What is Water Fasting: Dangers, Benefits, and Tips

by Marixie Ann Obsioma, MT, undergrad MD

Published on April 13, 2024

Fasting isn’t a brand-new idea. Even before folks started skipping meals to feel or look better, you probably heard about it in religious contexts. Way back in the day, even before the 1900s rolled around, fasting was a big deal. Take Ramadan, for example, when Muslims fast from sunrise to sunset for a whole month. It’s a special time for getting in touch with their spirituality and doing some serious thinking.

Such groups who promote fasting often use religious examples as an explanation as to why extended fasting is good and healthy. It is believed that our ancestors were able to sustain long periods of time without food access, so we should also be able to.

Fasting is indeed an excellent way to reset metabolism (1). Water fasting, in particular, can remove toxic substances from the body, promote healthy weight loss, lower blood pressure, and improve overall health (2).

However, human studies on water fasting are limited. Also, many paleo proponents forget that times are far different and there is no easy way to compare the eating habits of now to then. There is a fine line that separates its benefits from possible dangers. Read on to know more.

What is Water Fasting?

As its name implies, this type of fast restricts everything except water. There is no set time as to how long this should last for, but experts suggest anywhere from 24 to 72 hours. For your safety, you may wish to not try to fast for longer than 3 days without medical supervision.

Several popular cleanses are modeled after water fasting. Drinking water-based concoctions are perfect examples. Are you familiar with the lemon detox cleanse? It allows you to drink a good mixture of water, lemon juice, maple syrup, and cayenne pepper many times daily for up to 1 week (3).

Most people embark on water fasting or similar regimen for several reasons:

Religious or spiritual reasons
Weight loss
Detoxification
Preparation for a medical procedure
Other health benefits

Several studies have linked water fasting with a slew of impressive health benefits. It can help lower your risk of heart disease, diabetes, and some types of cancers. It can also induce autophagy, a process that helps the body break down and recycle old cell parts (4, 5, 6, 7).

Who Is It For?

Though generally healthy, water fasting is not for everyone. Patients with eating disorders, gout, diabetes, and chronic kidney disease are not allowed to water fast without medical supervision. Older people, pregnant women, and kids should seek medical consult too (8, 9).

How To Water Fast?

There are no specific guidelines on how to start water fasting. If this is your first time to do water fasting, it is better to prepare your body first. You should not go without food abruptly. Start eating smaller portions at each meal, 3-4 days before fasting.

Water Fasting (24 to 72 Hours)

During this time, you are not allowed to eat or drink anything aside from water. You can take as much as 2-3 liters of water daily. Water fasting should only last for 24 to 72 hours. Going beyond this time frame can be dangerous, especially without medical supervision.

Some people may complain of weakness or dizziness during fasting and may want to avoid operating heavy machines and driving to prevent accidents (10).

Post Fasting (1-3 Days)

After water fasting, you may wish to resist the urge to do binge eating. Consuming a big meal after a fast may cause uncomfortable symptoms. Instead, break your fast with smaller meals or a healthy smoothie. You can start taking larger meals throughout the day as you feel more comfortable.

This post-fast phase is very important after longer fasts to prevent the body from having rapid changes in fluid and electrolytes, which is also known as the refeeding syndrome (11).

This phase usually lasts a day, but those who fasted longer may need up to 3 days before they feel comfortable eating larger meals.

Potential Benefits of Water Fasting

As mentioned earlier, water fasting has been associated with a variety of health benefits. To know how a day or two of water fasting can benefit you, scroll down.

1. Promotes Autophagy

Autophagy is your cell’s natural process of excreting waste products caused by the degradation of cells or parts that are either dysfunctional or not needed by the body. Many studies found that autophagy can help protect against heart disease, Alzheimer’s disease, and cancer (12, 13, 14, 15). Animal studies also show that it can help promote longevity (6).

Fasting once or twice weekly can help your body promote autophagy and prevent the accumulation of damaged parts of your cells and toxins in the body (16).

2. Lowers Blood Pressure

Studies show that long, medically supervised water fasts can help patients with high blood pressure. A little less than 70 patients with borderline high blood pressure benefitted from water fasting for almost 2 weeks. More than 80% were able to lower their blood pressure to healthy levels, 120/80 mmHg (17).

Another study involving more than 170 patients diagnosed with high blood pressure water fasted for 10 to 11 days. Ninety percent (90%) of patients have lowered their BP to less than 140/90 mmHg, which is the limit for the diagnosis of high blood pressure (18).
However, no human studies have yet investigated the effect of short-term fasts, 24 to 72 hours, on blood pressure.

3. Prevents Oxidative Stress

Unhealthy lifestyle and eating habits may cause reactive oxygen species (ROS) accumulation. ROS affects cell structures and functions. Too much ROS can increase oxidative stress and inflammation in the body. Water fasting can help flush out ROS (19).

4. Protects Your Heart

Fasting for a day two weekly can help you consume fewer calories and lose weight. This, in turn, will help lower your risk of heart diseases. Experts found that fasting can help decrease your waist circumference, cholesterol, leptin, insulin-like growth factor, and tumor necrosis factor levels (20).

A study on 30 healthy individuals who water fasted for 24 hours showed a significant decrease in their blood cholesterol and triglyceride levels, which are important risk factors for heart disease (21). Animal studies also found that water fasting can protect the heart against free radicals (4, 22).

5. Improves Insulin and Leptin Sensitivity

Insulin and leptin are important hormones that greatly affects metabolism. Insulin can help store nutrients from the bloodstream. Leptin, on the other hand, makes you feel full (23, 24). Water fasting makes your body more sensitive to these hormones, which makes them much more effective (25, 26, 27, 28). With increased insulin sensitivity, your body can reduce blood sugar better. Also, by being more leptin sensitive can help process hunger signals more efficiently, thus lowering your risk of obesity (29, 30).

6. Decreases Risks of Other Chronic Diseases

Fasting is an effective way to keep chronic diseases at bay. Experts say that aside from heart disease and diabetes, fasting can also prevent cancer, chronic inflammation and rheumatoid arthritis, and decrease aging signals (31, 32). Evidence shows that water fasting on animals can help suppress genes that grow cancel cells. This may also boost the effects of chemotherapy (33).

You have to remember, however, that there are only a handful of studies about water fasting in humans. More research is needed before recommendations can be made.

7. Boosts Immunity

Water fasting can also help boost immunity by flushing out toxins. By making your cells healthy, your immune system will also start functioning better.

Possible Dangers of Water Fasting

While water fasting is healthy for a limited number of days weekly, doing it for an extended period of time can cause you harm. Here’s a list of what may happen:

1. Unhealthy Weight Loss

Water fast restricts calories, thus making you slim down quickly. Evidence shows that you may lose as much as 2 pounds daily during the 24 to 72-hour water fast (9). Unfortunately, most of the weight you lose may come from water, carbohydrates, and even muscle mass.

2. Dehydration

As strange as it may sound, water fasting can cause dehydration. This is because approximately 20-30% of your daily water intake comes from the food you eat (34). You will not enough water if you will just drink the same amount in the absence of food.

If dehydrated, one may complain of nausea, dizziness, headache, low blood pressure, constipation, and low productivity. To prevent these symptoms, one should drink more than usual (35).

3. Nutrient Deficiencies

Any fasting protocol can put you at risk for nutrient deficiencies. With calorie restriction, you are also limiting your intake of essential vitamins, minerals, amino acids, fatty acids, and electrolytes, which are all needed for the body to function properly.

4. Hyponatremia

Also known as water intoxication, hyponatremia occurs when one cannot replace the water and salt lost during perspiration because of dietary restrictions. Sodium will not be replenished by drinking water alone.

5. Binge Eating

Fasting or calorie restriction, in general, often leads to bingeing. This can cause obsessive or intrusive thoughts about food, which may cause you to eat more as soon as fasting is over.

6. Orthostatic Hypotension

Orthostatic hypotension is a common complication of water fasting. Defined as a drop in blood pressure that occurs when one suddenly stands up, it can make you dizzy, lightheaded, and at risk of fainting (9, 36, 37). If you experience these symptoms while water fasting, then this fast may not be appropriate for you. See a doctor immediately.

7. Other Medical Conditions

While water fasting is just relatively short, some patients are not allowed to water fast, as mentioned earlier. It can aggravate some medical conditions:

Gout: Water fasting increases uric acid production, which may cause gout attacks (9, 38).
Diabetes: Too much fasting may put you at risk of side effects in diabetes.
Chronic Kidney Disease (CKD): Water fasting may worsen damages to the kidneys (39).
Eating Disorders: Fasting may cause eating problems like bulimia in teenagers (40).
Heartburn or Gastroesophageal Reflux (GERD): Fasting triggers heartburn because your body may produce more acid in the absence of food (41).

More Helpful Tips

Water fasting can be physically and mentally challenging, so you may wish to carefully prepare yourself by:

Eat foods that are high in energy before water fasting
Do fasting on days when you are not at work so you’ll have time to rest
Do not fast if you are feeling tired or unwell
Avoid strenuous exercises
Prepare your body slowly by reducing your portion sizes a few days before water fasting
Drink more water all throughout the day
Do not eat too much at once after fasting to avoid stomach upset and being sick

Key Takeaway

While water fasting appears to have a slew of health benefits, most are seen in animal studies. More research is still needed to prove its claimed effects on humans. It also comes with risks, especially if you fast longer than recommended or have serious medical conditions.

If you choose to try water fasting, you may wish to fully understand the risks involved. You have to stop immediately if you feel fatigued, dizzy, or emotionally stressed. If water fasting is not suitable for you, you may try other safer methods like alternate-day and intermittent fasting. These are easier to follow long-term. A number of tasty cleansing foods can also help you flush out toxins and lose weight.

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