Practice · Updated Oct 13, 2026 · 4 min read
Dry mouth, bad breath and a pounding heart while fasting
The NHS names dehydration as a main cause of a dry mouth, and lists fasting or crash dieting as a cause of bad breath. In one small study, breath acetone rose the longer people stayed without food. Heart palpitations are usually harmless. During a fast, stop and get checked if a pounding heartbeat keeps coming back, lasts a long time, or comes with dizziness, chest pain, fainting, or shortness of breath.
Questions from the community · Part 9 of 9 · Fasting Library
People ask why their mouth goes dry and their breath changes around the first day of a fast, and whether a fast or pounding heartbeat at that point is normal.
A dry mouth
The NHS page on dry mouth says a dry mouth is rarely a sign of anything serious. Dehydration is a main cause it lists, for example from not drinking enough, sweating a lot, or being ill. The page says to sip plenty of cold water through the day, and to chew sugar-free gum. It says you are more likely to get tooth decay if you have a dry mouth, and not to drink lots of caffeine, such as tea and coffee. See a GP if it does not go away after a few weeks. Whether gum counts as food on your fast is in What usually ends a fast.
Bad breath
The NHS page on bad breath says bad breath is very common. Fasting or crash dieting is one cause it lists. Brush your teeth and gums at least twice a day for 2 minutes, and clean your tongue once a day. See a dentist if it does not go away after a few weeks.
In 2018, Fabian Bovey, Jérémy Cros, and colleagues studied 8 healthy adults. Breath acetone rose the longer they stayed without food, and it fell slightly when they ate high-carbohydrate meals. The study measured acetone in breath, not the smell, and not body fat lost.
A pounding heartbeat
The NHS page on heart palpitations says heart palpitations are when the heartbeat becomes more noticeable. They are usually harmless, but get help if you keep getting them or you also have other symptoms. The heartbeat may feel as if it is racing or beating very fast, irregular, with skipped or extra beats, pounding or thumping, or fluttering. Palpitations can last seconds, minutes, or longer. Causes it lists include stress and anxiety, lack of sleep, and caffeine.
Black coffee is common during a fast. The NHS lists caffeine among the causes of palpitations, and its dry mouth page says not to drink lots of caffeine.
See a GP if you have had heart palpitations and you feel well, but they keep coming back or they are happening more often, they last longer than a few minutes, you have a heart condition, or you have a history of heart problems in your family.
Call 999 or go to A&E if you currently have heart palpitations and they do not go away, or you also have chest pain, shortness of breath, or you feel faint or you faint. Do not drive to A&E. Ask someone to drive you, or call 999 and ask for an ambulance. If you have had those symptoms and they have now stopped, the page says to ask the GP surgery for an urgent appointment, call 111, or get help from 111 online.
During a fast, stop and get checked if the pounding keeps coming back, lasts a long time, or comes with dizziness, chest pain, fainting, or shortness of breath. The wider list of reasons to stop is in When to stop a fast.
Key takeaways
- The NHS dry mouth page names dehydration as a main cause. Sip cold water, and you can chew sugar-free gum. Do not drink lots of caffeine. A dry mouth makes tooth decay more likely. See a GP if it does not go away after a few weeks.
- Bad breath is very common. The NHS lists fasting or crash dieting as a cause. Brush your teeth and clean your tongue. See a dentist if it does not go away after a few weeks.
- Bovey, Cros, and colleagues studied 8 healthy adults in 2018. Breath acetone rose the longer they stayed without food, and it fell slightly with high-carbohydrate meals. The study measured acetone in breath, not the smell, and not body fat lost.
- The NHS says palpitations are usually harmless, and lists caffeine among the causes. During a fast, stop and get checked if a pounding heartbeat keeps coming back, lasts a long time, or comes with dizziness, chest pain, fainting or shortness of breath. Call 999 or go to A&E if it doesn't go away or comes with chest pain, shortness of breath or fainting.
Educational only. Not medical advice. See our disclaimer.