The scale moved a little, the tape measure a lot: in seven years Mexican waistlines grew almost three times faster than weight alone predicts

Body mass index is the number most of us know: weight divided by height squared, 30 or more counted as obesity. It is quick and cheap, but it cannot tell where the weight sits, and fat packed around the organs in the belly is the kind most closely tied to insulin resistance, type 2 diabetes and heart disease. A study published on 30 October in Frontiers in Nutrition used one country's survey data to ask how far apart the two pictures, weight and waist, can drift over a few years.

The researchers took three rounds of the Mexican Family Life Survey (ENNVIH), in 2002, 2005 and 2009, in which trained nurses from Mexico's National Institute of Public Health measured weight, height and waist, and people reported whether a doctor had ever diagnosed diabetes or high blood pressure. Because the same people could not be followed reliably from round to round, the team grouped everyone by sex and by age in 2002 (seven age bands from 12 to 74) and tracked these 14 'synthetic cohorts' over time, a method economists call a pseudo-panel. The pooled sample held 86,870 measurements.

Average BMI went from 26.6 to 27.8, a rise of 1.23 points, and the share of people with a BMI of 30 or more went from 24.0% to 30.2%. Waists moved much further: the average grew from 85.2 to 93.7 cm, and the share above the International Diabetes Federation cut-offs used in the study (more than 90 cm for men, 80 cm for women) climbed from 49.1% to 73.0%. Measured against how waist and BMI related to each other in 2002, the waist gain was 2.75 times larger than the BMI gain alone would predict. Over the same years self-reported diabetes rose from 7.4% to 11.4% and high blood pressure from 12.0% to 14.9%. The gap was sharpest in the oldest groups: people who were 55 to 74 in 2002 saw almost no change in BMI, yet their waists still grew by 4.7 cm on average.

The study has real limits, and the authors spell them out. The data stop in 2009, before Mexico's 2014 tax on sugary drinks, its 2020 warning labels on packaged food and the disruption of COVID-19, so the paper's straight-line projections to 2025 are illustrative only, not forecasts. The 2009 round could not be weighted to match the national population. Diabetes and blood pressure were self-reported, so undiagnosed cases are missed. Results describe groups, not individuals, and with only 14 cohorts the BMI rise became borderline in a stricter statistical check, while the waist rise held firm. What survives all of that is a simple point many clinicians already argue for: a tape measure catches a change that the bathroom scale and BMI can hide, especially in middle and later life.