How to Stay Hydrated While Fasting

By Jordan Rosenfeld

When it comes to fasting, a lot of emphasis is placed on what we eat (or don’t eat), which makes it easy to forget about another crucial element: staying hydrated. With the exception of people who fast from both food and water for the religious holiday of Ramadan, it’s generally not recommended to abstain from drinking water while fasting, and, in fact, dehydration can be a common side-effect of fasting even when you think you’re drinking enough water.

Remember there are numerous kinds of fasts. Here are just a few of them:

  • The 5:2 fast: You restrict your calorie intake for two days per week (to approximately 500 calories per day for women and 600 for men).
  • The 6:1 Pattern: Similar to the 5:2, you only reduce calorie intake to one day instead of two.
  • “Eat Stop Eat”: This is a 24-hour total — taking in no calories, only water, black coffee or unsweetened tea — one to two days per week.
  • The 16:8 fast: This fast involves only eating during an eight-hour window and fasting for 16 hours a day, every day of the week.
  • Prolonged fast: These fasts can be anywhere from 3 to 7 days and might include water only or a liquid diet. They should be done with the approval of your doctor. These run the greatest risk of greater dehydration if not vigilant.

Since you get approximately 20 percent of your fluid needs from food, if you don’t increase your fluids when you fast, you could easily become dehydrated. Whole carbs, such as oatmeal, whole grain pasta, and brown rice absorb water while cooking. Other foods such as soups, watermelon, celery and many other fruits and vegetables contain a significant amount of water naturally. So, eliminating these items might also drop your hydration levels.

In addition to the lost fluids from food, part of the reason for increased risk of dehydration is a surprising link between carbohydrates and hydration. Carbohydrates are stored in your body as glycogen and attached to that glycogen is water. For each gram of stored glycogen, about 3 grams of water are stored with it. This is one of the reasons why you tend to lose weight so quickly when you cut carbs — some of it is water weight, says dietitian Jaime Mass, RD. It’s also why people often seem to gain weight back so fast when they begin to reefed.

Estimating your fluid needs

While medical professionals use several methods to estimate fluid needs — the Weight Method (Holliday-Segar Method), the RDA Method (Energy Method), and the Fluid Balance Method — there isn’t enough research to validate whether one or the other is more effective, though you may choose one that feels right to you.

Zero’s nutrition expert, Nicole Grant, RD, recommends estimating fluid needs for healthy adults by 30–35 mL of water per kg of body weight to start and adjusting from there. If you continue exercising while fasting, increase fluids slightly, and take into consideration your age and any disease states you may experience. Additionally, some medications may require extra fluids to be taken with them. Finally, if you’re in an environment with high temperatures, always take in more fluids.

If you’re engaged in a prolonged fast of more than several days, you can be at higher risk for dehydration, so maintaining electrolyte balance during a fast will give you the best hydration outcome and limit risks.

Always pay attention to your thirst, which is an important cue for dehydration, but remember it can be easy to overlook thirst if you’re busy or distracted, so it may not be the most reliable way to stay hydrated.

Maintain electrolyte balance

Electrolytes are substances everybody needs to manage, regulate, and maintain important bodily functions. These are depleted through physical activity and fasting, so you want to be sure you’re getting enough of the following electrolytes in your feeding windows. Remember, too, that the best way to absorb electrolytes is through whole foods, not supplements, unless you are at an extreme deficit and require supplementation.

· Sodium. Sodium depletion leads to headaches and muscle spasms. Make sure to get the recommended daily allowance for your weight and gender. For prolonged fasts of three or more days, you may need to supplement with bouillon for extra sodium. Depending on the type and duration of your fast, you may also need to increase your sodium intake greater than the recommended daily amount (RDA). On the other hand, certain populations need to be more cautious about their sodium intake, such as those with congestive heart failure, renal disease, or hypertension and may need to consume less than the RDA. Consult with your doctor.

· Potassium. The main job of this mineral is to regulate fluid in muscles and help nerves send signals. You can thank potassium for keeping you free of painful leg spasms.

· Magnesium. Among many jobs, this helps regulate muscle and nerve function and can aid in sleep. For prolonged fasts of three or more days, consider adding a magnesium supplement.

· Zinc. This mineral is well known for supporting testosterone production and nerve function. Don’t take this on an empty stomach, however — it can upset your tummy.

· Calcium. Calcium is key to your bones and your musclesIt helps your blood clot, your heart to beat, and your muscles to contract. You should be fine getting this in the form of food during your feeding windows.

Some signs that your electrolytes are too low include such symptoms as headaches, heart palpitations, lightheadedness, muscle cramps, and fatigue to name a few.

Analyze your urine

If you’re not quite sure of your hydration level, and you aren’t taking any medication or supplement that changes the color of your urine, Grant recommends using the color of your urine to indicate your general level of hydration. A concentrated yellow color likely equals dehydrated; very clear urine can mean overhydrated; and a light straw color is what you’re aiming for — it reflects good hydration.

Whatever approach you take to hydration, don’t make it a last priority when fasting. Proper hydration will make you feel better while fasting and help smooth your transition back to eating.