Ultra-processed foods (UPFs) are industrially manufactured foods that use a variety of additives such as flavors, colorants, and emulsifiers to enhance taste, texture, appearance, and durability, with minimal to no inclusion of whole foods [1]. Although there is no accepted scientific definition of UPF, the most widely used method for evaluating diets according to the level of food processing is the Nova food classification system. The Nova system categorizes foods into four groups based on the degree of processing: unprocessed or minimally processed foods, processed culinary ingredients, processed foods, and UPFs [1]. UPFs typically have a higher caloric density and contain more fats, sugars, and sodium than unprocessed or minimally processed foods; they are considered to decrease the overall quality of the diet. UPFs include chicken nuggets, ham, ramen, sweetened breakfast cereals, ready to eat products, and fast food. Globally processed foods are gradually replacing home-cooked meals and the consumption of fresh fruits and vegetables. Since UPFs tend to be flavorful and convenient, offering quick and easily accessible food options, their consumption is on a steady rise [2].

Many epidemiological studies have warned for years about the link between increased UPF consumption and adverse health outcomes [3-5]. A very recent systematic umbrella review of existing meta-analyses indicated that 10% greater exposure to UPFs was associated with a 71% higher risk of adverse health outcomes [6]. The strongest associations were shown between UPF intake and all-cause mortality, cardiovascular disease-related mortality, mental disorder outcomes, obesity, and type 2 diabetes mellitus [6]. The potential mechanisms underlying these associations remain the subject of various hypotheses and include over-consumption due to energy density; fat, sugar, and salt contents; potential deleterious effects of certain additives; and contaminants from packaging [6].

In this issue of Diabetes & Metabolism Journal, Jung et al. [7] report associations of higher UPF intake with higher adiposity (percent body fat) and lower skeletal muscle mass among Korean adults. They used data from the Korea National Health and Nutrition Examination Survey (KNHANES) and found similar results to a recent study using the U.S. National Health and Nutrition Examination Survey (NHANES) [8]. A mechanism by which UPF intake leads to metabolic disease may involve a change in body composition with muscle loss, although there is no proven causal relationship. Previous research conducted in different populations has demonstrated negative correlations between UPF intake and lean body mass [9] or grip strength [10]. Interestingly and importantly, Jung et al. [7] showed that the impact of UPF intake on body composition was more common in rural residents and people with low levels of education.

The UPF defined by the Nova system reflects processing itself and not the nutrient profile of the food. With expansion of the processed food industry, UPF consumption has become widespread worldwide and has led to a wide variety of UPFs. Health risks may depend on UPF type. A 34-year U.S. observational study found that the link between UPF intake and mortality varied among specific food groups; meat, poultry, and seafood ready-to-eat products showed the strongest and most consistent associations with mortality, followed by sweetened beverages and dairy-based desserts [11]. The association was weaker when considering overall dietary quality [11]. People with low income, low education levels, or inadequate resources are likely to choose unhealthy UPFs. Therefore, the effects of UPFs on health cannot be solely based on consumption quantity but must also consider food quality. More detailed and diverse prospective studies using nutritionally refined evaluation tools based on extensive food-related data are essential.

In a previous study utilizing the Korean Nova food classification system, the calorie intake from UPFs among Koreans was 29.3% of the total energy intake (based on KNHANES 2018 data) [12]. This proportion is relatively lower than the 58.5% reported in the U.S. NHANES 2009–2010 data. However, even in Koreans, UPF consumption was associated with a 34% increase in the incidence of type 2 diabetes mellitus during 15 years of follow-up [5]. Moreover, the UPF intake among young adults aged 20 to 30 has been reported at 36.2% [12]. Despite higher UPF intake among young individuals, most studies have focused on adults. Therefore, there is a significant gap in research targeting children and adolescents. Evaluating the potential impact of UPFs on younger age groups is a crucial issue.

Some organizations such as the American Heart Association have cautiously advised people to choose unprocessed and minimally processed foods over UPFs [13]. Latin American countries adopted dietary guidelines that recommend avoiding UPFs [14]; these countries use public health measures promoting a reduction of UPFs including warning labels, taxes on sugar-sweetened beverages and UPFs, and bans in schools.

Though UPF is an increasingly available option, it is important to select foods that are not only low in calories, saturated fats, and sugars, but also to account for the level of processing. It is urgent to classify UPFs in more detail according to quality. Prospective studies are needed to clarify the biological mechanisms by which UPFs lead to poor health outcomes.