Very-low-calorie diets: who they suit

By the DieticiansWorldwide.com editorial team · 18 October 2026 · Weight management & GLP-1

What counts as a very-low-calorie diet

The terms are used loosely online, so it helps to start with the definitions used by guideline writers and regulators.

• Very-low-calorie diet (VLCD), also called a very-low-energy diet: under 800 kcal a day.
• Low-calorie diet (LCD) or low-energy diet: 800 to 1,200 kcal a day.
• Total diet replacement (TDR): all food replaced for a set period by nutritionally complete formula products such as shakes, soups and bars, usually at low or very low energy levels, followed by a planned return to food.

These definitions come from NICE's guideline on overweight and obesity management, updated in 2025 (NICE NG246). In practice, many clinical "800 calorie" programmes sit right on the line between the two categories. DiRECT used 825 to 853 kcal a day and DROPLET 810 kcal, so strictly they were low-energy rather than very-low-energy diets, although they are often described as VLCDs.

What EU rules require of formula products

In the European Union, products sold for total diet replacement must provide between 600 and 1,200 kcal and between 75 g and 105 g of protein across the daily ration, with at least 30 g of carbohydrate, under a regulation that has applied since 27 October 2022 and is based on EFSA's scientific advice (Commission Delegated Regulation (EU) 2017/1798). The label must state that the product should not be used for more than eight weeks, or repeatedly for shorter periods, by healthy adults with overweight or obesity without advice from a healthcare professional, and must stress the importance of an adequate daily fluid intake.

So a properly formulated product sets a floor on protein and nutrients. A home-made 600-calorie diet of ordinary food usually does not, which is one reason supervised programmes use formula products.

Who a very-low-calorie diet suits

NICE is specific about who should be offered these diets and where.

• Low-energy diets (800 to 1,200 kcal) belong inside a broader programme with long-term support, run by a specialist weight service or a long-term conditions service such as diabetes care, for people with obesity, or with overweight plus type 2 diabetes.
• Very-low-energy diets (under 800 kcal) are for specialist services only, for people with obesity who have a clinically assessed need to lose weight rapidly, for example to make surgery safer and more feasible.
• Neither should be used as a long-term way of managing obesity.

In everyday terms, the people who tend to benefit most are those with a clear medical goal and a team around them: adults diagnosed with type 2 diabetes in the last few years who want to try for remission, people who need to lose weight before an operation, and people for whom other approaches have not worked and who can be supported through the food reintroduction phase. Before any of this, a BMI calculation is one of the standard measures a clinician will use, alongside waist size, health conditions and medicines.

Can a VLCD put diabetes into remission?

For some people with type 2 diabetes, yes, and this is the best evidence for low and very-low-calorie programmes.

DiRECT

The Diabetes Remission Clinical Trial ran in 49 general practices in Scotland and Tyneside. Adults aged 20 to 65 who had been diagnosed within the previous six years, with a BMI of 27 to 45 and not using insulin, stopped their diabetes and blood pressure medicines and started a formula diet of 825 to 853 kcal a day for three to five months, followed by two to eight weeks of stepped food reintroduction and ongoing support. At 12 months, mean weight fell by 10.0 kg in the programme group against 1.0 kg with usual care, and 46% of participants were in remission against 4% (Lean and colleagues, Lancet 2018). Quality of life scores improved in the programme group and fell in the control group.

The five-year follow-up shows both the promise and the challenge. Of the programme participants who received continued low-intensity support, 13% were still in remission at year five, and the original programme group as a whole spent far more of the five years in remission than the control group (27% of visits against 4%) (Lean and colleagues, Lancet Diabetes & Endocrinology 2024).

The NHS programme in practice

England turned the approach into the NHS Type 2 Diabetes Path to Remission Programme, which starts with three months of total diet replacement within a 12-month programme. An evaluation of the first participants found mean weight loss of 10.3 kg among those who completed the programme. Among completers with the blood tests needed to judge remission, 32% were in remission at one year, with an average weight loss of 15.9 kg in that group (Valabhji and colleagues, Lancet Diabetes & Endocrinology 2024). Only 55% of starters completed the programme, a reminder that these diets are hard to sustain.

Remission depends on weight lost

In DiRECT, remission rose steeply with weight loss: no one who gained weight achieved it, against 7% with a loss of up to 5 kg, 34% with 5 to 10 kg, 57% with 10 to 15 kg and 86% with 15 kg or more. Remission is not a cure. Blood glucose can rise again if weight returns, so long-term support and regular checks remain essential. Our guide to working with a dietitian for diabetes covers the wider care plan.

How much weight people lose, and keep off

In DROPLET, 278 adults with obesity in Oxfordshire practices were randomised to weekly behavioural support plus eight weeks of 810 kcal formula products, or to usual nurse-led care. The programme group lost 10.7 kg at 12 months against 3.1 kg, and 45% lost at least 10% of their weight against 15%. Moderate or more severe adverse events were similar, at 11% and 12% (Astbury and colleagues, BMJ 2018).

Across randomised trials, a 2016 meta-analysis found that very-low-energy diets combined with a behavioural programme produced 3.9 kg more weight loss at one year than the behavioural programme alone, narrowing to 1.4 kg at two years and 1.3 kg at three to five years, where the confidence interval included no difference (Parretti and colleagues, Obesity Reviews 2016). Drop-out rates were similar in both groups. The pattern is consistent: a large early loss, then partial regain, leaving a smaller but real long-term advantage for many people.

Is an 800-calorie diet safe?

Under supervision, for a limited time, with nutritionally complete products, the trials suggest it is reasonably well tolerated. Outside those conditions, the risks rise. NICE asks professionals to explain the following before anyone starts.

Gallstones

Rapid weight loss raises the risk of gallstones. In a Swedish matched cohort of 6,640 adults in a commercial programme, symptomatic gallstones needing hospital care were about three times as common with a 500 kcal VLCD as with a 1,200 to 1,500 kcal diet over one year, though the absolute risk was low: one extra event for every 92 people treated (Johansson and colleagues, Int J Obes 2014). Get prompt medical advice for pain under the right ribs, a high temperature or jaundice.

Constipation, tiredness and hair loss

NICE lists constipation, fatigue and hair loss as risks of very-low-energy diets. Constipation is partly a consequence of eating very little bulk; fluids and, where the product allows, added fibre help. Hair shedding is usually temporary but distressing.

Hypoglycaemia and blood pressure

Eating 800 kcal a day while taking insulin or a sulfonylurea can cause dangerous hypoglycaemia. That is why DiRECT stopped diabetes and blood pressure medicines at the start under medical supervision, and why NICE asks for a medicines review before any low-energy diet. Never start one on your own if you take these medicines.

Weight cycling and regain

NICE wants people told plainly that regain is likely and that it does not mean they or their clinician have failed. It also stresses that these diets must be followed by lifelong attention to energy intake and activity.

Eating disorders

Rigid, highly restrictive diets can trigger or worsen binge eating and other disordered eating. NICE recommends offering assessment and counselling to anyone who may have an eating disorder before starting, so the diet is appropriate for them.

How long can you stay on a VLCD?

NICE says low and very-low-energy diets should last no more than 12 weeks and should be nutritionally complete, with ongoing clinical support and access to a registered dietitian or registered nutritionist. The EU labelling rule for formula products sets eight weeks as the point beyond which healthy adults should not continue without professional advice. DiRECT's formula phase ran for three to five months, but within a closely monitored trial.

The end of the formula phase is as important as the start. Programmes with good results reintroduce food in steps over several weeks, moving to a balanced diet. A calorie calculator can give a rough idea of maintenance needs at your new weight, but a dietitian should guide the transition.

Book a session with a dietitian

Who should not use a very-low-calorie diet

• Pregnant women. NHS pregnancy guidance says weight loss should not be attempted while pregnant, including for people with obesity (NHS).
• Children and teenagers, unless a specialist paediatric team advises it; the EU rules frame these products for adults with overweight or obesity.
• People with a current or past eating disorder, without specialist assessment.
• People who are already undernourished, for whom sudden changes in intake carry different risks, explained in our guide to refeeding syndrome.
• Anyone taking glucose-lowering or blood pressure medicines without a supervised medicines plan.

Medication-first or diet-first?

GLP-1 medicines have changed the conversation. In the main trials, semaglutide and tirzepatide produced average losses of about 15% to 21% over 68 to 72 weeks, larger than most diet programmes achieve at a year; our GLP-1 weight loss guide has the trial details. Both approaches share the same weakness: weight tends to return when the intervention stops.

The choice is not either-or. A supervised low-calorie programme may suit someone recently diagnosed with type 2 diabetes who wants to try for remission without long-term medicines, or who cannot access or tolerate a GLP-1. Medicines may suit someone for whom diet programmes have failed. Some people use both in sequence. The S-LiTE trial, for example, began with an eight-week low-calorie diet that produced a mean loss of 13.1 kg, and then tested liraglutide, exercise or both for maintenance (Lundgren and colleagues, NEJM 2021). Our comparisons of medical weight loss and weight management programs set out the routes in more detail.

People preparing for bariatric surgery are often asked to follow a short low-calorie diet beforehand; our guide to the bariatric diet covers what that involves. And if you are drawn to fasting rather than formula products, our article on intermittent fasting looks at a less restrictive option.

Weight-neutral care and outcome targets

Very-low-calorie diets are, by design, about weight. That sits uneasily with weight-neutral approaches, which focus on eating behaviours, fitness and blood results rather than the scale, and with the stigma many people in larger bodies experience in healthcare.

There is evidence on both sides. DiRECT showed that, for type 2 diabetes, how much weight people lost largely determined whether remission followed, which makes a weight target clinically meaningful. At the same time, NICE encourages people to improve their diet and increase physical activity even if they do not lose weight, because of the other health benefits. A good service holds both views: it is honest about what weight loss can achieve for a specific condition, respects the person's own goals, and never makes a restrictive diet the price of care. Our weight management hub covers the full range of options.

The practical takeaway

Very-low-calorie diets and total diet replacement are clinical treatments, not quick fixes. Delivered within a supervised programme, for up to 12 weeks, with nutritionally complete products, a medicines review and planned food reintroduction, they produce large early weight loss and, for some people with recent type 2 diabetes, remission. They carry real risks, including gallstones and hypoglycaemia, and regain is common without long-term support. They suit people with a clear medical goal and a team around them, and should not be attempted alone. If you are unsure whether you are a candidate, our guide to meal replacement shakes explains the less intensive options.