Q&A: Pros and cons to water fasting
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Key takeaways:
- Consuming nothing but water can cause significant weight loss but without sustained metabolic benefits.
- One researcher said the practice seems to be safe but comes with risks.
Water fasting offers clinically significant weight loss, but metabolic benefits seem to be short-lived, according to an expert.
Krista Varady, PhD, a professor of nutrition at the University of Illinois, Chicago, and colleagues wrote in Nutrition Reviews that water fasting — the practice of consuming little to nothing but water — has grown in popularity recently.
So, Varady and colleagues conducted a narrative review to summarize the effects that prolonged water fasting might have on various metabolic health measures like weight, glycemic control, blood pressure and more.
Healio spoke with Varady to learn more about water fasting, the difference medical supervision can make and more.
Healio: What is water fasting? Why do people decide to do this?
Varady: I didn't really realize how popular this was up until probably last fall, when I started getting a lot of journalists who were contacting me about prolonged water fasting. I think it might have gotten popular because intermittent fasting became popular 5-ish years ago, and intermittent fasting — where people have a shortened window of time per day [to eat] — definitely works for weight loss. But it’s not a crazy amount of weight loss. It’s usually 2% or 3% over the course of 6 months. So, I’m guessing maybe people are trying to boost their weight loss by just eating nothing for 5 to 20 days or so. In general, water fasting involves eating very little food, like 250 calories per day. Usually it’s a juice, or a soup, or no food at all, then just consuming water. That’s what’s been tested in the literature.
It seems like a lot of celebrities or athletes are doing this now, so since I was getting so many questions about it ... I thought I should do a formal data synthesis to figure out what’s really going on with these diets. So that’s what we did.
Healio: Is there any medical benefit to this?
Varady: I think in the short term, definitely, yes. People lose anywhere from 2% to 10% of their body weight and then blood pressure tends to almost always go down with these diets — probably because their salt intake has gone down. And then people sometimes see improvements with decreases in the bad cholesterol, LDL cholesterol. Sometimes, fats go down so triglyceride levels go down too. And it’s also been shown to help out with some diabetes risk indicators, so it decreases glucose levels, insulin levels and insulin resistance.
So yeah, I think there is benefit. But then in any of the studies that look at what happens after you’ve done the water fast ... they all found that none of those metabolic benefits were maintained, even if people maintain their weight loss, which was interesting. So short-term metabolic benefits? Yes. Long term? I don’t think so.
Healio: What are the pros and cons?
Varady: It definitely works for weight loss. I think the first 2 to 3 days ... are pretty difficult, but then once the ketone levels are high enough, that kind of shuts off your hunger response. I think it does become a little easier after 3 or 4 days of fasting, so I think that's one of the benefits. And if people can maintain the weight loss, that's great, but I think we need more studies showing what the long-term effects on body weight are.
It would be super difficult to socialize doing this. I don’t know what you do when you’re sitting with your family or going out with friends for dinner. Do you just sit there, drink water, tea or something? I’m assuming it would just make you kind of have to stay at home because, when you’re fasting, being around people who are eating is pretty difficult. It makes you want to eat, just being around that. So, I think socializing would be difficult. I think the hunger in the first couple of days would be really tough, too. It seems like it’d be really hard to cope with the hunger issues.
Healio: After completing this research, what is your conclusion? Would you recommend this any patients?
Varady: I probably wouldn’t, honestly. I'm a little bit biased though, because I study intermittent fasting ... and with that diet, we're noticing that people lose weight — we usually see about 5% of body weight loss over a year — and then they actually keep it off as well and then they tend to notice a few metabolic benefits as well that are sustained. So, I would probably do that instead.
I worry that if people are doing water fasting on their own and have any medical conditions, it could be really dangerous. People with type 2 diabetes and type 1 diabetes, if they do this without their doctor and without modifying all their medications ... they could pass out from hypoglycemia. So, I would probably not recommend it.
But you know, if somebody wants to do it for 2 to 3 days, I guess go ahead, but definitely not without medical supervision long term. Also, probably not just because we don’t even know the long-term effects and, right now, the minimal amount of research we have is pointing toward no sustained metabolic benefits.
Healio: What should primary care providers tell patients who are asking about this? What should they take away from your study?
Varady: It does result in weight loss, but it’s probably pretty difficult for people to do. Also, there’s not actually a ton of safety issues, which I was surprised by. A lot of these studies were medically supervised, so I think that’s why we didn’t see many adverse events. When they’re medically supervised, they can be safe. But I definitely wouldn’t recommend people do this on their own. And as I mentioned before, people can do it for weight loss if they wanted, but I probably wouldn’t do it if you’re looking to lower your cholesterol levels or blood pressure long term, because there’s no data showing that it works long term.
Healio: Is there anything else that you would like to add?
Varady: The study showed that it resulted in a lot of muscle mass loss even though this was only measured in a couple of different studies. That’s pretty concerning. How much muscle mass you have dictates your calorie needs. It’s the main thing that drives what your resting metabolic rate or your metabolism is, basically, so the fact that people lost so much muscle mass is definitely concerning.
Also, as people get older, they tend to lose muscle, which can make people kind of frail, so I probably wouldn’t recommend it for older people — people over the age of 50 or 60.