Fasting · Vialchemy Library

Before your first fast

Pick a plan you can repeat on work days and days off (for example 14:10 at home, OMAD at camp).
Set your protein goal on the Goals page, and plan the first meal before you start.
Keep water, and electrolytes for long or hot days, within reach.
Decide your style: clean (water, black coffee, plain tea) or dirty (small extras like a Monster Zero).
Ask your pharmacist whether any of your medicines need to be taken with food.
On a GLP-1 or another prescribed drug, tell your prescriber you plan to fast.

Does it break my fast?

Water, sparkling water or mineral water?
Clean. Drink plenty: fasting and GLP-1s both blunt thirst.
Black coffee?
Clean. Under 3 kcal a cup. Count the caffeine (see Caffeine below), and on an empty stomach it can upset a GLP-1 gut.
Plain tea (black, green or herbal), no sugar or milk?
Clean. Essentially no calories.
Monster Zero Ultra (white can) or another zero-sugar energy drink?
Dirty, not clean. About 10 kcal and 137 mg caffeine per 473 mL can, sweetened with erythritol, sucralose and acesulfame-K. The calories won't matter, and acute studies mostly show no insulin spike from sweeteners alone. It still isn't a clean fast, it counts toward your caffeine, and the fizz can add to GLP-1 bloating.
Diet pop (Coke Zero, Diet Pepsi and so on)?
Dirty. 0 to 1 kcal, but sweetened. Same reasoning as zero-sugar energy drinks.
Sweetener in coffee (Splenda, stevia packet)?
Dirty. A packet has a few calories from its filler and the sweet taste. Fine on a dirty fast; skip it on a clean one.
A splash of milk or cream in coffee?
Dirty if small. About 8 kcal for a tablespoon of 2% milk, about 50 kcal for a tablespoon of heavy cream. More than a splash and you've had a snack.
Electrolytes?
Plain salt, or an unsweetened mix: clean. Most zero-sugar sticks (Sqwincher Zero, sugar-free Liquid IV and the like) are sweetened, so they're dirty, but worth it on long or hot days. Check the label for sugar.
Sugar-free gum or mints?
Dirty. A few calories a piece from sugar alcohols. A piece or two won't change much; a pack will.
Apple cider vinegar or lemon in water?
Dirty but tiny (a few kcal). Acid on an empty stomach can bring on reflux, more so on a GLP-1. Dilute it well.
Creatine (plain, unflavoured)?
Clean. No calories. Flavoured or sweetened creatine products can break a fast; check the label.
Bone broth?
Breaks a clean fast: roughly 30 to 50 kcal and several grams of protein a cup. A common dirty-fast choice for long fasts because of the salt.
BCAAs or EAAs?
Breaks it. Amino acids (leucine especially) trigger insulin and muscle building. If your goal is the fast, take them with your first meal.
Collagen or a protein shake?
Breaks it. That's food. Great as the first thing you have when the fast ends.
Bulletproof coffee, MCT oil or butter in coffee?
Breaks it. Usually 100 to 250+ kcal of fat. Some keto plans call it a fast; by calories it's a meal.
Alcohol?
Breaks it, and drinking on an empty stomach hits harder, especially with a GLP-1 slowing your stomach.
My peptide or GLP-1 injection?
No calories, so it doesn't break a fast. Keep your usual dose day and time; the injection doesn't need food.
Prescribed pills and vitamins?
Never skip a prescribed medicine to protect a fast. Some must be taken with food: ask your pharmacist. Gummy vitamins have sugar; fat-soluble vitamins (A, D, E, K) absorb better with a meal.

Talk to your prescriber first if

You take insulin or a sulfonylurea (gliclazide, glyburide): fasting can drop your blood sugar too low.
You have type 1 diabetes, kidney disease, gallstones, or a history of low blood sugar.
You're pregnant, breastfeeding, under 18, or underweight.
You have or have had an eating disorder. Intermittent fasting is not recommended then, and young people are more vulnerable.
You're on a GLP-1 or similar and changing your dose. Ramadan guidance suggests finishing dose increases 4 to 6 weeks before a fasting period, and putting off an increase if a fast is coming. That's a conversation for your prescriber, not the app.

Ways to fast

Plan	Fast / eat	Good to know
12:12	12 h / 12 h	Overnight plus a later breakfast. The easy start.
14:10	14 h / 10 h	A common step up.
16:8	16 h / 8 h	The most studied daily plan (e.g. noon to 8 pm).
18:6 or 20:4	18-20 h / 4-6 h	Two meals or one big meal and a snack.
OMAD (23:1)	about 23 h / 1 h	One meal a day. Hard to fit enough protein into one meal.
24 h	one full day	Dinner to dinner, once or twice a week.
5:2	2 low days a week	About 500-600 kcal on 2 days, normal on the other 5.
4:3	3 low days a week	Fast days at roughly 400-700 kcal. Beat daily calorie counting in a 2025 trial.
Alternate-day	every other day	Very low or no food on fast days. Small edge in the 2025 BMJ review.
Longer than 48 h	2+ days	Medical supervision. See Long fasts below.

The app times the plan you choose. It doesn't recommend one.

What the research says

Fasting works about as well as eating less every day. A 2025 BMJ review of 99 trials (6,582 adults) found no fasting plan clearly beat daily calorie cutting. Alternate-day fasting had a small edge (about 1.3 kg), below what the authors call clinically important (2 kg). Certainty was low to moderate.
Calories still decide most of it. With the same calorie target, people eating in an 8-hour window lost 8.0 kg vs 6.3 kg over a year, a difference that could be chance (NEJM 2022). When calories were held exactly equal, eating in an early 10-hour window gave the same weight loss as a usual, later pattern (Annals 2024).
Structure helps. In a 2025 trial, 4:3 fasting with dietitian support lost 7.6% of body weight at 12 months vs 5.0% for daily calorie counting (Annals of Internal Medicine).
On its own, an eating window may do little. In two 12-week trials, 16:8 or a self-picked 8-hour window without other changes didn't clearly beat a control group (JAMA IM 2020; Obesity 2025).
Muscle is the watch-out. The 2020 16:8 trial found more lean mass loss in the fasting group. Protein and resistance training are what protect it, fasting or not.
Heart headlines: a 2024 conference abstract linked an 8-hour eating window with more heart deaths. It was based on two days of diet records, wasn't peer reviewed, and experts said it couldn't show cause. Worth knowing, not a reason to panic.

What happens in your body (what's actually known)

Somewhere around 8 to 12 hours without food, your body leans more on fat and blood ketones start to rise. It's gradual, not a switch at a set hour, and it depends on what and how much you ate last.
Hunger tends to come in waves around your usual meal times rather than building steadily. Riding out a wave with water or black coffee usually works.
Autophagy (cell clean-up) is well shown in animals. In people nobody knows how many hours it takes, and there's no good test for it. Don't fast longer chasing it.
Claims about longer life, sharper thinking and less inflammation come mostly from animal studies.

Clean vs dirty fasting

Clean fast: only zero-calorie, unsweetened drinks (water, sparkling water, black coffee, plain tea, plain electrolytes). The only way to be sure you're in a true fasted state.
Dirty fast: small amounts of calories or sweeteners during the window, usually kept under about 50 to 100 kcal. Think a Monster Zero, a splash of cream, a sweetener packet or a cup of bone broth.
"Dirty fasting" isn't a medical term and has never been studied directly. The 50 kcal rule is a rule of thumb, not science.
For weight loss, what matters is that the extras stay small and don't set off cravings. A 10 kcal Monster won't undo a fast. A latte will.
If you're fasting for a reason that needs a true fast (fasting bloodwork, ketone testing, a procedure), go clean and follow the clinic's rules.
Pick the style you can stick with. A dirty fast you keep beats a clean one you quit.

Sweeteners while fasting

In acute studies, zero-calorie sweetened drinks behaved like water for blood sugar and hormones (2022 review: 36 trials, 472 mostly healthy people).
One small study in people with obesity (17 people) found sucralose before a sugar load raised the insulin response to that sugar. So the sweetener matters more right before a meal than on its own.
WHO (2023, a conditional recommendation) advises against relying on sweeteners for weight control, based on long-term studies linking heavy use to diabetes and heart disease. Those links may come from who uses them rather than the sweeteners.
Erythritol (in Monster Zero Ultra and many "keto" products): a 2023 study linked higher blood levels to more heart attacks and strokes, and a single drink kept levels high for days and made platelets stickier. It's an association plus lab data, not proof, but worth weighing if you have heart risk factors or drink several a day.

Caffeine and energy drinks

Health Canada's limit for adults is 400 mg of caffeine a day from everything combined.
One 473 mL Monster Zero Ultra has 137 mg. Canadian energy drinks are capped at 180 mg per serving. Brewed coffee varies a lot; a large coffee can match or beat a can.
On an empty stomach caffeine hits faster: jitters, a racing heart, heartburn or a loose gut are more likely.
GLP-1-type drugs can raise resting heart rate. In the retatrutide trial it rose with dose, peaked around week 24, then eased. Caffeine adds to it. Watch your resting HR in the app on days you stack caffeine and fasting.
Health Canada says energy drinks aren't for people with heart rhythm problems or high blood pressure, people on stimulant medicines, or pregnant or breastfeeding people.
Energy drinks and coffee count as fluid, but don't let them be your only fluid on a long shift.

Fasting on a GLP-1 or other peptides

Your appetite is already turned down, so the risk flips: it's easy to under-eat, not over-eat. The app's low-intake note (under about 1,500 kcal for 3+ days) is there for this.
Protein first. The 2025 joint advisory suggests about 1.2 to 1.6 g per kg a day while losing weight, 25 to 40 g a meal, plus 2 to 3 strength sessions a week. A short eating window makes that harder to hit.
Fluids: GLP-1s blunt thirst, and fasting removes the water you'd get from food. Dehydration can strain the kidneys; watch for dark urine, dizziness on standing or unusual tiredness.
Nausea and constipation can get worse because the stomach empties slowly. Break the fast gently.
Low blood sugar is unlikely on a GLP-1 alone, but the risk goes up with insulin or sulfonylureas.
Long gaps without food plus fast weight loss raise the chance of gallstones. Pain under the right ribs, especially after a meal, needs checking.
Keep your injection day and time; it doesn't need food. If your dose day tends to make you queasy, plan long fasts away from it, and ask your prescriber before moving your dose.

Camp, shift work and heat

In heat, NIOSH advises about 1 cup (250 mL) of water every 15 to 20 minutes, before you're thirsty, and no more than about 1.4 L an hour.
Fasting through hard physical work is doable for many people, but low fuel plus heat plus heights is where it goes wrong. Break the fast if you feel lightheaded, shaky or foggy on the job.
On a long or hot shift, electrolytes beat plain water alone.
OMAD at camp: make the one meal count. Protein first, then vegetables and carbs, and enough of it. Keep a protein backup in the truck (jerky, a Core Power).
Rotation days shift your clock. A different plan on home days is normal; the app lets you schedule by weekday or by location.

Breaking your fast

Start with protein: a shake, Greek yogurt, eggs or lean meat. It helps fullness and protects muscle.
Keep the first meal moderate, especially on a GLP-1. A huge, greasy meal on a slow stomach is the classic recipe for nausea.
Drink with it, not just after.
Wings, pizza and fries are hardest to break a fast on. Have them later in the window, if at all.
The Food page's protein-first suggestions work well as a first meal.

Long fasts (over 24 to 48 hours)

Talk to your prescriber first, especially on a GLP-1.
After several days with little or no food, eating again too fast can cause refeeding syndrome: dangerous shifts in phosphate, potassium and magnesium. Multi-day fasts belong under medical supervision.
The app lets you plan up to 72 hours and never rewards going longer.

Stop the fast and eat or get help if

Shaky, sweaty, confused or about to faint: eat or drink something with sugar now.
You can't keep fluids down, or you're vomiting for more than a day.
Severe stomach pain, especially spreading to your back or under your right ribs: get medical care (pancreas or gallbladder).
Racing or irregular heartbeat, chest pain or shortness of breath: emergency care.
Very little or very dark urine, or dizziness when you stand up.

Sources

Researched 2026-10-26. Trial results and approval status change.