Dropping weight fast by drinking only water for a week may be enticing, especially if you have a special event coming up — but it's not a good idea. With a seven-day water fast, weight loss is probably inevitable, but it can be dangerous, especially if you do it without medical supervision.
The good news is that upping your water intake while following a balanced diet can help you lose weight in a healthy way. You may not reach your goal in seven days, but the weight loss will be more sustainable over the long term, and that's really what you should be striving for anyway.
There's no way to say exactly how much weight you would lose by drinking only water for a week, but the bottom line is that you shouldn't do it. Unsupervised water fasts can cause more harm than good, and you'll regain the lost weight once you start eating normally anyway. Aim for a weight loss of 1 to 2 pounds per week by eating healthy and exercising.
The Dangers of Water Fasting
You might be wondering, "Will I lose weight if I only drink water?" The short answer is: probably. But the risks of drinking only water for a week aren't really worth it, especially since any weight that you lose by eliminating food completely is likely to come right back once you start eating normally again.
According to a comprehensive review on water fasting that was published in BMC Complementary and Alternative Medicine in February 2018, there are several types of adverse events that can occur, even during a medically supervised fast. While most of these adverse reactions were classified as mild, some were considered moderate and a couple were severe. The report listed the following risks associated with water fasting:
- High blood pressure
- Severe lightheadedness
- Back pain
In two cases, there were also serious reactions to the water fasting that required hospitalization. One was characterized by headache, fever and high blood pressure from severe dehydration; and the other involved hyponatremia, which is a potentially fatal condition that develops when you drink too much water and the sodium levels in your body get too diluted. Luckily, both patients recovered.
The review does conclude that, since most adverse reactions were mild, water fasting can probably be safely done in a medical setting that allows intervention if something goes wrong. You shouldn't attempt to do it alone at home without supervision. Additionally, instead of using a seven-day water fast for weight loss, simply try upping your water intake while also following a healthy, balanced diet.
Read more: Water Fasting Side Effects
Water and Weight Loss
But just because drinking only water for a week isn't a good idea doesn't mean that water can't help you lose weight. A study published in the Journal of Human Nutrition and Dietetics in October 2016 looked at what could happen if you increase your water intake by just 1 percent, and they were surprised to find that this simple change had far-reaching effects.
Researchers from the study pointed out that this small increase in water consumption made participants consume fewer calories by decreasing their intake of sugar-sweetened beverages, fat and sugar. They also found that people who drank slightly more water took in less sodium, cholesterol and saturated fat over the course of the day. All of these factors together could not only help you lose weight, but also improve your health status, in general.
The researchers from another (very small) study that was published in Obesity in August 2015 wanted to see what would happen if participants drank water right before calorie-restricted meals — a strategy they called "pre-loading." They found that participants who drank a standard-size bottle of water (16.9 ounces) 30 minutes before eating lost about 4.5 pounds more weight over a 12-week period than participants who restricted calories but didn't "pre-load."
They weren't entirely sure why water had this effect, but speculated that it might have been due to participants eating less because they felt more satisfied going into the meal.
Read more: What is a Healthy Weight Loss Per Week?
Water and Body Fat
But is that weight loss simply from water weight? Not according to another small study published in the Journal of Natural Science, Biology and Medicine in December 2014. Researchers from this study didn't just look at total weight lost from drinking more water — they looked specifically at body composition too.
They instructed overweight females to drink about six extra cups of water (on top of their daily needs) per day. The researchers found that the extra water increased metabolism and helped the women lose weight in the form of body fat, reducing their body mass index (or BMI) by more than 3 points.
The participants also reported feeling less hungry on a daily basis, meaning they likely ate less, which partly contributed to their weight loss. Again, these studies were small, but they provide promising insight into how water can help you lose weight when it's part of a healthy diet.
Replacing Sugary Drinks With Water
Another strategy is to replace any sugar-sweetened (or calorie-containing) beverages, like soda, fruit juice, lemonade and sweetened iced tea with water. In a study published in the American Journal of Clinical Nutrition in March 2012, researchers looked at how much weight you could lose simply by making this switch alone.
They reported that in a six-month period, participants in the study lost 2 to 2.5 percent of their body weight. In theory, that means that someone who was 175 pounds would have lost between 3.5 and 4.5 pounds. That might not seem like a lot if you look at the numbers by themselves, but when you consider that the participants had to make only one small lifestyle change, it's significant.
But weight loss wasn't the only benefit. Participants also left the study with lower fasting blood glucose (which decreases the risk of developing Type 2 diabetes) and lower blood pressure. They were also better-hydrated than when they began.
Taking the patient approach by combining increased water intake with a healthy diet may not appeal to you as much as drinking only water for a week. But, it will be more beneficial in the long run.
- Journal of Human Nutrition and Dietetics: "Plain Water Consumption in Relation to Energy Intake and Diet Quality Among U.S. Adults, 2005-2012"
- BMC Complementary and Alternative Medicine: "Is Fasting Safe? A Chart Review of Adverse Events During Medically Supervised, Water-Only Fasting"
- American Journal of Clinical Nutrition: "Replacing Caloric Beverages With Water or Diet Beverages for Weight Loss in Adults: Main Results of the Choose Healthy Options Consciously Everyday (Choice) Randomized Clinical Trial"
- Journal of Natural Science, Biology and Medicine: "Effect of Excessive Water Intake on Body Weight, Body Mass Index, Body Fat, and Appetite of Overweight Female Participants"
- Obesity: "Efficacy of Water Preloading Before Main Meals as a Strategy for Weight Loss in Primary Care Patients With Obesity"
Lindsay Boyers is a holistic nutritionist with a Bachelor's degree in food and nutrition and a certificate in holistic nutrition consulting. She has a background in functional nutrition and 8 published books, including The Everything Guide to Gut Health, The Everything Guide to the Ketogenic Diet, and The Everything Guide to Intermittent Fasting.