Five-Day Fasting-Mimicking Diet Gains Attention

The five-day fasting-mimicking diet is attracting renewed attention as researchers continue investigating how nutrition and periods of calorie restriction may influence healthy aging.

The approach was developed by Dr. Valter Longo, a professor of gerontology and biological sciences at the University of Southern California. Unlike a traditional water-only fast, the plan allows people to consume a limited amount of food while attempting to reproduce some of the biological effects associated with fasting.

Longo says he has spent decades studying aging and believes that periodic fasting-mimicking cycles could become one component of a broader strategy for extending the years people spend in good health.

However, there is an important distinction between improving markers associated with healthy aging and proving that a particular diet will make people live longer.

The current research is promising, but it does not establish that the diet is a guaranteed longevity treatment.

What Is the Five-Day Fasting-Mimicking Diet?

The five-day fasting-mimicking diet is designed to make the body respond as though it were experiencing a prolonged fast while still providing some calories and nutrients.

According to the plan described by Longo, calorie intake starts at approximately 1,100 calories on the first day and falls to about 700 calories on days two through five. The diet is deliberately low in protein and carbohydrates and relatively high in fat.

The concept is based partly on the idea that reducing calorie and protein intake can influence metabolic pathways associated with growth, repair and aging.

Instead of simply stopping food completely, participants consume specially selected foods.

The program emphasizes foods such as:

• Vegetable-based soups
• Nuts
• Olives
• Healthy fats
• Vegetable ingredients
• Herbal teas
• Low-protein foods

The commercial version created by Longo’s team is sold as a five-day kit. The Telegraph reported that the kit costs £219 and contains drinks, soups, snack products and supplements designed around the fasting-mimicking approach.

The important point is that this is not simply a five-day low-calorie diet.

Its nutritional composition is specifically designed to encourage the body’s fasting-related responses.

How the Five-Day Fasting-Mimicking Diet Works

The scientific theory behind the diet is more complicated than simply eating less.

During a prolonged period of calorie restriction, the body changes how it uses energy. Insulin and other nutrient-sensitive pathways can change, while the body increasingly relies on stored energy.

Researchers studying fasting are particularly interested in pathways involved in cellular maintenance, metabolism and inflammation.

Longo’s research has also focused on what happens when food is reintroduced after fasting.

His hypothesis is that temporary nutritional stress may activate processes that help tissues recover when normal nutrition resumes. Research from his group has investigated changes involving stem cells and other biological pathways.

However, many of the more dramatic explanations remain hypotheses rather than established clinical facts.

For example, Longo has suggested that temporary changes in organ size during fasting could be connected with stem-cell activity and tissue regeneration. His team is continuing to investigate whether fasting and subsequent refeeding could produce meaningful regenerative effects.

That distinction matters.

The scientific community can observe changes in biological markers without being able to conclude that those changes translate directly into longer human lifespans.

Five-Day Fasting-Mimicking Diet and Biological Age

One of the most interesting findings associated with the diet concerns biological age.

Biological age is different from the number of years a person has been alive. It attempts to measure how quickly or slowly the body’s cells and tissues appear to be aging.

A 2024 study reported improvements in several health-related measures among people following repeated cycles of the fasting-mimicking diet.

According to the Telegraph’s report, participants who followed the diet once a month for three months experienced reductions in abdominal and liver fat, improvements in markers related to metabolism and immune function, and a reported reduction of about 2.5 years in a measure of biological age.

That finding generated considerable interest.

Still, biological-age measurements should not be interpreted as proof that someone has literally become 2.5 years younger.

Different biological-age tests can produce different results, and researchers are still studying how strongly these measurements predict future disease and lifespan.

Therefore, the finding is better understood as evidence of a potentially meaningful change in certain markers of aging rather than a proven reversal of chronological aging.

The Diet May Improve Metabolic Health

Another reason researchers are interested in the five-day fasting-mimicking diet is its potential effect on metabolic health.

In earlier research, repeated fasting-mimicking cycles were associated with improvements in measures including blood glucose, insulin resistance and body composition.

Animal experiments have also produced striking results.

In one study described by Longo, mice fed a Western-style diet developed metabolic problems and gained weight. Researchers then introduced periodic fasting-mimicking cycles and observed improvements in several metabolic measures.

Human evidence is more important when evaluating whether the approach could eventually have a role in preventive health.

Patient.info, in a clinically reviewed overview of Longo’s longevity diet, notes that studies have reported reductions in some risk factors associated with aging and chronic disease. However, it also emphasizes that more research is needed to determine whether the specific diet actually extends human lifespan.

That is an important limitation.

A reduction in a risk marker does not necessarily mean a person will live longer.

Weight Loss Is Another Potential Benefit

The five-day fasting-mimicking diet can also lead to weight loss because calorie intake is substantially restricted during the five-day cycle.

Longo told the Telegraph that six cycles over six months had helped overweight people lose approximately 20 pounds, or about 9 kilograms. He also argued that much of the weight lost was fat rather than lean tissue.

The potential advantage is therefore not simply the number on the scale.

Researchers are interested in whether a structured short-term intervention could reduce abdominal fat while minimizing unwanted loss of muscle.

However, weight loss results vary substantially between individuals.

A person who loses weight during five days of severe calorie restriction may also regain some weight after returning to normal eating. Long-term results depend heavily on the overall dietary pattern maintained between fasting cycles.

For that reason, the fasting-mimicking diet should not be viewed as a substitute for a consistently nutritious diet.

The Longevity Diet Goes Beyond Five Days

Longo’s broader philosophy is not based exclusively on fasting.

His longevity diet emphasizes a predominantly plant-based eating pattern inspired partly by traditional diets associated with areas known for exceptional longevity.

Recommended foods include vegetables, fruits, whole grains, nuts, seeds and legumes, with relatively modest amounts of animal protein.

This resembles several established healthy-eating patterns, particularly Mediterranean-style diets.

The broader message is therefore less about repeatedly starving the body and more about maintaining a nutritious dietary pattern most of the time while potentially using short fasting-mimicking periods as an additional intervention.

Patient.info describes the longevity diet as a mainly plant-based eating pattern combined with periodic fasting-mimicking periods.

That distinction is important for anyone considering the approach.

What Longo Eats for Longevity

The Telegraph article also describes several of Longo’s personal eating habits.

For breakfast, he reportedly eats whole-grain bread with an almond-and-cocoa spread and an apple.

His afternoon snack may consist of fruit such as melon, peaches or blueberries.

For dinner, he favors a large vegetable-and-legume meal, including minestrone with pasta, vegetables and legumes.

He also drinks several cups of tea throughout the day.

Longo recommends keeping daily eating within approximately a 12-hour window. He reportedly eats between around 9 a.m. and 9 p.m. and advises leaving several hours between the last meal and bedtime.

These habits highlight an important aspect of his approach.

The goal is not necessarily extreme fasting every day.

Instead, the emphasis is on food quality, moderation, meal timing and periodic fasting-mimicking cycles.

Five Foods Commonly Associated With the Plan

Although the precise five-day program has specific nutritional requirements, several food categories repeatedly appear in descriptions of the approach.

1. Vegetables

Vegetables provide fiber, vitamins, minerals and other compounds while generally offering relatively few calories.

They are central to the broader longevity diet and feature prominently in the soups used during fasting-mimicking periods.

2. Nuts

Nuts provide healthy fats and are calorie-dense, allowing a relatively small portion to provide substantial energy.

They are also a major component of the fasting-mimicking approach.

3. Olives

Olives provide fat while fitting naturally into Mediterranean-style eating patterns.

They are included among the foods used during the five-day program.

4. Legumes

Beans and other legumes are important components of Longo’s broader dietary recommendations.

They provide plant protein and fiber and can contribute to satiety.

5. Whole Grains

Whole grains are another important part of the long-term eating pattern.

Longo has described foods such as whole-grain bread and pasta as part of his regular diet.

Who Should Avoid the Five-Day Fasting-Mimicking Diet?

This is perhaps the most important part of the discussion.

A five-day diet providing only around 700 to 1,100 calories is restrictive.

It should not automatically be considered safe simply because it is based on published research.

Patient.info warns that fasting components of the longevity diet may not be appropriate for people who are pregnant or breastfeeding, underweight, nutrient deficient or living with certain medical conditions. It also specifically advises people with diabetes to consult their medical team before attempting fasting approaches.

People with a history of eating disorders should also be especially cautious.

Anyone taking medication that affects blood glucose, blood pressure or other metabolic functions should seek professional medical advice before making major dietary changes.

Children and adolescents generally should not undertake restrictive fasting diets unless specifically directed and monitored by an appropriate medical professional.

The same caution applies to older adults who may be vulnerable to muscle loss or inadequate nutrient intake.

Is the Five-Day Fasting-Mimicking Diet Proven to Make You Live Longer?

Not yet.

This is the biggest qualification surrounding the diet.

There is evidence that fasting, calorie restriction and dietary quality can influence several biological processes associated with health and aging. Studies of the specific fasting-mimicking diet have also reported encouraging changes in metabolic health, body composition and biological-age measures.

But demonstrating that people actually live longer requires much longer studies.

Researchers would need to follow large numbers of people for many years and compare mortality and disease outcomes with appropriate control groups.

That evidence does not currently exist at the level required to say that the five-day diet is an established life-extension treatment.

Patient.info similarly notes that relatively little research directly demonstrates that the specific longevity diet increases human lifespan. Some of the research has also involved scientists affiliated with the approach.

Therefore, headlines suggesting that five days of dieting can definitively add years to a person’s life go further than the evidence currently allows.

The Bigger Lesson About Longevity

The most useful takeaway may not be the five-day fast itself.

Longo’s broader dietary recommendations emphasize many principles already associated with healthy eating: vegetables, fruits, whole grains, legumes, nuts, seeds and healthy fats.

Those foods can form the foundation of a sustainable diet.

The five-day fasting-mimicking diet is better viewed as a specialized intervention that remains under scientific investigation rather than a guaranteed shortcut to a longer life.

People interested in healthy aging may benefit more from improving everyday food quality, maintaining a healthy weight, exercising regularly, sleeping adequately and avoiding smoking than from focusing exclusively on a short fasting cycle.

Five-Day Fasting-Mimicking Diet: What to Remember

The five-day fasting-mimicking diet represents an intriguing intersection between nutrition, fasting and aging research.

Studies suggest that periodic cycles may influence body fat, insulin sensitivity, immune-related markers and other measures associated with metabolic health. Some research has also reported changes in biological-age measurements.

Yet the evidence does not prove that the diet can make humans live to a specific age or reverse aging in a literal sense.

For healthy adults who are interested in the approach, medical guidance is sensible before beginning a highly restrictive five-day program.

For everyone else, the broader lesson is simpler: a longevity-focused diet built around vegetables, fruits, whole grains, legumes, nuts and healthy fats may be a more sustainable place to start.

The search for longer life may ultimately depend less on a single dramatic diet and more on consistently improving the everyday habits that influence health over decades.