Fasting troubleshooter
Tell us what is going wrong and we will give you the most likely cause, the specific fix, and a plain safety line for when to stop. Most fasting side effects come down to sodium, under-eating, or too big a jump too soon. Some do not, and this tool says so.
What is happening?
Headache
Doctors recognise a 'fasting headache': a dull, usually mild to moderate headache after eight or more hours without food that eases once you eat. Its exact cause isn't known. Low fluid and salt, your usual caffeine and short sleep are the common things to check first.
Common
Low fluid and salt. When insulin falls during a fast, your kidneys pass more sodium and water.
Fix: Sip a large glass of water with a quarter to half a teaspoon of salt in it (roughly 500 to 1,150 mg of sodium). That counts towards the under 2,000 mg of sodium a day adults are advised to stay below, so it is part of your daily allowance, not extra. If the headache eases, low salt may have been part of it.
Common
Caffeine. Missing or delaying your usual coffee can bring on a withdrawal headache.
Fix: Keep your caffeine roughly the same from day to day. We suggest not quitting caffeine and starting fasting in the same week.
Common
Not drinking enough. Food normally supplies about a fifth of your water, so a fast leaves a gap.
Fix: Drink steadily through the day so your urine stays pale yellow. Official reference amounts for total water are about 2 litres a day for women and 2.5 for men, counting food.
Break the fast now if
- • The headache is sudden and severe, unlike any you have had before
- • It comes with confusion, vision changes, weakness on one side, or a stiff neck
See a doctor if
- • You get a headache every fast even after checking fluids, salt, caffeine and sleep
- • You have high blood pressure, kidney disease or heart failure, or take water tablets, and are unsure about adding salt
Dizziness or light-headedness
Feeling light-headed when you stand up during a fast is usually about fluid and blood pressure rather than blood sugar: with less fluid and salt on board, there is less pressure when you stand.
Most common
Low fluid volume from sodium loss, made worse by standing up fast.
Fix: Drink water with a little salt in it, and stand up slowly. Sit straight back down if the room greys out.
Common
Blood pressure medicine. Fasting can change how you feel on your usual dose.
Fix: If you take blood pressure medicine and feel dizzy or faint while fasting, stop the fast and speak to your doctor. Do not change your dose yourself.
Less common
Genuinely low blood sugar, more likely if you take glucose-lowering medication.
Fix: If you are on diabetes medication and feel shaky, sweaty and dizzy, treat it as hypoglycaemia and break the fast.
Break the fast now if
- • You feel faint while sitting or lying down, not just on standing
- • It comes with chest pain, a racing or irregular heartbeat, or confusion
See a doctor if
- • You take medication for blood pressure or diabetes and get dizzy on fasts
- • Dizziness keeps happening even with salt and water and slow standing
Not losing weight
Intermittent fasting does not burn fat on its own. It is a way to make a calorie deficit easier to hold. If the scale has not moved in a month, check the calorie deficit first.
Most common
The eating window quietly got bigger, or liquid calories crept in (milky coffees, a glass of wine, a healthy smoothie).
Fix: Write down everything you eat and drink for seven honest days, including tastes while cooking. The leak is often here.
Common
Window creep. Dinner drifts later, turning a 16-hour fast into a 14-hour one without you noticing.
Fix: Anchor your last meal to the same time each night, within 30 minutes. Anchor the last meal, not the first.
Common
You are losing fat but holding water, from new training, a salty weekend, or your cycle.
Fix: Measure your waist and take a monthly photo. Weigh daily but only read the seven-day average.
Worth checking
A medical cause such as thyroid issues or PCOS, or a medication that affects weight.
Fix: If you have tracked carefully for several weeks with no change, see your doctor rather than trying harder.
See a doctor if
- • Eight weeks of a genuine, tracked deficit with no change in weight or waist
- • Unexplained weight change alongside fatigue, hair loss or feeling cold
Fatigue and low energy
Timing gives a clue. Tiredness that lifts when you eat may be about fluids and salt. Tiredness that carries into your eating window and the next day often means you are eating too little overall.
Common
Low fluid and salt, if the tiredness lifts once you eat.
Fix: Drink steadily and try a little salt in water during the fast. Eat foods rich in potassium, such as vegetables, beans and fruit, in your window.
Common
Under-eating, if the tiredness carries into your eating window and beyond. A short window quietly cuts total intake too far.
Fix: Work out your maintenance calories with the TDEE calculator. We suggest staying within about 20% of them and not stacking a long fast on top of a large deficit.
Common
Poor sleep, which fasting can worsen, especially with late caffeine.
Fix: Stop caffeine well before bed (a sleep study found coffee still disturbed sleep when drunk six hours before bed) and protect your sleep before you lengthen your fast.
Break the fast now if
- • The fatigue is sudden and severe, with a racing heart or breathlessness: stop the fast and get medical help (call your local emergency number if it is severe)
See a doctor if
- • Deep fatigue that does not improve when you eat and sleep normally
- • Fatigue with feeling cold, hair shedding or low mood, which can point to under-eating or thyroid issues
Trouble sleeping
Fasting and sleep can pull against each other. Some people find it hard to sleep when they go to bed very hungry. Where you place your eating window matters.
Common
Going to bed very hungry.
Fix: Move your window a little later so you are not fasting through the whole evening, or make your last meal more filling with protein and fibre.
Common
Caffeine's long tail. Its half-life averages about four hours, and in some people up to eight.
Fix: Try stopping caffeine at least six hours before bed (eight is our cautious suggestion) for a week and see if sleep improves.
Worth checking
Alcohol, late heavy meals or screens in the hour before bed.
Fix: Finish eating a few hours before bed and keep the last hour calm. If you take magnesium, the evidence that it helps sleep is limited.
See a doctor if
- • Insomnia that persists for weeks and affects your daytime functioning
- • Sleep problems with a low or anxious mood that is not lifting
Irritability (hangry)
Feeling hangry is real. For many people it eases as they get used to a new routine. When it does not, it is often the same culprits as the physical symptoms: too large a deficit, low fluids, or a window that fights your day.
Most common
Adjustment. In the first week or two, hunger hormones have not yet settled into the new pattern.
Fix: Give a gentler window such as 14:10 a couple of weeks before judging. Many people find the edge fades.
Common
Too aggressive a jump, going straight to a long fast from three meals a day.
Fix: Step down. Shorten the fast, stabilise, then extend slowly once it feels easy.
Common
A long gap without enough fluid, or an eating window that is not filling enough.
Fix: Drink steadily, and make sure your eating window is genuinely well fed, with protein at each meal.
See a doctor if
- • Mood changes that feel out of proportion or are affecting your relationships
- • You find yourself thinking about food constantly or dreading eating
Constipation
Eating less food, and less often, simply means less material moving through, plus fasting can leave you mildly dehydrated. It is common early on and usually easy to fix without stopping.
Most common
Less food volume and fibre in a shorter eating window.
Fix: Prioritise fibre in your window: vegetables, fruit, legumes, and consider a little ground flax or psyllium.
Common
Dehydration from fasting.
Fix: Drink steadily through the day so your urine stays pale yellow; the gap from eating less food needs filling.
Worth checking
Too little fibre and fluid together, or a big change in what you eat.
Fix: Add fibre gradually, with more water as you do. A pharmacist can suggest a gentle laxative if you need one.
Break the fast now if
- • Severe abdominal pain, vomiting, or a hard, swollen belly
See a doctor if
- • Constipation lasting more than a week or two despite fibre and fluids
- • Blood in your stool, or a sudden lasting change in your normal pattern
Binge when breaking the fast
How you break a fast decides whether the next few hours are controlled or chaotic. Breaking a fast ravenous with fast carbohydrate almost guarantees overeating. This one is about the first meal, not the fast.
Most common
Breaking with sugary food or refined carbohydrate, which is easy to overeat.
Fix: Break the fast with protein first and something with fibre. Eat the protein and vegetables before the starch.
Common
Too long or too aggressive a fast for you right now, leaving you overly hungry.
Fix: Shorten the window. A fast you can end calmly beats a longer one that ends in a binge.
Worth taking seriously
A restrict-then-binge cycle, which fasting can feed for some people.
Fix: If breaking a fast reliably becomes a binge, that is a signal to step back from structured fasting and speak to a professional.
See a doctor if
- • Breaking a fast reliably turns into a binge you feel you cannot control
- • You are hiding your eating, or food and weight are affecting your mood daily
Hair shedding
More hair than usual in the brush a few months after a big change, such as eating much less, is common. It usually starts around two to four months after the cause and settles within six to nine months once the cause is fixed.
Most common
Too large a calorie deficit, or too little protein, stressing hair follicles into a shedding phase.
Fix: Eat closer to your maintenance calories and make sure each meal has protein. The TDEE calculator gives a protein range.
Common
Another trigger at the same time, such as an illness, an operation, serious stress or stopping birth-control pills.
Fix: Shedding from these triggers usually settles once the cause passes. If it carries on, ask your doctor to look into it.
See a doctor if
- • Shedding that continues beyond a few months, or comes with fatigue and feeling cold
- • Patchy hair loss rather than general thinning, which needs a proper look
Period changes
The menstrual cycle is sensitive to energy availability. If fasting has tipped you into too large an energy deficit, your cycle can change or pause. This is a signal worth respecting, not pushing through.
Most common
Low energy availability, from too aggressive a deficit combined with fasting.
Fix: Ease off. Move to a gentler window, eat closer to maintenance, and make sure you are getting enough protein and fat.
Worth taking seriously
Fasting may simply not suit your body at this life stage, especially if you are very active or lean.
Fix: Consider whether structured fasting is right for you now. It is not the only path to health.
See a doctor if
- • Your period stops for three months or more
- • Any change in your cycle that worries you, or if you could be pregnant
How to use the fasting troubleshooter
Pick the problem closest to yours from the 10 buttons. They cover headache, dizziness, not losing weight, fatigue, trouble sleeping, irritability, constipation, bingeing when you break the fast, hair shedding and period changes.
Each panel opens with a short explanation. Below it are two to four likely causes, each with a fix to try. Labels such as “Most common” and “Worth checking” are our own ranking, not rates measured in a study. Every panel ends with a “See a doctor if” box. Panels where a symptom can signal an emergency also have a “Break the fast now if” box.
When intermittent fasting is not working for weight loss
If intermittent fasting is not working because the scale has stopped moving, look at calories first. In a 12-month trial, 139 adults with obesity cut calories, and one group also kept to an 8-hour eating window. That group lost 8.0 kg (17.6 lb) against 6.3 kg (13.9 lb) for calorie cutting alone, a difference that was not significant. So the window matters mainly as a way to eat less.
The “Not losing weight” panel checks for drinks with calories, a later last meal, water hiding fat loss and medical causes. For more on each cause, read our guide on why the scale stalls. To compare your intake with your maintenance calories, use the TDEE calculator.
When to stop and get help
Some problems mean the fast should end now. Chest pain and fainting need urgent medical help. So do confusion, a heart that races or beats unevenly, vomiting that keeps coming back and changes in your sight. Call your local emergency number if they are severe.
Other signs call for a doctor’s appointment. Missing three periods in a row is one. Extra hair shedding can follow a big weight loss by a few months, and it usually settles within 6 to 9 months. See a doctor if it lasts longer than that. If breaking a fast keeps turning into a binge you cannot control, pause fasting and see a doctor. The site findahelpline.com lists free helplines for many countries.
Who should be careful
Fasting on insulin or a sulfonylurea risks low blood sugar, so ask your doctor before you start. Never change a diabetes medicine yourself. Dizzy spells while fasting on blood pressure tablets mean ending that fast and phoning your doctor. Pregnancy and an age under 18 both call for a doctor’s advice before you fast. With any eating disorder in your past, our advice is to stop fasting rather than troubleshoot it.
Side effects of intermittent fasting lists what trials report and who should not fast. How much salt to take for headaches and dizziness is covered in electrolytes while fasting.
Frequently asked
Why is intermittent fasting not working for me?
If the scale has not moved, open the troubleshooter's "Not losing weight" panel. It lists the usual causes, from drinks with calories to a window that has crept later. In a 12-month trial with calories matched, adding an 8-hour eating window made no significant difference to weight loss. The calories you eat over the week matter most.
What are the signs intermittent fasting is not working?
No change in your weight or waist over several weeks is the plainest sign. Other signs are tiredness that lasts into your eating window, more hair in the brush and changes to your period. First meals that turn into binges count too. Each of these has its own panel in the troubleshooter.
When should I stop intermittent fasting?
For chest pain, fainting or confusion, stop the fast and get urgent help. A heartbeat that races or skips, repeated vomiting and any change in your sight need the same. See a doctor if you miss three periods in a row.
Can the troubleshooter tell me what is wrong?
No. It gives general information about common fasting problems and cannot diagnose anything. If a symptom is severe, keeps coming back or worries you, see a doctor.