Intermittent Fasting on a 75-Day Challenge
How to run 16:8 through a 75-day challenge with two-a-day workouts: real eating window schedules, a protein floor, and the signs you're underfueling.
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It’s Day 12. You finished the outdoor workout at 6am on an empty stomach, your window opened at noon, and the second session is at 5pm. The window shuts at 8pm and you have eaten twice. Something is going to give, and it probably isn’t the schedule.
Quick note before the numbers: this is general information, not medical advice. Fasting plus two daily workouts is a real physiological load, and if you’re pregnant or breastfeeding, managing diabetes on medication, taking anything that has to be dosed with food, or you have any history of disordered eating, this combination is not for you. Talk to a clinician before you start. That last one matters more than people admit: a rigid eating window bolted onto a no-cheat-meal rule with a restart penalty is a structure that can make disordered patterns worse.
Whether fasting is “allowed” was never the interesting question. The program (75 Hard, created by Andy Frisella) doesn’t hand you a meal plan. It tells you to pick a diet and hold it for 75 days with no cheat meals and no alcohol. Fasting qualifies. (If you haven’t picked yet, start with our 75-day challenge diet plan guide instead of this post.)
The real question is whether you can fuel 90 minutes of daily training inside an eight-hour window for 75 straight days. That has an answer, and it’s mostly arithmetic.
The two conflicts, and only one of them is real
Search “75 Hard intermittent fasting” and you’ll find the same two objections repeated everywhere: the gallon of water and the two workouts won’t fit in an eight-hour window. One of those is a misunderstanding.
Plain water does not break a fast. It has no calories and triggers no insulin response, so the gallon spreads across all 24 hours. In practice the fasting block is the best place to put most of it, because you’re awake for hours with nothing else to consume and you’re not competing with food volume at meals. Most people find the morning fast is where three quarters of the gallon disappears without effort. (More tactics in our gallon of water a day guide, and you can log it with the free gallon water tracker.)
One caveat: drink to thirst rather than hitting a clock. Forcing volume past thirst is the main driver of exercise-associated hyponatremia, and the fasted state is when you’re least likely to be replacing sodium. Get electrolytes with your meals.
The workout objection is also weaker than it looks. A small 2025 randomized clinical trial (28 young adults, twice-weekly sessions) compared 12 weeks of resistance training performed fasted against the same training performed fed, with daily energy intake matched. Quadriceps thickness rose 1.21 cm in the fasted group and 1.18 cm in the fed group. Maximal strength and training workload came out the same. Training fasted is not what breaks this.
The squeeze is total calories and total protein, compressed into three feedings. You need a full day of energy and a full day of protein on a day where you burned 90 minutes of training, and you have eight hours to do it in. Everything below is about solving that.
Three schedules that hold up
Nobody writing about this gives you a clock, so here are three, each with the trade-off stated plainly. All assume two 45-minute sessions at least three hours apart, one of them outdoors.
Fasted-first: window noon to 8pm
Workout one at 6am, fasted, outdoors. Workout two around 5pm, inside the window and about 90 minutes after your second meal. Meals at 12pm, 3:30pm and 7:30pm.
Best for early risers who want the evening free of training pressure. The failure mode is a small first meal: if noon is a salad, you have six hours left to make up an entire day of food and you won’t. The 12pm meal should be the biggest of the three.
Split-straddle: window 11am to 7pm
Meal one at 11am, workout one at 11:30am, meal two at 2pm, workout two at 5:30pm, meal three at 6:45pm.
Both sessions are fed, which is the setup if you’re lifting heavy or doing hard intervals and you care about session quality. The catch is obvious: two midday-ish training blocks are hard to fit around a standard workday. This one works for shift workers, remote workers and anyone with a flexible lunch.
Early-window: window 8am to 4pm
Workout one at 7am fasted, break the fast at 8am immediately after, meal two at noon, workout two at 3pm, meal three at 3:45pm.
Sleep quality tends to be better with an earlier last meal, and your evenings are completely free. The downside is social: you’re done eating before most people start dinner, which makes 75 days of that window awkward.
The rule of thumb across all three: put the harder session inside the eating window and the easier one in the fasted block. Heavy lifting and intervals go where the fuel is. The outdoor walk, easy cardio and mobility work go fasted.
Two tools help here. The fasting timer runs your window so you stop doing mental math at 4pm, and the two-a-day workout planner will space your sessions around it. Our two-a-day workout schedule guide covers session spacing in more detail, and the intermittent fasting calculator will map window options against your wake time.
The protein floor that decides whether this works
A 2018 meta-analysis pooling 49 randomized trials and 1,863 participants found that training-induced gains in fat-free mass plateau at roughly 1.6 g of protein per kg of body weight per day. Past that, more protein bought nothing.
When you’re training twice a day inside a compressed window, treat 1.6 g/kg as the floor, not the ceiling to aim near.
Do the arithmetic out loud. An 80 kg (176 lb) person needs about 130 g of protein. Split across three feedings in eight hours, that’s 43 g per meal. Run your own number through the protein intake calculator.
What does 43 g look like on a plate?
- 6 oz of chicken breast (about 50 g)
- 6 whole eggs, or 3 whole eggs plus 5 whites (about 36 g)
- 200 g of Greek yoghurt plus a scoop of whey (about 45 g)
- 6 oz of salmon plus a cup of lentils (about 52 g)
- 8 oz of lean ground beef (about 46 g)
None of these are snacks. You cannot graze your way to 130 g in an eight-hour window. Three substantial feedings is the only shape that works, and each one has to be built around a protein anchor you decided on in advance. Compressed windows make meal prep more important, not less, because there is no room to improvise a 43 g meal at 3pm.
Getting the calories in, where most people slip
Time-restricted eating works largely by producing an involuntary calorie deficit. You eat less because you have less time to eat. That’s the mechanism, and on a 75-day program with 90 minutes of daily training, that mechanism can overshoot badly.
A 2025 randomized controlled trial (54 participants, 10-hour window, 8 weeks) split this cleanly. Time-restricted eating alone cost participants 2.3 kg of fat-free mass. Time-restricted eating combined with resistance training held fat-free mass essentially stable (+0.2 kg) while dropping 3.2 kg of fat mass.
The lifting is what protects your muscle, and it is also what made the fat loss happen. Time-restricted eating on its own didn’t move fat mass to any meaningful degree in that trial (−0.3 kg, not statistically significant); it just cost lean tissue. You need both running at once. Drop the resistance training and the window starts eating your muscle instead of your fat.
Most trials in this area run 8 to 12 weeks. A 75-day challenge is 10.7 weeks, right at the edge of that evidence, and a deficit that’s fine for a month can quietly compound into something else by Day 60.
Tactics that move calories in a short window:
- Front-load the first meal. It should be the largest of the three, not a light “breaking the fast” gesture.
- Use calorie-dense whole foods. Olive oil, whole eggs, nuts, avocado, rice, potatoes. A tablespoon of olive oil on your first two meals is 240 calories you won’t notice eating.
- Put liquid calories at the window edges. A smoothie with whey, oats, nut butter and milk lands 600 calories in three minutes when solid food won’t fit.
- Don’t clean-eat your way into a 1,400-calorie day. This is the single most common accident. Chicken and vegetables three times is a beautifully compliant day and nowhere near enough fuel for two workouts.
Get a baseline from the TDEE calculator and add your training expenditure on top. If you land more than about 500 calories under that for weeks on end while training twice daily, something is going to break.
Warning signs the pairing is too aggressive
The useful lens is low energy availability: the energy left over for your body’s basic functions after training takes its cut. Roughly 30 kcal per kg of fat-free mass per day is the commonly cited threshold, though current consensus treats it as a spectrum with individual variation rather than a hard line. Below it, reproductive, bone, immune and metabolic function start to degrade.
You don’t need to calculate it. You need to notice these:
- Workout performance sliding for seven or more consecutive days. Weights down, pace down, sessions dragging.
- Fatigue that sleep doesn’t fix. Waking up unrefreshed most mornings.
- Menstrual cycle changes: skipped, late, or noticeably lighter periods.
- Getting sick repeatedly, or small injuries that won’t resolve.
- Feeling cold all the time.
- Flat mood, irritability, poor concentration.
- Disrupted sleep and intrusive thoughts about food.
One or two of these in a rough week is noise. Several of them together for two weeks is a signal, and the answer is not to tough it out. Widen the window to ten hours. Add a fourth feeding. Raise calories.
Adjusting your eating window does not restart your challenge. The rule is that you follow a diet, not that your diet is frozen on Day 1. Moving from 16:8 to 14:10 because you’re underfueling is a deliberate, written revision to your own protocol, which is a completely different thing from an unplanned cheat meal. This scares people more than it should. On the diet side, what restarts you is eating outside your chosen diet or drinking alcohol. Missing any of the program’s other daily tasks sends you back to Day 1 too.
If symptoms stick around after you’ve eaten more for a couple of weeks, see a clinician. Our post on whether the 75-day challenge is healthy covers the wider risk picture.
Making the window survive 75 days
The physiology is solvable. The logistics are what end most attempts.
A fixed eating window plus two workouts is a schedule with no slack in it. A 7am meeting eats workout one. A delayed flight puts you at the gate at 6pm with your window closing at 7. A friend’s birthday dinner starts at 9pm on Day 41.
Decide the fallback now, before it happens: shift the window, don’t skip the food. Moving your eight hours later on a travel day costs you nothing. Skipping a feeding costs you 43 g of protein and 700 calories you cannot get back tomorrow.
There’s also the tracking load. Two workouts, a gallon of water, window adherence, protein, plus the reading and the photo, every day, for 75 days. That’s a lot to hold in your head at 9pm when you’re tired, and the thing you forget to log is usually the thing you forget to do. Reset75 checks all of it off in one screen, with a per-task reminder you can set for the top of your eating window so the first meal never gets skipped. Its statistics dashboard breaks compliance out by category, so a nutrition slide shows up around Day 20 instead of ambushing you at Day 60.
And if you read the warning signs section and already decided that 75 days of 16:8 plus two-a-days is more than you want to sign up for, that’s a reasonable read. Build a custom challenge with a shorter duration or a wider window instead. A protocol you finish beats a perfect one you abandon on Day 19.
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Frequently Asked Questions
Can you do intermittent fasting on 75 Hard?
Yes. The program, created by Andy Frisella, doesn't prescribe a specific diet. It requires you to choose one and follow it for 75 days with no cheat meals and no alcohol, and intermittent fasting qualifies. Write your protocol down on Day 1 so "following my diet" has a definition you can check against. Define it as food rules plus a minimum fast (for example: whole foods, no added sugar, no alcohol, at least 14 hours fasted daily) rather than one fixed clock, so a legitimate schedule change later doesn't read as breaking your own rule.
Does drinking a gallon of water break your fast?
No. Plain water has no calories and does not break a fast, so the gallon spreads across all 24 hours instead of getting crammed into the eating window. Most people find the fasting block is the easiest place to drink the bulk of it. Drink to thirst rather than forcing volume, and take your electrolytes with meals.
Should you work out fasted or inside the eating window?
Either works. A 2025 randomized clinical trial found that 12 weeks of fasted resistance training produced essentially identical muscle thickness and strength gains compared with fed training when daily nutrition was matched. Practical rule for two-a-days: put the harder session (heavy lifting or intervals) inside the eating window and the easier one (the outdoor walk or easy cardio) in the fasted block.
Will you lose muscle doing intermittent fasting with two workouts a day?
Not if you lift and hit your protein. In an 8-week randomized trial, time-restricted eating alone cost participants 2.3 kg of fat-free mass, while time-restricted eating combined with resistance training held fat-free mass stable and stripped 3.2 kg of fat. The resistance training is what protects the muscle. In that trial the fasting-plus-lifting combination is also what produced the fat loss, not the window on its own.
How much protein do you need on intermittent fasting during a 75-day challenge?
A meta-analysis of 49 trials found that training-induced gains in fat-free mass plateau around 1.6 g of protein per kg of body weight per day. For an 80 kg person that is roughly 130 g, which works out to about 43 g in each of three feedings across an 8-hour window. Treat it as a floor to hit, not a target to beat.
What eating window works best with two 45-minute workouts?
Three setups cover most people: noon to 8pm with a fasted morning workout and an evening session inside the window, 11am to 7pm with both workouts fed, or 8am to 4pm training fasted at 7am and again at 3pm. Pick the one that lets you eat three real meals, not the one with the longest fast.
What are the signs intermittent fasting is too aggressive with this challenge?
Performance declining for a week or more, fatigue that sleep doesn't fix, feeling cold all the time, getting sick repeatedly, missed or lighter periods, disrupted sleep, low mood, and poor concentration. Together these point to low energy availability, meaning sustained intake below roughly 30 kcal per kg of fat-free mass per day. If they show up, eat more and see a clinician if they persist.
Does changing your eating window restart the challenge?
No. The rule is to follow a diet, not to never adjust it. Widening from 16:8 to 14:10 because you're underfueled is a deliberate revision to your written protocol, which is a different thing from an unplanned cheat meal. Make the change on purpose, write down the new version, and hold it. On the diet side, what restarts you is eating outside your chosen diet or drinking alcohol. (Missing any of the program's other daily tasks also sends you back to Day 1.)