- Religious or spiritual reasons
- To lose weight
- For “detoxing”
- For its health benefits
- Preparing for a medical procedure
Summary: Water fasting is a type of fast in which you are not allowed to consume anything except water. It is linked with a lower risk of chronic diseases and autophagy but also comes with many health risks.
Water Fast (24 to 72 hours)
Post-Fast (1 to 3 days)
Summary: A water fast usually lasts 24 to 72 hours and is followed by a post-fast phase. If you’re new to water fasting, you might want to spend three to four days preparing your body to be without food by reducing your portion sizes or fasting for part of the day.
It May Promote Autophagy
It May Help Lower Blood Pressure
It May Improve Insulin and Leptin Sensitivity
It May Lower the Risk of Several Chronic Diseases
Summary: Research shows that water fasting may lower the risk of many chronic diseases and promote autophagy. However, most research is from animal studies or are short-term. More studies are needed before recommending it.
You May Lose the Wrong Type of Weight
You May Become Dehydrated
You May Suffer From Orthostatic Hypotension
Water Fasting May Worsen Several Medical Conditions
- Gout: Water fasting may increase uric acid production, a risk factor for gout attacks (
7 , 34). - Diabetes: Fasting may increase the risk of adverse side effects in type 1 and type 2 diabetes (35).
- Eating disorders: There is some evidence that fasting may encourage eating disorders like bulimia, especially in teenagers (
36 ). - Heartburn: Fasting may trigger heartburn, as your body may continue to make lots of stomach acid without any food to digest (
37 ).
Summary: Although water fasting may have some health benefits, it also comes with many risks and dangers. For example, water fasting could make you prone to muscle loss, dehydration, blood pressure changes and a variety of other health conditions.
Summary: A water fast is not an effective way to burn fat, as it takes several days before your body starts to burn fat for fuel. However, other types of fasting can offer you the benefits of fasting and weight loss with fewer risks.
Diet advice, including fasting-based diets, can be found all over the Internet, and much of it is nonsense, so I was very skeptical about these latest claims.
This time, though, there might be something to it. The scientific study that my colleague told me about was published back in June by USC’s Valter Longo, who studies aging and longevity. In this paper, Longo and colleagues described remarkable metabolic changes that occurred as a result of prolonged fasting. They found that fasting for 3 days or longer–drinking only water and eating less than 200 calories per day–can truly “reset” some components of your immune system.
The research looked at both mice and humans. (It’s far easier to run the experiments in mice, of course, but we can’t always trust that the same effects will occur in humans.) In both species, fasting lowered white blood cell counts, which in turn triggered the immune system to start producing new white blood cells. White blood cells (or lymphocytes) are a key component of your body’s immune system.
Longo’s hypothesis is that fasting (or starvation) forces your body to “recycle a lot of the immune cells that are not needed” which explains the drop in the white blood cell count. Two of the key mechanisms are an enzyme called PKA and a hormone called IGF-1, both of which are reduced by fasting. Once you start eating again, your stem cells kick back into high gear to replenish the cells that were recycled.
The human part of the study was much more limited: a group of cancer patients fasted for 1, 2, or 3 days prior to chemotherapy. The idea is that fasting might reduce the harmful side effects of chemotherapy, particularly the immunosuppression caused by some chemotherapeutic drugs. These results are very preliminary: the patients are participating in a phase I clinical trial, which is designed to assess safety, not effectiveness. Nonetheless, the results indicate that a 3-day fast (but not a 1-day fast) was beneficial for these patients.
A key finding in this research is that you have to fast for several days to get any benefit: basically, you have to fully deplete your energy reserves (in the form of glycogen), and it takes your body at least 24 hours, and probably 48 hours or more, to do this. This is much harder than a 1-day fast, which many people do routinely.
On the other hand, Valter Longo has compared the effects of periodic fasting to long-term caloric restriction, which has been shown to prolong lifespan in mouse and other animals. In a separate review article, Longo wrote:
Caloric restriction is extremely difficult to achieve for humans: you have to nearly starve yourself for years. Compared to this, an occasional 3-day fast should be a snap.
Caveats: fasting can be harmful, especially for people who have other health problems. If you’re seriously thinking of trying this, you should consult your doctor first. And this preliminary evidence, though encouraging, is primarily based on mice, not people. We might eventually learn that the benefits of fasting are outweighed by other problems. Fasting for more than two days isn’t easy, either: you’re going to get really hungry.