Obesity is a major public health burden worldwide. In 2005, two in five adults worldwide were overweight or obese [1]. High body mass index (BMI) is an important risk factor for diabetes, hypertension, cardiovascular disease, cancer, and premature death [2,3]. Despite ongoing efforts to prevent or treat obesity, there has been a continuously rising prevalence of obesity among adults and children both in developed and developing countries [1,4-6]. It is projected that 57.8% of the world’s adult population will be either overweight or obese by 2030 [1].

Basically, obesity reflects energy imbalance wherein consumption exceeds expenditure; however, diverse factors are involved in the etiology of obesity, including genetic, biological, individual, and social factors [2]. Recently, there have been dramatic changes in the global food system [7,8] and the shifts in the food environment have substantially increased opportunities to purchase or consume highly processed foods [9-11]. These foods are typically high energy-dense, poor in nutrient content (high in sugars and fat, and low in dietary fiber), are easily consumed anytime and anywhere, leading to an excess energy intake [12,13]. The consumption of ultra-processed foods (UPFs) has been indicated as an emerging risk factor affecting human health [14,15]. Growing evidence suggests that higher household availability or consumption of UPF is associated with obesity and abdominal obesity [16-26]. However, mostly, these results were of studies conducted among European and American countries, where relatively high UPF is consumed [16,18,20-26]. Little is known regarding whether the consumption of UPF is associated with obesity among populations in Asian countries who consume relatively low levels of UPF.

According to our recent study, Koreans’ consumption of about one-fourth of the total energy from UPF in 2016 to 2018 [27] was lower than those for other countries, with UPF accounting for half of the total energy intake [16,28-30]. However, the past decade revealed a distinctive increase in UPF consumption in all subgroups regardless of sex, age, and economic status, among Koreans [27]. The food environment and sociodemographic structure (e.g., income, urbanization, household composition) have continuously changed and individual preference has changed towards the pursuit for convenience and taste [7,8,10]. In consideration of these findings, the proportion of UPF in the Korean diet is likely to increase. Thus, it seems necessary to provide evidence on the impact of the consumption of these foods on health.

This study aimed to investigate the association between consumption of UPF and obesity in Korean adults using a cross-sectional analysis of the Cardiovascular and Metabolic Diseases Etiology Research Center (CMERC) cohort study baseline data.

This cross-sectional analysis of the CMERC cohort study baseline data showed that higher consumption of UPF was associated with higher BMI and WC and higher odds of having obesity and abdominal obesity among Koreans after adjusting for sociodemographic characteristics, health-related behaviors, and family history of diseases. The strength of the association between UPF consumption and each obesity indicator was similar regardless of sex and age group, but there was a subtle difference in subgroup analysis by overall diet quality. The positive association of UPF consumption with each obesity indicator was more clearly observed in those with low quality diet. Furthermore, the association with each obesity indicator was attenuated after further adjustments for total energy intake and overall diet quality score, in addition to sociodemographic characteristics, health-related behaviors, and family history of diseases. Our findings, indicating that higher consumption of UPF was associated with higher obesity indicators, are in line with those of previous studies. An ecological study using nationally representative data from 19 European countries showed that household availability of UPF is positively associated with prevalence of obesity among adults [18]. Another study that analyzed associations between ultra-processed products sales per capita and population-level BMI trajectories across 80 countries found that increases in ultra-processed products volume sales per capita are positively associated with adult BMI trajectories [40]. Similar associations were also found in other studies using individual dietary assessment data [17,19-26]. A cross-sectional study of the Unite States adults aged 20 to 64 years participating in the 2005 to 2014 National Health and Nutrition Examination Survey demonstrated that those in the highest quintile of energy contribution of UPF for total energy (≥74.2%) had 1.61 kg/m² higher BMI, 4.07 cm greater WC, 1.48 times higher odds of having obesity (BMI ≥25.0 kg/m²), and 1.62 times higher odds of abdominal obesity, compared with those in the lowest quintile (≤36.5%), after adjusting for sex, age, race, education, income, marriage, smoking, and physical activity [20]. Another study of Brazilians with relatively low dietary energy contribution of UPF found similar results of significantly higher BMI and WC (0.80 kg/m² and 1.71 cm, respectively), and greater chances of having obesity (BMI ≥30.0 kg/m²; OR, 1.41) and abdominal obesity (OR, 1.41), when those in the lowest quartile (<16%) were compared with adults in the highest quartile of UPF consumption (>29%), independent of sex, age, race, income, smoking, physical activity, and comorbidities [19]. Evidence of the detrimental effects of UPF on obesity indicators were further strengthened by cohort studies [22,24-26] and clinical trials [41]. Higher consumption of UPF was strongly associated with increased weight gain and showed higher risks of overweight and obesity [22,24-26]. Some cohort studies investigated whether these associations with obesity indicators were independent of other dietary factors of obesity (e.g., dietary intake of total energy, dietary fiber, ω-3 fatty acid, fruit and vegetable, and overall diet quality) [22,24,26]. They showed that the associations remained unchanged even after further adjusting for other dietary factors. The adverse impact of UPF on obesity was also found in a 2-week crossover randomized clinical trial of healthy adults [41]. In this trial, participants’ energy intake was higher, and their body weight and body fat mass increased during the ultra-processed diet but decreased during the unprocessed diet.

The strength of the associations between UPF consumption and obesity was smaller than those observed in the United States, Canadian, and Australian adults [20,21,23]. This can be partially explained by that the UPF consumption in our study has a lower and narrow distribution, compared to those in the previous studies. In addition, in our study, the associations between UPF consumption and obesity were attenuated after adjusting for total energy intake and overall dietary quality. Total energy intake and overall dietary quality may, at least partially, medicate the association between UPF consumption and obesity.

The mechanisms explaining the association between UPF consumption and obesity are not fully elucidated, but it may be partly understood by their obesogenic nutritional features, physical and structural characteristics, food additives, and packaging materials. As widely known, the design of UPF as industrial products, are focused on profitability, convenience, and high palatability, and they are made from derived food and additive substances, with none or little whole food [12,13]. Thus, these foods generally have poor nutritional value, are high in energy, refined carbohydrate, added sugars, total fat and saturated fat, and low in dietary fiber, minerals, and vitamins [12,42]. Consumption of these foods is associated with excess caloric intake, poor dietary intakes, as well as a low diet quality [39,41,43]. Furthermore, UPF themselves may contribute to overeating; that is, foods that are easily consumed anywhere and at any time, and that may alter the consumers’ eating behavior, promote inattentive eating, thereby leading to overconsumption [44]. Furthermore, chronic consumption of foods high in refined sugars and fat causes changes in the brain’s reward pathway, leading to food addiction and overeating [45]. Moreover, through industrial processing, including moisture removal and heating, these foods undergo physical and structural changes (i.e., reduced total volume and softened textured). Therefore, these changes may lead to rapid eating rate, which interrupts the mechanisms controlling satiety, inducing excess caloric intake [44]. A recent review suggested that dietary exposure of food additives (e.g., flavor, sweetener, emulsifier, stabilizer, thickener, preservative) can increase glucose intolerance, and alter the gut microbiota, resulting in increased weight gain and increased risk of metabolic syndrome [46]. Another review demonstrated that food contaminants which are leached out from food packaging materials (e.g., phthalates, bisphenol-A) might disrupt the hormonal control regarding hunger and satiety, alter individual’s dietary intake, and thus lead to weight gain and fat storage [47].

The present study has several strengths. Most previous studies which examined the association between UPF consumption and obesity assessed dietary intake using 24-hour recall [17,20,21,23,24] or used food purchase or sales data as a proxy indicator of food consumption [18,40]. The dietary exposures measured in those studies may not represent individuals’ usual consumption of UPF, with the possibility of biased result. However, the present study collected dietary information using a validated FFQ [34] which is generally recommended for long-term diet assessment [48], and thus could investigate the association of the usual consumption of UPF with obesity. Moreover, some previous studies used estimates of obesity prevalence from national surveys [18], or defined obesity using national BMI estimates derived from population-based studies [40], or partially used self-reported height and weight [21]. However, we used anthropometric values measured by a standardized method, and the stadiometers and scales used during the entire study period at the two research clinics were calibrated regularly according to a standardized protocol [32]. Third, we identified UPF among FFQ items, according to the NOVA food classification. The NOVA classification system had been widely applied in the research of highly processed foods [12,13]. Finally, in most epidemiologic studies [17,18,20, 21,23,25], potential confounders were considered when examining the association of UPF and obesity, but no independent association was determined for other dietary factors known to be associated with obesity. Our study could examine whether the associations between UPF consumption and obesity persisted after further adjustment for total energy and overall dietary quality, and also provide the subgroup analysis results by sex, age groups, and overall diet quality.

Nevertheless, several limitations must be considered. As a cross-sectional study, our findings cannot support the causal relationship between UPF consumption and obesity indicators. Next, although the FFQ used is acceptable to assess usual dietary intake, it was not designed to classify foods based on the food processing status. Based on the Food and Agriculture Organization of the United Nations guidelines [49], diverse information such as ingredient list, preparation methods (for foods prepared at home or outside-the home), as well as brand and product name (for foods produced in industrial settings) are needed, in order to classify foods by the nature, extent, and purpose of food processing. Open-ended dietary assessment such as the 24-hour recall have been suggested as the most appropriate method to obtain information on food processing. However, the CMERC cohort study assessed dietary intake using FFQ, and thus UPF was identified by applying the NOVA classification to each food item description in the FFQ. Therefore, the present study cannot rule out the possibility of misclassification in assessing UPF consumption. In addition, self-reported dietary data are subject to misreporting from social desirability bias. This tendency is more frequently observed for foods considered unhealthy and in individuals with obesity [21], which would result in attenuation of the association of UPF consumption with obesity indicators among our population. Finally, although we considered diverse confounders, the potential of residual confounding exists.

In summary, our findings add to the growing evidence indicating that a higher consumption of UPF is associated with higher BMI and WC as well as higher odds of having obesity and abdominal obesity after adjusting for potential confounders, with a dose-response relationship. When additionally adjusting for dietary factors, such associations were attenuated. These findings show that even in a population with relatively low consumption of UPF, the consumption of UPF is associated with obesity. Currently, the UPF types are diverse, with increasing sales and transition towards more highly processed diets observed worldwide [7,8,11,27,28], although with variations across regions and countries. Further studies are needed to better understand the underlying mechanisms of UPF consumption and health outcomes as well as the impact of UPF on various health outcomes. Furthermore, strategies and actions to reduce UPF consumption and enhance the consumption of less processed foods should be considered.