Intermittent Fasting, What Is It?
Intermittent Fasting (IF) is an eating pattern that cycles between periods of eating and periods of fasting. Instead of focusing primarily on what you eat, it focuses on when you eat. It has gained popularity for weight loss, metabolic health, and longevity, but the concept is much older than modern diet culture.
Where Did Intermittent Fasting Come From?
Fasting is not a new concept. Fasting has roots in religious traditions, evolutionary biology, and more recently in modern science.
Humans have historically experienced natural cycles of feast and famine. Our metabolism evolved to function during periods without food by switching from glucose (sugar) to fat derived energy (ketones).
What Is Intermittent Fasting?
IF involves restricting food intake for specific periods of time while allowing normal eating during designated windows. Common IF approaches include:
Time-restricted eating (TRE): Eating within a daily window (e.g., 8 hours eating / 16 hours fasting).
5:2 method: Eating normally 5 days per week and consuming ~500–600 calories on 2 non-consecutive days.
Alternate-day fasting: Alternating between normal eating days and fasting or very low-calorie days.
Extended fasting: Fasting longer than 24 hours (less commonly recommended without supervision).
The most popular approach today is 16:8 time-restricted eating.
What Happens In the Body During Fasting?
Typically, our bodies break down foods into glucose (sugar). The hormone insulin then acts like a key opening the door into our cells allowing glucose to enter and be utilized as an immediate energy source or store it for later use. When you stop eating for several hours, the body shifts metabolic pathways.
For the first 0-4 hours after eating the body uses glucose (sugar) from the recent meal.
4-12 hours without eating the body uses glycogen stores (a storage form of glucose) in the muscles and liver as its main energy source.
12-24 hours into a fast the body increases fat oxidation, the metabolic process where stored fat is broken down to produce energy in the mitochondria. Insulin decreases, this not only enables fat oxidation to occur but it also allows for lipolysis to release fat from adipose tissue (the storage form of fat) to be burned.
24+ hours of fasting increases the production of ketones. Ketones are produced by the liver when the body breaks down fat for energy instead of glucose. Ketones act as an alternative fuel source for the body and the brain. The body utilizes them and then excretes them in urine. It is normal to have some ketones in your blood however, too many ketones can make your blood acidic and toxic, this is called ketoacidosis.
Newer research has found after 24+ hours of fasting there is an activation of cellular repair pathways such as Autophagy. This is when the cell will clean up damaged components which aids in optimizing the function of the cell. This research is very new and ongoing studies need to be done.
What Does the Research Say About Fasting?
Weight Loss
Most studies show intermittent fasting leads to similar weight loss compared with traditional calorie restriction. The key mechanisms are reduced calorie intake overall (a calorie deficit) and for some people improved appetite regulation. However, fat loss and most metabolic improvement come primarily from a calorie deficit and not timing of meals.
Metabolic Health
Evidence suggests potential improvements in insulin sensitivity (our bodies ability to respond to insulin in order to open doors into cells) and as a result improve blood glucose control.IF may improve triglycerides, LDL cholesterol, and blood pressure through several interconnected metabolic changes. By extending periods without food, insulin levels drop, which reduces fat production in the liver and encourages the body to burn stored fat for energy. This shift can lower triglycerides and improve overall lipid metabolism, especially in individuals with insulin resistance or conditions like Nonalcoholic Fatty Liver Disease. Fasting may also support reductions in inflammation and enhance blood vessel function, contributing to lower blood pressure. Importantly, many of these benefits are driven by overall improvements in metabolic health and, in many cases, a calorie deficit, meaning similar outcomes can be achieved through other nutrition approaches as well.
Cellular Repair and Longevity
Animal studies show fasting can activate pathways linked to longevity, including: autophagy, improved mitochondrial function, and reduced oxidative stress. However, human longevity evidence is still very limited.
Brain Health
IF may support brain and metabolic health by influencing several key processes. Some research suggests it can increase Brain-Derived Neurotrophic Factor (BDNF), a protein that supports brain cell growth, learning, and memory. It may also enhance neuronal resilience, helping the brain better adapt to stress and maintain function over time. Additionally, fasting can improve Metabolic Flexibility, or the body’s ability to switch efficiently between using carbohydrates and fat for energy. While these effects are well-supported in animal studies, human research is still emerging.
Potential Risks or Downsides:
IF is not a one size fits all, it does not work well for everyone. Some individuals compensate for fasting by overeating large amounts of foods during eating window. Initially, during early adaptation energy and concentration issues may occur. Very aggressive fasting disrupts hormones, particularly in women with low body fat or high stress. Shorter eating windows can also make it harder to meet nutrient needs. Training fasted may reduce performance in some athletes as fat is not the optimal fuel source.
Who May Benefit Most from It ?
IF may be helpful for:
Individuals with overweight or obesity
Those with insulin resistance or prediabetes
Individuals who struggle with late-night snacking.
Who Should Be Cautious or Avoid It ?
Certain populations should be cautions or avoid IF, these populations include:
Type 1 Diabetes
Individuals on glucose-lowering medication
People with a history of Eating Disorders
During Pregnancy
During Breastfeeding
Adolescents
Individuals with high caloric needs
Endurance athletes
The Bottom Line
IF is an eating schedule, not a diet. Research suggests it can support weight loss and metabolic health for some individuals, but it is not inherently superior to traditional calorie restrictions.
The effectiveness of IF largely depends on:
Total calorie intake
Composition of foods
Individual lifestyle and preferences.
For many people, the best approach is the one that is sustainable and nutritionally adequate.
References:
Song, Y., Almutairi, M., Almutairi, A., Jamilian, M., & Abu‑Zaid, A. (2025). The effect of intermittent fasting on insulin resistance, lipid profile, and inflammation in adults with metabolic syndrome: A systematic review and meta‑analysis with GRADE evaluation. Journal of Health, Population and Nutrition.https://pubmed.ncbi.nlm.nih.gov/41459076/
Zhang, S., Sun, B., Sun, L., Zou, S., Chen, Q., & et al. (2025). Effect of intermittent fasting on obesity and metabolic indices in patients with metabolic syndrome: A systematic review and meta‑analysis. BMC Endocrine Disorders, 25, 130. https://doi.org/10.1186/s12902-025-01952-x
(Authors not individually listed). (2025). The impact of intermittent fasting on body composition and cardiometabolic outcomes in overweight and obese adults: A systematic review and meta‑analysis of randomized controlled trials. PubMed.https://pubmed.ncbi.nlm.nih.gov/40731344/
(Authors not individually listed). (2025). The effect of intermittent fasting on insulin resistance, lipid profile, and inflammation in adults: A systematic review and meta‑analysis. Journal of Health, Population and Nutrition.https://pubmed.ncbi.nlm.nih.gov/40826125/
Almabruk, B. A., et al. (2024). The role of intermittent fasting on metabolic syndrome: A systematic review and meta‑analysis. Cureus.https://pubmed.ncbi.nlm.nih.gov/39553053/
Alkurd, R., et al. (2024). Effect of calorie restriction and intermittent fasting regimens on brain‑derived neurotrophic factor (BDNF) and cognitive function in humans: A systematic review. Medicina (Kaunas).https://pubmed.ncbi.nlm.nih.gov/38276070/
Bastani, A., & co‑authors. (2017). The effects of fasting during Ramadan on plasma brain‑derived neurotrophic factor (BDNF). PubMed.https://pubmed.ncbi.nlm.nih.gov/28713531/
Riat, A., et al. (2021). Ramadan fasting and its association with cortisol and BDNF levels in healthy adults. PMC.https://pubmed.ncbi.nlm.nih.gov/34458302/
Medina Rodríguez, M., Muñoz, Á. M., & Franco Hoyos, K. (2025). Intermittent fasting strategies and cardiometabolic risk: Systematic review and network meta‑analysis. BMJ, 389, r1156. https://doi.org/10.1136/bmj.r1156
