How long should you stay on the Cambridge diet?

December 25, 2025 •

3 min read

According to nutrition experts, a very low-calorie diet of under 1000 kcal per day should not be followed for more than 12 weeks continuously. Therefore, the most restrictive phases of the Cambridge diet, now known as the 1:1 Diet, have a defined maximum duration for safety and effectiveness.

Quick Summary

The Cambridge diet's initial low-calorie phases have a strict time limit, with the 'Sole Source' phase lasting up to 12 weeks. Safe duration is determined by your chosen step and a personal consultant, with risks associated with prolonged extreme restriction.

Key Points

Initial Phase Limit: The very low-calorie 'Sole Source' phase of the Cambridge diet should not exceed 12 weeks for safety.
Staged Progression: The diet moves through several stages, with calories and regular food gradually reintroduced to prevent health risks.
Professional Supervision: Due to its highly restrictive nature, the diet requires guidance from a qualified consultant to ensure safety and effectiveness.
Long-Term Maintenance: For sustainable results, dieters must transition to a maintenance phase with healthy eating habits, using meal replacements sparingly.
Risk of Side Effects: Prolonged restriction can lead to nutrient deficiencies, a slower metabolism, and other side effects like fatigue and gallstones.
Focus on Re-feeding: A gradual re-feeding process is critical to avoid rebound weight gain and manage health as regular food is added back.

Understanding the Cambridge Diet Program

The Cambridge diet, now rebranded as the 1:1 Diet by Cambridge Weight Plan, is a structured weight loss program that uses meal replacements to control calorie intake. It is divided into several steps, with total calories gradually increasing as you progress from the initial, most restrictive phase towards a long-term maintenance plan. A personalized consultant guides you through each stage, helping determine the appropriate duration for your goals.

The Initial Sole Source Stage: A Short-Term Approach

The most restrictive part of the diet is the 'Sole Source' or 'Sole Source Plus' phase, where daily calorie intake can be as low as 400-600 kcal. Due to this extreme calorie deficit, this phase is strictly limited. Experts advise against following any diet of under 1000 kcal per day for more than 12 continuous weeks, and those on fewer than 600 calories should be under constant medical evaluation. This initial phase is designed for rapid weight loss and is meant to be a short, intense period. Prolonging this stage increases the risk of side effects and metabolic slowdown.

Transition and Re-feeding: Gradually Increasing Calories

After the initial rapid weight loss phase, the diet gradually reintroduces regular food. The subsequent steps are longer in duration and less restrictive, preparing your body for a balanced diet. This re-feeding process is crucial for preventing rebound weight gain and refeeding syndrome.

Example of a step-by-step calorie increase:

Step 1 (Sole Source): 400-600 calories for up to 12 weeks.
Step 2: Around 800 calories, adding protein-rich foods.
Step 3: Around 1000 calories, including a light breakfast, lunch, and dinner.
Step 4: Around 1200 calories, adding a snack.
Step 5: Around 1500 calories, incorporating more regular meals.
Step 6 (Maintenance): Long-term healthy eating, potentially with occasional meal replacements.

The Long-Term Maintenance Phase

The maintenance phase is the final and indefinite stage of the Cambridge diet. The goal is to transition to a regular, healthy diet, using Cambridge products only as needed to maintain weight. Without this crucial long-term strategy, there is a high risk of regaining the lost weight.

Comparison of Short-Term and Long-Term Program Phases


Feature	Initial Phases (Sole Source)	Later Phases & Maintenance
Primary Goal	Rapid weight loss	Sustainable weight management
Duration	Short-term, maximum 12 weeks	Indefinite
Calorie Intake	Very low (e.g., 400-600 kcal)	Higher, gradually increasing (e.g., 1200+ kcal)
Meal Composition	Exclusively Cambridge meal replacements	Combination of regular food and meal replacements
Consultant Role	Intensive guidance and monitoring	Continued support for long-term habits
Potential Risks	Nutrient deficiencies, dizziness, muscle loss	Risk of rebound weight gain without proper habits

Potential Risks of Prolonged Restriction

Extending the very low-calorie phases of the Cambridge diet beyond the recommended period can have serious health consequences. A sluggish metabolism, nutrient deficiencies, and increased risk of gallstones are all potential risks. The restrictive nature of the diet can also negatively impact your mental well-being and relationship with food. It is crucial to follow the program's structure and consultant's guidance to mitigate these risks.

The Importance of Professional Guidance

Because the Cambridge diet is a very low-calorie plan, professional supervision is paramount. Your consultant helps personalize your plan, ensuring you transition safely between steps and adopt healthy, long-term eating habits. It is not a DIY diet and should not be attempted without structured support. In some cases, a medical professional's oversight may be necessary, especially for individuals with significant weight to lose or pre-existing health conditions.

Conclusion

The question of how long you should stay on the Cambridge diet has a clear answer: the duration depends on the phase you are in. The initial, very low-calorie stages should not be followed for more than 12 weeks continuously without medical supervision. Transitioning through the program's later, higher-calorie steps and focusing on the maintenance phase is crucial for achieving sustainable, long-term results. Adherence to the structured program with guidance from a consultant is the only safe way to complete this diet. For those concerned about long-term sustainability or potential health risks, alternative approaches focusing on balanced, whole foods may be more beneficial and safe. For comprehensive program details, visit the official 1:1 Diet website.

Frequently Asked Questions

The most restrictive phase of the Cambridge diet, known as 'Sole Source,' should not be followed for more than 12 consecutive weeks without medical supervision, according to modern guidelines for very low-calorie diets.

No, it is highly recommended not to extend the very low-calorie phase beyond 12 weeks, even if you feel fine. Risks such as nutrient deficiencies, muscle mass loss, metabolic slowdown, and gallstones increase with prolonged, extreme calorie restriction.

The maintenance phase is the final stage, where the goal is to sustain your weight loss by transitioning to a healthy, balanced diet. It teaches long-term eating habits, with occasional use of Cambridge products as needed for weight management.

The very low-calorie initial phase is not sustainable long-term. The overall program's success depends on effectively transitioning to the maintenance phase, which focuses on healthier eating habits rather than relying on meal replacements.

Staying on the highly restrictive phases too long increases the risk of side effects like dizziness, fatigue, nutrient deficiencies, digestive issues, and potential health problems such as gallstones. It also makes you more susceptible to rebound weight gain.

The diet incorporates a step-by-step re-feeding process. After the initial phase, calories are gradually increased, and regular, healthy meals are slowly introduced alongside meal replacements to normalize eating patterns.

It is highly advisable to seek medical supervision, especially during the very low-calorie phases of the diet. For individuals consuming fewer than 600 calories, constant evaluation by a healthcare professional is recommended.