Fasting, the voluntary abstinence from food for defined periods, is gaining traction as a therapeutic tool. Beyond its historical and cultural roots, fasting is being re-examined for its potential to support metabolic health, cognitive function, and cellular resilience. This blog reviews common fasting approaches, key health benefits—including liver health in MASLD—and important functional medicine considerations.

What Is Fasting?

Fasting is a natural metabolic state in which the body shifts from using glucose to using fat as its primary fuel. Unlike calorie restriction, which reduces intake daily, fasting involves defined periods of not eating. Physiologically, fasting decreases insulin, increases glucagon, enhances fat oxidation, and stimulates ketone production. It also activates autophagy, a cellular repair mechanism triggered by nutrient deprivation.

Common Types of Fasting

Fasting approaches can be tailored to individual needs. Common types include:

• Time-Restricted Eating (TRE): Limits food intake to a daily window, such as 12:12 (12 hours fasting, 12 hours eating), 16:8, or even 23:1 (one meal a day). TRE aligns with circadian rhythms and has shown benefits in metabolic markers.
• Alternate-Day Fasting (ADF): Alternates between fasting days (no or very low intake) and regular eating days. Modified versions allow ~500 calories on fasting days.
• Modified Fasting (5:2 Diet): Involves two non-consecutive days per week of significant calorie reduction (~500–600 kcal), with normal eating the other five days. The 5:2 diet has demonstrated benefits for weight loss, insulin sensitivity, and liver health.
• Fasting-Mimicking Diet (FMD): A 5-day, plant-based, low-protein, low-carb diet that provides around 700–1,100 kcal per day. It’s designed to mimic the metabolic effects of water-only fasting while providing some nourishment. Clinical trials have shown improvements in visceral fat, inflammation, and metabolic biomarkers with monthly FMD cycles.

Potential Benefits of Fasting

Fasting can be a powerful adjunct to personalized functional medicine care:

• Metabolic Health: Fasting reduces insulin levels and improves insulin sensitivity, aiding in blood sugar regulation and reducing risk of type 2 diabetes.
• Fat and Weight Loss: Periodic fasting often results in reductions in visceral adiposity and body weight while helping preserve lean mass.
• Cellular and Mitochondrial Health: Fasting may enhance autophagy and mitochondrial resilience, supporting longevity and repair.
• Cognitive Function: Ketone metabolism during fasting improves brain energy efficiency and may increase BDNF, supporting neuroplasticity.
• Cardiovascular Markers: Fasting has been shown to reduce triglycerides, blood pressure, and markers of inflammation.
• Liver Health (MASLD): Of particular interest, fasting reduces liver fat, improves AST/ALT, and enhances hepatic autophagy. In patients with MASLD, strategies like ADF, 5:2, and FMD have been associated with significant reductions in steatosis and liver enzymes.

Cautions and Contraindications

Fasting isn’t appropriate for everyone. Fasting is contraindicated or cautioned in the following cases:

• Pregnancy or breastfeeding
• Underweight individuals
• History of eating disorders
• Children or adolescents
• Significant adrenal dysregulation
• Reactive hypoglycemia or certain thyroid conditions

Each case should be evaluated individually before initiating a fasting protocol.

Fasting Implementation

Fasting should be introduced gradually and strategically. Starting with a 12:12 TRE schedule allows patients to ease into fasting without stress.

Hydration is essential. Water and electrolyte support is needed, especially during longer fasts. During eating windows, nutrient-dense meals should be consumed with high-quality protein, fiber, and healthy fats to sustain energy.

Patients should be encouraged to listen to their bodies. Headaches, fatigue, or sleep disturbances may occur early but often resolve. Flexibility is key; fasting should align with the patient’s lifestyle, stress levels, and circadian rhythm. 

Conclusion

Fasting offers a low-cost, evidence-based strategy to enhance metabolic, cognitive, and liver health—especially in patients with MASLD. A functional medicine lens allows for tailored fasting protocols that work with each patient’s physiology. By starting gently, focusing on nutrient quality, and individualizing plans, fasting becomes a sustainable, empowering tool in clinical practice.

-Carey Kunz, ND, IFMCP

Director of Education at FMP Essentials

References

1. de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med. 2019;381(26):2541-2551.
2. Wilkinson MJ, et al. Ten-hour time-restricted eating reduces weight, blood pressure, and atherogenic lipids in patients with metabolic syndrome. Cell Metab. 2020;31(1):92-104.e5.
3. Hoddy KK, et al. Alternate day fasting and chronic disease prevention: a review of human and animal trials. Am J Clin Nutr. 2015;102(5):1072–1080.
4. Johari MI, et al. Modified alternate-day calorie restriction in improving the activity of NAFLD: a randomized controlled trial. Sci Rep. 2019;9(1):11232.
5. Brandhorst S, et al. A periodic diet that mimics fasting promotes multi-system regeneration, enhanced cognitive performance, and healthspan. Cell Metab. 2015;22(1):86-99.
6. Sutton EF, et al. Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell Metab. 2018;27(6):1212-1221.e3.
7. Mattson MP, et al. Meal frequency and timing in health and disease. Proc Natl Acad Sci U S A. 2014;111(47):16647-16653.