in relation to high, we assumed zero
covariance between BMI and height squared. Secondly, we assumed symmetrical (normal)
distributions of BMI in each population, when in reality many population distributions will be skewed, with a tail to the right of the distribution comprising a relatively small proportion of people with very high body mass. We may therefore have underestimated total biomass. Finally, we did not estimate biomass in children who comprise a significant proportion of the population in many countries, nor in countries with population less than 100,000. Future work in this area should account for population age structure, as well as education levels and urbanisation.
There are also limitations in our estimates of energy requirements. We have used FAO data to estimate the BMR but the extent to which they can be applied to all populations is open to question. The assumption of similar physical activity levels in all countries is clearly
unrealistic with higher physical activity levels in low income countries. As a result, we will
have underestimated energy requirements in some countries. However, this approach is
appropriate for comparing different scenarios of BMI distribution and its implications on
relative changes in energy requirements.
Increasing biomass will have important implications for global resource requirements,
including food demand, and the overall ecological footprint of our species. Future work will investigate the extent to which food demand and carbon emissions are likely to increase with increasing biomass.
Although the concept of biomass is rarely applied to the human species, the ecological
implications of increasing body mass are significant and ought to be taken into account when evaluating future trends and planning for future resource challenges. Our scenarios suggest that global trends of increasing body mass will have important resource implications and that unchecked, increasing BMI could have the same implications for world energy requirements as an extra 473 million people. Tackling population fatness may be critical to world food security and ecological sustainability