Intermittent Fasting: The Complete Beginner's Guide

What is intermittent fasting?

Intermittent fasting is an eating pattern that cycles between periods of fasting and eating. It doesn't specify which foods to eat — it specifies when you eat them. The most common approach is the 16:8 method: fast for 16 hours, eat within an 8-hour window.

During the fasting period, insulin levels drop significantly, triggering fat burning. After about 12–14 hours of fasting, your body depletes liver glycogen and begins burning stored fat for energy — a state called ketosis.

The main protocols compared

What does the science say?

A 2022 meta-analysis in Annual Review of Nutrition found that IF produces comparable weight loss to continuous calorie restriction over 6–12 months — roughly 0.8–1.3% of body weight per week when combined with a modest calorie deficit.

Beyond weight loss, the research shows benefits including improved insulin sensitivity (particularly in type 2 diabetics), reduced inflammatory markers, lower LDL cholesterol, and — in animal studies — activation of autophagy (cellular cleanup). The human autophagy data is promising but not yet conclusive.

Importantly, IF does not offer a metabolic advantage over equivalent calorie restriction. Its power is simplicity: fewer eating occasions means naturally fewer calories for most people.

What can you consume during the fast?

How to break your fast

What you eat first matters. Coming off a 16+ hour fast, your insulin response is heightened. Starting with protein and vegetables before carbohydrates blunts blood sugar spikes and keeps you fuller longer. A good break-fast meal: eggs, a protein shake, Greek yoghurt, or a grilled chicken salad.

Avoid breaking your fast with a large, carbohydrate-heavy meal — this triggers a significant insulin spike that can cause an energy crash within 1–2 hours.

Side effects and who should avoid IF

Common early side effects include hunger (peaks around day 3–5 then reduces), headaches (usually dehydration — drink more water and add electrolytes), low energy, and irritability. Most resolve within 1–2 weeks.

IF is not recommended for: pregnant or breastfeeding women, people with a history of eating disorders, type 1 diabetics on insulin, those with certain medical conditions, or anyone under 18. Always consult a GP before starting.