A nationwide analysis of US grocery shopping shows highly processed foods make up more than 60 per cent of calories in food purchased – with these items tending to contain more fat, sugar and salt than less-processed foods.

The study, conducted by researchers at the University of North Carolina at Chapel Hill and published by the Federation of American Societies for Experimental Biology (FASEB), saw 157,142 households use barcode scanners to record all foods and beverages they purchased from grocery stores for at least one year between 2000 and 2012. Households participated in the study for an average of four years and collectively purchased 1.2m items. The researchers then linked each item to its nutritional information, product description and ingredient list, allowing them to rank each product’s degree of food processing.

Foods given the ‘highly processed’ classification included multi-ingredient, industrially formulated mixtures, such as fizzy drinks, cookies, crisps, white bread, sweets and prepared meals. Fresh or frozen fruits and vegetables, milk, eggs, dried beans and fresh meat were labelled ‘unprocessed’ or minimally processed.

The data showed the proportion of calories in highly processed food and beverages by these US households remained stable at 61.0 per cent to 62.5 per cent between 2000 and 2012.

The proportion of calories purchased in ‘ready-to-heat’ foods increased significantly to reach 15.2 per cent in 2012. More than 80 per cent of calories were purchased in ‘ready-to-eat’ or ‘ready-to-heat’ form in 2012. The items tended to be higher in fat, sugar and salt than food purchases that required preparation.

The researchers are continuing to track household purchases to see how nutrition level and processing changes over time. They are also using the data to examine whether purchasing habits vary based on race or socio-economic status.

Jennifer M Poti, PhD, lead researcher for the project, highlighted how the term ‘processed food’ needs to be used more carefully: “It is important that when we discuss processed foods, we acknowledge that many processed foods, such as canned vegetables or wholegrain breakfast cereals, are important contributors to nutrition and food security. However, it is the highly processed foods – those with an extensive degree of processing – that might be related to obesity.”