The advice has been drummed into us for decades. Crash diets don’t work. Lose it slowly, keep it off forever. Go too fast and your body will snap back, undoing weeks of effort and leaving you worse off than when you started. It is the kind of received wisdom that feels too settled to question, the sort of thing GP surgeries have printed on leaflets since roughly forever. But a new randomised controlled trial out of Norway suggests the conventional wisdom is, at best, incomplete, and possibly just wrong.
The trial, led by Dr Line Kristin Johnson at Vestfold Hospital Trust in Tønsberg, followed 284 adults with obesity through a year-long programme comparing two approaches to weight loss: rapid and gradual. Same food quality, same maintenance support afterwards. Just different speeds of getting there.
The rapid weight loss group started on fewer than 1,000 calories a day for the first eight weeks, stepping up slowly to 1,500 by week sixteen. The gradual group cut roughly 800 to 1,000 calories below their estimated daily expenditure, averaging around 1,400 calories throughout, which is not exactly generous either but is a long way from what most people would call a crash diet. Both groups then entered an identical 36-week maintenance programme, with regular group sessions, video check-ins, and personalised calorie adjustments as their weights stabilised or continued to drift downward.
The Numbers That Challenge Everything
After sixteen weeks, the gap was stark. The rapid group had lost an average of 12.9% of their body weight; the gradual group, 8.1%. A four point eight percentage point difference, which sounds modest until you consider that at this scale, for people at real risk of type 2 diabetes and cardiovascular disease, it isn’t.
Then comes the part that really matters, because this is where the old assumption about rebound was supposed to kick in. At one year, did the rapid loss group regain more? No. At twelve months, the rapid group had lost 14.4% of body weight, the gradual group 10.5%. The gap narrowed somewhat (from 4.8 to 3.9 percentage points) but held. Both groups continued losing during maintenance, actually, which the researchers hadn’t necessarily expected given that most participants were free to choose additional weight loss rather than stabilisation. Most opted for more.
Crucially, the study wasn’t just tracking the numbers on a scale. Johnson and colleagues used two clinically meaningful benchmarks drawn from a large 2025 population cohort: a BMI at or below 27, and a waist-to-height ratio at or below 0.53. Hit those targets, the earlier cohort data suggests, and your ten-year risk of developing type 2 diabetes, hypertension, cardiovascular disease, and hip or knee osteoarthritis drops substantially. At one year, 28.3% of the rapid loss group had reached the BMI target. In the gradual group: 9.7%. The waist-to-height target: 33% versus 18.4%.
A Long-Standing Belief, Largely Built on Shaky Evidence
Why did the slow-and-steady consensus take hold? The authors point out, perhaps a bit pointedly, that the concerns about rapid weight loss are “largely based on observational data, historical assumptions, or small, methodologically limited studies.” In other words, the kind of evidence base that rarely survives contact with a properly designed randomised trial. Johnson herself put it plainly: “Our results clearly challenge the prevailing belief that slow and steady gradual weight loss is necessary to prevent weight regain and reduce obesity-related complications.”
There are caveats worth holding onto. The trial participants were predominantly women (90%), all with a BMI of 30 or above, and all going through a structured commercial programme with regular professional contact. The results may not translate to people dieting alone, or to different demographics. And 284 participants, while reasonable for a year-long RCT, isn’t enormous. Replication in larger and more diverse populations would sharpen the picture considerably.
Still, the direction of travel is clear enough to be uncomfortable for a lot of existing guidance. That the rapid group achieved more than three times the rate of reaching a clinically meaningful BMI target is, by any measure, a meaningful finding. So is the fact that the dreaded rebound, the thing everyone warns you about, simply did not materialise at twelve months in any distinguishable way.
What This Means for People Who Can’t Access Ozempic
The timing matters here. We are living through what might fairly be called a pharmaceutical revolution in weight management. GLP-1 receptor agonists like semaglutide and tirzepatide produce weight loss that would once have been considered achievable only through bariatric surgery. But these drugs are expensive, supply-constrained, and not accessible to most people with obesity globally. Johnson notes that the trial’s findings are “particularly relevant given the urgent need for effective weight-loss and weight-maintenance strategies,” adding that commercially available programmes could help reduce pressure on overstretched healthcare systems for people who cannot access or afford medical options.
That framing shouldn’t be dismissed as promotional gloss (the trial did involve Roede AS, a Norwegian commercial weight-loss company, and two of its employees were among the authors, which is a conflict the paper discloses). The underlying scientific question is genuine: what works, for most people, in the real world, without a prescription? If supervised rapid weight loss produces better outcomes and does not cause more regain, the field needs to grapple with that rather than continue repeating advice that may simply be incorrect.
What the study cannot yet tell us is why the rapid group did better. Was it the stronger early momentum, the psychological boost of visible results? Was there some metabolic shift in the first eight weeks that set a better trajectory? Or does the higher initial loss simply give people a bigger buffer against the inevitable drift upward that comes with any programme over time? Those questions are open. But the old answer, the one that said slow and steady wins the race, is looking shakier than it has in years.
Frequently Asked Questions
Is rapid weight loss actually bad for you, or is that a myth?
The evidence that rapid weight loss leads to worse long-term outcomes is weaker than most people assume. According to a new Norwegian randomised trial, people who lost weight quickly not only kept the weight off just as well at one year as those who lost it gradually, but actually lost more overall and hit clinically meaningful health targets at roughly three times the rate. The concern appears to be based largely on older, smaller, or observational studies rather than high-quality trial data.
Why do people regain weight after dieting, and does it matter how fast you lost it?
Regain after weight loss is common and driven by a mix of hormonal, metabolic, and behavioural factors that persist long after the diet ends. The speed of initial loss appears to be less predictive of regain than previously thought: in this trial, the rapid and gradual groups showed similar regain patterns during the maintenance phase, suggesting the “lose it fast, put it back faster” idea may not hold up under controlled conditions. Ongoing structured support during the maintenance phase likely plays a bigger role than the speed of initial loss.
Could very low calorie diets work for someone without medical supervision?
The trial involved fewer than 1,000 calories a day in the first eight weeks, which is quite severe and not without risk, particularly for people with underlying conditions. Critically, participants had weekly in-person professional contact throughout, with careful calorie adjustment during the maintenance phase. Whether similar outcomes would emerge without that level of support is genuinely unknown, and most clinicians would caution against very low calorie approaches without professional oversight.
How does this compare to weight-loss drugs like Ozempic?
GLP-1 receptor agonists like semaglutide typically produce larger total weight loss than either approach in this trial, often in the range of 15 to 20% of body weight. But they are expensive, not universally available, and carry their own side-effect profiles. The significance of this trial is partly that it points to an accessible, drug-free option that may be substantially more effective than the gradual approaches many programmes currently recommend, which matters enormously for people who cannot access pharmaceutical interventions.
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