16/8 Fasting: Who It Suits, When to Eat and What the Trials Measured

16/8 fasting means eating all of your meals within an 8-hour window, then taking in nothing for the remaining 16 hours. Since 2018, about ten randomized trials have measured its effects on weight, insulin and blood pressure in adults who were overweight, diabetic or healthy. The results are more nuanced than the promises circulating on social media: weight loss is real but modest, and the timing of the window changes a great deal. Here is who stands to benefit, when to eat and what the numbers actually show.

16/8 fasting: definition and place among intermittent fasting methods

The 16/8 pattern belongs to the family of time-restricted eating (TRE), where only the timing of meals is constrained, never their content. Someone who eats a first meal at 10 a.m. finishes the last one by 6 p.m., then drinks only water, tea or unsweetened coffee until 10 a.m. the next day. The night’s sleep counts toward the 16 hours, which makes the protocol easier to sustain than alternate-day fasting or the 5:2 approach. The review published in Endocrine Reviews by Manoogian et al. (2022) points out that most adults spread their eating over more than 12 hours a day: moving to 8 hours cuts daily nutrient exposure time by a third.

The 16/8 rule says nothing about what goes on the plate, and that is both its strength and its limit. It combines with any eating pattern, Mediterranean or otherwise, but it does not fix an unbalanced diet. The other intermittent fasting formats rest on different logics, detailed in our complete guide to intermittent fasting. The table below places them side by side.

How 16/8 fasting acts on metabolism

The 8-hour window works first through a spontaneous drop in calories eaten, then through a longer daily period in which the body draws on its reserves. These two effects add up and explain most of the results observed. The mechanisms measured in humans are the following:

• Fewer calories, no counting. In practice, the 8-hour window eliminates evening snacking. The measured reduction reached 350 kcal a day in the study by Gabel et al. (2018), 425 kcal a day in Lin (2023) and 313 kcal a day in people with type 2 diabetes (Pavlou 2023).
• A shift toward fat. After 12 to 14 hours with no intake, the liver has used part of its glycogen and the body mobilizes more fatty acids, with moderate production of ketone bodies. This daily shift maintains metabolic flexibility.
• Better insulin sensitivity. The crossover trial by Sutton et al. (2018), run at constant weight in 8 prediabetic men, shows improved insulin sensitivity and beta-cell response with a 6-hour window, with no weight loss at all. The effect therefore does not depend solely on calorie balance.
• Alignment with the body clock. Insulin sensitivity peaks in the morning and declines in the evening, as melatonin rises. Eating early in the day respects this rhythm, a topic covered in our article on the circadian clock.
• More active autophagy? In mice, time-restricted eating stimulates cellular recycling. In humans, only indirect markers have been measured after 16 hours of fasting: the effect on longevity remains a hypothesis, not a result.

What randomized trials say: the measured effects of 16/8 fasting

In adults with obesity, the 8-hour window produces a 3 to 5% loss of starting weight over 3 to 12 months, a result equivalent to classic calorie restriction, not superior to it. The reference trial remains the one by Liu et al. (2022), published in the New England Journal of Medicine: 139 adults with obesity followed calorie restriction for 12 months, with or without an 8-hour window (8 a.m. to 4 p.m.). Weight loss reached 8.0 kg (17.6 lb) with the window versus 6.3 kg (13.9 lb) without, a non-significant difference. Waist circumference, fat mass, blood pressure and metabolic markers evolved the same way in both groups.

Without calorie counting, the method still holds up against no intervention at all. In the trial by Lin et al. (2023), 90 adults with obesity were assigned to a noon to 8 p.m. window with no calorie instructions, a 25% calorie restriction, or a control group. At 12 months, the window group had lost 4.61 kg (10.2 lb) more than the control group, and the restriction group 5.42 kg (11.9 lb) more. The gap between the two interventions (0.81 kg, or 1.8 lb) was not significant. Time-restricted eating therefore delivers the same result as a counted diet, with a rule that is easier to remember.

Two results call for caution. In the TREAT trial by Lowe et al. (2020), 116 overweight adults followed a noon to 8 p.m. window for 12 weeks with no support: weight loss (0.94 kg, or 2.1 lb) did not differ from the three-meals group (0.68 kg, or 1.5 lb), and appendicular lean mass fell more in the fasting group. In resistance-trained men, by contrast, Moro et al. (2016) observed after 8 weeks of 16/8 (1 p.m. to 9 p.m.) a 16.4% drop in fat mass with muscle mass and strength preserved, but also a decrease in testosterone and IGF-1.

In people with type 2 diabetes, the trial by Pavlou et al. (2023) in 75 patients compared, over 6 months, a noon to 8 p.m. 16/8 fast, calorie restriction and a control group. Weight fell by 3.56% in the window group and glycated hemoglobin by 0.91 points, a result identical to calorie restriction, with no severe hypoglycemia reported. This type of protocol nonetheless requires close medical follow-up to adjust treatments.

When should the 16/8 fasting window fall?

An early window, closed by mid-afternoon, produces better metabolic effects than a window shifted toward the evening, at equal calories. In the trial by Xie et al. (2022), published in Nature Communications, 82 healthy adults without obesity were assigned to a 6 a.m. to 3 p.m. window, an 11 a.m. to 8 p.m. window or a control group for 5 weeks. Only the early window improved insulin resistance, fasting glucose and inflammation markers. The midday window did not produce these effects.

The same gap appears with the 16/8 protocol in adults with obesity. Jamshed et al. (2022) compared, over 14 weeks, a 7 a.m. to 3 p.m. window with a window of 12 hours or more, both groups following the same calorie restriction. The early window produced an extra 2.3 kg (5.1 lb) of weight loss, the equivalent of 214 fewer kcal a day, lowered diastolic blood pressure by 4 mmHg and improved mood and fatigue. Conversely, the two late-window trials run with no support (Lowe 2020, noon to 8 p.m.) gave the weakest results in the entire time-restricted eating literature.

The reason lies in the internal clock. Insulin secretion and glucose tolerance are better in the morning, then decline in the evening as melatonin rises. A late dinner therefore arrives when the body handles sugars least well. In France, where dinner rarely starts before 7:30 p.m., a realistic compromise is to open the eating window around 11 a.m. and close it at 7 p.m., by moving dinner earlier rather than skipping breakfast.

Who should try 16/8 fasting, and who should not?

16/8 fasting is aimed first at adults who are overweight or obese, prediabetic or living with metabolic syndrome, and who cannot sustain calorie counting. These are the populations in which trials show weight loss and improved metabolic markers. In 19 patients with metabolic syndrome already treated with statins or antihypertensives, Wilkinson et al. (2020) observed, with a 10-hour window only, a 3% drop in weight, waist circumference and blood pressure in 12 weeks. The 14/10 format is often enough to start. This link between meal timing, insulin and excess weight explains why the topic falls under the deregulated nutrient sensing pillar, one of the mechanisms of aging.

The profiles that gain the most from a 16/8 schedule share three features: frequent evening snacking, a spontaneous eating window longer than 14 hours and signs of insulin resistance (large waist circumference, borderline fasting glucose, high triglycerides). For a normal-weight person who already eats within 10 to 12 hours, the expected gain is small, unless they adopt an early window as in the Xie (2022) trial.

The practice is not advised, or should be reserved for medical supervision, in the following situations:

• Diabetes treated with insulin or sulfonylureas: risk of hypoglycemia, doses must be adjusted by the physician.
• Pregnancy, breastfeeding, children and growing adolescents.
• History of an eating disorder: time restriction can reactivate a restriction-binge cycle.
• Underweight, malnutrition or progressive chronic disease.
• Frail older adults: the pilot study by Anton et al. (2019) in 10 adults over 65 shows a 2.6 kg (5.7 lb) loss in 4 weeks with no way of knowing how much came from muscle, a critical point given sarcopenia.
• Medications that must be taken with a meal, at fixed times.

The review of intermittent fasting by Harvard’s Nutrition Source reaches a similar conclusion: the human data do not establish a proven preventive or therapeutic effect, and people with a medical condition need supervision. The trials published since then clarify the effects on weight and insulin, but none has measured an effect on mortality or lifespan.

Protocol: 7 tips to practice 16/8 fasting without mistakes

16/8 fasting only pays off if it is sustained for several months, and the most common mistakes concern the content of the window more than the fast itself. The trials that worked share the following rules:

• Start with 12 hours, then 14, then 16. Wilkinson’s (2020) participants got results with a 10-hour window: no need to force 16 hours in the first week.
• Close the window as early as possible. Moving dinner earlier brings more than delaying breakfast. Aim to finish eating by 7 p.m. at the latest, 3 hours before bedtime.
• Keep the same window every day. In the trials, participants adhered 6 days out of 7. Schedules that change daily desynchronize the internal clock.
• Reach 1.2 to 1.6 g of protein per kilogram of body weight a day (0.55 to 0.73 g per pound). Two meals are enough, provided each contains 30 to 40 g of protein to limit the lean mass loss reported in the TREAT trial.
• Do not compensate. An 8-hour window filled with ultra-processed foods cancels out the calorie effect. A Mediterranean plate remains the foundation, fasting or not.
• Drink during the fast. Water, tea, coffee with no sugar or milk, herbal teas. Sweetened drinks were tolerated in the trials but keep the taste for sweetness alive.
• Maintain muscle. Two strength sessions a week, placed preferably within the window or just before the first meal, protect lean mass and strength, as in Moro (2016).

One signal to watch: dizziness, persistent fatigue, sleep problems or binge urges at the end of the window indicate that the protocol is not right for you, or not yet. In that case, go back to 12 hours and see a doctor.

Frequently asked questions about 16/8 fasting

How much weight do you lose with 16/8 fasting?

In randomized trials, 16/8 fasting produces a 3 to 5% loss of starting weight over 3 to 12 months in adults with obesity, or 3 to 5 kg (6.6 to 11 lb) for a 100 kg (220 lb) person. Lin (2023) measured 4.61 kg more than the control group in one year, and Liu (2022) 8.0 kg when the window is added to calorie restriction. With no change to what is on the plate, the effect remains modest.

When should the 16/8 fasting window start and end?

The most effective window ends early: 7 a.m. to 3 p.m. or 8 a.m. to 4 p.m. in the Jamshed (2022) and Sutton (2018) trials, which obtained the best results on insulin and blood pressure. A noon to 8 p.m. window remains effective compared with no intervention but loses part of the metabolic benefits. The most realistic compromise for a working adult sits between 11 a.m. and 7 p.m.

Does 16/8 fasting make you lose muscle?

It can if protein intake drops. The TREAT trial (Lowe 2020) observed a decrease in appendicular lean mass with a late window and no nutritional advice. By contrast, in trained men who kept up their protein and strength training (Moro 2016), muscle mass and strength were preserved. The safeguard comes down to two points: 1.2 to 1.6 g of protein per kilogram of body weight and two strength sessions a week.

What is the difference between 16/8 fasting and classic calorie restriction?

Calorie restriction sets a quantity (for example 25% below energy needs) with no time constraint. 16/8 fasting sets a schedule with no quantity constraint. Over 12 months, the two approaches give the same weight loss (Liu 2022, Lin 2023), because the window spontaneously cuts intake by 300 to 450 kcal a day. Its advantage is a single rule, easier to follow than daily counting.

Can you drink coffee during 16/8 fasting?

Yes, as long as it has no sugar and no milk. Black coffee, tea and herbal teas provide virtually no calories and do not trigger any notable insulin secretion. They were allowed during the fasting period in most trials, including those by Lin (2023) and Pavlou (2023). A coffee with milk or sugar, on the other hand, breaks the 16/8 fast in the metabolic sense.

Is 16/8 fasting suitable after 60?

With caution. After 60, the priority is to preserve muscle, and the data in older adults are limited to a pilot study of 10 people (Anton 2019) that did not measure body composition. A 10- to 12-hour window, two protein-rich meals, strength training and medical advice for anyone on treatment form a reasonable framework. Any unwanted weight loss should lead to stopping the practice.