Physical complaints, such as headache, bellyache, tiredness, eczema and sleep complaints, are common comorbid problems in children with ADHD [10
], with a prevalence of sleep complaints up to 50% [39
]. In contrast to comorbid psychiatric conditions, relatively little is known on the comorbidity of ADHD and physical complaints. In this study, we examined whether physical and sleep complaints in 24 children with ADHD were improved by an elimination diet using a randomised controlled design. We previously described that the diet significantly reduced the ADHD symptoms in this group of patients [31
]. In the current study, in which the subjects were not preselected for somatic symptoms, 23/24 (96%) children had one or more physical complaints, indicating that comorbidity between ADHD and physical complaints is high, thus underlining the importance of studying physical complaints in ADHD.
The results of this pilot study should be interpreted in the light of several limitations. First, in this study, a very restricted elimination diet was used, thus making it impossible to compose a reliable placebo diet. Furthermore, parents had to be aware of the intervention and had to pay attention to what the child should eat. Therefore, we had to choose for an open RCT. Although a blinded RCT should be given preference to, open RCTs are commonly used and accepted when blinding is difficult and when no placebo is available, e.g. in studies into the effects of cognitive behaviour therapy, eczema, obesity, autism or other medical intervention trials [7
]. Also, the well known and highly cited Multimodal Treatment Study of Children with ADHD, the MTA-study, was not blinded [26
]. To compensate for the absence of a placebo diet, the parents in the control group, like the parents in the diet group, had to monitor and to observe their child intensively, writing down the behaviour and the physical and sleep complaints of their child conscientiously in a diary. It is conceivable that the child's behaviour and somatic complaints might improve because of the special attention which parents had to pay to their child. In our study, the reduction of the total numbers of complaints in the diet group (77%) was 4.6-fold compared to the reduction in the control group (17%; p
0.001), indicating that the effect of an increase of attention may be small, when compared to the effect of an elimination diet. Second, the trial lasted only 5 weeks, which is a short period of time. Follow-up studies should include a follow-up period of at least 1 year. Finally, the sample size of the study was relatively small; consequently, the data reported here should be considered exploratory. Nevertheless, due to the considerable effect sizes in this study, statistically significant differences between diet and control were obtained.
The effect of the intervention on physical and sleep complaints did not differ significantly between children who did or did not show ADHD symptom reduction after following the diet. The adjusted difference between both groups amounted to 0.82 (p
0.10), suggesting the diet is equally effective in reducing physical complaints in responders and non-responders. However, the power of this analysis is low (0.26), as the non-responder group consisted of four children only. Correlation analyses revealed that ADHD symptom reduction and the reduction of physical complaints were correlated significantly. We hypothesise, considering the effect size of an elimination diet on both ADHD and physical complaints, that there may be a common underlying mechanism for both conditions. This mechanism may be a hypersensitivity reaction to food, which could be an etiological factor of both conditions [30
]. This hypersensitivity mechanism might either be allergic, i.e. related to the induction of IgE or IgG antibodies or of a cell-mediated response [30
], or not allergic, i.e. related to a toxic or pharmacologic mechanism. When there is no effect of an elimination diet on one or more of the complaints, other etiological mechanisms are likely and should be considered.
In this study, 71% of the ADHD children had an atopic constitution. This high prevalence may be related to the possibility that parents acquainted with allergic disorders are more willing to let their child follow an elimination diet than parents unfamiliar with allergies. On the other hand, atopy is a widespread condition, found in many children. A UK study reported that 39% of children in the UK had been diagnosed with one or more atopic conditions [17
], and positive skin prick tests to at least one allergen was found in 63.7% of urban children [23
]. Our study shows that in atopic and in non-atopic children, the number of physical and sleep complaints did not differ significantly before (p
0.081) as well as after (p
0.32) the elimination diet. We did find, although not statistically significant, that at the start of the trial more physical complaints were reported in atopic children (average, 3.5 per child) than in non-atopic children (average, 2.0 per child). The results of this study indicate that the presence of an atopic constitution is not a moderator of the effect of an elimination diet on physical complaints and sleep complaints in children with ADHD, but do suggest atopy is an important condition co-occurring with ADHD.
The subjects in our study were young, but children of 4 years and older are generally expected to be able to tell that it hurts and where it hurts. Therefore, headache, abdominal pains and pain in the legs or arms (growing pains) are probably reliably reported. However, restless legs or breathing difficulties may be more difficult for a child to describe, so it may be conceivable that the number of physical complaints is underestimated. We would like to emphasise that the sleep complaints were reported by the parents, not by the child. These complaints are generally well visible to the parents and have a large impact on family life.
As ADHD has an increased association with sleep-related movement disorders such as restless legs syndrome [44
], the relationship between food, ADHD and sleep complaints should be investigated more thoroughly in follow-up studies.
Although we do not know the mechanisms in which an elimination diet exerts its effects on physical and sleep complaints in ADHD, our findings indicate that the results of this study may be important for children with physical complaints or sleep complaints and ADHD. They even may be important for children with physical conditions without ADHD [4
] and for children with functional somatic symptoms, as these are common health complaints in 5–7-year-old children [33
More research on the effects of foods and on the underlying mechanism is advised to investigate whether children with ADHD and co-occurring physical complaints may represent a specific ADHD subgroup. We hypothesise that there may be a common underlying genetic mechanism contributing to both medical conditions, comparable to the mechanism found by Campbell et al., in children with co-occurring autism and gastrointestinal conditions [8
]. Consequently, the further unravelling of the genetic architecture of ADHD is very important to identify a common genetic pattern or genetic vulnerability in children with ADHD and physical complaints. Also, it is important to segregate between non-allergic or allergic mechanisms involved. This includes analysis of the role of IgE and IgG antibodies being specific for the food and the possible involvement of T cell-mediated hypersensitivity.
In studies specifically asking for physical complaints in children with ADHD, it turns out that comorbidity is high [9
]. This high comorbidity between physical symptoms and ADHD does not reflect clinical practice, which may be due to the fact that in children with ADHD, it is not current practice to ask for physical complaints specifically. A general question like ‘are there any physical complaints’ may not be sufficient, generating too little information. Many of the physical symptoms investigated in this trial would not have been mentioned by the parents if we had not asked for them.
Because diets are not without its limitations (socially handicapping, putting a strain on the whole family), they should only be applied after responsiveness has been individually and carefully tested by means of an elimination diet, supervised and administered by trained staff [34
]. If a child following the diet shows beneficial behavioural or physical effects, sequential introduction of foods is necessary to identify the incriminated foods [9
], so that the eventual diet of the child will be as comprehensive as possible. If a child who responds favourably to the diet will not proceed with this provocation period and returns to its usual diet, consequently, the problems are likely to return.
Further controlled studies are needed to verify the efficacy of an elimination diet in children with physical complaints and to provide a feasible algorithm for treatment, especially for children with behavioural or physical complaints triggered by foods. We will pursue this issue in a large (N
100) sample of ADHD children using an RCT (the Impact of Nutrition on Children with ADHD study) currently underway, the protocol of which can be found on the website of The Lancet (http://www.thelancet.com/protocol-reviews/06PRT-7719
Clinical implications and conclusion
Our study shows that hypersensitivity to food may play an etiologic role in physical and sleep complaints in children with ADHD and suggests that an elimination diet may be a valuable tool to manage these problems in ADHD children. As functional somatic symptoms are common health complaints in 5–7-year-old children [33
], the results of this study may be important for all children. Still, the sample size was small, and we cannot rule out expectation effects. Therefore, more research is needed to determine the effects of food on physical and sleep complaints in children with and without ADHD.