A UK study found that obese patients lost more than two stone (15kg) within one year on the diet, which costs 80% less than bariatric surgery.
Researchers said the intervention, which is provided by GP practices, was a 'highly acceptable and attractively costed alternative' to bariatric surgery.
One in 30 British adults is severely obese, and 10% have a BMI over 35kg/m2 with complications from their weight.
But losing 2.4 stone (15kg) or more can return blood glucose levels to normal, improve life expectancy and reverse type 2 diabetes.
Weight-loss surgery is a common treatment for obese patients but it is relatively expensive and patients can suffer complications.
Researchers led by Professor Michael Lean of the Glasgow Royal Infirmary ran a trial of a low-energy, nutrient-rich liquid diet containing around 820 calories per day as a potential alternative to bariatric surgery.
The liquid diet was given to 91 patients for 12 weeks as part of a service delivered by practice nurses. The diet was stopped if weight loss reached 20kg before this time.
After this time, normal food was reintroduced slowly over 6-8 weeks, and followed by routine appointments to monitor weight loss until 12 months after the diet began. The weight-loss drug orlistat was taken by around half of participants.
Before the trial, the average patient was aged 46, weighed 131kg and had a BMI of 46kg/m2.
At 12 months after the liquid diet and associated care, patients had lost an average of 12.4kg, with 33% losing 15kg or more.
Researchers estimated that the diet would cost the NHS £2,611 for each patient who lost 15kg or more - far less than the £12,500 price of bariatric surgery. They calculated that for £1m of NHS funding, 380 patients could lose 15kg or more on the liquid diet compared with 80 patients through surgery.
Authors concluded that although the study was not an RCT, results were 'strikingly better' than other weight-management interventions in primary care.
They said: 'The programme therefore offers a highly acceptable and attractively costed alternative to bariatric surgery for the increasingly common clinical problem of severe obesity.'