Intermittent Fasting: A Review ★★★★☆

By R.Davies, PhD・Diet Atlas
Published on May 05, 2026

Intermittent fasting is a dietary pattern that alternates between periods of eating and periods of fasting; you focus on when to eat rather than what to eat. Unlike traditional diets that tend to focus on reducing the amount of calories you eat, intermittent fasting limits the time that you eat, which creates extended periods of time when you aren’t eating. 

These types of fasts aren't new; they have roots in a number of religious and cultural practices. However, they’ve recently gained attention from scientists and (then) mainstream popularity for their potential health benefits.

There are many types of intermittent fasts. ‘Time-restricted eating’ limits daily food intake to a specific window (e.g., 6-10 hours), while fasting for the rest of the day. ‘Alternate-day fasting’ involves regular eating days and fasting (or severe calorie restriction) on alternate days. An example is the 5:2 diet that permits normal eating for five days a week while restricting food intake to 500-600 calories a day on two non-consecutive days.

During the fasting period, the body shifts from using carbohydrates for fuel to fat (and then ketones during longer fasts). This metabolic ‘shift’ typically occurs after 12–16 hours into the fast. Some scientists have suggested that this process may activate cellular repair processes (called ‘autophagy’), normalise blood sugar levels, and reduce inflammation. Many people find fasting appealing because it doesn’t overtly restrict specific foods or require calorie counting.

Pros

Intermittent fasting is associated with several health benefits. Studies show that it results in weight loss, comparable to traditional calorie-restriction diets, alongside improved body composition and metabolic health markers. 

Studies have shown that it can help normalise blood pressure, blood sugar and blood lipid profiles in people who have high levels already. Improved control of appetite and beneficial changes to the gut microbiome have also been reported. The structured eating schedule may help some people control their calorie intake without the constant vigilance required by other (more traditional) calorie-restriction diets.

Cons

While the health benefits are obvious, intermittent fasting doesn’t seem to be superior to other standard calorie-restriction diets. Different people also seem to respond differently to fasts, with some people meaningfully improving their health, while others don’t seem to respond as well. 

There are no clear indicators at the moment for who may (or may not) benefit from intermittent fasts. There have also been questions about the long-term sustainability of intermittent fasts (and their health benefits), and whether some people may have problems getting all the essential nutrients they need in shorter eating windows.

There are also some practical challenges. A lot of people struggle with hunger when starting intermittent fasts; fatigue, irritability, headaches and gut problems have also been reported. A strict fasting schedule can also conflict with family meals, social gatherings, work tasks, or some cultural practices. Shift workers, people with irregular work patterns, and those who’re exercising a lot may find intermittent fasting difficult.

Intermittent fasts are also not recommended for certain people, including pregnant or breastfeeding women, people with histories of eating disorders (where rigid eating rules may trigger disordered eating patterns), children and adolescents, and people with certain medical conditions requiring regular food intake.

Bottom Line

Intermittent fasting is (in most cases) a safe, effective approach for weight loss and improving metabolic health, with benefits comparable to traditional calorie-restriction diets. However, the sustainability and the variable responses suggest it is one effective option among several, rather than the sole dietary solution to health.

Sources

Jefcoate PW et al. Exploring Rates of Adherence and Barriers to Time-Restricted Eating. Nutrients. 2023 May 16;15(10):2336. PMID: 37242218

Patterson RE, Sears DD. Metabolic Effects of Intermittent Fasting. Annu Rev Nutr. 2017 Aug 21;37:371-393. PMID: 28715993

Rynders CA et al. Effectiveness of Intermittent Fasting and Time-Restricted Feeding Compared to Continuous Energy Restriction for Weight Loss. Nutrients. 2019 Oct 14;11(10):2442. PMID: 31614992

Stockman MC et al. Intermittent Fasting: Is the Wait Worth the Weight? Curr Obes Rep. 2018 Jun;7(2):172-185.PMID: 29700718

Teong XT et al. Intermittent fasting plus early time-restricted eating versus calorie restriction and standard care in adults at risk of type 2 diabetes: a randomized controlled trial. Nat Med. 2023 Apr;29(4):963-972. PMID: 37024596

Varady KA et al. Clinical application of intermittent fasting for weight loss: progress and future directions. Nat Rev Endocrinol. 2022 May;18(5):309-321. PMID: 35194176

Further Reading

Minimalist Training for Muscle Growth

3 Key Principles of Muscle Growth

Building Muscle: The Basic Blueprint for Beginners

Published: May 05, 2026

Lead Author: R.Davies, PhD | Author Bio

Alphabet Guides provides independent, evidence-based information you can trust. Learn more about our Editorial Standards.

Disclaimer: The information on this website is for educational purposes only. It should not be used as a substitute for medical advice from a qualified healthcare professional.