Fasting, a method of restricting food intake, has been practiced for thousands of years for religious, ethical, or health reasons. In recent years, water fasting, a type of fast that restricts everything except water, has gained popularity as a quick way to lose weight. While studies suggest that water fasting may offer several health benefits, including weight loss and a reduced risk of some chronic conditions, it is also associated with several risks and may not be suitable for everyone.
What is Water Fasting?
Water fasting is a type of fast during which you cannot consume anything besides water. Most water fasts last 24-72 hours, and it is not recommended to follow a water fast for longer than this without medical supervision. The main reason why people try water fasting is to improve their health, lose weight, for “detoxing”, for religious or spiritual reasons, or in preparation for a medical procedure. Popular diets like the lemon detox cleanse are modeled after the water fast. The lemon detox cleanse only lets you drink a mixture of lemon juice, water, maple syrup, and cayenne pepper several times per day for up to 7 days.
How to Prepare for and Undertake a Water Fast
There are no specific scientific guidelines on how to start water fasting. However, if you have never water fasted before, it’s a good idea to spend 3-4 days preparing your body for being without food. You can do this by eating smaller portions at each meal or by fasting for part of the day.
Water Fast (24-72 hours)
During a water fast, you are not allowed to eat or drink anything besides water. The water fast lasts for 24-72 hours. You should not water fast for longer than this without medical supervision because of health risks. Some people may feel weak or dizzy during a water fast and may want to avoid operating heavy machinery and driving to avoid causing an accident.
Post-Fast (1-3 days)
After the water fast, you should resist the urge to eat a big meal, as eating a large meal after a fast may cause uncomfortable symptoms. Instead, break your fast with a smoothie or smaller meals. You can start introducing larger meals throughout the day as you feel more comfortable. The post-fast phase is especially important after longer fasts because you may be at risk of refeeding syndrome, a potentially fatal condition in which the body undergoes rapid changes in fluid and electrolyte levels. This phase normally lasts a day, but people who fast for 3 or more days may need up to 3 days before they feel comfortable eating larger meals.
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Potential Health Benefits of Water Fasting
Both human and animal studies have linked water fasting to a variety of health benefits.
May Promote Autophagy
Autophagy is a process in which old parts of your cells are broken down and recycled. Several animal studies suggest that autophagy may help protect against diseases like cancer, Alzheimer’s, and heart disease. For example, autophagy may prevent damaged parts of your cells from accumulating, which is a risk factor for many cancers, which may help prevent cancer cells from growing. Animal studies have consistently found that water fasting helps promote autophagy. Animal studies also show that autophagy may help extend life span. That said, there are very few human studies on water fasting, autophagy, and disease prevention, so more research is needed before recommending it to promote autophagy.
May Help Lower Blood Pressure
Research shows that longer, medically supervised water fasts may help people with high blood pressure lower their blood pressure. One study in 48 people with overweight or obesity found that water fasting for an average of 17 days under medical supervision led to reductions in systolic blood pressure. It was also associated with improvements in levels of LDL (bad) cholesterol and inflammation. Similarly, another review concluded that water fasting could significantly reduce blood pressure levels and body weight in people with high blood pressure. Still, more research is needed to evaluate the link between short-term water fasts (24-72 hours) and blood pressure.
May Improve Insulin and Leptin Sensitivity
Insulin and leptin are important hormones that affect the body’s metabolism. Insulin helps the body store nutrients from the bloodstream, while leptin helps the body feel full. Research shows that water fasting could make your body more sensitive to leptin and insulin. Greater sensitivity makes these hormones more effective. For example, being more insulin sensitive means your body is more efficient at reducing its blood sugar levels. Meanwhile, being more leptin sensitive could help your body process hunger signals more efficiently, and in turn, lower the risk of obesity.
May Lower the Risk of Several Chronic Diseases
There is some evidence that water fasting may lower the risk factors of chronic diseases like diabetes, cancer, and heart disease. In one 2013 study, 30 healthy adults followed a water fast for 24 hours. After the fast, they had significantly lower blood levels of triglycerides - a risk factor for heart disease. Some research also suggests that water fasting may protect the heart against damage from free radicals. Free radicals are unstable molecules that can damage parts of cells and are known to play a role in many chronic diseases. Moreover, studies in animals and humans have found that water fasting may suppress genes that help cancer cells grow and may also improve the effects of chemotherapy. Keep in mind, only a handful of studies have analyzed the effects of water fasting in humans, so more research in humans is needed before making recommendations.
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Risks and Dangers of Water Fasting
Although water fasting may have some benefits, it comes with health risks.
May Lose the Wrong Type of Weight
Unfortunately, a lot of the weight you lose in the beginning may come from water, carbs, and even a small amount of muscle mass.
May Become Dehydrated
Although it sounds strange, a water fast could make you dehydrated because roughly 20-30% of your daily water intake comes from the foods you eat. If you’re drinking the same amount of water but not eating foods, you might not be getting enough water. Symptoms of dehydration include dizziness, nausea, headaches, constipation, low blood pressure, and low productivity. To avoid dehydration, you may need to drink more than usual.
May Experience Orthostatic Hypotension
Orthostatic hypotension is defined as a drop in blood pressure that happens when you suddenly stand up, and it can leave you dizzy, lightheaded, and at risk of fainting. If you experience orthostatic hypotension while fasting, you may need to avoid driving or operating heavy machinery because the dizziness and risk of fainting could lead to an accident. If you experience these symptoms during a water fast, this type of fasting may not be a good option for you.
Water Fasting May Worsen Several Medical Conditions
Although a water fast is relatively short, there are a few conditions that may be aggravated by water fasting. People with the following medical conditions should not water fast without first seeking advice from a healthcare professional:
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- Gout: Water fasting may increase uric acid production, a risk factor for gout attacks.
- Eating disorders: There is evidence that fasting may encourage disordered eating behaviors for some people.
Other risks of water fasting
Water fasting carries risks, including side effects such as mood changes, migraine headaches, and disordered eating. It can make you feel physically and mentally drained, too.
And even after you’ve finished an extended water fast, there’s another risk - refeeding syndrome. This is a potentially fatal complication that can develop when people reintroduce food too quickly after a prolonged fast. This can trigger severe shifts in levels of fluids and electrolytes.
Water Fasting vs. Other Types of Fasting
Like other types of fasting, water fasting can help you lose weight. However, if you want to reap the benefits of fasting but also want to lose weight, intermittent fasting and alternate-day fasting are probably more effective approaches than water fasting for 24-72 hours. These fasts provide similar health benefits but can be followed for much longer periods, as they allow you to eat food, decreasing the risk of nutrient deficiencies.
Intermittent Fasting
Intermittent fasting is a safer alternative to water fasting because it involves focusing on when you eat rather than what you eat. Several studies have shown that intermittent fasting can lead to weight loss and may also have helpful metabolic effects, like improving your blood sugar control and levels of blood fats. Although it’s still early days, emerging research suggests that intermittent fasting may positively affect brain health. It may, for example, improve mental health and memory. There are several types of intermittent fasting, and one is time-restricted eating (TRE). It involves eating within a set window of time, which may last for 4, 6, or 8 hours, for example. The rest of the day you fast. The good thing about TRE is that it’s flexible - you pick the time frame that suits you. And unlike water fasting, TRE allows you to meet your daily nutrient requirements. But, as with all fasting approaches, there’s not much evidence about the long-term effects, and there are still many questions about how sustainable intermittent fasting is.
Potential Downsides of Intermittent Fasting
Depending on the length of the fasting period, people may experience headaches, lethargy, crankiness, and constipation. Two common effects of calorie-restricted diets-a slowed metabolism and increased appetite-are just as likely when people practice intermittent fasting as when they cut calories every day. While intermittent fasting shows promise, there is even less evidence about the benefits or how fasting might affect older adults, and intermittent fasting could be risky in some cases.
Long-Term Complete Fasting
As an extreme dietary method, long-term complete fasting still requires an in-depth discussion of its health risks and safety assessment before its promotion and application. The influence of fasting on energy consumption is related to factors such as fasting mode, duration, and frequency etc. Long-term fasting refers to complete fasting (CF) for more than 2 consecutive days, usually ranging from 3 to 21 days. Studies have demonstrated that prolonged fasting and serious calorie restriction can significantly reduce the resting metabolic rate. Prolonged fasting markedly reduces the 24 -hour total energy consumption, and the respiratory quotient approaches 0.7, indicating that fat is the main source of energy substrate.
Study on 10-Day Complete Fasting
One study aimed to investigate the safety, time effects of metabolic homeostasis and health indexes during prolonged fasting. Thirteen participants were recruited to conduct a 10-day complete fasting (CF) in a controlled health research building under medical supervision including 3-day Baseline (BL), 10-day CF, 4-day calorie restriction (CR) and 5-day full recovery (FR). Body healthy status was assessed by surveying pulse, blood pressure, body weight (BW), blood glucose and ketones, body composition and nutritional and biochemistry indexes at different times. BW declined about 7.28 kg (−9.8%) after 10-day CF, accompanied by increased pulse and decreased systolic blood pressure, but there were no changes to the myocardial enzymogram. Body composition analysis showed fat mass was constantly lost, but lean mass could recover after CR. The energy substrate switch from glucose to ketone occurred and formed a stable dynamic balance between 3-6 days of CF. The lipid metabolism presented increased total cholesterol, LDL-C, ApoA1 and almost no changes to TG and HDL-C. Prolonged CF did not influence liver function, but induced a slight decrease of kidney function. The interesting results came from the marked increase of lipid-soluble vitamins and a significant decrease of sodium and chlorine. Adults could well tolerate a 10-day CF. A new metabolic homeostasis was achieved. No vitamins but NaCl supplement should be considered.
Study on 21-Day Complete Fasting
One study showed that healthy humans can tolerate a water-only fasting for up to 21 days without structural damage to major organs such as the liver and heart and provide experimental data and theoretical support for the application of long-term fasting to develop a rescue strategy for long-term food shortages under extreme conditions such as long-duration manned spaceflight etc. The study was a 34-day experiment protocol structured into four distinct phases: 3 days for baseline (BF) examination, 21 days of CF, 5 days with gradual refeeding diet characterized by calorie restriction (CR), and 5 days with full recovery diet (FR). During the 21-day CF phase, the volunteers were only permitted to consume Nongfu Spring mineral water ad libitum, which contains a mineral profile including calcium (Ca ≥ 400 µg/100 mL), magnesium (Mg ≥ 50 µg/100 mL), sodium (Na ≥ 80 µg/100 mL), potassium (K ≥ 35 µg/100 mL), and metasilicate (≥ 180 µg/100 mL). They were also encouraged to engage in light lifestyle activities within a health research building, such as watching TV, walking and reading. However, it was inhibited to engage in high-intensity, energy-consuming activities, as they may increase energy expenditure and pose unknown risks.
Impact of extended fasting on body weight and composition
During the CF period, the BW of the volunteers continued to decrease substantially. The average BW loss was 10.00 ± 1.66 kg (14.96%±1.55%) at the end of the 21st day of fasting (CF21), and was still significantly lower than baseline by 4.53 ± 1.61 kg at the 4th day of the recovery phase (FR4). In the early stages of fasting, the rate of BW loss was rapid, with an average daily decrease of approximately 1.12 kg during the first three days of CF compared to the previous day’s BW. In the later stages of fasting (from CF11 to CF21), the daily BW loss stabilized at an average of 0.31 ± 0.068 kg.