Short answer: intermittent fasting works as well as a regular calorie deficit for weight loss, but not better. In the largest network meta-analysis to date, covering 99 randomised trials, time-restricted eating such as 16:8 did not produce more weight loss than the same calorie deficit without an eating window. For people who lift, intermittent fasting works when two things are in place: protein at roughly 1.6-2.2 g per kg of body weight per day, and total energy set for your goal. Under those conditions, muscle mass and strength have held up in studies just as well as on a normal meal pattern. Fasting is a way to limit eating, not a fat loss mechanism of its own.
A familiar January routine: black coffee in the morning, the first meal at lunch, and the gym after five. The scale is moving, but the last set of bench press feels heavier than it did before the holidays, and you start to wonder whether fasting is eating your muscle. This guide covers what the research says about exactly this combination, when to schedule your workout and how to fit enough protein into an eight-hour window.
What is intermittent fasting? 16:8, 14:10 and 5:2
Intermittent fasting means limiting when you eat, not what you eat. The most common form is time-restricted eating: all of the day's meals go inside a set window and the rest of the day is a fast. 16:8 means a 16-hour fast and an 8-hour eating window, for example noon to 8 pm. During the fast you drink water, black coffee or tea.
The most common intermittent fasting methods
| Method | Fast | Eating window | For lifters |
|---|---|---|---|
| 14:10 | 14 h | 10 h, e.g. 9 am-7 pm | Easy start: basically no late-night eating and a later breakfast. |
| 16:8 | 16 h | 8 h, e.g. noon-8 pm | The most common method. 3-4 meals fit in the window and training is easy to schedule. |
| 18:6 | 18 h | 6 h, e.g. noon-6 pm | Stricter. Spreading protein gets harder. |
| One meal a day (OMAD) | about 23 h | 1 meal | Tough: the whole day's protein in one meal. |
| 5:2 | 2 days a week with about 70% less energy than usual | 5 days as normal | No daily window. Put heavy sessions on normal days. |
Intermittent fasting does not decide what or how much you eat. If you eat the same amount inside the window that you used to eat across the whole day, your weight won't change. That is the single most important point in this whole topic, and the next section explains why.
Does intermittent fasting work better than a regular calorie deficit?
No. A network meta-analysis by Semnani-Azad and colleagues (2025, BMJ) pooled 99 randomised trials with 6,582 adults. Every fasting approach and a regular calorie deficit reduced weight compared with unrestricted eating, but time-restricted eating did not stand out from a regular calorie deficit. The only fasting approach that beat a continuous deficit was alternate-day fasting, and even that difference was about 1.3 kg.
The longest head-to-head comparison is a one-year trial by Liu and colleagues (2022, New England Journal of Medicine). 139 people with obesity followed the same calorie target, and half of them ate only between 8 am and 4 pm. After a year, the eating window group had lost 8.0 kg on average and the regular deficit group 6.3 kg, and the difference was not statistically significant. Changes in body fat, waist size and lean mass did not differ either.
The TREAT trial (Lowe et al. 2020, JAMA Internal Medicine) shows the other side. 116 adults with overweight or obesity ate between noon and 8 pm for 12 weeks with no calorie counting or other guidance. Weight change did not differ between the groups (a 0.26 kg difference), but in the 50-person in-person cohort, the fasting group lost more appendicular lean mass than the control group. An eating window on its own, without strength training and without paying attention to protein, is not a muscle-friendly way to lose weight.
The practical takeaway: weight loss comes from the calorie deficit, and intermittent fasting is one way to create it. If skipping breakfast is easier for you than shrinking every portion, fasting can work well. If the window makes you binge in the evening, a regular meal pattern is the better choice. Find the size of your deficit with the calorie deficit calculator and your daily burn with the TDEE calculator.
Do you lose muscle with intermittent fasting? Studies on lifters
This question has been tested in trials that combined fasting with supervised resistance training. The results are surprisingly consistent:
Intermittent fasting and resistance training: key studies
| Study | Participants | Design | Result |
|---|---|---|---|
| Moro et al. 2016 | 34 resistance-trained men | 16:8 (meals at 1, 4 and 8 pm) vs. normal pattern, same calories, 8 weeks | More fat loss in the fasting group. Lean mass, arm and thigh muscle and maximal strength maintained in both. |
| Tinsley et al. 2019 | 40 resistance-trained women | Eating noon-8 pm vs. normal pattern, protein 1.6 g/kg, 8 weeks | Lean mass up 2-3% and muscle growth in all groups. Strength improved with no differences. |
| Keenan et al. 2022 | 34 untrained adults | 5:2 fasting vs. 20% daily deficit, protein at least 1.4 g/kg, 12 weeks | Lean mass +3.7% and fat -24% with no group difference, strength gains similar. Muscle cross-sectional area (pQCT) grew more with the daily deficit. |
| Ashtary-Larky et al. 2021, meta-analysis | 8 studies, 221 participants | Fasting + resistance training vs. normal eating + resistance training | Fasting groups lost on average 1.4 kg more fat, no difference in lean mass. |
| Ho et al. 2024, meta-analysis | 7 studies, 164 participants | Time-restricted eating + resistance training vs. resistance training alone | 1.5 kg less fat and about 175 kcal/day lower energy intake. Muscle mass and strength unchanged. |
| Zhang et al. 2026, meta-analysis | 11 studies, 303 participants | Fasting + resistance training vs. resistance training alone | No clear difference in maximal strength, muscular endurance, muscle growth or explosive performance. |
The pattern is clear: when calories and protein are in place, the eating window itself does not eat muscle. The small extra fat loss in the meta-analyses goes hand in hand with lower energy intake. In the data pooled by Ho and colleagues, the fasting groups ate about 175 kcal less per day. In Moro's study, fat loss was greater even with matched calories, but a single 34-person trial doesn't prove a metabolic advantage specific to fasting.
Two honest caveats. In Moro's study, testosterone and IGF-1 dropped in the fasting group, even though muscle and strength were maintained over eight weeks. What that means over a longer period is unknown. The studies are also small and short, most of them 8-12 weeks. A competitive strength athlete should weigh that uncertainty, but for a regular gym-goer the evidence is consistent enough.
When should you train while intermittent fasting?
Workout timing is the most common practical question for people who fast. It has barely been studied head to head, so practice and how the session feels decide. Three options that work:
- Train in the middle of the window, the best fit for most people. For example, a noon to 8 pm window and a workout at 4-5 pm: lunch fuels the session and dinner comes right after. This is the easiest way to combine 16:8 and lifting.
- Train fasted in the morning. This works for many people when it's an hour of lifting rather than a long endurance session. Open your window right after the workout with a protein-rich meal instead of fasting until noon. If you always train in the morning, move the whole window earlier, for example 8 am to 4 pm.
- Train in the evening at the end of the window. A 6 pm workout with a window that closes at 8 pm: make the last meal of the window a solid protein serving after training. According to the ISSN position stand, the effect of exercise on muscle protein building lasts at least 24 hours but fades over time, so don't push your post-workout meal to the next day.
What about performance on an empty stomach? A meta-analysis by Aird and colleagues (2018) pooled 46 studies: eating before exercise improved prolonged endurance performance, but no difference was found for shorter endurance efforts. There is less direct evidence for strength training. Most people can handle an hour of lifting without breakfast, but if your working weights drop or you feel dizzy, move the session inside the window. Drink water before training, because the first fluid of the morning is often just coffee.
SisuLift data: 1 in 20 chooses fasting, and many eat before noon
SisuLift asks everyone for their meal rhythm, and those who pick time-restricted fasting also set their eating window. We looked at the saved choices, anonymised:
- Of 1,330 users who set a meal rhythm, 65, or about 5 percent, chose time-restricted fasting: 16:8 (37 users), one meal a day (15) or 18:6 (13).
- A clear majority eats without a window: 63 percent chose regular meals and 32 percent eat whenever they have time.
- Of the 50 users on 16:8 or 18:6, 26 start eating before noon, 14 at exactly 12 and 10 only in the afternoon. The default window starting at noon is not what most people actually do.
An earlier window is often a smart choice for lifters: a morning or lunchtime workout fits inside it, and the post-workout meal comes naturally. In Liu's one-year trial the window was 8 am to 4 pm, and the result matched a regular calorie deficit. Pick a window that fits your training time and your social life, not a clock time someone else came up with.
Protein in your eating window: 3-4 meals and the right amount
The only real risk fasting poses to muscle is protein distribution: 16 hours without protein is a long gap. The International Society of Sports Nutrition (ISSN) position stand recommends 1.4-2.0 g of protein per kg of body weight per day for people who exercise, up to 2.3-3.1 g for resistance-trained people in a calorie deficit, and spreading intake into 20-40 g servings every 3-4 hours. An eight-hour window fits exactly 3-4 such servings, so distribution works as long as you plan it.
Schoenfeld and Aragon (2018) concluded in their review that a good target for muscle growth is about 0.4 g/kg per meal across at least four meals. The table shows a daily intake of 1.6-2.2 g/kg split into three and four meals:
Protein in your eating window by body weight (1.6-2.2 g/kg per day)
| Body weight | Protein per day | Per meal, 3 meals | Per meal, 4 meals |
|---|---|---|---|
| 60 kg | 96-132 g | 32-44 g | 24-33 g |
| 70 kg | 112-154 g | 37-51 g | 28-39 g |
| 80 kg | 128-176 g | 43-59 g | 32-44 g |
| 90 kg | 144-198 g | 48-66 g | 36-50 g |
| 100 kg | 160-220 g | 53-73 g | 40-55 g |
With three meals the servings get large, especially for heavier people. That's why on 16:8 it pays to add a fourth protein serving, for example Greek yogurt or quark after training or cottage cheese as the last meal of the window. On 18:6 and one meal a day this becomes hard, which is a good reason for lifters to prefer 16:8 or 14:10. Get your own target from the protein calculator, and the background is in the guide how much protein per day.
Sample day: 16:8 with a 4:30 pm gym session
Take an 80 kg lifter who is cutting fat. The target is about 160 g of protein (2.0 g/kg) and a calorie deficit, with an 11 am to 7 pm window and an afternoon workout:
- 7:00-11:00: water, black coffee or tea.
- 11:00 lunch, about 45 g protein: chicken, fish or beans, rice or potatoes and plenty of vegetables.
- 14:30 snack, about 30 g protein: Greek yogurt or quark, berries and oats. The carbs fuel your workout.
- 16:30-17:30 gym session.
- 17:45 dinner, about 50 g protein: for example beef mince sauce with pasta or tofu with rice, vegetables on top.
- 18:45 last meal before the window closes, about 35 g protein: cottage cheese and rye bread or a protein shake.
The same structure works for morning lifters by moving the window to 7 am to 3 pm: train before work and eat breakfast right after. The rule is the same in every version: make the first and last meals of the window protein-rich, because the longest gap of the day sits between them.
Who should not try intermittent fasting
According to Terveyskirjasto, the Finnish medical library by Duodecim, fasting is not suitable for most people with chronic conditions such as diabetes or heart disease, or for people who are pregnant or breastfeeding, underweight, have or have had an eating disorder, children or older adults. From a lifter's point of view, the most important ones are:
- A history of eating disorders. A strict window and watching the clock can reinforce restrictive eating. If eating has been difficult for you, skip fasting.
- Diabetes and glucose-lowering medication. Especially with insulin, fasting can push blood sugar too low, and Terveyskirjasto states fasting is not suitable for type 1 diabetes. Change your meal pattern only together with your doctor.
- Pregnancy and breastfeeding. Energy and protein needs go up, and there is no reason to fast at this stage.
- Children and growing teenagers.
- Very high training volume or a competition season. If you train twice a day, getting enough energy into a narrow window is hard.
If you have a chronic condition or take regular medication, talk to your doctor before you start.
Intermittent fasting in SisuLift: what the app does and doesn't do
SisuLift asks for your meal rhythm when you start (for example regular meals, 16:8, 18:6, one meal a day or 5:2) and, for time-restricted fasting, your eating window. You can change both later under Profile → Goal & limits → Diet & meals.
The information doesn't just sit in settings. The Grind coach sees your meal rhythm and window when you ask it about nutrition or workout timing, and the AI chef fits recipes to your eating window. Your calorie and protein targets are calculated from your profile: for fat loss, the starting point is about a 20 percent calorie deficit and 2.2 g of protein per kg of body weight, and the food diary shows how much of the day's protein is still missing. On the training side, automatic progression adjusts your working weights based on how your last session went, so you can quickly see whether your strength is holding up in a deficit.
What the app doesn't do: there is no fasting timer, and the nutrition view doesn't show your eating window. Your phone's clock is enough for a timer. Basic nutrition tracking and workout logging are free, the Grind chat includes 3 free messages a month, and the AI chef is part of Premium.
Sources
- Semnani-Azad Z et al. (2025). Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials. BMJ 389:e082007.
- Liu D et al. (2022). Calorie restriction with or without time-restricted eating in weight loss. New England Journal of Medicine 386:1495-1504.
- Lowe DA et al. (2020). Effects of time-restricted eating on weight loss and other metabolic parameters in women and men with overweight and obesity: the TREAT randomized clinical trial. JAMA Internal Medicine 180:1491-1499.
- Moro T et al. (2016). Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. Journal of Translational Medicine 14:290.
- Tinsley GM et al. (2019). Time-restricted feeding plus resistance training in active females: a randomized trial. American Journal of Clinical Nutrition 110:628-640.
- Keenan SJ et al. (2022). Intermittent fasting and continuous energy restriction result in similar changes in body composition and muscle strength when combined with a 12 week resistance training program. European Journal of Nutrition 61:2183-2199.
- Ashtary-Larky D et al. (2021). Effects of intermittent fasting combined with resistance training on body composition: a systematic review and meta-analysis. Physiology & Behavior 237:113453.
- Ho Y et al. (2024). Synergistic effects of time-restricted feeding and resistance training on body composition and metabolic health: a systematic review and meta-analysis. Nutrients 16:3066.
- Zhang M et al. (2026). Effects of intermittent fasting combined with resistance training on training adaptations: an exploratory multilevel meta-analysis. Frontiers in Nutrition 13:1879031.
- Aird TP, Davies RW & Carson BP (2018). Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: a systematic review and meta-analysis. Scandinavian Journal of Medicine & Science in Sports 28:1476-1493.
- Jäger R et al. (2017). International Society of Sports Nutrition position stand: protein and exercise. Journal of the International Society of Sports Nutrition 14:20.
- Schoenfeld BJ & Aragon AA (2018). How much protein can the body use in a single meal for muscle-building? Implications for daily protein distribution. Journal of the International Society of Sports Nutrition 15:10.
- Hussein I (2023). Paasto (Fasting). Terveyskirjasto, Duodecim. In Finnish.
→ Read also: How much protein per day | Calorie deficit calculator