Substantial mean weight losses were achieved in all studies following VLCD/LELD, varying from 8 to 21 kg in total over 4–52 weeks. That total depended on a variety of factors, including duration of diet, and clinical characteristics of groups, such as age, gender and status of health. In all the studies, the mean rate of weight loss fell below that that predicted for 100% compliance with the estimated energy deficit.

Weight losses were similar for participants with T2DM and participants without diabetes, with the exception of one study,16 where baseline weight was significantly greater in participants without diabetes (126.1±8.5 vs 99.7±3.7 kg, P<0.05), which is likely to have contributed to greater weight loss in the non-diabetic group.

The data generated on weight loss per week, as studies varied widely in duration, also showed similar rates of weight loss in those with T2DM and those without. As with overall weight loss this was with the exception of the study by Henry et al.16 in which a greater rate of weight loss was observed in those without diabetes, again reflecting the higher baseline weight.

The mean weight loss in people with T2DM reported here, in studies which compared people with T2DM and those without T2DM, is similar to a previous meta-analysis of VLED in T2DM, which reported substantial weight loss of 9.6% by people with T2DM over a 6 week period, at a rate of around 1.6 kg per week.13 Studies using similar diets among only non-diabetic participants report similar mean weight losses.21, 22, 23

Similar weight losses with VLED and LELD have been reported previously in the study by Christensen et al.21 which compared VLED and LELD, with weight losses of around 11 kg was observed in both study groups over 8 weeks. Greater energy restriction also conferred no additional benefit in the study by Lin et al.22 with a mean weight loss of 8 kg observed in both groups (450 vs 800 kcal per day) after 12 weeks. Wikstrand et al.23 reported mean weight losses of 20 and 16 kg in free living non-diabetic, male and female participants respectively.

Amongst the studies included in the present review, rates of weight loss were consistent, at 1.2–3.2 kg per week except for much lower rates in studies by Li et al.17 0.3 kg per week and Baker et al.18 0.7 kg per week. The study by Li et al. was over a much longer period of 52 weeks, and the duration of the exclusive formula diet phase was variable amongst participants over the 1 year period, as they could opt to include a meal of ~300 kcal at different time points, which may have contributed to the lower rate of weight loss. Lower weight loss in comparison to other studies was acknowledged by Baker and colleagues. Factors suggested by the authors to explain this were a ‘calendar effect’ and also that the study population were well established patients at a tertiary referral centre and may have represented a more ‘difficult to treat group’ than that of other studies.

The metabolic benefits of bariatric surgery appear to be reproducible by VLCD/LELD. The Newcastle Counterpoint study12 achieved reversal of T2DM with mean weight loss of 15.3 kg in 11 people with T2DM within 4 years of diagnosis, using a 600 kcal per day low-energy liquid diet. The normalisation of fasting plasma glucose persisted for up to 3 months after return to normal diet. Normalisation of HbA1c, or fasting blood glucose, following VLCD was reported in one study included in the present review, Paisey et al.19 with mean absolute weight loss similar to that achieved by Lim et al.12 However normalisation of metabolic status relates both to ongoing energy restriction and on overall weight loss. Acute energy restriction has been shown to improve plasma glucose values and insulin sensitivity even before significant weight loss occurs.12, 13

Formula diets, with a period of total diet replacement, are widely used and popular outside healthcare settings. There is some published evidence that they are successful, with little or no evidence for serious safety problems,24 although large-scale safety testing has not been undertaken. The serious adverse events reported in one study included in the present review were not considered directly related to the diet. Despite clear evidence for efficacy and lack of safety concerns, there is a continuing reluctance by medical staff and in clinical guidelines to support the use of formula diets. Guidelines published in the USA and Australia25, 26 advocate the use of very low calorie/energy diets for weight management, but recommend these should be delivered in medical settings, with close medical monitoring.

At present formula diets are not funded directly within routine UK NHS care, whereas bariatric surgery is. In the 1970 s there was concern about safety of ill-designed modified fasting diets.27 Beliefs that more intensive interventions and rapid weight loss lead to greater weight regain, are not supported by evidence from controlled trials. Comparison of weight regains, following either a 12 week rapid weight loss programme or a 36 week gradual programme, found no difference in the proportion of weight regained at 144 week follow-up.28 Effective strategies for long-term weight-loss maintenance have been described.29, 30 A study conducted within routine NHS primary care29 found that an 810 kcal per day LELD was well accepted by 91 severely obese participants (mean BMI 48 kg/m2), and by GP’s and nurses. Those who attended had a mean weight loss of around 17 kg in 12 weeks. Critically around 33% of all 91 entrants maintained >15 kg weight loss at 12 months. The study was in people without diabetes, but the present review would suggest that results in those with T2DM would not differ greatly. One year follow-up of patients with knee osteoarthritis also found that a structured weight maintenance programme resulted in good maintenance of weight loss.30

It is estimated that by 2025, without new interventions for prevention of weight gain and obesity, five million people in the UK will have diabetes, mostly T2DM (DiabetesUK.org). Despite the recommendations in current clinical guidelines, there is no realistic prospect of bariatric surgery being offered to most obese people because of surgical and follow-up resource limitations. In addition, many people will not agree to surgery. New weight management approaches are required for obese people with T2DM that can be implemented in routine care, where most of these people are managed. The present results suggest that the weight loss phase of weight management can be achieved by a period of ‘Total Diet Replacement’ (TDR), using micronutrient-complete formula diets. This approach towards the SIGN weight loss target of >15 kg (enough to reverse most recent onset T2DM), in many cases is more cost-effectiveness than bariatric surgery.29

Most published studies do not adequately describe whether adverse events were sought systematically, or simply reported on an ad-hoc basis. Future research should include systematic data collection. At present there is no evidence for serious adverse events, and no specific medical monitoring is necessary except for withdrawal or dose reduction of hypoglycaemic, diuretic or anti-hypertensive drugs.