Intermittent Fasting Diet: Complete Guide

Last updated: 20 February 2026

An intermittent fasting diet (or fasting diet) is an eating pattern that alternates periods of eating with periods of fasting–no or very few calories. It focuses on when you eat rather than exactly what you eat. Common methods include the 16:8 diet (eat in an 8-hour window, fast 16 hours), the 5:2 diet (very low calories on 2 days per week), and OMAD (one meal a day diet). Many people use an intermittent fasting diet plan or fasting diet plan for weight loss or convenience. This guide explains what intermittent fasting is, how it works, the main methods, benefits and risks, and who should avoid it. Always discuss a fasting diet with your doctor, especially if you have diabetes, an eating disorder, or are pregnant.

Disclaimer: This page is for education only and does not replace advice from your doctor or dietitian. Intermittent fasting is not safe for everyone. Do not start a fasting diet if you are pregnant, breastfeeding, have an eating disorder, or have diabetes on insulin or other glucose-lowering medication without medical approval.

Below you will find the definition of an intermittent fasting diet, how a fasting diet plan works, the 16:8 diet, 5:2 diet, OMAD diet, and other methods, what to eat during eating windows, benefits and risks, and answers to common questions. You can use our calorie calculator and TDEE calculator to set your calorie target if you combine intermittent fasting with a weight goal.

What is the intermittent fasting diet?

The intermittent fasting diet (or fasting diet) is a pattern of eating that cycles between periods of eating and periods of fasting. During the fasting window you consume no calories (or very few–e.g. on the 5:2 diet you might eat 500–600 kcal on “fast” days). Water, black coffee, and unsweetened tea are usually allowed during the fast. The idea is to restrict when you eat, not necessarily what you eat–though many people still choose nutritious foods during eating windows.

There is no single official intermittent fasting diet plan; methods vary. The 16:8 diet (or 16/8) means 16 hours of fasting and an 8-hour eating window each day. The 5:2 diet means eating normally 5 days and restricting to about 500–600 calories on 2 non-consecutive days. The OMAD diet (one meal a day diet) means one meal per day within a short window. Other approaches include the warrior diet (small amounts during the day, one large meal at night), the fasting mimicking diet (very low calories for a few days, e.g. Prolon diet), and the Daniel diet (plant-based, often with fasting elements). You can follow an intermittent fasting diet plan or fasting diet plan that fits your schedule and goals.

Core principles

  • Set an eating window and a fasting window (or, for 5:2, normal days and low-calorie days).
  • During the fast: no calories (or only the allowed amount on 5:2); water, black coffee, and unsweetened tea are usually fine.
  • During the eating window: eat enough to meet your needs; for weight loss, stay in a calorie deficit.

How the intermittent fasting diet works

Intermittent fasting can support weight loss when it helps you eat fewer total calories. By shortening the time you eat (e.g. 16:8 diet) or cutting calories on some days (5:2 diet), many people naturally consume less. Weight loss still requires a calorie deficit–burning more than you eat over time. An intermittent fasting diet is one way to achieve that; it does not magic away calories. Some research suggests that fasting may affect hormones (e.g. insulin) and metabolism in ways that support fat loss, but the main driver is still total calorie intake. Use our TDEE calculator to estimate your needs and plan your eating window so you do not overeat in compensation.

There are no mandatory phases. You choose a method (e.g. 16:8 diet, 5:2 diet, one meal a day diet) and follow it. You can combine an intermittent fasting diet plan with other eating patterns (e.g. Mediterranean, low-carb) during your eating window. The fasting mimicking diet and Prolon diet are structured programmes that provide very low-calorie, specific foods for a few days to mimic fasting while still eating; they are often done periodically (e.g. a few times a year) under guidance.

What to eat on an intermittent fasting diet

During your eating window, what you eat is up to you–the intermittent fasting diet does not prescribe specific foods. For health and satiety, focus on whole foods: vegetables, fruits, lean protein, whole grains, legumes, and healthy fats. That helps you get enough fibre, protein, and nutrients in a shorter time. Avoid “saving” all your calories for junk food in the window; you can still gain weight or feel unwell if you overeat or eat poorly. For weight loss, keep total calories in a deficit–use our calorie calculator as a guide.

During the fast: Stick to zero-calorie or very low-calorie drinks (water, black coffee, unsweetened tea). Anything with calories (milk, sugar, cream, diet drinks–depending on your strictness) can break the fast for metabolic purposes. On the 5:2 diet, your 2 “fasting” days typically allow one or two small meals (e.g. 500–600 kcal total). On the fasting mimicking diet or Prolon diet, you follow the programme’s specific low-calorie meals. The warrior diet allows small amounts of raw fruit or vegetables during the day and one large meal at night; the one meal a day diet (OMAD) compresses all intake into one meal.

Potential benefits of the intermittent fasting diet

Benefits are often described as “may” or “in studies”–individual results vary.

  • Weight loss: An intermittent fasting diet plan or fasting diet plan can help when it leads to a calorie deficit. Short-term trials show weight loss with 16:8 and similar methods; long-term comparison with other diets is mixed. Adherence matters.
  • Convenience: Some people find it easier to skip breakfast and eat in a shorter window (16:8 diet) than to count calories all day.
  • Metabolic markers: Some studies suggest improvements in insulin sensitivity or blood lipids with intermittent fasting; more research is needed. Do not use fasting as a substitute for medical treatment of diabetes or other conditions.

Quality of evidence: short-term weight loss is supported; long-term and comparative data are limited. Talk to your doctor about whether an intermittent fasting diet is appropriate for you.

Risks and who should avoid intermittent fasting

Intermittent fasting is not safe for everyone.

  • Pregnant or breastfeeding women: Not recommended–adequate nutrition and regular eating are important.
  • Children and adolescents: Not recommended without medical supervision.
  • History of eating disorders: Fasting can trigger or worsen disordered eating. Avoid a fasting diet if you have or have had an eating disorder unless your care team explicitly supports it.
  • Diabetes (especially on insulin or sulphonylureas): Fasting can cause dangerous hypoglycaemia. Do not start an intermittent fasting diet without your doctor’s approval; medication may need to be adjusted.
  • Other conditions: If you have low blood pressure, take medication that requires food, or have a condition that affects energy or nutrients, check with your doctor first.

Side effects when starting can include hunger, irritability, headache, or fatigue. The one meal a day diet (OMAD) can make it hard to get enough nutrients in a single meal. Before starting any fasting diet plan, consult your doctor or a registered dietitian.

Sample intermittent fasting day (16:8 example)

This is one example of how a 16:8 diet day might look–eating window noon to 8 p.m. Adjust to your schedule and calorie target.

  • Fasting (e.g. 8 p.m. to noon): Water, black coffee, or unsweetened tea only.
  • First meal (e.g. noon): Balanced lunch–e.g. salad with chicken or chickpeas, olive oil, and whole grain; or a substantial bowl with vegetables, protein, and a small portion of carbs.
  • Snack (optional): Fruit, nuts, or yogurt if needed.
  • Second meal (e.g. 6–8 p.m.): Dinner–e.g. grilled fish or meat with vegetables and a side; or a legume-based dish with vegetables. Finish eating by 8 p.m.

For the 5:2 diet, on your 2 low-calorie days you might have one or two small meals (e.g. 250–300 kcal each). For the one meal a day diet (OMAD), you would eat one larger meal within your chosen hour. For a structured fasting mimicking diet or Prolon diet, follow the programme’s instructions. Use our TDEE calculator and calorie deficit calculator to tailor intake. See our weight loss diet and calorie deficit diet guides for the role of total calories.

Frequently asked questions about the intermittent fasting diet

Intermittent fasting is an eating pattern that alternates periods of eating with periods of fasting (no or very few calories). It is not about what you eat but when you eat. Common methods include the 16:8 diet (fast 16 hours, eat in an 8-hour window), the 5:2 diet (eat normally 5 days, very low calories 2 days), and OMAD (one meal a day diet).

Intermittent fasting can support weight loss when it helps you eat fewer calories overall (a calorie deficit). By shortening the eating window, some people naturally eat less. Weight loss still depends on total calories; an intermittent fasting diet plan can be one way to achieve a deficit. Use our calorie calculator and TDEE calculator to set your target.

The 16:8 diet means you fast for 16 hours each day and eat all your meals within an 8-hour window (e.g. noon to 8 p.m.). During the fast you can have water, black coffee, or unsweetened tea. It is one of the most popular intermittent fasting diet methods.

The 5:2 diet means you eat normally for 5 days of the week and restrict calories to about 500–600 on the other 2 non-consecutive days. It is a form of intermittent fasting (fasting diet). The 2 low-calorie days are sometimes called “fasting” days even though you still eat a small amount.

OMAD stands for one meal a day. You eat a single meal per day (typically within a 1-hour window) and fast the rest of the time. It is a strict form of intermittent fasting. It can be hard to get enough nutrients in one meal and is not suitable for everyone–check with your doctor first.

Intermittent fasting is not recommended for pregnant or breastfeeding women, children, people with a history of eating disorders, or those with certain medical conditions (e.g. diabetes on insulin or other glucose-lowering medication) unless approved by a doctor. Always speak to your doctor before starting a fasting diet.

Summary

An intermittent fasting diet (or fasting diet) alternates eating windows with fasting windows. Popular methods include the 16:8 diet, the 5:2 diet, and the one meal a day diet (OMAD). Others include the warrior diet, fasting mimicking diet, and Prolon diet. Weight loss still requires a calorie deficit; an intermittent fasting diet plan or fasting diet plan can help by limiting when you eat. Intermittent fasting is not safe for everyone–avoid it if you are pregnant, have an eating disorder, or have diabetes on medication unless your doctor agrees. Before starting, talk to your doctor and use our calorie calculator and TDEE calculator for weight goals. For related approaches, see our weight loss diet and calorie deficit diet guides.

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Sources and further reading

This article is based on current scientific and clinical sources. For more detail, see:

  • NIH and systematic reviews on intermittent fasting and weight loss, metabolic health
  • American Heart Association and similar bodies: intermittent fasting and cardiovascular risk
  • Clinical guidelines on fasting in diabetes (caution and medical supervision)

Last updated: 20 February 2026. Content is for educational purposes only and does not replace medical advice.

Content quality

Written by DietaBest Editorial Team

Medically reviewed by Dr. Alex Novak, MD (Internal Medicine)

Last updated:

This guide is for general education only and does not replace personal medical advice. Always talk to your doctor or a registered dietitian before making major diet or lifestyle changes, especially if you have a medical condition or take medication.