How does the 1 to 1 diet work? A guide to the Cambridge Weight Plan

November 22, 2025 •

4 min read

The 1 to 1 Diet, originally developed at Cambridge University, has reportedly helped millions worldwide achieve weight loss since 1984. So, how does the 1 to 1 diet work to produce rapid results, and what is involved in its structured, multi-step program?

Quick Summary

The 1 to 1 Diet employs a stepped plan using specially formulated, nutritionally complete meal replacements, along with dedicated one-on-one consultant support to facilitate weight loss through calorie restriction and ketosis.

Key Points

Core Mechanism: The diet operates on a Very Low-Calorie Diet (VLCD) principle, restricting calories to induce ketosis, where the body burns fat for energy.
Structured Steps: It follows a flexible 6-step program, starting with intense meal replacement and gradually reintroducing conventional food over time.
Meal Replacements: Dieters consume proprietary, nutritionally complete meal replacements (shakes, soups, bars) to simplify calorie control and ensure nutrient intake.
Personal Consultant: A dedicated, one-on-one consultant provides ongoing support, motivation, and guidance through the entire weight loss journey.
Rapid Results: The significant calorie deficit and ketosis aim to deliver quick and noticeable weight loss in the short term.
Sustainability Challenge: The restrictive nature can make long-term adherence difficult and does not teach general food selection skills, potentially leading to rebound weight gain.
Health Considerations: The diet is not suitable for everyone and requires medical consultation, especially during the very low-calorie phases, due to potential side effects.

Understanding the 1 to 1 Diet

Formerly known as the Cambridge Diet or Cambridge Weight Plan, the 1 to 1 Diet is a weight-management program that uses a Very Low-Calorie Diet (VLCD) approach, featuring meal replacement products and personal one-to-one support from a consultant. The core principle is significant calorie reduction, often below 1000 kcal per day, which forces the body into a metabolic state called ketosis. During ketosis, the body burns stored fat for energy instead of carbohydrates, leading to rapid weight loss.

The Role of Meal Replacements

A key component of the diet is the consumption of proprietary meal replacement products, which are sold exclusively through registered consultants. These products, which include shakes, soups, bars, and meals, are designed to be nutritionally complete, providing essential vitamins, minerals, protein, and trace elements, despite their low-calorie content. The pre-packaged nature of these products simplifies the process, removing the need for calorie counting or complex meal preparation, especially during the initial, most restrictive phase.

The 6-Step Program

The 1 to 1 Diet follows a flexible, six-step system that progressively increases daily calorie intake and reintroduces conventional food. The specific starting step is determined with a consultant based on individual weight, health, and weight loss goals.

Here is a general overview of the steps:

Step 1 (Sole Source): The most intense phase, consisting of 3-4 meal replacement products daily, providing 440 to 600 calories. This phase can last for a maximum of 12 weeks and should be medically supervised.
Step 2: A less restrictive phase, combining 2 meal replacements with a protein-rich meal, skimmed milk, and some vegetables, totaling 800 calories per day.
Step 3: Dieters consume 2 meal replacements, a healthy breakfast, and salads for lunch and dinner, bringing the daily intake to around 1,000 calories.
Step 4: This step involves 2 meal replacements alongside regular breakfast, lunch, and dinner.
Step 5: The transition continues with one meal replacement, plus breakfast, lunch, dinner, and a snack.
Step 6 (Maintenance): The final step focuses on a healthy, balanced diet, supplemented by occasional meal replacement products, to maintain the achieved weight loss.

The Importance of a Consultant

The name “1 to 1 Diet” emphasizes the crucial one-on-one support provided by a personal consultant. This consultant guides the dieter through each step of the plan, offering weekly weigh-ins, motivation, and practical advice. This personalised support system is considered a key factor in accountability and success, especially for navigating the challenges of a very low-calorie diet.

Comparison: 1 to 1 Diet vs. Conventional Calorie Counting


Feature	1 to 1 Diet (Meal Replacement)	Conventional Calorie Counting
Mechanism	Uses prepared meal replacements to ensure precise calorie control and induce ketosis for rapid fat burning.	Requires calculating and tracking the calories of everyday foods, aiming for a consistent deficit.
Rate of Loss	Designed for very rapid, short-term weight loss due to extreme calorie restriction.	Typically slower and more gradual, promoting sustainable, long-term habit formation.
Nutritional Control	Nutritionally complete products prevent deficiencies during the restrictive phases, but relies heavily on proprietary formulas.	Requires diligent planning to ensure all macro and micronutrient needs are met.
Support System	Offers dedicated, weekly one-on-one support from a trained (but not necessarily accredited nutritionist) consultant.	Relies on self-motivation, external apps, or private consultation with a registered dietitian or nutritionist.
Sustainability	Restrictive nature can be hard to sustain long-term, and may lead to rebound weight gain if old eating habits return.	Teaches skills for portion control and healthy eating with real food, making it more sustainable for lifelong changes.
Cost	Can be expensive, as products are purchased directly from consultants, but may be cheaper than a weekly food shop during intensive phases.	The cost is equivalent to the normal grocery budget, but with healthier choices.

Potential Downsides and Considerations

While the 1 to 1 Diet can provide fast weight loss, it comes with risks and challenges. The severe calorie restriction can lead to short-term side effects such as headaches, dizziness, fatigue, and constipation. Some health professionals raise concerns about the long-term sustainability and potential for developing an unhealthy relationship with food. Furthermore, critics argue that the reliance on proprietary products does not adequately prepare individuals for independent healthy eating habits, increasing the risk of regaining weight after stopping the plan. Medical supervision is strongly recommended, especially for the very low-calorie steps, and the diet is not suitable for certain individuals, such as those with a low BMI or who are pregnant.

How to Get Started with the 1 to 1 Diet

For those interested, starting the 1 to 1 Diet is a straightforward process. First, find a local consultant via the official website or a referral. Your consultant will then conduct an initial consultation to discuss your health, goals, and lifestyle before recommending a suitable starting step from the plan's six variations. The consultant will order and provide the meal replacement products you need, with regular check-ins to monitor your progress and provide support.

Conclusion

The 1 to 1 Diet works by leveraging a very low-calorie, meal-replacement-based system to achieve rapid weight loss, supported by personalized, one-on-one guidance from a consultant. The stepped program gradually reintroduces regular food to aid in long-term maintenance. While it offers a structured and effective short-term solution, potential dieters must be aware of the restrictive nature, potential side effects, and the importance of professional medical advice. For more detailed information on the diet's mechanics and product range, you can visit the official website: The 1:1 Diet by Cambridge Weight Plan.

Important Disclaimer

Before embarking on any very low-calorie diet, it is essential to consult with a healthcare professional to ensure it is safe and appropriate for your individual health circumstances.

Frequently Asked Questions

The main difference is the rebranding to emphasize the one-to-one relationship between the dieter and their dedicated consultant, which provides personalized support. The core principles of the stepped, meal replacement program remain consistent.

Yes, the 1 to 1 Diet products are formulated by nutrition experts and are designed to provide a complete balance of vitamins, minerals, and trace elements, even during the most restrictive, low-calorie phases.

Most dieters experience some hunger and side effects like tiredness or irritability, especially during the first few days as their body adjusts to the calorie restriction. These feelings often subside as the body enters ketosis.

A consultant provides personalized guidance, helping you choose the right step for your goals, weighing you in regularly, and offering motivational support through the diet's various phases.

During the very low-calorie phases (Steps 1-3), only gentle activity, such as walking, is recommended. More strenuous exercise can be incorporated during later steps with higher calorie intake, typically above 1000 kcal.

Yes, some studies have shown that a VLCD approach like the 1 to 1 Diet can lead to significant weight loss and even remission of Type 2 diabetes in some adults, but it requires medical supervision.

Common side effects include headaches, fatigue, constipation, dizziness, and bad breath due to the drastic calorie and carbohydrate restriction. It is important to stay well-hydrated to help manage some of these symptoms.