Autism spectrum disorder (ASD) is a heterogeneous and complex group of neurodevelopmental disorders. These are characterized by alterations in social interaction, cognitive functions, oral communication, social–emotional interaction, intellectual impairment, and motor and learning dysfunction, as well as self-restrictive obsessive–repetitive behaviors that manifest in early childhood and remain throughout an affected person’s life. The main manifestations of ASD include limitations in social communication and interaction, the inability to control or understand their own and others’ emotions, and alterations in proprioceptive and sensorial anomalies, such as the perception of pain and organic sensations (e.g., smell, taste, and sight). There are also limited interests in certain daily life activities, and a decreased intellectual capacity. It should be noted that the symptoms do not always present themselves in a linear and homogeneous chronology, and that their presentation varies according to the phenotype and severity of the ASD ^[1][2][1,2].

The worldwide prevalence of ASD is estimated at 1.13% (1:88 children), regardless of sex, race, socioeconomic status, or geographic determinants ^[2][3][2,3]. In addition to behavioral disturbances, several morbidities coexist with ASD, such as gastrointestinal disturbances/dysfunctions, which affect 8 out of 10 children with ASD [4]. The management of gastrointestinal symptoms (GIS) is relevant in the treatment of ASD because a relationship is observed between the progressive worsening of behavior and cognition with the severity of GIS [5]. The most frequently reported GIS in children with ASD are constipation, abdominal pain, and diarrhea, followed by chronic diarrhea, gastroesophageal reflux, nausea/vomiting, achalasia, and bloating. It should be noted that constipation has an important impact on the exacerbation or increase in behavioral and neurological symptoms ^[6][7][6,7].

Eating problems are common in children with ASD. Although most children have eating disorders in the first 2 years of life, those in children with ASD tend to be more severe, and persist throughout life. Food selectivity (FS) was hypothesized to play an important role in exacerbating GIS. FS, commonly referred to as picky/fussy eating, refers to a limited food repertoire in children [8]. This FS can be accompanied by a sensory aversion to food, characterized by a rejection of certain textures, temperature, flavors, colors, and smells; therefore, they tend to have preferences for a single type or brand of food, with unique characteristics of these sensory features. The diet of children with ASD who have FS is heterogeneous, and depends on the severity of the FS and concomitant GIS. When there is FS in ASD, the diet is restricted to the consumption of simple carbohydrates, and foods low in fiber and high in saturated fat. Children with ASD–FS prefer ultra-processed, calorie-dense foods, and food additives that impact their nutritional and gastrointestinal health status [9].

2. Eating Problems and Eating Behavior in Children with ASD

During early childhood, children are exposed to new experiences with food. These experiences are important sensory interactions that include new tastes, colors, temperatures, smells, and textures. Commonly, there is evidence of feeding problems in this period of new sensory experiences. More than a decade ago, it was reported that in neurotypical (NT) children (with normal development), there are about three times fewer feeding problems than in children with ASD (25–35% vs. 80% for NT and ASD, respectively) [10]. About 58% to 68% of parents or caregivers of children with ASD report problems with eating, such as selective food eating and food neophobia (Table 1) [11]. FS and feeding problems increase the risk of malnutrition, significant nutritional deficiencies, and GID in children with ASD (Table 1). In addition to the risks, it is reported that children with ASD have a 4–5 times higher risk of having eating disorders than NT children [19]. Parents and caregivers of children with ASD frequently report a concern regarding some of their eating behavior, such as very selective eating patterns restricted to only some food groups, even limiting themselves to consuming only four or five food types [20]. Unfortunately, these alterations in eating behavior usually persist throughout the lives of subjects with ASD [21]. The clinical study by Wallace et al. (2018) [22] reports a significant correlation in the severity of ASD and food neophobia with malnutrition risk, being overweight, and obesity, corroborated by BMI. This correlation depends on the foods included in the FS of children with ASD. Food neophobias are expected at the beginning of exposure to foods other than milk in infants. Once complementary feeding begins, children are exposed to many sensory stimulations, flavors, colors, smells, and textures other than breast milk or infant formula. This incorporation leads to a sensory aversion of a physiological nature, characterized by food neophobia. This problem is usually transient in most NT children. However, it is reported that in children with ASD, most of these food neophobias persist [20]. Unfortunately, eating disorders in subjects with ASD do not seem to be transient, instead chronically affecting nutritional status. Metabolic alterations associated with modified intake patterns and eating behavior are common, with comorbidities associated with being overweight, obesity, and malnutrition the most common in subjects with ASD aged between 14 and 30 years. Without proper treatment, systemic problems related to malnutrition persist chronically [23]. The reality is that information about eating disorders in children with ASD is limited, even today. This limitation is mainly because these eating disorders do not represent a high risk to the health of subjects with ASD. Even with a diet restricted to several food groups, most children with ASD meet their caloric requirements at the expense of accepted (usually calorie-dense) foods ^[24][25][24,25].

Food Neophobia in ASD

Although refusal of food and novel tastes is typical in NT children, they tend to disappear as the child is subjected to continuous exposure to tastes; however, children with ASD tend not to improve this acceptance over time. Food neophobia or “food rejection”, defined as a fear of trying new foods, is a restrictive eating behavior disorder [26]. Food neophobia is a key component of FS, generating a significant reluctance to try new foods. Children with high levels of food neophobia are more likely to avoid certain foods, and even avoid food groups altogether, negatively impacting their nutrient intake and diet quality. Food rejection debuts in most children aged between two and six years. In NT children, this neophobia is transitory. From the sixth year, this eating behavior decreases while the child is more exposed to food, and the variety of foods in their diet increases. At this moment, the food begins to have a meaning beyond the nutritional state, involving the socio-emotional component of food. Regardless of the age at which children, and even adults, present food rejection, exposure to a greater variety of foods (even if they are not accepted, only exposed) reduces the severity of food rejection [27]. In addition to continuous exposure to a greater variety of foods, physical food transformation (such as bananas into banana ice cream, sweet potatoes into chips, carrot into juice, and kiwi into popsicles) also reduces the severity of food neophobia, and increases the consumption of fruits and vegetables [28]. In 2022, Marlow and Forestell, looking for factors associated with low consumption of fruits and vegetables in children with food rejection, found that children with parents who exhibit food neophobia of fruits and vegetables demonstrate food rejection of the same foods, even to a greater variety; that is, parents’ food-related behavior indirectly affects the eating behavior of their children [29]. The etiology of food rejection is still unknown; however, it is identified that some physiological, psychological, behavioral, and social components influence food neophobia development, duration, and severity [26]. Food rejection is extremely common in children with ASD, although its true prevalence is not reported ^[22][30][22,30]. Both conditions (ASD and food neophobia) have different characteristics and etiologies, for which they should be classified as different, not related, disorders. Studies report the prevalence of eating disorders or FS in children; however, food rejection is included, and not selectively. Besides, there are discrepancies in the concepts of food rejection because children are usually identified as “selective eaters”. More studies are needed to better classify food rejection, and other eating disorders, for the above. Some studies focus on evaluating the role of sensory sensitivity in children with food rejection. The altered sensory sensitivities in the above-described ASD syndrome can negatively impact the interaction with food and the food environment [31].

3. Sensory Sensitivity in Children with ASD

It is common for individuals with ASD to have atypical sensory responses. These sensory alterations are a key point in clinical and diagnostic features, due to their high prevalence. Leekam et al. (2007) conducted a clinical study to evaluate sensory symptoms in children with ASD [32]. The study includes two groups of children with ASD, divided by functionality according to IQ (range IQ of 66–140: high-functioning children with ASD, n = 17; range IQ of 12–80: low-functioning children, n = 16), and one typically developing group (15 typical developing children with an IQ range of 81–138). The Diagnostic Interview for Social and Communication Disorders (DISCO) survey was conducted on the parents of children with ASD, to identify sensory disturbances in the study subjects. This first study reports that about 90% of individuals with ASD have alterations in sensory response compared to the typical development group children (33%). In addition, it is observed that IQ differences might impact sensory responses. High-functioning children with ASD show less atypical sensory responses than low-functioning children. A meta-analysis by Ben-Sasson et al. (2009) reports a lower prevalence of impaired sensory response in individuals with ASD (45–46%) [33]. Unfortunately, these crucial differences in the estimated prevalence of sensory alterations may be due to the heterogeneity of the samples and subjectivity of the evaluation, since parents answer the surveys. Children with ASD exhibit abnormal or atypical visual, tactile/oral, olfactory, and auditory sensory responses; the entire somatosensory system that provides information from the environment, and allows its interaction with it, presents alterations in the generation and processing of information [34]. This atypical sensory response to environmental stimuli makes the somatosensory pathway a stressor for ASD subjects. Several studies report a relationship between atypical sensory responses and various behavioral alterations in children with ASD [35]. Hence, sensory hyper- and hyposensitivity in the different sensory pathways can moderately or severely influence the ASD phenotype and its severity. In addition, those atypical sensory responses may persist throughout the life of people affected with ASD ^[36][37][38][36,37,38]. Sensory sensitivity strongly interferes with the daily activities of subjects with ASD [39]. For example, hyper-reactions to tactile stimuli, such as a hug or a particular scent, may result in rejection or a spontaneous negative response in the ASD subject. Furthermore, not only social interactions are affected by sensory sensitivity; sensory hypersensitivity is reported to be a significant component of feeding problems or disorders in children with ASDs [40]. Atypical sensory processing results in an over-response to food stimuli, perhaps leading to increased food rejection and food neophobia. The process of eating is quite complex, and includes the integration of various sensory aspects that influence individual acceptance and preference for food or its groups ^[41][42][41,42]. There are various characteristic patterns of ASD, but sensory sensitivity is significantly affected, leading to hyper or hypo-reactivity in these patients [43]. Their feeding struggles may be due to rituals at mealtime, chewing problems, vomiting, saturating themselves with food in the mouth, keeping food in the mouth for long periods without swallowing, food sensitivity, or neophobia [44]. Therefore, they tend to have preferences for a single type or brand of food, with unique characteristics such as texture, temperature, smell, color, or flavor [45]. The preferred texture is uniform or crunchy, with neutral colors, hot temperatures, and sweet flavors. The systematic review by Page et al. (2022) reports a positive correlation between FS and impaired sensory processing [46]. Children with ASD who exhibit sensory over-response tend to respond to stimuli with greater intensity, longer duration, and even faster than NT subjects or typical sensory responses. Some studies report a relationship between atypical oral sensory over-response and a rejection of the consumption of fruits and vegetables ^[21][47][21,47]. On the other hand, it is reported that a hyporesponsive sensory state can lead the child with ASD to a greater search for sweet, salty, or spicy foods to reach an adequate stimulus. Both processes make feeding difficult, and can lead to propitiating alterations in the feeding of children with ASD [48]. The clinical study by Dudova et al. (2011) identifies that children with ASD have a lower olfactory capacity than NT children, indicating a lower sensitivity and food identification in children with ASD [49]. Its identification is not established, nor is it very clear. However, research associates it with the severity of autism, the heterogeneity of populations, and screening tools such as clinical history or physical examination. For this reason, it is proposed that the research be carried out in the future, including large samples, autism severity classification, and homogeneous tools for its evaluation, in order to find statistically significant levels of association. Another clinical study, conducted by Sena et al. (2019), identifies that children with ASD have an olfactory dysfunction, particularly in the threshold olfactory capacity and odor identification [50]. Unfortunately, this study also has some critical limitations in terms of sample size (20 ASD children), although it does provide important insights into the nature of olfactory dysfunction in ASD. Psychological studies by Bennetto et al. (2007) report a lower detection of sugars, and a greater preference for drinks such as juices [51]. Kral et al. (2015) conducted a clinical trial on adolescents with autism and NTs [52]. They indicate that adolescents with ASD show less precision in identifying sour and bitter tastes than NT adolescents. Regarding vision, children with ASD were presented with various images of different foods, showing a pattern of food stimuli compared to NT children. These children visualize complex dishes with greater preference and time than simple dishes: carrot vs. peas mix. However, children with ASD explored all dishes (complex and simple) similarly. Children with ASD are more likely to reject food because of its texture and consistency, due to the atypical oral sensitivity they present, without forgetting smell and taste. Although visual and olfactory sensory sensitivities are reported to impact eating behaviors in children with ASD, they do not affect them as severely as tactile hypersensitivities, especially atypical oral responses. Oral defensiveness turns out to be a great trigger for neophobia and FS; oral over-response causes rejection of food textures. The study carried out by Muratori et al. (2015) identifies that children with ASD present a more severe food neophobia when they present an oral sensory over-response [53]. In addition, only a few studies evaluate the impact of oral sensory hyper-responsiveness in children with ASD. However, the authors agree that oral sensory hyper-responsiveness is the leading cause of food neophobia and FS in children with ASD. It is not surprising that the foods least consumed by children with ASD are usually fruits, vegetables, legumes, meats, and some soft cereals. Generally, fruits and vegetables (in addition to their flavor and odors) are usually prepared in complex consistencies, which are generally rejected by children with ASD who present an oral sensory over-responsiveness. Therefore, children with ASD who exhibit this atypical sensory response often prefer crunchy, uniform, and semi-dry textures ^[45][47][45,47]. The etiology of oral sensory over-responsiveness in children with ASD is currently unknown. However, it is hypothesized that alterations in neurobiology and synaptic structure, characteristic of ASD, may significantly affect the transduction of these oral/tactile sensory stimuli. In addition, the assessment of oral sensory sensitivity also has several limitations. One of the main limitations is that the tools to assess this atypical sensory response are based on reports from parents and caregivers of children with ASD. These reports generate a necessary subjectivity that does not allow truthful conclusions to be made.