As fad diets go, the Cambridge Diet is pretty old school. Devised as a hospital weight loss programme in the sixties – before the inventors of most modern day boom-and-bust diets were born – the Cambridge Diet went public in 1984, and since then, millions have gulped down its meal replacement products in the hope of shedding their spare tyre.
Now known as the 1:1 Diet, the Cambridge Diet kicks off with a 500-calorie phase made up of shakes, soups and bars that are (supposedly) designed to fulfil your daily nutritional requirements. For those who can stomach such an extreme calorie deficit, fat loss is said to come thick and fast. Whether it lasts is another question altogether.
Forgoing food has long been the reigning dietary trend, either by doing away with it entirely – i.e. fasting – or prioritising one ingredient (the Coffee Diet, the Cabbage Soup Diet, the Egg Diet...) at the expense of entire food groups. Spoiler alert: this diet is no different.
Can a programme invented more than half a century ago – at a time, let’s not forget, when cigarettes were still considered healthy – still hold weight in 2021? With expertise from nutritionists and dieticians, we explain what the Cambridge Diet is, how it works, and whether it’s a safe way to drop pounds. Scroll on to ingest our bite-sized guide.
What Is the Cambridge Diet?
The Cambridge Diet was developed for people with clinically severe or ‘morbid’ obesity, which involves a body mass index (BMI) of 40 or higher. It was invented by nutritionist Dr Alan Howard at the University of Cambridge, the co-founder of the International Journal of Obesity, a peer-reviewed medical journal, and a pioneer of obesity research.
His diet follows six staggered ‘steps’, starting with Step 1 or ‘Sole Source’ – where you exclusively eat pre-packaged branded products – to Step 6 or ‘Maintenance’, which involves following a healthy diet with occasional meal replacements. What you eat depends on the ‘step’ you’re in. The first five include at least one meal replacement product – bars, soups and shakes – that contain 200 calories or less:
Step 1: You exclusively consume meal replacement products.
Step 2: Three meal replacements a day, one healthy meal of your choice.
Step 3: Two meal replacements a day, healthy breakfast, lunch and dinner.
Step 4: Two meal replacements a day, lunch, dinner, and a snack.
Step 5: One meal replacement, breakfast, lunch, dinner and a snack.
Step 6: The maintenance phase, which has no set structured plan.
“The Cambridge diet encourages participants to engage in a very low-calorie meal replacement diet in the initial phase,” explains nutritionist Jenna Hope. “During phase one, calorie recommendations are around 500kcal per day. This step can last for up to 12 weeks.” Foods are re-introduced over time, and “calories gradually increase to around 1,000 to 1,200 kcals per day,” she says, before the maintenance phase is reached.
How Does the Cambridge Diet Work?
The Cambridge Diet works by slashing calories, plain and simple. “This relatively extreme weight loss plan works by putting its participants into a very large calorie deficit,” says nutritionist Mike Molloy, founder of coaching company M2 Performance Nutrition. “Anytime you are in a large deficit, you will lose substantial amounts of weight.”
While the plan doesn’t necessarily aim to put you into ketosis – altering your energy source from carbohydrates to stored fat – it does so by default. Ketosis occurs when you deplete your glycogen stores. This happens when you reduce your carb intake below 50 grams per day, which isn’t hard when you’re eating the same number of calories as a newborn baby.
Slashing carbs is no walk in the park, and results in side effects including fatigue, muscle cramps, irritability and low mood (carbohydrates are needed for the production of feel-good chemical serotonin). What’s more, carbs help you maintain muscle strength and fuel your workout – cutting down makes HIIT hard – so a low-carb existence can be draining.
What Are the Pros of the Cambridge Diet?
Once you’ve wrapped your head around the various stages of the Cambridge Diet, executing the plan is pretty straightforward. There’s no meal-prep, no macro-counting, no portion control, no shopping lists, no grazing – so it’s easy to work into your daily routine – and if you’re trapped in an endless loop of Costa for breakfast, Uber Eats for lunch and Deliveroo for dinner, it’ll help you break the habit and give your bank account some respite (you can expect to pay around £2.63 per meal, on average). The Cambridge Diet also claims to be nutritionally complete, so you’ll reach the minimum benchmark for vitamins and minerals that your body needs to function.
And, given the extreme calorie deficit involved, you’re likely to lose a lot of weight quickly. “However, it’s not a sustainable, long-term or particularly healthy approach,” says Hope, who advocates for whole foods over meal replacements. “It’s important to consume adequate micronutrients to support the physiological demands of the body and around 30g of fibre per day to support optimal gut function,” she adds. “There are more enjoyable, safer and healthier methods of weight loss than the Cambridge Diet.”
Is the Cambridge Diet Safe?
If the Cambridge Diet’s exceptionally-low calorie recommendations aren’t already ringing cathedral-sized alarm bells, they should be. “There are stages of the diet that operate well below 1,000 calories per day,” says Molloy. “This is considered by many medical professionals to be a starvation-based state, and should only be attempted under close supervision of a medical professional. As such, I have concerns about the diet’s safety.”
Restricting calories too severely can cause lasting health issues. Extreme low-calorie diets catabolise muscle, which stunts your metabolism – decreasing the number of calories your body burns by up to 23%, studies show. The effects can persist long after you ditch the diet, and in fact, researchers reckon this newly-sluggish metabolism is the main reason eight out of 10 people regain their lost weight in the future.
Does the Cambridge Diet Work?
While there is some science behind the plan, the trial – published in The BMJ – was undertaken exclusively on people with a BMI of 30 and above. Oxford University researchers put 138 people on a diet of Cambridge Diet products for eight weeks – consuming 810 calories per day – followed by a four-week food re-introduction phase and a 24-week maintenance period. After a year, they’d lost 10.7kg on average.
So it works, then? Not quite. “It depends on how you define the term ‘work’,” says Molloy. “If you follow it, will you lose weight? Yes, almost certainly. If you determine whether or not a diet ‘works’ based on whether or not it will produce sustainable weight loss that you can keep doing for the rest of your life, I’d say the Cambridge Diet is destined for failure, just like any other extremist approach to nutrition.”
You might shed your paunch, but it’ll come at a cost. Alongside the study’s reported side effects like constipation, headache and dizziness, “You’ll likely have very large energy lulls from the low daily calorie intake,” Molloy continues. “You’ll likely have very large hunger spikes. Extreme calorie deficits typically cause an increased loss of lean muscle mass, which will decrease your resting metabolic rate.”
Chasing gains becomes impossible in such a vast calorie deficit. Over time, it burns away your hard-earned muscle, crashes your hormones – including testosterone – and causes your 1RM to plummet. Your immune system takes a battering too, which increases your risk of infections and illness. And then there’s the monotony of it all. “It’s extremely restrictive in the foods you’re able to eat, a trait that often leads to boredom,” Molloy says.
The Cambridge Diet: A Nutritionist’s Verdict
Tried-and-tested weight loss tool or diet to discard? Tempting as the lure of rapid weight loss may be, we’d warn against attempting the Cambridge Diet. The highly restrictive nature of the programme can negatively affect your relationship with food, your social life and mental wellbeing, says Hope – not to mention your health.
“Consuming 500 calories per day for 12 weeks can pose a risk of nutrient deficiencies and impaired intakes of proteins, complex carbohydrates and healthy fats,” Hope adds. “Additionally, due to the very low calorie intake, there is a high risk that individuals will re-gain the weight and some once they return to a normal eating pattern.”
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