Reported calories fell year after year in China while BMI kept rising. A 135,235-observation comparison shows why national trends cannot explain one waistline

What happened to food intake and body size while China and the United States changed over two decades? Researchers put two national survey series side by side: the China Health and Nutrition Survey from 1993 to 2015 and the US National Health and Nutrition Examination Survey from 1997 to 2016. After restricting the analysis to adults aged 20 to 64, they had 80,448 observations from China and 54,787 from the United States.

The Chinese curves pull in opposite directions. Age-standardised reported energy intake fell by an average 0.92% a year in men and 1.08% a year in women. Yet BMI rose by 0.46% and 0.25% a year, respectively, while waist circumference rose by 0.45% in men and 0.21% in women. Those percentages describe the average annual movement of population measures, not the weight change of a typical participant.

The US pattern was different but no tidier. Reported daily energy intake was broadly stable. BMI did not show a statistically clear overall rise in the study's trend model, yet waist circumference increased by 0.31% a year in men and 0.49% a year in women. Both countries also shifted the share of energy away from carbohydrate and towards fat. Protein increased clearly only in China.

The researchers found that energy and macronutrient trends moved with BMI and waist trends in the Chinese data, but not in the US data. That does not mean carbohydrate or fat caused the changes. It is an analysis of population-level survey curves. People entering one survey round are not necessarily the people in the next, and an association between two national trends can differ from the relationship between diet and weight within an individual.

Food surveys also have a familiar weak spot: people may forget foods, misjudge portions or report differently as food supplies and eating habits change. The Chinese and US surveys did not use identical dietary methods, years or social settings. Physical activity, sleep, smoking, medicines, ageing, urbanisation and the mix of people sampled can also move body size without appearing in a simple calories-versus-BMI chart.

So the honest lesson is not that energy balance has stopped mattering. It is that a falling line for self-reported calories cannot, by itself, settle why a country's waistlines rise. For a person, food quality, portions and movement still matter, while weight change can also reflect health conditions and medicines. For public health, repeated measurements of diet, activity and body composition in the same people would tell a stronger story than two national trend lines alone.