A systematic review and meta‐analysis of the effectiveness of meal replacements for weight loss

1Baseline observation carried forward

2Haemoglobin A1C

3High‐density lipoprotein cholesterol

4Low‐density lipoprotein cholesterol

5Meal replacements

6Randomized controlled trial

7Obesity is a condition of excess accumulation of body fat that causes premature mortality. It also causes substantial morbidity, including significantly increased risks of type 2 diabetes, cardiovascular disease, and several nonsmoking‐related cancers, as well as physical impairments linked to excess weight such as breathlessness, joint problems, and back pain.1, 2 Nevertheless, the health risks of obesity are offset by weight loss,3, 4, 5, 6 with greater weight losses associated with greater health benefits.7 The prevalence of excess weight is high and rising throughout the world;8 thus, unless effective obesity treatment and prevention strategies are implemented, the increasing incidence of preventable diseases will place a growing burden on health systems and the global economy.9 There is a pressing need to identify effective interventions to treat obesity that can be delivered at scale.

8Weight loss programmes that incorporate self‐management strategies, such as the use of meal replacements (MR), may be a particularly useful approach, as they can be delivered within the community, potentially without support by health care professionals. For the purposes of this review, we defined MR as discrete foods, food products, or drinks that are used to replace foods usually consumed at one or more meals with the intent to reduce daily energy intake for the purposes of achieving weight loss or weight maintenance following weight loss. MR programmes replace at least one meal per day and include at least one meal comprising conventional foods. This definition not only includes products purposefully marketed as MR for weight loss such as soups, shakes, and bars but also includes portion‐controlled ready meals as well as discreet portions of readily available conventional foods such as breakfast cereal or rice. This review does not include very‐low‐energy diets (VLED) providing less than 3347 kJ (800 kcal)/day, or those that use low‐energy total diet replacement (TDR), which use formula food products to replace all meals and snacks, but the impact of these types of programs on weight has been reviewed by others recently.10, 11

9A previous systematic review and meta‐analysis of interventions that incorporated MR for weight loss, using a similar definition as here, identified only six studies of MR programmes versus comparator. It is reported that the use of MR significantly increased short‐term weight loss (3 months) compared with control. However, at 1 year, there was no statistically significant difference in weight change between the groups, though the point estimate favoured the intervention.12 Uncertainty over the long‐term effectiveness of MR may explain why many national guidelines do not reference MR,13, 14, 15, 16 or advise against their use17 for the routine management of overweight and obesity.

10However, in recent years, several further studies of the effects of MR with longer term follow‐up have been conducted. We therefore set out to conduct an up‐to‐date systematic review of randomized controlled trials to examine the effectiveness of MR in people with overweight and obesity, compared with interventions that do not include MR, on weight loss at 1 year. The effects on cardiometabolic risk and any adverse effects were also investigated. We included interventions intended for the treatment of overweight or obesity and considered suitable for use in the community, without medical supervision.

11A protocol for this review was published in advance and implemented without changes.18

12MEDLINE, Embase, PsycINFO, CINAHL, Web of Science, and the Cochrane Controlled Register of Trials (CENTRAL) were searched from database inception to August 2018. We also screened references from systematic reviews identified through our search and requested papers from authors for those we were unable to obtain or for which we had abstracts only. The searches were not restricted by country or language. The search strategy (MEDLINE) is included in Supplementary Table S1.

13Studies were included if they recruited adults (≥18 years) with a body mass index (BMI) ≥25 kg/m2. Studies in pregnant women, people with eating disorders, those who had undergone bariatric surgery, or in those that included concomitant use of pharmacotherapies for the purposes of weight loss were excluded. We included randomized controlled trials of interventions incorporating the use of one or more MR daily, as part of a hypoenergetic diet intended for weight loss. We excluded interventions in which daily energy intake was restricted to <3347 kJ (800 kcal)/day, as these diets (VLEDs) require medical supervision and have been systematically reviewed previously.10 Studies that used formula TDR were also excluded. To be included, studies must have had as a comparator either a weight loss intervention that did not include MR or offered no or minimal intervention. Studies which compared different types of MR but did not include a control arm were excluded. Studies were required to report participants' weight at least 1 year after enrolment.

14Covidence (Cochrane) was used for abstract and full‐text screening and data extraction.19 Data were extracted on weight and adverse events at all reported time points as well as on fasting glucose; insulin; HbA1c; total, LDL, and HDL cholesterol; triglycerides; and systolic and diastolic blood pressure at 1 year. Studies were excluded if they did not present sufficient information to allow data extraction or quality assessment and if this information was not available in a trial protocol or provided to us on request by the authors. Initially, titles and abstracts were assessed for inclusion by two independent reviewers with disagreements resolved by discussion or by a third reviewer. Full papers of the included studies were obtained and independently assessed for inclusion by two reviewers, with disagreements resolved by discussion or by consultation with a third reviewer.

15Two reviewers independently used a prespecified and piloted data extraction form to characterize the population, intervention, control groups, and outcomes and to assess risk of bias. Disagreements were resolved by discussion or by a third reviewer. Authors were contacted for missing data and clarifications as required. Risk of bias was assessed using the Cochrane risk of bias tool,20 consistent with the methods used in previous reviews of weight loss interventions.21 If the outcome assessors were not blinded, we judged studies at low risk of detection bias where the outcome was objective (eg, body weight measured by study staff). Studies were judged at high risk of attrition bias if fewer than 50% of participants were measured at follow‐up or if there was a difference of ≥20% drop‐out rate between groups, for the primary outcome of weight at 1 year from baseline.

16We categorized interventions including MR, and their comparators, to ensure that we compared effectiveness within meaningful groups. There were three types of active intervention. The first was stand‐alone dietary advice which recommended the use of MR to replace one or more eating occasions with the remainder of the diet provided by conventional food (MR diet). The second included interventions which recommended the use of MR and also provided participants with additional support to lose weight (MR diet + support). If the “support” offered was at a higher intensity than the support offered to the comparator group, we called this enhanced support, and this made up the third intervention group (MR diet + enhanced support). There were three types of comparators. The first comprised advice on dietary changes to aid weight loss that did not include the use of MR (diet only). The second was broader support for weight loss, which include advice on diet and other behavioural strategies, though the details may not be specified (diet + support). The third type of comparator intervention offered a nominal intervention for weight loss, either in the form of a printed leaflet or one‐off educational lecture (minimal intervention).

17Meta‐analyses were conducted in Review Manager 5.2 (Cochrane)22 to examine the difference between intervention and comparator groups for weight change at 1 year (primary outcome). Additionally, where available, we extracted interim weight change (~3 months, typically at the end of the main intervention) and weight change at 2 and 4 years after baseline. Further analyses were conducted to examine the changes in biomarkers of diabetes and cardiovascular risk at 1 year. Studies varied in whether and how they imputed data for participants lost to follow‐up, and as the practices adopted for this can impact on the reported effect size, we recalculated weight change using the method of baseline observation carried forward (BOCF) to reduce spurious heterogeneity.23

18In studies that did not report baseline weight for the randomized participants, we assumed that there was no difference in baseline weight between those randomized and those who completed the study unless it was stated otherwise. In one study, it was unclear how many participants were lost to follow‐up in each group, and we assumed equal drop‐out for the purposes of the BOCF calculation.24 A sensitivity analysis was conducted removing this study from the comparison.

19Because the definitive numbers of participants achieving ≥5% or ≥10% weight loss were inconsistently reported in the included papers, we estimated these numbers based on the mean and standard deviation of the reported weight loss, assuming a normal distribution as a post hoc analysis.

20All meta‐analyses used a random effects model to account for differences in the intervention programmes and populations. Pooled results were calculated as mean differences in kilograms (kg) with 95% confidence intervals (CI) for continuous variables; weighted means were used to report individual group means. For the proportion of participants achieving ≥5% or ≥10% weight loss, pooled results are reported as odds ratio with 95% confidence intervals. The I 2 statistic was used to quantify statistical heterogeneity.25 We used Cochrane guidance for interpretation of I 2: 0% to 40% might not be important, 30% to 60% may represent moderate heterogeneity, 50% to 90% may represent substantial heterogeneity, and 75% to 100% considerable heterogeneity.20 Where a study contributed more than one intervention arm to a meta‐analysis, the control group was divided equally between interventions to avoid double counting in the pooled result.

21In the preregistered protocol, we planned to use meta‐regression to investigate whether characteristics of the MR interventions impacted the observed effect. However, there were few studies in each meta‐analysis, and the data reporting the details of the programmes were limited such that we judged that the outcomes would not be robust, and this was omitted from our analysis.

22The literature search identified a total of 2924 papers. One hundred eighteen full papers were retrieved, and 24 papers reporting the results of 23 randomized controlled trials (RCTs) with 8253 participants were eligible for inclusion. The main reason studies were excluded is that they did not report body weight at 1‐year follow‐up or longer, and this information could not be obtained from authors. For some studies, additional data were obtained from the authors.26, 27, 28 The PRISMA flow chart is displayed in Figure 1.

23There were 4411 people allocated to an intervention including MR and 3852 people to a comparator in the included studies. One study was conducted in Australia, 1 in China, 3 in Germany, 1 in Malaysia, 1 in Singapore, 1 in Thailand, and 15 in the United States. Most studies were small; the median number of participants per study was 110 [IQR 92.5 to 165.5]. Over three quarters of participants were women, median 79.0% [IQR 54.2 to 94.2]; the mean participant age was 47.7 years [IQR 41.9 to 52.6], and mean baseline BMI was 34.5 kg/m2 [IQR 31.3 to 36.1] (Table 1). There was no significant difference in baseline body weight between intervention and control groups (mean ± SD; http://93.7.kg ± 12.5 and 98.4 ± 12.3 kg in MR and control groups, respectively).

24Fourteen studies were conducted in people who were overweight but with no specific clinical condition,24, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39 five in people with type 2 diabetes,40, 41, 42, 43, 44 two in participants with metabolic syndrome or impaired glucose regulation,45, 46 one in men with type 2 diabetes and erectile dysfunction,47 and one in adults with developmental disabilities and obesity.48

25From the 23 studies reporting weight change data at 1 year (Figure 2), 17 of these studies reported weight change at a point earlier than 1 year (Figure 3), which was collected at a median 18 weeks (range 8 to 34 weeks). Six studies also reported weight outcomes at 2 years (range 64 to 108 weeks) (Figure 4), and two studies reported weight outcomes at 4 years (range 204‐208 weeks) (Figure 5). Twelve studies reported information on biomarkers of cardiovascular and diabetes risk at 1 year (Table 3 and Figures S1‐S9).

26In 15 studies (65%), the MR used consisted of specially formulated shakes, soups, or bars.24, 27, 29, 30, 32, 33, 34, 37, 38, 39, 41, 43, 44, 45, 47 In four studies (17%), the MR consisted of prepackaged portion‐controlled meals which participants reconstituted or cooked at home,28, 35, 36, 42 and in four studies (17%), the MR was a combination of prepackaged portion‐controlled meals and specially formulated shakes, soups, or bars.31, 38, 40, 48

27The median duration of the active weight loss intervention in all 23 included studies was 20 weeks [IQR 12 to 52 weeks]. Twelve studies also included a second MR phase for continued weight loss or weight maintenance where continued use of MR was recommended, generally at a reduced level. The median duration of this phase was 42 weeks [IQR 30 to 52 weeks].28, 30, 31, 32, 33, 34, 38, 42, 44, 46, 47, 48

28During the weight loss phase, 13 studies advised participants to replace two eating occasions each day with a MR.24, 27, 28, 29, 30, 32, 33, 34, 41, 43, 44, 45, 47 One study advised participants to replace one eating occasion,39 and one, two, and four studies advised participants to replace three,37 four,26, 48 or five31, 38, 40, 42 eating occasions (meals or snacks) with one MR, respectively. Two studies (four intervention arms) did not specify the number of MR products participants were advised to consume, but rather recommended that participants replace a proportion of their daily energy intake with the MR products.35, 36

29In six of the included studies, the active intervention consisted of advice to use and/or provision of MR products along with dietary guidance on the quality and quantity of food to be consumed in the remainder of the diet (MR diet).4, 12, 19, 33, 47, 49 In 15 studies, in addition to the MR diet, participants also received support for weight loss during the active intervention. This support consisted of 12 sessions [IQR 7 to 19] over 24 weeks, ranging from 12 to 52 weeks [IQR 12 to 52 weeks] (MR diet + support).27, 28, 29, 30, 31, 34, 36, 38, 39, 41, 43, 44, 45, 46, 48 In two studies (four intervention arms), an enhanced level of support was offered to the MR group (73 sessions [IQR 57.5 to 88.5 sessions] over 78 weeks [IQR 65 to 91 weeks]) compared with the support provided to the control group which consisted of two sessions [IQR 2 to 2] over 78 weeks [IQR 65 to 91 weeks]) (MR diet + enhanced support).35, 42

30In eight studies, the control intervention was dietary advice or a diet plan providing guidance on what to eat to achieve weight loss (diet only).24, 32, 33, 36, 37, 38, 40, 47 In a further 12 studies, the control diet was combined with support for weight loss, consisting of 11.5 sessions [IQR 4.75 to 18.25] over 52 weeks ranging from 12 to 52 weeks [IQR 45 to 52 weeks] (diet + support).26, 27, 28, 29, 30, 31, 34, 35, 42, 43, 45, 48 In three studies, the control intervention consisted of an information leaflet, educational lecture, or usual care (minimal control).39, 41, 44 Accordingly, the included studies fell into one of five distinct comparisons for analysis: MR diet versus diet only24, 32, 33, 37, 40, 47 MR diet + support versus diet + support27, 28, 29, 30, 31, 34, 43, 45, 46, 48 MR diet + support versus diet only36, 38 MR diet + enhanced support versus diet + support35, 42 MR diet + support versus minimal intervention39, 41, 44

31MR diet versus diet only24, 32, 33, 37, 40, 47

32MR diet + support versus diet + support27, 28, 29, 30, 31, 34, 43, 45, 46, 48

33MR diet + support versus diet only36, 38

34MR diet + enhanced support versus diet + support35, 42

35MR diet + support versus minimal intervention39, 41, 44

36A full explanation and examples of the interventions that make up the comparison groups are provided in Table S2. Details of the individual interventions in the included studies by comparison group are presented in Table 1.

37Summary risk of bias for the included studies is provided in Table 2, and justifications for any unclear of high judgments are provided in the supplementary material (Table S3). Six of the included studies were judged to be at low risk of bias across all domains.33, 38, 41, 44, 45, 48 Five studies were judged to be at high risk of bias in at least one domain,24, 26, 31, 34, 40 with all other studies judged to be at unclear risk of bias in at least one domain.27, 28, 29, 30, 32, 34, 35, 36, 37, 39, 40, 42, 47 Twelve studies were judged to be at low risk of sequence generation bias,28, 32, 33, 34, 38, 40, 41, 42, 43, 44, 45, 48 with the remaining studies at unclear risk,24, 26, 27, 29, 30, 31, 35, 36, 37, 39, 47 mainly due to insufficiently detailed reporting. Six studies were judged to be at low risk of allocation concealment bias,33, 38, 41, 44, 45, 48 with the other 17 studies judged to be at unclear risk of this bias because they did not report whether allocation was concealed or not. Five studies were judged to be at high risk of attrition bias,24, 31, 34, 40, 46 mainly because of unequal follow‐up rates between groups, with all other studies judged to be at low risk of attrition bias (Table 3).27, 28, 29, 30, 32, 33, 35, 36, 37, 38, 39, 41, 42, 43, 44, 45, 47, 48

38Twenty‐three studies were included in the primary analysis (weight change at 1 year) across the five comparisons, defined based on intervention and comparator type (Figure 2). Where reported, the difference in weight change reported earlier than 1 year consistently favoured the MR treatment (Figure 3). Results for 1 year or longer follow‐up are reported narratively below. Odds ratios of achieving weight change ≥5% and ≥10% baseline weight at 1 year were calculated for each comparison (Table 4).

39Six studies compared a MR diet with diet only.24, 32, 33, 37, 40, 47 Mean weight change among participants randomized to an intervention incorporating the use of MR (n 229) was −3.0 kg at 1 year compared with participants in the diet only intervention (n 225) who had weight change of −1.5 kg (mean difference = −1.44 kg; 95% CI −2.48, −0.39, I 2 = 38%)24, 32, 33, 37, 40, 47 (Figure 2).

40Two of the studies in this comparison also reported weight change at 2 years32, 40 where mean weight change in participants randomized to the MR diet intervention (n 81) was −3.4 kg while those randomized to the diet only intervention (n 67) lost −3.0 kg (mean difference = −0.39 kg; 95% CI −2.07, 1.29, I 2 = 0%) (Figure 4). One of these studies32 reported that 4 years after randomization, mean weight change in those allocated to the MR diet intervention (n 50) was −7.2 kg, while in the comparator group (n 50), it was −3.0 kg (mean difference = −4.2 kg; 95% CI −6.59, −1.81), reflecting continued weight loss in the MR group but little change in weight since 1 year in the comparator group (Figure 5).

41Ten studies (12 intervention arms and 10 control arms) were included in the MR diet + support versus diet + support comparison.27, 28, 29, 30, 31, 34, 43, 45, 46, 48 One study was omitted from the meta‐analysis as weight data were not normally distributed, were reported in the paper as median and interquartile ranges, and we could not obtain further data.45 In the nine studies included in the meta‐analysis, mean weight change among participants randomized to a MR diet plus support programme (n 622) was −2.4 kg at 1 year, compared with −1.34 kg in a comparator group (n 576) with a diet but no MR and a similar support programme (mean difference = −2.22 kg; 95% CI −3.99 to −0.45, I 2 = 81%) (Figure 2). Statistical heterogeneity was high, but attributable to one small outlier,27 which showed greater weight loss favouring the intervention. In the study omitted from the meta‐analysis, weight change was consistent with the other included studies, in that the median weight change was greater in the MR group (median −1.8 kg, IQR −4.13, 0.43) compared with the control group (median −0.8 kg, IQR −4.0, 1.05).45 Two of the included studies also reported weight at 2 years,27, 30 with weight change favouring MR over control interventions (mean difference −2.2 kg, 95% CI −4.02 to −0.38, I 2 = 67%) (Figure 4).

42Two studies compared interventions involving a MR diet and support programme (n 95) with a diet‐only comparator (n 95).36, 38 Mean weight change at 1 year was −5.3 kg for the MR diet + support and −1.5 kg in the diet only group (mean difference = −3.87 kg; 95% CI −7.34 to −0.40, I 2 = 60%) (Figure 2). Neither of these studies reported weight beyond 1 year.

43Two studies tested a MR diet together with an enhanced support programme.35, 42 At 1 year, mean weight change among participants randomized to MR diet plus enhanced support (n 482) was −8.5 kg while among those receiving the diet + support (n 187), it was −2.4 kg (mean difference = −6.13 kg; 95% CI −7.35, to −4.91, I 2 = 19%) (Figure 2). One study (two intervention arms) followed up participants at 2 years,35 with weight change (−5.5 kg MR with in person support, −6.3 kg MR with telephone support, and −4.5 kg in control group) continuing to favour the MR diet (n 331) over the comparator (n 111) (mean difference = −1.37 kg; 95% CI −2.64 to −0.10) (Figure 4).

44Three studies (four intervention arms) compared a MR diet plus support with a minimal intervention control.39, 41, 44 Statistical heterogeneity was considerable (I 2 = 98%); therefore, we do not present pooled results here. In all three studies, participants in the intervention arm lost significantly more weight than those in the control arm at 1 year. In a study in China (n = 88), the mean difference was −1.10 kg (95% CI −2.09 to −0.11), favouring the MR group. In a study in Germany (n = 409), mean difference in weight change from baseline was −7.01 kg (95% CI −9.87, to −4.15) in the moderate use of MR group and −6.82 kg (95%CI −9.81 to −3.83) in the stringent use of MR groups compared with control group, respectively.44 In the Look Ahead study in the USA (n = 5145), mean difference in weight change between participants in the MR diet + support group compared with the control (usual care) group was −7.70 kg (95% CI −8.07 to −7.33) (Figure 2). In the latter study, the mean difference in weight change was also significantly greater for MR diet plus support group at 2 years (mean difference = −5.20 kg, 95% CI −5.25 to −5.15) and 4 years (mean difference −3.4 kg, 95% CI −3.45 to −3.35) (Figures 4 and 5). The other two studies did not report weight change beyond a year.

45For each comparison, we calculated the odds ratio that participants would achieve a weight loss ≥5% and ≥10% from baseline at 1 year in the MR group compared with the control (Table 4).

46For weight change of ≥5% baseline weight, all comparisons showed that the odds of achieving this were statistically significantly greater in the MR group compared with control. For weight change ≥10%, four out of the five showed that participants allocated to MR intervention had statistically greater odds of achieving the cut‐point compared with those assigned to the control.

47There were very limited data on biochemical outcomes with only 12 studies reporting at least one biochemical outcome.28, 32, 33, 34, 38, 39, 40, 41, 42, 44, 46, 47 Across all studies that reported outcomes, results consistently favoured meal replacement groups for HbA1C. For the results for all other biochemical outcomes (glucose, insulin, lipids, and blood pressure), results were mixed and rarely reached statistical significance. Further detail can be found in Table 3 and Figures S1 to S9.

48Only two studies reported information on adverse events (AE).26, 41 Neither of these studies gave details on the total number of AEs reported by intervention arm, but the Look Ahead Research Group 2007 reported that there were no between‐group differences in the frequency of hypoglycaemia, fractures, amputations, congestive heart failure, or occurrence of gallstones among 5145 participants.50 Flechtner‐Mors et al reported that there were no adverse events that could be attributed to the intervention.26 We also considered nutritional deficiencies to be an AE. Five studies assessed diet quality during the interventions, and these studies reported that diet quality was improved in those randomized to MR.29, 30, 32, 46, 48

49The use of discrete foods, food products, or drinks a replacement for usual foods on some occasions during the day leads to significantly greater weight loss at 1 year than comparator interventions based wholly on conventional foods, with commensurate improvements in HbA1c and mixed findings for markers of diabetes and cardiovascular disease risk. All of the included studies individually reported that interventions including MR products were either equally as effective or superior to the comparator interventions for interim weight change as well as weight change at 1 year. Only two studies reported outcomes at 4 years, but both reported that weight change from baseline to 4 years favoured the MR groups over control. Only two studies assessed adverse events; there was no evidence from these studies of any excess adverse events associated with the use of MR products.26, 41

50A strength of this review is the rigorous methodology, including use of a consistent approach to address missing data (BOCF).23 Given the scope of this review, the included studies are inherently clinically diverse. We tried to overcome this by grouping studies according to a predefined protocol to offer a meaningful insight into some of the variation in weight losses seen in studies using different treatment or comparator protocols. However, there is still moderate to substantial statistical heterogeneity in some of the groups. It is plausible that the effect of MR on weight could be affected by the factors that vary between studies, most obviously variations between interventions (nature, frequency, duration, or composition of the MR used) or differences in participant characteristics. However, there were insufficient studies in each comparison to fully explore study characteristics which may explain the observed heterogeneity between studies through meta‐regression or subgroup analyses.

51For each domain of bias, the majority of studies were judged at low or unclear risk of bias. Five studies were judged at high risk of attrition bias.24, 31, 34, 40, 46 These studies were judged at high risk of bias because fewer than 50% of participants were measured at follow‐up or if there was a difference of ≥20% drop‐out rate between groups, and in these cases, it is possible that there is a plausible intervention effect among participants with missing data.

52We endeavoured to identify unpublished studies by searching study registries and grey literature and contacting experts in the field, but we cannot rule out publication bias. We are not aware of unpublished studies with negative results, but given the nature of this research and the fact that many studies were funded or supported by industry, it is possible that smaller studies with nonsignificant results may be less likely to be published. Reporting of the content of the interventions was poor, and there were insufficient studies within each comparison to enable us to conduct a meta‐regression to identify the effective components of the interventions. There was no information reported on the cost‐effectiveness of these programmes or the costs to individuals compared with diets based on conventional food.

53This review expands on the indicative findings of a previous review.12 By including more studies, and reducing the confidence interval around the point estimate, the present findings demonstrate that the weight loss at 1 year in interventions that include MR products is statistically significantly greater than in interventions that do not use these products. Other recent reviews have included weight loss interventions incorporating meal replacements.49, 51 but these reviews have considered the use of meal replacements alongside other weight loss interventions, and have therefore been unable to determine the specific effect of meal replacements per se. TDR have also been reviewed in various populations.10, 11, 52 Although TDR products are like MR in that they are designed to replace usual food‐based meals, TDR products are formulated to be used as the sole source of nutrition, that is, to replace all food in the diet. MR aims to replace one or more usual meals, but the instructions for use suggest eating a food‐based meal at least once a day to ensure adequate intake of nutrients and vitamins. MR can be purchased over the counter and used without guidance from a health professional, whereas weight loss programmes incorporating TDR are advised to be undertaken under supervision. Given these differences between MR and TDR programmes, it is important to consider their effectiveness independently.

54The key finding of this review is that participants assigned to a MR diet compared with a diet only approach (akin to a self‐directed weight loss attempt) lose an additional 1.44 kg at 1 year, and this difference appears to be maintained up to 4 years. Likewise, many people join behavioural weight loss programmes to provide support to lose weight. The results of the present study suggest that all other things being equal, incorporating meal replacement products into such behavioural weight loss programmes enhances their effect (reflected in the MR diet + support versus diet + support comparison), with a 2.22 kg greater weight loss at 1 year in the MR group. Although these differences may appear to be modest, currently around 70% of the UK population with a BMI of 30 kg/m2 are trying to lose weight,53 and a sustained weight loss of even 1 kg will bring substantial benefits to public health.54, 55 Moreover, since the comparative interventions also lead to weight loss, by and large incorporating MR into weight loss programmes increases the proportion of participants who achieve ≥5% and ≥10% weight loss at 1 year.

55Meal replacement products are consumer food products (as opposed to medical devices or drugs), widely available to be purchased over the counter without prescription, but at present are infrequently used by those attempting weight loss in their weight loss attempts.56 Advice and guidance provided by clinicians to encourage their use could deliver benefits at minimal cost to health care providers. Few studies included in the review reported adverse events, including the impact on the nutritional quality of the data. Two studies found no evidence of adverse events arising from the use of these products,26, 41 and five studies report that they may improve the overall nutritional quality of the diet.5, 6, 18, 19, 35

56The greater weight loss observed in programmes incorporating MR suggests that this approach makes it easier to adhere to a reduced energy diet. However, there has been little detailed study of eating behaviours. It is possible that these fixed‐energy, portion‐controlled, or prepacked foods contain less energy than most self‐selected meals and snacks, or that the structure and external control associated with their use facilitate adherence. Further work could usefully examine the behavioural processes that facilitate weight loss which would be of use when designing effective weight loss programmes to enhance effectiveness and reach.

57These findings provide important new evidence to inform clinical guidelines. At present in many countries, clinical recommendations for the treatment of obesity advise that individuals attempting weight loss should be advised to aim for an energy deficit of 2092 to 4184 kJ/day (500‐1000 kcal), but they do not suggest that MR could be used to help individuals achieve this deficit nor do they recommend MR as an effective dietary strategy for weight loss.13, 15 Guidelines in some countries do refer to meal replacement programmes. In the United States, the guidelines state that the strength of evidence on the longer term effect on weight of MR is low.57 Singaporean guidelines note that meal replacements may induce greater acute reductions in weight but are not advised for the long‐term management of overweight and obesity.17 Australian guidelines do not recommend meal replacements for weight loss beyond their use as part of a VLED.16

58In conclusion, this review provides evidence that MR is an effective intervention for the treatment of overweight and obesity at a year, especially when used together with a support programme. MR could be recommended for inclusion in weight management programmes offered by professionals or as part of a self‐management strategy for people with overweight or obesity.

59NMA, PA, and SAJ are investigators on a research grant from Cambridge Weight Plan UK Ltd. PA and SAJ are investigators on a research grant from Weight Watchers Inc.