Monthly 5-Day Fasting Diet Reduces IGF-1, Body Weight, and Inflammation: What the Research Shows

Medical disclaimer: This article summarises published research for informational purposes only. It is not medical advice and is not a substitute for guidance from a qualified health professional. Always consult your doctor before starting any fasting protocol, especially if you have an existing health condition or take medication.

Study at a Glance

What This Study Looked At

Researchers wanted to know whether periodically mimicking the effects of prolonged fasting — through a structured five-day, low-calorie diet — could produce the same regenerative and metabolic benefits seen in animal models without the risks of complete water fasting. The study tested a “fasting-mimicking diet” (FMD) that dramatically reduces calories for five consecutive days each month, then returns to normal eating for the rest of the month. Understanding this question matters for anyone interested in the benefits of intermittent fasting or how fasting promotes autophagy — periodic prolonged fasting may amplify these processes in ways daily 16:8 fasting cannot.

Who Was Studied

Participant profile: Adults in the study ranged across age groups; the sample included both men and women without severe chronic disease at baseline. Body weights ranged from normal weight to overweight.

How the fasting-mimicking diet worked in this study: The FMD was not a water fast. On Day 1, participants consumed approximately 1,090 calories with a macronutrient split of 10% protein, 56% fat, and 34% carbohydrates. On Days 2–5, intake dropped to approximately 725 calories per day (9% protein, 44% fat, 47% carbohydrates). The diet consisted of structured foods including soups, energy bars, crackers, and supplements. After five days, participants resumed their normal diet for the remaining 25 days of each month.

What the Researchers Found

IGF-1 and Growth Factor Markers

Reduced IGF-1 is significant because high IGF-1 is associated with accelerated aging, increased cancer risk, and reduced cellular repair signalling. The 13% reduction in IGF-1 after just three monthly FMD cycles is one of the study's most clinically meaningful findings. IGFBP-1 rising alongside this indicates more IGF-1 was being bound and inactivated — a beneficial pattern consistent with the cellular effects of longer fasting.

Metabolic and Cardiovascular Markers

• Body weight: significantly reduced in the FMD group versus control
• Waist circumference: measurably reduced, indicating visceral fat reduction
• Blood pressure (systolic): reduced in the FMD group
• Fasting blood glucose: reduced in participants who had elevated baseline levels
• Triglycerides: reduced
• CRP (C-reactive protein, an inflammation marker): reduced

What Did Not Change

• Lean body mass (muscle) showed no significant reduction in the FMD group — an important finding suggesting the FMD preserved muscle despite the caloric deficit
• Total cholesterol did not change significantly in this small pilot
• No serious adverse events were reported during the study

What the Researchers Concluded

The researchers concluded that a monthly five-day fasting-mimicking diet is feasible, safe, and produces meaningful improvements in metabolic markers — including IGF-1 reduction, visceral fat loss, and reduced inflammation — in a healthy adult population. They noted that these changes occurred without requiring daily caloric restriction or a complete water fast, making periodic prolonged fasting a potentially practical approach for people who cannot sustain continuous dietary intervention.

What This Means If You Fast

• A five-day monthly fasting cycle may add benefits beyond daily IF. Daily 16:8 fasting produces significant metabolic improvements, but monthly periodic fasting appears to trigger cellular repair and IGF-1 reduction at a deeper level — effects linked to aging, cancer protection, and immune regeneration.
• IGF-1 reduction matters for long-term health. Lower IGF-1 is consistently associated with reduced cancer risk and slower biological aging in research. A simple dietary intervention reducing IGF-1 by ~13% in three months is a meaningful result. Read more about how intermittent fasting promotes longevity.
• Muscle was preserved. The finding that lean mass did not decrease despite five days of low calories is encouraging. Combined with adequate protein during the 25 days of normal eating, this supports the case that periodic fasting does not cause muscle loss. Learn more at does intermittent fasting destroy muscle?
• The inflammation reduction adds up. Reduced CRP is a direct indicator of lower systemic inflammation. The connection between fasting and inflammation is one of the most well-supported mechanisms for fasting's health benefits.
• Visceral fat — the dangerous kind — was specifically reduced. Waist circumference reduction indicates loss from the abdominal region, where fat accumulation most directly elevates cardiovascular and metabolic disease risk.
• This approach is distinct from daily fasting. If you already do 16:8 daily, adding an occasional 5-day FMD or extended fast once per month may produce benefits that daily fasting alone does not reach.

Study Limitations

• The human arm was a pilot trial with a small sample (n=38), limiting statistical power and generalisability
• Duration was only three months — long-term effects beyond this window are unknown from this study alone
• The study used a proprietary FMD food kit; results may differ with self-constructed low-calorie protocols
• Gender breakdown and specific age distribution of participants were not uniformly reported in the abstract
• The control group was not matched for caloric restriction — some effects may reflect overall calorie reduction rather than the fasting-mimicking pattern specifically
• No data on what participants ate during the 25 days of normal eating (diet quality outside the FMD was not controlled)

Source

Brandhorst S, Choi IY, Wei M, Cheng CW, Sedrakyan S, Navarrete G, Dubeau L, Yildirim BO, Pena-Rodriguez M, Gaber T, Park JH, Thompson R, Yancy WS Jr, Wang T, Longo VD. “A Periodic Diet that Mimics Fasting Promotes Multi-System Regeneration, Enhanced Cognitive Performance, and Healthspan.” Cell Metabolism. 2015 Jul 7;22(1):86-99. PMID: 26094889

Frequently Asked Questions

What is the fasting-mimicking diet and how is it different from water fasting?

The fasting-mimicking diet (FMD) is a structured five-day protocol that reduces calories to approximately 1,090 on Day 1 and 725 on Days 2–5, with specific macronutrient ratios designed to replicate many of the cellular effects of water fasting while still allowing some food. It is significantly less demanding than complete water fasting and was designed specifically for periodic monthly use.

How much weight can you lose on a 5-day fasting-mimicking diet?

The Brandhorst 2015 study found statistically significant reductions in body weight in the FMD group after three monthly cycles. Individual weight loss during a single 5-day FMD cycle varies depending on baseline weight and diet quality during normal eating days. The reduction in waist circumference suggests a portion of this loss comes specifically from visceral (abdominal) fat.

Does the fasting-mimicking diet reduce IGF-1 the same way full fasting does?

The study found a ~13% reduction in IGF-1 after three cycles, which is consistent with the direction of change seen in water fasting studies, though the magnitude may differ. Whether the FMD fully replicates the IGF-1 reduction of complete prolonged fasting in humans has not been directly compared in a single trial.

Is muscle loss a risk on a 5-day calorie-restricted fast?

In this study, lean body mass did not significantly decrease in the FMD group. This suggests that with appropriate protein intake during the non-fasting days of the month and moderate caloric intake during the FMD itself, muscle preservation is achievable. This aligns with findings from research on whether intermittent fasting burns muscle.

Can the fasting-mimicking diet reduce inflammation?

Yes — this study found reduced CRP (a key inflammation marker) in the FMD group after three monthly cycles. This is consistent with broader evidence that fasting reduces inflammation through multiple pathways including autophagy, reduced oxidative stress, and lower insulin.

Related Research and Articles

• Intermittent fasting and inflammation: the research explained
• How intermittent fasting promotes autophagy
• Does intermittent fasting slow aging?
• Intermittent fasting and longevity: what the science says
• Does intermittent fasting destroy muscle?
• Intermittent fasting benefits: the complete science-backed guide
• What is autophagy and when does it start during fasting?

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