Data sources

For each country, we obtained estimates of the population in 2005 by age and sex from the United Nations population database [6]. We obtained estimates of mean (and SD) body mass index (BMI) from the WHO SURF2 report [7] and estimates of mean height (and SD) for 190 countries from national health examination surveys, primarily the Demographic and Health Surveys[5]. Because surveys were not conducted in every country, height data were not available by age and sex in some countries. To estimate mean height (and SD) by age and sex in every country using the available data, we built a linear regression model (of age-sex group, average height, WHO region and sub-region) using R open access statistical software. Some countries and territories were excluded from the analysis due to insufficient data on BMI (see Table 1 for a list of these).

Table 1 List of excluded countries due to insufficient data on BMI Full size table

Biomass estimation

Total biomass by age-sex group was estimated as the product of the number of people in the group and their average body mass. The formulae for the estimation of body mass are given in the appendix. We also estimated total biomass due to overweight in each age-sex group. We assumed that BMI is normally distributed in the group and estimated the number of people overweight (using prevalence of BMI > 25) and their average BMI. Using their average BMI, we then estimated their average body mass. The biomass of overweight people was calculated as the product of the number of overweight people and their average body mass. Biomass due to overweight was calculated by estimating the biomass of overweight people assuming they had BMI of 25 and subtracting this from their actual biomass. Using a similar method we estimated the biomass due to obesity. We calculated the total biomass of obese people in each age-sex group and subtracted their estimated biomass assuming that they all had a BMI of 30. For each country, we calculated total human biomass, biomass due to overweight and biomass due to obesity by adding the estimates for each age-sex group. Global totals were calculated by summating across countries.

Extreme case scenarios

We estimated global biomass under two hypothetical scenarios. Specifically, we assumed that each country had the same BMI distributions as that of [1] Japan and [2] USA. We used the method outlined above but applied the BMI of the relevant age-sex group from Japan or USA instead of the actual BMI for that age-sex group. These countries were chosen because despite being high income countries with adequate nutrition, they have average BMI values close to global extremes. For each scenario, we calculated the global biomass and biomass due to overweight and obesity.

Population and energy equivalents

We calculated the food energy required to sustain human biomass using formulae and values from the FAO [8]. Physical Activity Level (PAL) values for each age-sex group are based on non-overweight adults in the USA. Total Energy Expenditure (TEE) is estimated as the product of Basal Metabolic Rate (BMR) and PAL (see Table 2). The energy required to sustain the biomass due to overweight, obesity or the change in biomass that would be seen under hypothetical scenarios, was estimated by multiplying the number of kg by weight dependent component of BMR and by the PAL. We did all calculations by country and age-sex group applying the corresponding coefficients. Then we summed across age-sex groups to obtain total energy requirements for each country and for the world. To calculate the number of average adults that could be sustained with a given quantity of biomass we divided the amount of energy required to sustain that biomass by the average food energy requirement of one human.