For many dieters, the “fantastic results” of a new NHS diet focused on soups and shakes won’t come as a surprise. 

More than 2,000 people with type 2 diabetes were put on a very low calorie diet restricting them to just 800 calories per day for three months. Within one month, dieters had lost, on average 7.2kg (just over one stone) and in three months they’d lost 13.4kg (just over two stone.) The average starting weight of those taking part in the first trials was 17 stone.

Meal replacements, liquid diets and very-low-calorie-diets have been around for decades, and the NHS one is based on a meal replacement diet, Oviva, that is available commercially to anyone. So does the study vindicate these diets once and for all? 

Perhaps not, says nutritionist and founder of The Food Doctor, Ian Marber. 

“This diet is for a purpose: to avoid becoming fully type 2 diabetic,” he explains. “It’ll work in theory, but for most of us, if we went on a liquid diet or a partial liquid diet, they’re just not sustainable. If we see food as just being calories, a battle to be won, it’s not as if you’re going to win the battle and then just go back to normal eating. Unless we understand what our habits are and develop different ones so we can exist in the real world, then you’ll always be a slave to your diet.” 

Unlike the Cabbage Soup Diet, which became popular in the 1980s, modern very low calorie diets (VLCDs) are not food-based at all but chemically manufactured to provide the vitamins and nutrients people need. 

“The majority of VLCDs are made with skimmed milk powder,” explains Lucy Perrow, dietician and spokeswoman for the British Dietetic Association. “Added to this is some kind of fat such as vegetable oil. Then there are lots of various added vitamin and mineral supplements and thickeners, flavouring and stabilisers.” Soylent, for example, a popular American meal replacement diet, is made with potassium gluconate, calcium carbonate, monosodium phosphate, maltodextrin, and olive oil mixed with water.

These are the same chemicals found in traditional food, albeit broken down in advance. Even so, Perrow says she would “never” recommend staying on a liquid diet for more than the recommended 12 weeks as she often sees side effects such as “constipation, headaches, and cramps.”

One reason for this is that liquid diets can leave people deficient in fibre. “We need around 30g of fibre per day; most of us get 18g” says Marber. “Fibre reduces bowel cancer risk, but it also provides a good environment for gut bacteria to thrive, which helps you digest food.”

The lack of fibre means the breaking down of food into glucose, which powers the cells of the body, happens faster. When that breakdown happens quickly, the feeling of fullness doesn’t last very long, leading to an increased desire to snack. 

“Imagine eating a carrot,” explains Marber. “There’s only so much you can eat at once because it’s thick, it’s fibrous. All the sugars and carbohydrates are bonded to the fibre, so in order to break down and release the potential energy, you have to chew. That takes time and the same goes for when it gets into your stomach. The more fibre in the product, the longer it takes to break down. If you then put a carrot through a juicer and break down the fibre, its sugars are no longer bound to fibres, so its journey from food to glucose is quite rapid and spends less time in your stomach, making you feel full.” 

You won’t notice that in the moment because “studies have suggested that consuming food in a liquid form may trick the stomach into thinking it is more full than it is,” explains Perrow. However, the feeling won’t last long. She adds: “If the water can be separated quickly because your entire meal is liquid, your stomach shrinks faster, and you feel hungry sooner.”

Once you’ve finished your breakfast shake, then, you’ll be looking forward to your lunchtime option more than if you ate a fibre-filled breakfast, such as porridge. 

Another factor to consider when it comes to liquid diets is the absence of the sensation and experience of eating. Various studies have found that the physical act of chewing food correlates with decreased levels of ghrelin, the hormone which sends hunger signals to the brain. Therefore, a liquid diet absent of physical eating might well see increased levels of the hunger hormone. 

Marber also notes that an 800 calorie per day diet, like the one the NHS has trialled, would be immensely difficult to maintain over long periods of time. “If you carried on with 800 calories per day from liquids or anywhere else, you’d continue losing weight, but with any diet, if you go back to your normal eating and your normal eating hasn’t been addressed for the long term, of course, you’ll regain all the weight,” he warns. A more sustainable option, like the popular 5:2 diet, which also asks people to restrict their calorie intake but only twice a week, might be more likely to yield long-term results. 

Perhaps the most important point is found buried elsewhere within the NHS’s advice website, which notes that diets like this are ‘not the first option to manage obesity and should not be routinely used’ and advises that ‘while very low-calorie diets can lead to short term weight loss, it is likely that the weight will come back on after the diet ends.’ 

“On a diet where the number of calories consumed is dramatically cut, a person’s metabolism starts to slow down,” explains Perrow. “The body gets used to surviving on fewer calories. When they start eating ‘normally’ again they are susceptible to gaining weight quickly. The problem I see most often in clinic is that people may lose a significant amount of weight, and then return to their ‘old’ eating habits that led to their weight gain in the beginning, due to eating or drinking the wrong foods or liquids, consuming larger portions than required, emotional eating due to stress or anxiety, lack of knowledge around food or cooking, or lack of activity to burn calories.”

The NHS study was noteworthy because in the three months after completing it the participants managed to keep the weight off. However, they have done so with the aid of clinicians and ‘a carefully managed plan [reintroducing] healthy, nutritious food… virtual one-to-ones, group sessions and digital support – to help them maintain a healthier weight’. 

The shakes and soups may have done their work in helping dieters lose weight initially, but the headline £10bn per year saving to the NHS, from combating obesity in the long term, will only come from the slower, more difficult re-evaluation of their relationship with food. Perhaps for that reason, the NHS has also just rolled out its Digital Weight Management Programme, which those with a BMI over 30 and high blood pressure can self-refer to here. For anyone else who is keen to lose weight, there’s a more sustainable 12-week programme here.