## 1 Introduction  
In 2016, the prevalence of being overweight in the adult world population was 39% in persons over 18 years old (39% of men and 40% of women). The prevalence of obesity was 13% (11% of men and 15% of women). The prevalence of obesity throughout the world almost tripled between 1975 and 2016 [1]. The prevalence of obesity has been reported at about 20% in the United States of America [2]. Being overweight or obese is related to overall mortality [3] and particular causes of death, including cardiovascular and respiratory diseases and cancer [3]. Being overweight or obese augments the risk of numerous chronic diseases, especially cardiovascular diseases, type 2 diabetes mellitus, and some cancers [4]. Inadequate dietary habits have been shown to cause obesity prevalence among the population [5]. In the age of digital technology, different ways of trying to improve health with innovative technologies have been developed [6]. Health information via social media, with nutrition [7] and fitness counseling [8], wearable devices to track physical activity [9], and applications to record food intake--or even more specific platforms for different types of users, such as patients with food allergies [10], older adults [11] or young children [12]--are frequently used by the population. Thus, nowadays, digital applications or platforms are replacing traditional ways of recording our food intake, such as written food diaries or food frequency questionnaires [13; 14]. Some of these applications have been developed for different populations, such as the Irish [15], British [16; 17; 18], German [19; 20], French [21], Swedish [22], Italian [23], Arabian [24], Canadian [25], Australian [26] or United States of America [27] populations, with the aim of addressing the epidemiological challenges regarding health and weight loss. On the contrary, a large number of smartphone applications have been created by private institutions, with a mainly commercial purpose [28]. Usually, they have not been validated (only around 0.8% of the apps registering food intake have been scientifically evaluated [29]); they have not involved nutrition professionals during their development (only around 0.05% have been created with identifiable professional advice [30]) [31], and they have not been adapted to the different cultural food habits [32]. These apps are developed to be used by the general population, with the main objective of weight management [31]. They have been shown to report acceptable energy intake and fat proportions [33], despite micronutrients being predominantly underrated [33]. For example, MyFitnessPal has been shown to be accurate for calculating total energy intake and fiber [34] but underrated sodium intake [35]. The accuracy of the apps in registering the consumption of saturated and polyunsaturated fatty acids, a relevant aspect of cardiovascular health, has been evaluated as poor [36]. It has been stated that the greater source of error might reside in the estimation of the portion size [37], in the use of non-specific food composition data for each country, and in the modification of a food list by the user [38]. Widely used applications, such as MyFitnessPal, Lose It!, or FatSecret [31], are designed to capture dietary data and even to provide personalized nutritional advice, and the majority are used without professional support [39]. The quality of the information provided by some of these apps (Yazi, FeelEat, and Bonne App) has been evaluated by dieticians and nutritionists, showing high-quality scores, although other widely used applications, such as Lose It!, obtained worse marks [40]. Specificity of the content has been shown as a deficit topic in general, although FeelEat has also been evaluated as being correct in this issue [40]. The experiences that favor the use of apps to track food consumption have been elucidated.