Sauna While Fasting (Dry, Water, and Intermittent)

Fasting and saunas are both popular, and many people try to combine them. Both put strain on your fluid balance, though, so whether it is a good idea depends on the type of fast you are doing.

Is it safe to use a sauna while fasting?

For healthy adults, a short session during a fast that allows water is generally fine. Pay attention to how you feel. A sauna makes you lose fluid fast: Harvard Health says the average person sweats out about a pint during a short session. Your pulse rises by 30% or more, and more blood flows to your skin. Add a fast, little fluid and no food, and you are more likely to feel dizzy, weak or faint.

Sauna while intermittent fasting

Johns Hopkins Medicine describes two common types of intermittent fasting. With 16:8 you eat within an eight-hour window and fast for 16 hours. With 5:2 you eat normally five days a week and have one 500 to 600 calorie meal on the other two. While fasting you can have water and zero-calorie drinks such as black coffee and tea.

Because you can drink, an intermittent fast and a sauna can work together. A few practical points:

• Drink water before and after. Harvard Health suggests two to four glasses of cool water after each session.
• Keep it short. Harvard suggests no more than 15 to 20 minutes. Cleveland Clinic suggests beginners start at about 110°F for five to 10 minutes in an infrared sauna.
• If you feel light-headed when you sauna on an empty stomach, move the session into your eating window.
• Cool down gradually and stand up slowly when you leave.
• Skip alcohol before and after. See can you drink alcohol in the sauna.

Hopkins also says some people should not try intermittent fasting: children and teens under 18, people who are pregnant or breastfeeding, people with type 1 diabetes who take insulin, and anyone with a history of eating disorders. For timing around meals, see sauna before or after eating.

Sauna during a water fast

A water fast means only water for a day or more. Johns Hopkins warns that fasts of 24, 36, 48 or 72 hours “are not necessarily better for you, and may be dangerous”. Sweat also takes salt out of your body. The NHS suggests a sports drink or oral rehydration solution after heavy sweating for that reason, and a water fast replaces neither the salt nor the energy. We recommend skipping the sauna during multi-day water fasts. If a doctor supervises your fast, ask them first.

Sauna during a dry fast

A dry fast means no food and no water. Cleveland Clinic advises against dry fasting because it can lead to dehydration and kidney problems, with symptoms such as fatigue, headaches and less urination. A sauna adds even more fluid loss, so do not combine the two.

A steam room is not a safer choice. Steam rooms run cooler: the MrSteam iTempo control sets 80 to 120°F. But the air is saturated, and the CDC explains that in humid air sweat cannot evaporate well, which makes it harder to stay cool. If you are on a religious fast without water, wait until you have eaten and drunk again.

Does a fasted sauna boost autophagy, detox or fat burning?

These claims are common, but the evidence does not support them:

• Autophagy: we found no human studies showing that a sauna increases autophagy (the body’s cell recycling) during a fast. Claims that you can “stack” the two are guesswork.
• Detox: sweat is mostly water and salts. Researchers estimated in Environment International (2018) that sweat removes less than 0.024% of the pollutants (PCBs) a person takes in each day, and called it negligible.
• Fat loss: the weight you lose in a sauna is water, and it comes back when you drink. Cleveland Clinic calls weight lost by dehydrating yourself “water weight and not real body weight”. See does a sauna get rid of water weight and how many calories a sauna burns.
• Blood sugar: a lower blood sugar after a sauna is not a reason to use one while fasting. For people on diabetes medicine it can be a risk (see below).

Diabetes, medicines and fasted saunas

If you take insulin or other diabetes medicines, a fasted sauna needs care. In a small BMJ study, eight people with insulin-dependent diabetes took two 25-minute saunas at 85°C (185°F). Insulin was absorbed 110% faster, and blood sugar was 3.0 to 3.3 mmol/L (54 to 60 mg/dL) lower than on a control day. The CDC says high temperatures can change how your body uses insulin and that people with diabetes get dehydrated more quickly.

Symptoms of low blood sugar, such as sweating, dizziness and shaking, are easy to mistake for normal sauna sweating. Talk to your diabetes team before combining fasting and saunas.

Warning signs: when to get out

Leave the sauna right away if you notice any signs of heat exhaustion: tiredness, dizziness, headache, nausea, heavy sweating with pale, clammy skin, cramps, or feeling very thirsty. Move to a cool place, lie down, drink water or a rehydration drink, and cool your skin. The NHS says you should start to feel better within 30 minutes. If not, or if you are confused, have hot skin that is not sweating, or pass out, call emergency services. If you feel faint, the NHS advises lying down with your legs raised. More on this: dizzy, weak or cramping after a sauna and is a sauna good for nausea.

Who should skip the sauna while fasting

• People who are pregnant (see ACOG) or breastfeeding.
• Anyone on insulin or other diabetes medicines, unless their doctor agrees.
• People with heart disease or uncontrolled high blood pressure. Harvard Health says they should check with their doctor before using a sauna.
• Anyone who is unwell, has been vomiting or has diarrhoea.
• Anyone with a history of eating disorders.

After the sauna, break your fast gently and rehydrate. See what to eat and drink after the sauna and how often and how long to use a sauna.

Frequently asked questions

Can you use a sauna while intermittent fasting?

Usually yes, if you are healthy, drink water during the fast and keep the session short. Harvard Health suggests no more than 15 to 20 minutes. Get out if you feel dizzy, sick or shaky, and try a session during your eating window instead.

Does a sauna break a fast?

No. A sauna has no calories, so it does not break a fast. What you drink can: Johns Hopkins lists water and zero-calorie drinks such as black coffee and tea as fine while fasting.

Is it safe to use a sauna on a water fast?

We advise against it. Johns Hopkins warns that fasts of 24 hours or more are not necessarily better and may be dangerous. Sweat also loses salt, which a water-only fast does not replace. If a doctor supervises your fast, ask them first.

Can you use a sauna during a dry fast?

No. Cleveland Clinic advises against dry fasting because it can lead to dehydration and kidney problems. A sauna adds more fluid loss through sweat. A steam room is not safer, because humid air makes it harder for sweat to cool you.

Does a sauna while fasting burn more fat?

There is no good evidence that it does. The weight you lose in a sauna is mostly water and comes back when you drink. Cleveland Clinic says weight lost by dehydrating yourself is water weight, not real body weight.

Should people with diabetes use a sauna while fasting?

Only after talking to their doctor. In a small BMJ study of people using insulin, a sauna sped up insulin absorption and lowered blood sugar. The CDC says heat can change how your body uses insulin. Fasting adds to the risk of low blood sugar.

Sources

• Harvard Health, Saunas and your health (a pint of sweat, pulse +30%, 15–20 minutes, 2–4 glasses of water)
• Johns Hopkins Medicine, Intermittent fasting: what is it, and how does it work? (16:8, 5:2, long fasts may be dangerous, who should not fast)
• Cleveland Clinic, Dry fasting: why you should avoid it
• Cleveland Clinic, Infrared sauna benefits (start at 110°F for 5–10 minutes)
• Koivisto 1980, BMJ: sauna-induced acceleration in insulin absorption (PubMed 7000239)
• CDC, Managing diabetes in the heat
• Imbeault et al. 2018, Environment International: Can POPs be substantially popped out through sweat? (PubMed 29197670)
• NHS, Heat exhaustion and heatstroke
• NHS, Low blood sugar (hypoglycaemia)
• NHS, Fainting
• MrSteam iTempo control manual (80–120°F)
• ACOG, Can I use a sauna or hot tub early in pregnancy?