Also for women aged 5059 years and a mean BMI of 25 kgm 2, the prevalence of obesity and severe obesity would both have been the highest in Oceania and the lowest in the high-income Asia Pacific region, with a difference of 12 and 6 percentage points, respectively, for the two outcomes; a unit increase in mean BMI would have been associated with a relative change ranging from 21 to 46 for obesity and from 30 to 59 for severe obesity, the smallest change for both being in Oceania and the largest in East and Southeast Asia.

We investigated how much change in mean body mass index (BMI) explains changes in the prevalence of underweight, obesity, and severe obesity in different regions using data from 2896 population-based studies with 187 million participants.

Changes in the prevalence of underweight and total obesity, and to a lesser extent severe obesity, are largely driven by shifts in the distribution of BMI, with smaller contributions from changes in the shape of the distribution.

In East and Southeast Asia and sub-Saharan Africa, the underweight tail of the BMI distribution was left behind as the distribution shifted.

There is a need for policies that address all forms of malnutrition by making healthy foods accessible and affordable, while restricting unhealthy foods through fiscal and regulatory restrictions.

In contrast, changes in the shape of BMI distribution for example, widening or narrowing of the BMI distribution, becoming more or less skewed, or having a thinner or thicker tail would affect the prevalence of underweight and obesity with only small impacts on the population mean, as shown schematically in Figure 1.

Understanding these two mechanisms is essential as they may require different public health and clinical responses ( Penman and Johnson, 2006 ).

But it is unclear how much the two mechanisms have contributed to the observed decline in underweight and rise in obesity in different world regions.

In this example, the change (shown as the difference between blue and gray) results in a small decrease of underweight and a large increase in obesity.

B ) Change in the prevalence of underweight and obesity when only mean BMI changes (shown as the difference between orange and gray), without a change in the shape of the distribution.

To understand whether weight gain occurs across all BMI levels or disproportionately affects the underweight or obese segments of the distribution, and how this phenomenon varies geographically, there is a need for a population-based study that simultaneously investigates both underweight and obesity in relation to mean BMI in different regions of the world.

We used a comprehensive global database to investigate how much change in mean BMI can explain the corresponding changes in prevalence of adults with underweight (defined as BMI 2 ), total obesity (BMI 30 kgm 2 ), and severe obesity (BMI 35 kgm 2 ) over three decades from 1985 to 2016 in different regions of the world.

Of these, 2033 studies had measurements of height and weight on 132.6 million participants aged 2079 years.

The number of studies with participants aged 2079 years in different regions ranged from 53 in Oceania to 637 in the high-income western region.

The list of data sources and their characteristics is provided in Supplementary file 4.

The prevalence of underweight decreased or stayed unchanged and that of obesity and severe obesity increased from 1985 to 2016 in all regions, with the exception of an increase in the prevalence of underweight in younger women in the high-income Asia Pacific region.

The largest absolute decrease in underweight prevalence from 1985 to 2016 was seen in South Asia; East and Southeast Asia; and sub-Saharan Africa, where it declined by 1435 percentage points in different agesex groups ( Figure 4 ).

Nonetheless, underweight prevalence remained higher in these three regions than elsewhere in 2016.

Women in these regions also experienced the largest increase in severe obesity prevalence, along with men in the high-income western region.

In these regions and sexes, obesity prevalence increased by 1624 percentage points in different age groups, and severe obesity increased by 513 percentage points.

The increase in obesity was less than five percentage points in the high-income Asia Pacific region; South Asia; and in men in sub-Saharan Africa; in the same regions, along with East and Southeast Asia, the increase in severe obesity was less than two percentage points.

These indicate that 93 (men) and 96 (women) of variation in obesity, and between 83 and 92 of variation in underweight and severe obesity, were explained by mean BMI and other variables (year, region, and age group) in the regression models.

The coefficients of the mean BMI terms represent the changes in (probit-transformed) prevalence associated with a unit change in mean BMI, and their interactions with region represent variations in this association across regions.

For all three outcomes, the association of prevalence with mean BMI varied across regions.

For example, in the year 2016, for women aged 5059 years, at a mean BMI of 25 kgm 2 (which was approximately the global age-standardised mean level of BMI) ( NCD Risk Factor Collaboration (NCD-RisC), 2017a ), prevalence of underweight would have varied by seven percentage points across regions, being lowest in Central and Eastern Europe and highest in sub-Saharan Africa; a unit increase in mean BMI would have been associated with a relative change in prevalence ranging from 49 in the high-income Asia Pacific region to 14 in Oceania.

Marius Et Jeannette Torrent