Intermittent fasting 16:8: schedules, what breaks a fast and what the studies say

What 16:8 fasting is (and what it is not)

16:8 is a form of time-restricted eating: you eat everything for the day within an 8-hour window and spend the other 16 hours with nothing that has calories. It does not tell you what to eat, only when. In practice almost everyone skips either breakfast or dinner, and since you sleep 7 or 8 hours, half of the fast happens in bed. A 2021 review from Krista Varady's group (Annual Review of Nutrition) summarises what the different forms of intermittent fasting achieve in trials: losses of 1 to 8% of starting weight and an intake that drops by 10 to 30%. Put another way: when it works, it works because you end up eating less, not because of some special effect of the clock.

Example schedules for your shift

The noon to 8 pm window is the most studied because it is the one used by trials such as TREAT (Lowe, 2020) and Lin (2023), but there is nothing magic about it: the best window is the one you can repeat almost every day. These are example schedules; fine-tune the exact hours with the intermittent fasting calculator. If your shift changes every week, bear in mind that the only solid data on long shifts come from 137 firefighters working 24-hour shifts: with a 10-hour window (not 8) for 12 weeks, they cut their real eating window from about 14 to about 11 hours with no adverse effects (Manoogian, Cell Metab 2022). On rotating shifts, 14:10 or 12:12 may be a more realistic goal than 16:8.

What breaks a fast and what does not

For the goal almost everyone has (losing weight or eating less), the practical question is whether something has calories. Whether a sweet taste or 10 kcal "breaks" some hormonal effect has not been measured directly in trials, so here we separate what is known from what is not:

• Does not break it: water (still or sparkling), black coffee, unsweetened tea and herbal tea. Brewed coffee has about 1 kcal per 100 ml (USDA FoodData Central, FDC 171890): a large cup is about 2 kcal. EFSA considers up to 400 mg of caffeine a day to raise no safety concerns for healthy adults, and up to 200 mg during pregnancy and breastfeeding
• Grey zone (does not break your deficit, but technically it is no longer a fast): a splash of milk in your coffee (20 ml of whole milk is about 12 kcal, FDC 171265) and drinks or gum with calorie-free sweeteners. There is no firm evidence that sweeteners stop you losing weight on 16:8; if you notice the sweet taste opens your appetite, save them for the window
• Does break it: any food, juices (green ones included), smoothies, plant drinks or milk in any real amount, a latte or a cappuccino, sugary soft drinks, alcohol and supplements with calories, such as protein powder or collagen
• Medication: if a treatment has to be taken with food or at a set time, do not move it because of the fast. The window adapts to the treatment, never the other way round, and it is worth checking with your doctor first

What the trials actually show

16:8 is sold as if it had something special beyond eating less. The randomised trials that compare it with normal meal timing or with good old calorie restriction tell a different story:

• TREAT (Lowe, JAMA Intern Med 2020): 116 adults with overweight or obesity for 12 weeks, eating whatever they wanted from noon to 8 pm, versus three meals a day. The 16:8 group lost 0.94 kg and the control group 0.68 kg, with no significant difference between them. In the subgroup whose body composition was measured, 16:8 lost 1.10 kg of lean mass and 0.51 kg of fat (the latter not significant): about two thirds of what was lost was not fat
• Liu (N Engl J Med 2022): 139 people with obesity for 12 months, all on the same diet of 1,500-1,800 kcal (men) or 1,200-1,500 kcal (women); half of them also ate only between 8 am and 4 pm. They lost 8.0 kg with the time window and 6.3 kg without it, a 1.8 kg difference that was not statistically significant. Adding the window to the diet added nothing clear
• Lin (Ann Intern Med 2023): 90 adults with obesity for 12 months. Eating from noon to 8 pm without counting calories versus cutting calories by 25%. Compared with the control group, 16:8 lost 4.61 kg and calorie restriction 5.42 kg, with no significant difference between them. Both cut intake by about 400 kcal a day (425 and 405 kcal)
• Cienfuegos (Cell Metab 2020): with even shorter windows of 4 and 6 hours, people ate about 550 kcal less a day without counting anything and lost around 3% of their weight in 8 weeks. More fasting hours translate into a bigger spontaneous cut, not into a different effect
• Meta-analysis (Semnani-Azad, BMJ 2025): 99 trials and 6,582 adults. Every form of fasting beats eating without any control, but 16:8-style time-restricted eating does not outperform continuous calorie restriction. Only alternate-day fasting gained a small edge (1.29 kg), and in trials lasting 24 weeks or more that edge was no longer there
• At the same calories, fasting does not burn more: in the Templeman trial (Sci Transl Med 2021), in lean adults, alternate-day fasting with the same deficit as daily restriction led to less fat loss (0.74 kg versus 1.75 kg in 3 weeks)

What about the study linking it to cardiovascular deaths?

In March 2024 an abstract presented at an American Heart Association meeting (abstract P192, not peer reviewed at the time) linked eating within less than 8 hours a day to a 91% higher risk of cardiovascular death, and, among people who already had cardiovascular disease, eating within 8 to 10 hours to a 66% higher risk. In 2025 it was published as a paper in Diabetes & Metabolic Syndrome: with 19,831 adults from the NHANES survey and a median follow-up of 8.1 years, the association with cardiovascular mortality held (HR 2.35 compared with eating over 12-14 hours), but not with overall mortality. Be clear about what it is: an observational study in which the window was calculated from two days of dietary recall, with no way of knowing whether people were fasting on purpose or what they ate. The authors themselves say it is unclear whether the risk comes from the short window or from other factors. It does not prove that 16:8 causes anything, but if you already have a heart condition, it is one more reason to ask your doctor first.

What it may offer beyond weight (and what it does not)

Beyond the scale, these are the effects attributed to 16:8 and what lies behind each one:

• Blood pressure and insulin sensitivity: the Varady review (2021) reports improvements in blood pressure, insulin resistance and oxidative stress. But the most cited study (Sutton, Cell Metab 2018) was not 16:8: it was a 6-hour window with dinner before 3 pm, for 5 weeks, in men with prediabetes, and they were fed just enough NOT to lose weight, precisely to isolate the effect of timing. It is a clue about eating early, not proof that a noon to 8 pm 16:8 does the same
• Autophagy: Yoshinori Ohsumi won the 2016 Nobel Prize in Medicine for uncovering its mechanisms, studied in yeast. What nobody knows is how many hours of fasting people need for a clinically meaningful effect: the "16-18 hours" figure that gets passed around does not come from human trials. Treat it as a hypothesis, not as a reason to fast
• Metabolism: fasting does not "speed it up". In TREAT resting energy expenditure did not change, and in the Templeman trial fasting did not burn more fat than daily restriction with the same deficit
• The practical part: fewer food decisions each day. For many people it is easier to say "I do not eat before noon" than to weigh portions, and that is its real value

Common side effects

In trials the side effects of 16:8 are few and mild. A 2025 meta-analysis in women with overweight (13 trials, 612 participants) found only minor complaints, mostly hunger and headache, that resolved on their own, and the firefighter trial reported no adverse effects. The Varady review (2021) also found no energy slumps and no increase in disordered eating behaviours. What is worth watching:

• Hunger and headaches in the first weeks: they usually fade. If they do not, widen the window (14:10 or 12:12) rather than gritting your teeth
• Overeating when the window opens: in TREAT the 16:8 group did not eat less than the control group according to estimated intake, and that is why it did not lose more
• Losing muscle: that is what happened in TREAT, with no protein or exercise guidance. In resistance-trained men eating their maintenance calories on 16:8, lean mass and maximal strength were maintained over 8 weeks while fat went down (Moro, J Transl Med 2016)
• Performance: if you train hard or long, place the session inside the window or close to a meal rather than after 16 hours without food

How to start in 3 weeks

If you currently eat every 3 hours, do not jump straight to a 16-hour fast. Hunger tends to show up at the times you are used to eating, and narrowing the window gradually usually makes it easier to bear:

• Week 1, close the kitchen after dinner: no snacking after 9 pm. If you eat dinner at 8 pm and breakfast at 8 am, you are already doing a 12-hour fast without effort
• Week 2, push breakfast back to 10 or 11 am. In the morning, water, black coffee or unsweetened tea
• Week 3, reach 16:8: first meal at noon and the last one before 8 pm. Have 2 main meals and, if you like, a snack inside the window
• After that: check the scale and your food log at 3 or 4 weeks. If nothing has changed, the window is not reducing what you eat

Other protocols (16:8 is not the only one)

16:8 is the most popular, but not the only option. The best one is whichever fits your life, because for weight loss the differences between them are small:

• 14:10 and 12:12: you fast for 14 or 12 hours. In practice it means no more late-night snacking. It is the easiest starting point and the most realistic option on changing shifts
• 18:6 and 20:4: shorter windows. They cut intake more (like Cienfuegos's 4- and 6-hour windows), but they are more demanding and have not been shown to cause more weight loss than an equivalent calorie restriction
• OMAD (one meal a day): barely studied and hard to fit enough protein into. Not recommended for beginners or for anyone prone to eating disorders
• 5:2: you eat normally on 5 days and drop to 500-600 kcal on 2 non-consecutive days. It was popularised by the journalist and doctor Michael Mosley. Some people find it easier than restricting every day

The mistakes that ruin 16:8

• Compensating inside the window: skipping breakfast is not a licence to eat 3,000 kcal across lunch and dinner. Eat the same amount in less time and you will not lose weight
• Obsessing over the exact time: if one day you eat at 11:30 instead of noon, nothing happens. Flexibility is what lets you keep it up for months
• Forgetting protein: with two meals a day it is easy to fall short, and without protein or strength training part of what you lose is muscle
• Drinking too little: part of each day's water comes with food; if you skip a meal, drink a bit more in the morning
• Thinking fasting makes up for a poor diet: the quality of what you eat in the window still matters (vegetables, protein, healthy fats, whole-grain carbohydrates)

Who should not do 16:8 (or only with their doctor)

This is general information, not medical advice. Check with a doctor before starting if any of these apply to you:

• Pregnancy or breastfeeding: fasting trials usually exclude these women, so there are no data to back its safety
• Diabetes treated with insulin or with drugs that can lower blood glucose (such as sulfonylureas): fasting without adjusting the medication can cause hypoglycaemia. A clinical review (Grajower and Horne, Nutrients 2019) notes that the evidence in diabetes is almost non-existent and that, if it is done, it should be under medical supervision with glucose monitoring
• A history of eating disorders (anorexia, bulimia, binge eating): rigid rules about when to eat can trigger them again. Trials in the general population see no rise in disordered eating, but these people are usually excluded from them
• Under-18s, people who are underweight (BMI below 18.5) and anyone taking medication that must go with food at fixed times
• Existing cardiovascular disease: because of the observational study in the section above, ask first