Fasting Tracker

Fasting, hour by hour

What happens after 18 hours of fasting?

Around 18 hours, you're two hours into Fat Burning: stored fat is a major fuel and ketones are starting to rise

  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!

What's happening in your body

Insulin has been low for many hours, so your fat cells keep releasing fatty acids that your muscles and heart burn directly 1,2. Your liver keeps your blood sugar steady, now mostly by making new glucose 3. Ketones are rising, but they're still a minor fuel at this point; Fasting Tracker marks Ketosis at around 24 hours.

At 18 hours, in Fasting Tracker
StageFat Burning (16 to 24 h)
Main fuelMostly stored fat
InsulinLow

How you might feel

Many people feel lighter and more stable once hunger settles, and energy often comes back. Hunger tends to arrive in waves around your usual meal times and pass on its own 4. If you feel flat or get a headache, check your water first: even mild dehydration can bring headaches and low mood 5.

What helps

  1. Aim for a glass of water every couple of hours.
  2. A great window for a walk or a gentle workout.
  3. 16:8 reaches this stage at its goal; 18:6 and 20:4 end inside it.

Does 18 hours do more than 16?

A little more of the same. The extra two hours keep you in Fat Burning for longer, but they don't unlock a new stage. What a shorter eating window does reliably is help you eat less: adults with obesity who ate within 6 hours, from 1 to 7 p.m., ate around 550 fewer calories a day without counting and lost about 3% of their weight in 8 weeks compared with a control group 6.

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

So if you fast for 18 hours, finishing your eating earlier in the day may be worth more than adding hours. See 16:8 vs 18:6 for how the two plans compare.

Breaking an 18-hour fast

A 6-hour 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 per kilo of body weight a day 8: about 110 g for a 70 kg person, or 55 g in each of two meals. Your body uses a large serving well, so two generous meals are fine 9.

A good first meal

  1. Start with a glass of water, then a normal meal: no need for anything special after 18 hours.
  2. Put protein first: eggs, fish, meat, tofu, yoghurt or legumes.
  3. Add vegetables and fibre so your window carries you to the next fast 10.

Sources

  1. 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
  2. de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019;381(26):2541-2551. doi.org/10.1056/NEJMra1905136
  3. Rothman DL, Magnusson I, Katz LD, et al. Quantitation of hepatic glycogenolysis and gluconeogenesis in fasting humans with 13C NMR. Science. 1991;254(5031):573-6. doi.org/10.1126/science.1948033
  4. Natalucci G, Riedl S, Gleiss A, et al. Spontaneous 24-h ghrelin secretion pattern in fasting subjects: maintenance of a meal-related pattern. Eur J Endocrinol. 2005;152(6):845-50. doi.org/10.1530/eje.1.01919
  5. Armstrong LE, Ganio MS, Casa DJ, et al. Mild dehydration affects mood in healthy young women. J Nutr. 2012;142(2):382-8. doi.org/10.3945/jn.111.142000
  6. 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
  7. 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
  8. 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
  9. 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
  10. Leidy HJ, Clifton PM, Astrup A, et al. The role of protein in weight loss and maintenance. Am J Clin Nutr. 2015;101(6):1320S-1329S. doi.org/10.3945/ajcn.114.084038
  11. 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.