Fasting-Mimicking Diets Explained: How They Differ from Intermittent Fasting

Can you eat food and still produce some of the metabolic changes associated with fasting?

That is the question behind fasting-mimicking diets. These structured programmes have attracted attention through research into metabolic health and ageing, often alongside headlines about a five-day “reset”.

Understanding the approach starts with knowing what it involves. A fasting-mimicking diet changes both the amount and composition of food for several consecutive days. Intermittent fasting covers a broader range of patterns that alternate eating with periods of fasting or restricted intake.

Neither is essential for eating well. Before choosing a programme, it helps to understand what researchers have tested, what the findings mean and how the approach might fit your circumstances.

What is a fasting-mimicking diet?

A fasting-mimicking diet, often shortened to FMD, is a structured eating programme designed to produce selected fasting-related responses while still providing food.

Much of the human research has used five-day cycles of a plant-based diet restricted in energy and protein, followed by a return to usual eating. The food composition is part of the intervention.

ProLon is a commercial programme used in this research. Results from a specific formulation and schedule cannot automatically be applied to homemade versions or other products. Read the study’s description of the programme.

How does it differ from intermittent fasting?

Intermittent fasting is an umbrella term. It includes time-restricted eating, alternate-day approaches and patterns such as 5:2.

Time-restricted eating is the version many people recognise: eating within a daily window and fasting for the remaining hours. A 5:2 pattern usually involves two days of restricted energy intake each week. Dietitians Australia explains the common approaches.

The useful question is always: what exactly did participants eat, for how long and under what supervision?

Two programmes can both use the word “fasting” while involving very different food intakes and practical demands.

What have human studies found?

Human research has reported changes in several health markers. Interpreting those findings means separating measurements taken during a study from long-term outcomes such as disease prevention or lifespan.

The 2017 trial: changes in risk markers

A randomised trial enrolled 100 generally healthy adults and compared usual eating with five-day FMD cycles over three months.

Researchers reported reductions in body weight, body fat, blood pressure and IGF-1, a hormone involved in growth signalling. After the crossover phase, 71 participants had completed three cycles.

The study measured risk markers over a relatively short period. It did not establish that participants subsequently experienced fewer heart attacks, developed fewer diseases or lived longer. Incomplete participation also matters when assessing the results. Read the 2017 trial.

The 2024 paper: estimated biological age

A 2024 paper reported secondary and exploratory analyses involving insulin-resistance measures, liver fat and estimated biological age.

The widely shared “2.5 years younger” finding referred to a median change in a biomarker-based estimate in 52 participants after three FMD cycles. It did not measure years of life gained.

The paper also disclosed commercial interests involving some authors and L-Nutra. Independent replication and longer follow-up remain important when interpreting the findings. Read the 2024 paper.

How should you assess programme claims?

When an advertisement describes a programme as “clinically studied”, look for the details behind that phrase:

• The intervention: Was the actual product and schedule tested?
• The participants: Were they similar to you in age and health?
• The comparison: What did the other group do?
• The outcomes: Did researchers measure laboratory markers, symptoms or disease events?
• The duration: How long were participants followed?
• The disclosures: Who funded the work, and what commercial interests were reported?

A webinar or product page can introduce an idea. The underlying research is where you can check what was actually measured.

What about autophagy?

Autophagy is a natural process through which cells break down and recycle components. It is frequently mentioned in fasting discussions.

However, researchers do not have a universal fasting duration that reliably tells an individual when a meaningful increase will occur. Much of the research behind popular claims comes from animals. Dietitians Australia discusses these limitations.

Hunger or tiredness should not be treated as a measure of cellular recycling. When assessing a programme, look for demonstrated outcomes rather than using how difficult it feels as evidence that it is working.

Is a fasting-mimicking diet better than intermittent fasting?

The studies described above do not answer that question.

Showing a change compared with usual eating does not establish that an approach works better than time-restricted eating, a Mediterranean-style diet or another supported eating pattern.

Practical fit matters too. Consider how a programme would work around family meals, shift work, physical activity and medication schedules.

Start with the goal you want to address. Improving meal quality, managing a medical condition and finding a sustainable eating routine can involve different decisions. A GP or Accredited Practising Dietitian can help you assess the options.

Who needs professional advice before considering fasting?

Multi-day food restriction requires careful consideration.

Fasting is unsuitable for some people, including those who are pregnant or breastfeeding, adolescents and people with a history of eating disorders. Being underweight or having an underlying medical condition also warrants individual advice. See Dietitians Australia’s suitability guidance.

If you have diabetes, speak with your treating team before changing your food intake. Fasting can require medication adjustments, particularly with insulin and sulfonylureas, which can cause low blood glucose. Do not stop or adjust medication yourself. NIDDK explains why clinician involvement matters.

For a multi-day programme, take the full programme details and your medication list to your clinician so they can assess your circumstances.

Practical questions to ask before buying a programme

What will it cost?

Consider repeat cycles, appointments and recommended monitoring alongside the initial purchase price.

How will it fit your week?

Think about physical work, concentration, caring responsibilities, training and social meals.

What support is available?

Check whom you can contact with concerns, what symptoms require medical advice and when the programme should be stopped.

What happens afterwards?

Look for clear transition instructions and an achievable plan for ordinary eating days.

A programme should make its requirements understandable before you commit to it.

Can you add bone broth or collagen during an FMD?

Do not assume that bone broth or collagen peptides are compatible additions to a researched fasting-mimicking programme.

Both can contribute protein and energy. Adding them changes the intake of a programme designed around a particular nutrient composition. Replacing programme foods with homemade broth, vegetable soup or collagen drinks also does not reproduce the studied intervention.

Broth & Co bone broth can bring flavour to everyday cooking, and collagen peptides can fit some everyday routines. Their role in ordinary meals does not establish their suitability within an FMD.

If you are following a clinician-supported programme, discuss any changes with that clinician.

Returning to everyday eating

Follow the programme’s transition instructions and any individual advice from your healthcare professional.

After prolonged fasting or significant undernutrition, returning to food can involve risks, including refeeding syndrome. People at risk may need a carefully managed clinical plan. A general recipe collection cannot provide that assessment. NICE outlines the circumstances requiring nutrition support and monitoring.

Once you have returned to ordinary eating, practical meal ideas can make everyday cooking easier.

Meal ideas for ordinary eating days

For the days when you are back to your usual meals, try:

• Chicken, vegetable and barley soup: Use prepared ORIGINAL bone broth as a savoury cooking base, with chicken, barley and vegetables.
• Lentil and roasted vegetable bowls: Combine lentils, roasted vegetables and quinoa, using broth to cook the quinoa if desired.
• Berry, oat and yoghurt smoothie: Blend fruit, oats, yoghurt and milk, with an optional labelled serving of citrus-flavoured Healthy Glow.

Find the recipes in Eating Well Between Fasts: Nourishing Soups, Bowls and Everyday Meals. These are everyday meals, separate from an FMD or a clinical refeeding plan.

For savoury cooking options, explore Which Broth + Co Bone Broth Is Right for You? A Complete Comparison Guide.

To understand how different protein sources fit into everyday eating, read Functional Proteins Explained: Why Whey, Collagen & Bone Broth All Have Different Roles.

Frequently asked questions

Is fasting mimicking the same as eating soup for five days?

No. A researched FMD uses a specified food composition and schedule. Eating soup alone does not reproduce that programme.

Is ProLon the same as intermittent fasting?

ProLon is a commercial fasting-mimicking programme. Intermittent fasting covers a broader range of eating patterns, many of which do not involve packaged foods.

Does fasting mimicking reverse ageing?

The research discussed here does not establish that. A change in estimated biological age is not evidence that someone has gained years of life.

Can I add collagen to maintain my protein intake during an FMD?

Discuss this with your clinician before changing the programme. If its protein restriction is unsuitable for your needs, the programme itself may need to be reconsidered.

How often should you do a fasting-mimicking diet?

A schedule used in a study is not automatically a personal recommendation. Suitability and frequency require individual assessment.

Do you need to fast to eat well?

No. You can build a varied, enjoyable eating routine without following a fasting programme.

Choosing an approach that fits your needs

If you are considering a fasting-mimicking diet, ask your GP or dietitian what benefit is realistic for you, how it would be assessed and what other approaches could address the same goal.

Take the programme details with you. Understanding its food restrictions, practical demands and evidence will help you make an informed decision.