Similar to other reviews [3–6], a limited number of studies in lifestyle weight management for adults with ID and obesity were found. In general, over the years people with ID and obesity have had a minimal involvement in research  despite expressing their interest to participate . There is no research examining the specific reasons of exclusion of individuals for ID from weight management studies. However, this can be explained by the already identified challenges in developing research for adults with ID, especially in relation to ethics. Several studies and ethics committees have looked at the ethical issues related to the types of interventions provided to people with ID, reporting the necessity of interventions tailored to the needs of the participants and reviewing the principles and procedures that need to be followed when individuals with ID have not the capacity to consent their participation in a study .
What components are included in weight loss interventions for adults with ID?
Inconsistency in the methodology of the studies and insufficient information regarding the components of the interventions used made their classification into a specific category difficult. The physical activity and behaviour change components of the interventions were more clearly described in most of the studies in comparison with the dietary aspects of the interventions. This limitation can affect the reproducibility of the studies and has been also identified in weight management studies for adults without ID .
Several clinical guidelines recommend that obesity management interventions should use a multi-component model that incorporates advice on dietary behaviour and physical activity patterns [7–9, 14] and should also include behaviour change techniques to help individuals achieve sustainable changes in these lifestyle areas [7–9, 14]. However, few studies (n = 8) were classified as multi-component interventions in this review.
A 600 kcal energy deficit is identified as a realistic amount of energy deficit that can lead to a loss of adipose tissue and sustained weight loss of 0.5 kg per week, ensuring a better compliance from individuals with obesity [42, 43]. However, very few studies in this review used energy deficit diets with Melville et al.  being the only study that offered a 600 kcal energy deficit diet to the participants. The absence of studies examining the effectiveness of energy deficit diets in this population group may be related to the challenging issues that may arise implementing a significant change in the routine of an individual with ID, especially when the individual has autism . It is possible that researchers and carers may consider that a healthy balanced diet will not disturb the dietary patterns of an individual with ID to a great extent and will not cause distress. However, a 600 kcal energy deficit diet can be based on the same principles as a healthy balanced diet requiring small changes for a small sustained weight loss. This issue has not been investigated by other studies or reviews but a qualitative investigation on the opinions and beliefs of researchers and carers could provide an insight into this.
The benefit of physical activity in the management of obesity depends on the amount and the intensity of the intervention [45, 46]. Clinical guidelines for the treatment of obesity recommend more than 225-300 min per week of moderate intensity physical activity [8, 9]. None of the studies provided an exercise programme that followed these recommendations. However, this amount of exercise may not be realistic for adults with ID, a population group with a very sedentary behavior  and resistant to change daily routines . This means that adults with ID may require longer periods to reach and sustain this amount of daily physical activity than adults without ID.
Behaviour change techniques in weight management aim to support and maintain changes in cognitive behaviour in relation to eating habits or activity patterns of individuals with obesity . Most common behaviour change techniques used in studies for adults with ID in this review are the same with those identified in interventions for adults without ID: self-monitoring, goal setting, reward strategies and relapse prevention [13, 49–51]. However, contrary to the behaviour change techniques used in weight management interventions for adults without ID , the intervention for adults with ID did not state if they were based on a specific theory (e.g. stages of change of the Transtheoretical model of change or the Social Cognitive theory).
Several studies in this review reported that carers were involved at different levels with poor description of their role and with only three of them describing the impact of their involvement on weight loss [15, 16, 31]. Willner et al.  reported that carers can have a vital role in motivating individuals with ID in the process of cognitive therapy and readiness to change. This finding was supported by Spanos et al.  that explored in depth the role and the experiences of the paid and family carers that participated in Melville et al.  According to the qualitative study the carers provide encouragement and praise to the participants in a weight loss intervention and assist in the process of goal setting, essential mechanisms for behaviour change in obesity management.
The majority of the interventions were delivered in group sessions, which could be regarded as more preferable potentially offering improved cost effectiveness . However, there is insufficient evidence to support the effectiveness of group therapy for weight management versus individual therapy [50, 55]. No studies in this review explored or commented on which method is the most suitable way of delivering a weight loss intervention for adults with ID.
To reduce health inequities that adults with ID frequently experience while using health services  weight loss interventions should be made accessible by tailoring the intervention to the cognitive, communication and literacy abilities of adults with ID . Some of the reviewed studies highlighted the importance of developing an intervention based on the needs of the people with ID by describing the resources and the adaptations that had to be followed [19, 27, 35, 37].
Are weight loss interventions for adults with ID associated with a clinically significant weight loss?
Even though there were studies that did not report robust statistical analysis, the majority of the studies reported weight loss based on weight or BMI. Some studies reported changes in waist circumference [28, 32, 35, 37, 38] or waist hip ratio [22, 24] but the results are not reported in this review.
According to clinical guidelines for obesity and weight management, for individuals with BMI 25–35 kg/m2 with no comorbidities present a 5-10% weight loss (approximately 5-10 kg) is required for the reduction of obesity related health risks [7–9]. Three studies reported a clinically significant weight loss within six months: one behaviour change and physical activity intervention , and two multi-component interventions [31, 38]. Other studies reported a clinically significant weight loss at nine months  and at 12 months [22, 28]. Limitations and the differences in methodology and intervention components do not allow comparisons or support of the effectiveness of these studies. However, the absence of use of energy deficit diets and the lack of recommended levels for physical activity, may partly explain the poor weight loss outcomes in these studies.
This review focused only on first line treatment of obesity and did not examine pharmacotherapy and surgery in adults with ID, treatments that could potentially be effective in this population group. However, to our knowledge no studies have examined the effectiveness of this type of weight management in adults with ID and obesity and this can be explained by the ethical issues related with such type of weight management for this population.
Do interventions include a weight loss maintenance component?
Weight loss maintenance following a weight loss intervention is important, showing that individuals who have lost weight and maintained their weight have made sustainable lifestyle changes that will prevent future weight gain or health risks [8, 9]. However, research for weight management in the general population has mainly focused on the development and evaluation of weight loss strategies and has not examined extensively the effectiveness of weight maintenance interventions that follow a weight loss phase . Only four studies out of the 22 in this review offered a structured weight loss maintenance intervention [15, 16, 19, 38], with weight loss being still promoted in two of these studies [15, 19].
A major limitation of this literature is the absence of sample justifications making it likely that these studies are under powered given the small sample sizes (ranging from 6 to 192). A review of 20 studies in this population group showed that lack of direct contact when inviting individuals with ID to participate in a study, inclusion of invasive procedures such as blood testing and the procedures of taking consent may discourage poor participation in the studies for adults with ID .
Only two studies recruited participants from institutional settings [21, 25] and the rest from community settings. Samples were usually heterogeneous, especially in relation to the level of ID. Level of ID was reported in different ways including as mean IQ scores  or percentage of mild, moderate and profound ID  or not reported . In addition, some studies used strict inclusion criteria and offered an intervention only to participants that had mild to moderate ID and others offered an intervention to participants with a variety of levels of severity of ID. This may have had an impact on the level of support from the carers leading the studies to making inappropriate generalisations of the effectiveness of their intervention.
The same pattern of sample heterogeneity was also seen in relation to the weight status of the participants. For example Melville et al.  delivered a multi-component weight loss intervention to obese participants only but Chapman et al.  offered a diet and physical activity intervention to a group of participants who ranged from a healthy weight to the overweight or obese and it was even more surprisingly that Wu et al.  included normal weight and underweight participants in their study. Most of the studies provided the same intensity of intervention to participants that were obese, overweight and sometimes normal weight. According to clinical guidelines [8, 9] the intensity of a dietary intervention (600 kcal energy deficit) can be the same for overweight and obese individuals but the intensity of the physical activity intervention and the targets of weight loss may need to change based on the BMI and the associated health risks of their weight to an individual.
Only four studies reported using randomised allocation [19, 20, 23, 31]. Allocation concealment to the intervention or control groups was unclear for all these studies. RCTs are regarded as the most “powerful tool” in research, especially for the evaluation of healthcare interventions . However, it is essential for these studies to explain the process of random allocation because a detailed description ensures that these studies are truly randomized aiming to reduce the limits for bias [10, 60]. For example, studies that report being randomized but not reporting using a method of concealment and have allocated participants by using the date of birth (odd and even numbers) are not regarded as randomized .
There was no consistency in the duration of the interventions varying from two months to 12 months. According to a recent clinical guideline  most individuals are able to lose weight actively for about three to six months and so studies reporting ‘weight loss’ at 12 months actually measure a mixture of weight loss and weight maintenance.
According to the clinical guidelines [7–9], the effectiveness of weight loss interventions is also associated with the duration that the weight loss is maintained. This aspect of weight management can be evaluated with long term follow up measurements after the intervention. However, the longest follow up measurements reported in this review were by Chapman et al.  at six years followed by one study reporting measurements at 18 months  and four at 12 months [16, 19, 31, 32].
High attrition levels are common incident among weight loss interventions, with a usual attrition rate range of 30%-60% . Attrition is used to judge the acceptability of interventions, as it often reflects participants’ high weight loss expectations and low initial weight loss . The majority of the studies included in this review did not report a high dropout or attrition rate with the exception of one dietary intervention and a multi-component intervention [22, 38].
Limitations of the review
One of the great difficulties in the review of weight management interventions is the classification of an intervention to a category (e.g. multi-component, physical and dietary interventions) but also to provide a description of their components (e.g. behaviour change). The process can be seen as quite biased and subjective and it has been seen in other reviews where different or unclear definitions have been used, especially in the case of the multi-component interventions. However, this review described and evaluated the components of each intervention using the specific recommendations of national and international guidelines, a method that has not be used in other reviews of this area of research.