The 18:6 and 20:4 Windows Compared

Intermittent fasting pairs naturally with a ketogenic diet. Both approaches work by extending the body’s fat-burning state. The 18:6 (18-hour fast, 6-hour eating window) and 20:4 (20-hour fast, 4-hour window) protocols are among the most sustainable for UK adults managing work and family commitments.

How fasting windows affect ketosis

Shorter eating windows typically deepen ketosis by prolonging glycogen depletion. A 20:4 schedule may elevate blood ketones slightly higher than 18:6, particularly when combined with strict keto macros. However, research suggests the difference is marginal after adaptation. The 2013 Sumithran study found ketosis primarily depends on carbohydrate restriction rather than fasting duration alone.

Appetite regulation differences

Both windows reduce ghrelin (the hunger hormone) after the initial adaptation period. The 20:4 schedule shows a slight edge in appetite suppression, according to data from the Sumithran paper. Many find they naturally eat fewer calories on 20:4 without conscious restriction. Tesco’s £2.50 ready-made keto salads make shorter eating windows practical for British lunch breaks.

What this means in practice

An 18:6 window (e.g. eating between 12pm-6pm) suits those who:

Need breakfast alternatives like bulletproof coffee
Exercise in mornings
Work traditional UK office hours

20:4 (e.g. 2pm-6pm) works better for those who:

Have afternoon energy slumps
Prefer one large meal
Can tolerate black coffee until lunch

Sainsbury’s sells £3.50 electrolyte tablets that help combat fasting headaches during the adjustment period.

Metabolic flexibility over time

Longer fasting windows may improve insulin sensitivity slightly faster. A 2019 study in Nutrients (Kosinski & Jornayvaz) noted greater improvements in HbA1c with 20:4 versus 18:6 in people with metabolic syndrome. However, adherence rates were higher in the 18:6 group after six months.

Frequently asked questions

Is 20:4 harder than 18:6?

The first 3-5 days present similar challenges. Most adjust to 20:4 within two weeks. Having keto-friendly emergency snacks like nuts or cheese can ease the transition.

Can I switch between windows?

Yes. Many use 18:6 on workdays and 20:4 on weekends. Consistency matters more than perfect adherence to one schedule.

Which burns more fat?

Studies show negligible differences in fat loss between the two when calories are matched. The key advantage of 20:4 is automatic portion control for many people.

The bottom line

Both 18:6 and 20:4 fasting windows complement a ketogenic diet effectively. The 20:4 schedule may offer slightly better appetite control and metabolic markers, while 18:6 proves more sustainable long-term for most British lifestyles. If you’d rather not do the macro maths yourself, the Keto Dieting app does it for you on Google Play and the App Store.

Educational only — not medical advice. This article is for general information. Speak to your GP before changing your diet, especially if you have type 1 or type 2 diabetes, kidney or liver disease, are pregnant or breastfeeding, or take medication for blood pressure, cholesterol, or blood glucose.

References

Sumithran P, Prendergast LA, Delbridge E, et al. (2013). Ketosis and appetite-mediating nutrients and hormones after weight loss. European Journal of Clinical Nutrition. https://doi.org/10.1038/ejcn.2013.90
Kosinski C, Jornayvaz FR (2017). Effects of Ketogenic Diets on Cardiovascular Risk Factors: Evidence from Animal and Human Studies. Nutrients. https://doi.org/10.3390/nu9050517