Fasting Tracker

Fasting plans · Intermediate

18:6 intermittent fasting

An 18-hour fast and a 6-hour eating window: a step beyond 16:8 for experienced fasters

What is 18:6?

18:6 means 18 hours of fasting and a 6-hour eating window, usually two meals, such as lunch at noon and dinner before 6 p.m. Fasting Tracker calls it Fat Burner: your fast spends a couple more hours in the Fat Burning stage than 16:8 does. It's best tried once 16:8 feels easy.

Sample 18:6 schedules
Eating windowFastGood for
9:00 a.m. to 3:00 p.m.3:00 p.m. to 9:00 a.m.An early window, as in the early-eating trial 2
12:00 p.m. to 6:00 p.m.6:00 p.m. to 12:00 p.m.Lunch and an early dinner
2:00 p.m. to 8:00 p.m.8:00 p.m. to 2:00 p.m.Late lunches and later dinners

What the research shows

Key study

A 6-hour window cut calories without counting

  • Randomized trial
  • 58 adults with obesity
  • 8 weeks

People who ate only within 4 or 6 hours lost about 3% of their weight compared with a control group, because they ended up eating around 550 fewer calories a day.

Cienfuegos et al., Cell Metab 2020

Key study

Eating early may help, even without weight loss

  • Randomized crossover trial
  • 8 men with prediabetes
  • 5 weeks each

The same food, eaten within 6 hours ending before 3 p.m. instead of across 12 hours, improved insulin sensitivity, blood pressure and oxidative stress.

Sutton et al., Cell Metab 2018

What your body may do in 18 hours

By 18 hours you're well into Fat Burning: glycogen is low, fat is a major fuel and your liver is making some ketones 4. Fasting Tracker's tip for this stage: 18:6 and 20:4 end inside it.

  1. 0 hBlood Sugar RisingYour body is digesting your last meal and using that energy. Insulin is elevated.
  2. 4 hBlood Sugar FallingBlood sugar settles back to normal and insulin starts to drop.
  3. 12 hFuel SwitchYour liver's glycogen is getting low, so your body starts turning to fat for fuel.
  4. 16 hFat BurningYour body starts burning stored fat for energy. Stay hydrated!

Eating well in 6 hours

A short window makes it easy to fall short on protein, and protein is what protects your muscle while you lose fat. Gains from strength training level off at about 1.6 g of protein per kilo of body weight a day 3: about 110 g for a 70 kg person, or 55 g in each of two meals. Your body can use a large serving well, so two generous meals are fine 5.

  1. Get comfortable with 16:8 first.
  2. Build both meals around protein: eggs, fish, meat, tofu, yoghurt or legumes.
  3. Keep a bottle of water with you; some people find a pinch of salt in water helps on longer fasts 6.
  4. If you train, schedule strength sessions near your eating window.

Sources

  1. Cienfuegos S, Gabel K, Kalam F, et al. Effects of 4- and 6-h Time-Restricted Feeding on Weight and Cardiometabolic Health: A Randomized Controlled Trial in Adults with Obesity. Cell Metab. 2020;32(3):366-378.e3. doi.org/10.1016/j.cmet.2020.06.018
  2. Sutton EF, Beyl R, Early KS, et al. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metab. 2018;27(6):1212-1221.e3. doi.org/10.1016/j.cmet.2018.04.010
  3. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018;52(6):376-384. doi.org/10.1136/bjsports-2017-097608
  4. Anton SD, Moehl K, Donahoo WT, et al. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity (Silver Spring). 2018;26(2):254-268. doi.org/10.1002/oby.22065
  5. Trommelen J, van Lieshout GAA, Nyakayiru J, et al. The anabolic response to protein ingestion during recovery from exercise has no upper limit in magnitude and duration in vivo in humans. Cell Rep Med. 2023;4(12):101324. doi.org/10.1016/j.xcrm.2023.101324
  6. Sigler MH. The mechanism of the natriuresis of fasting. J Clin Invest. 1975;55(2):377-87. doi.org/10.1172/JCI107941
  7. Grajower MM, Horne BD. Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. Nutrients. 2019;11(4):873. doi.org/10.3390/nu11040873

A note on health

Fasting Tracker is designed for general wellness and is not a medical device. Fasting is not right for everyone. If you are pregnant, diabetic, have a history of eating disorders or take medication, talk to your doctor first, and before any extended fast.