Dehydration and Blood Sugar: How Much Water Should a Diabetic Drink?

Dehydration and blood sugar affect each other, and people with diabetes are more exposed to both sides. The NHS says dehydration can happen more easily if you have diabetes.^[13] High glucose pulls fluid out through the kidneys, and there is some evidence that being short of water can push glucose up. This guide sets out what the studies found, how much to drink and which drinks to choose.

This guide is general education. If you use insulin or other glucose medicines, follow the plan your diabetes team gave you.

Does high blood sugar cause dehydration?

Yes. This is the better established side of the link. Diabetes UK explains that when there is too much glucose in the blood, the body tries to remove the excess through urine, which can make you lose more fluid and feel thirsty.^[2] It adds that your kidneys work harder to get rid of the extra glucose, so you may need to wee more often.^[2]

Doctors call this osmotic diuresis. A 2024 consensus report endorsed by the ADA and the EASD states that "hyperglycemia leads to an osmotic diuresis, leading to volume depletion and hemoconcentration."^[1] If fluid intake does not keep up, the report says this can lead to a hyperosmolar state and kidney problems.^[1] The report is about hyperglycaemic crises, not everyday glucose levels. The authors report industry ties, listed in the report's Duality of Interest statement.^[1]

That is why thirst and peeing a lot are classic signs. The NHS lists feeling very thirsty and peeing a lot among the symptoms of high blood sugar.^[11] The ADA-endorsed consensus report also notes that older adults are at particular risk of dehydration, partly because thirst weakens with age.^[1] Our guide to high blood sugar symptoms covers the other warning signs.

Can dehydration raise blood sugar?

Possibly, but the evidence is small and mixed. The main studies point in different directions.

The trial in people with type 2 diabetes

In a 2017 crossover trial, 9 men with type 2 diabetes spent 3 days on low water intake and 3 days on normal intake, then took a glucose tolerance test.^[3] Low intake left them hypohydrated by 1.6% of body weight. Their fasting glucose was 10.4 mmol/L (about 187 mg/dL) after low intake against 9.5 mmol/L (about 171 mg/dL) after normal intake. At 2 hours it was 21.0 against 19.1 mmol/L (about 378 against 344 mg/dL).^[3] The authors linked the effect to cortisol.^[3]

The study was small, short and included no women. It did not receive a specific grant, and two authors declared separate research grants from Danone Research. One author is a scientific consultant for Quest Diagnostics.^[3]

The trials in healthy adults

A 2019 crossover trial in 16 healthy adults found mild dehydration raised copeptin, a marker of the water hormone vasopressin. The authors "found no effect on glycemic regulation." Fasting glucose was 5.10 against 5.02 mmol/L (about 92 against 90 mg/dL), which was not a significant difference.^[4] It was funded by the Economic and Social Research Council and a European Hydration Institute grant, and several authors declared fees, consultancy or funding from Danone and other drink or food companies.^[4]

Another 2019 trial gave 60 healthy adults a salty drip to trigger the same hormone, then a glucose drink. Their glucose response was higher than after a plain saline drip.^[6] A salt infusion is a lab model, not everyday dehydration. The trial was funded by the Novo Nordisk Foundation and Danone Research, and four authors were listed with a Danone Research affiliation.^[6]

What the meta-analysis found

A 2020 meta-analysis of 21 studies on short-term water loss found cortisol rose with dehydration. The authors still concluded that it "has a negligible effect on markers of glycemic regulation."^[5] Most of the people in those studies were healthy and active, so it may not reflect people with diabetes. The authors received no external funding and declared no conflict of interest.^[5]

What happens to glucose when people drink more water?

No study shows that water works as a glucose treatment. In a 2019 pilot, 31 adults without diabetes who drank little water and had high copeptin added 1.5 litres (about 51 fl oz) a day for 6 weeks. Fasting glucose differed by 0.2 mmol/L (about 4 mg/dL) between the start and the end.^[14] There was no control group and the change was small. The pilot had Danone Research funding, and four authors were employed by Danone Research.^[14]

A controlled trial in 39 healthy adults who drank an extra 3 litres (about 101 fl oz) a day for one week found copeptin fell by 15%. In the people whose copeptin fell most, usually low drinkers with high copeptin, "glucose and insulin were unaffected."^[15] It was funded by Swedish foundations, research councils and hospitals and the Novo Nordisk Foundation, among others, and three authors declared grants or consultancy from Danone Research.^[15]

A French cohort followed 3,615 adults for 9 years. People who said they drank 0.5 to 1 litre (about 17 to 34 fl oz) of water a day had smaller odds of developing high blood sugar than those who drank under 0.5 litres (about 17 fl oz). The top group, over 1 litre (about 34 fl oz), did not show a significant difference.^[16] This is an association from self-reported intake, so it cannot show cause. The cohort was supported by INSERM contracts with the French national health insurer and drug companies including Lilly, Novartis Pharma and sanofi-aventis.^[16]

How much water should a diabetic drink a day?

When we read the main diabetes guidance side by side, we found no diabetes body that sets a separate number. The ADA tells people with diabetes to choose water over other drinks but gives no daily volume.^[8] So the general guides apply.

The US numbers look higher because they include water from food. The panel chair said they do not offer a rule of thumb for how many glasses to drink each day.^[17] The NHS gives a simpler check: drink enough so your pee is a clear pale yellow.^[7] It says you may need more in hot weather, during long exercise, when ill or when pregnant or breastfeeding.^[7] Our guide to weather and diabetes covers hot days.

When your team sets a fluid limit

Some people are told to drink less, not more. NICE says people with heart failure should not routinely be told to restrict fluid, but some may be, for example those with low blood sodium caused by too much water in the body (dilutional hyponatraemia).^[9] The NHS says people with kidney disease and swelling may be advised to reduce their daily salt and fluid intake.^[10] If you have been given a limit, follow it and ask your team before you change it.

Which drinks suit people with diabetes besides water?

Water comes first. The ADA calls it "the primary beverage of choice" and grades its advice to choose water at the top evidence level.^[8] Diabetes UK also puts water first as an everyday drink.^[18] For people who find plain water dull, the ADA suggests adding lemon, lime, berries or cucumber, or trying sparkling or flavoured no-calorie water.^[8]

Other drinks that count, according to the NIDDK and the NHS:

• Unsweetened tea and coffee.^[19][7]
• Low-fat or non-fat milk.^[19]
• Sparkling water.^[19]
• Diet, sugar-free or no added sugar drinks in place of sugary ones.^[7]

Fruit juice is the one to watch. The NHS says to limit fruit juice and smoothies to one small glass of 150 ml (about 5 fl oz) a day, with a meal.^[7] The CDC notes that fruit juice raises blood sugar faster than whole fruit.^[20] Diabetes UK says to save fizzy sugary drinks for hypos and choose sugar-free ones the rest of the time.^[18] Lows are more likely if you use insulin or a sulfonylurea such as gliclazide or glimepiride, and low blood sugar usually means below 4 mmol/L (72 mg/dL) in the UK or below 70 mg/dL (3.9 mmol/L) in the US.^[21][22] In the UK, the NHS says to call 999 if someone may have very low blood sugar and is not responding normally or is unconscious, and a glucagon injection is not available or you do not know how to use it, or they have not recovered within 10 minutes of a glucagon injection, or they have been drinking alcohol.^[21] In the US, the ADA says to call 911 right away if someone is unconscious and glucagon is not available or no one knows how to use it.^[22] The NIDDK adds that sports drinks are often high in sugar and are not needed for most moderate exercise.^[19]

What are the signs of dehydration?

The NHS defines dehydration as your body losing more fluid than you take in.^[13] Its list of signs includes:^[13]

• Feeling thirsty.^[13]
• Dark yellow, strong-smelling pee.^[13]
• Peeing less often than usual.^[13]
• Feeling dizzy or light-headed, or having a headache.^[13]
• Feeling tired.^[13]
• A dry mouth, lips and tongue.^[13]
• Sunken eyes.^[13]

Thirst can mean two things when you have diabetes. You may simply need a drink, or your glucose may be high. The NHS lists feeling very thirsty and peeing a lot among the signs of high blood sugar.^[11] If you notice both, check your levels if your team has shown you how, and speak to your care team if they stay high. Our guide to checking your glucose explains how.

When are dehydration and blood sugar problems an emergency?

The NHS says to get an urgent GP appointment or call NHS 111 if you feel unusually tired, feel dizzy when you stand up and it does not go away, have dark yellow pee or are peeing less than normal, or are breathing quickly or have a fast heart rate.^[13] Call 999 or go to A&E for signs of shock, such as blue, grey, pale or blotchy skin, cold skin, trouble breathing, confusion or being sleepier than normal.^[13]

For high blood sugar, the NHS says to call 999 or go to A&E if you are being sick or have stomach pain, are breathing faster than usual, have breath that smells fruity, feel drowsy or confused, or have a high level of ketones. It says not to drive yourself to A&E.^[11]

In the US, the CDC says to call 911 or go to the emergency room right away if your blood sugar stays at 300 mg/dL (about 16.7 mmol/L) or above, your breath smells fruity, you are vomiting and cannot keep food or drinks down, or you are having trouble breathing.^[12] These can be signs of diabetic ketoacidosis, which is a medical emergency.^[12]

How to manage dehydration and blood sugar day to day

1. Keep a glass or bottle of water where you can see it.^[8][18]
2. With meals, choose water or a low-calorie drink such as unsweetened iced tea.^[20]
3. Check the colour of your pee and aim for a clear pale yellow.^[7]
4. Drink more in hot weather, when exercising for a long time or when you are ill, unless your team has set a fluid limit.^[7]
5. Swap sugary drinks for sugar-free ones.^[7]
6. If you have a fluid limit for heart or kidney disease, follow it.^[9][10]

Questions to ask your diabetes team

• Is there a reason I should drink more, or less, than the usual 6 to 8 glasses a day?^[7][9][10]
• Do any of my diabetes, blood pressure or kidney medicines make dehydration or hypos more likely?^[1][13][21]
• What should I drink, eat and check when I am ill with vomiting, diarrhoea or fever?^[1][11][13]
• At what high or low blood glucose level should I call you, NHS 111 or emergency services?^[11][12][21]

Frequently asked questions

Can dehydration raise your blood sugar?

It may. One small trial in 9 men with type 2 diabetes found higher glucose after 3 days of low water intake,^[3] but a trial in healthy adults and a meta-analysis found little or no effect.^[4][5] High blood sugar causing dehydration is much better established.^[1][2]

How much water should a diabetic drink a day?

There is no separate number for diabetes. The NHS suggests 6 to 8 cups or glasses of fluid a day, enough for pale yellow pee.^[7] If your team has set a fluid limit, follow it.^[9][10]

What happens to blood sugar if you drink more water?

Water is not a treatment for high glucose. One small uncontrolled pilot in adults without diabetes found only a small change in fasting glucose with extra water.^[14] Water is still the drink the ADA recommends for people with diabetes.^[8]

Why am I so thirsty with diabetes?

High blood sugar makes the kidneys pass more urine, which takes fluid with it and makes you thirsty.^[2] If you are often thirsty and peeing more, speak to your care team, and check your levels if they have shown you how.^[11]

Where to go next

Dehydration and blood sugar are easiest to manage together: drink regularly, choose water first and take constant thirst as a reason to talk to your care team.

• High blood sugar symptoms, for the warning signs and when to act.
• Weather and diabetes, for hot days and cold days.
• Is Diabec legit?, for what is in our supplement and what it does not do.

Note: If you are pregnant, breastfeeding, or considering giving any supplement or herbal product to a child, consult your healthcare provider before making any changes to your diet, supplement routine, or prescribed medications.