Drinking Enough Water May Lower Cortisol Levels

• Hydration is important for optimal health, but most of us are at least slightly dehydrated.
• Low hydration status may increase cortisol, the stress hormone, in your body. 
• Begin where you’re at and slowly increase your water intake over time.

About 50% to 60% of your body is made of water, so it makes sense that hydration is important for our health. According to recent statistics, most of us don’t consume enough water, with the average U.S. adult coming in at just 44 fluid ounces of water a day. This falls far short of the recommended 15.5 cups per day for men and 11.5 cups per day for women (or 124 fluid ounces and 92 fluid ounces respectively).

Water is involved in just about every process that takes place inside your body. From regulating your body temperature through sweat to transporting nutrients and oxygen to cells, hydration plays a key role in many of the body’s essential functions. That means that when you’re dehydrated, your joints may have a hard time staying lubricated and you may even end up feeling constipated.

Researchers from the U.K. wanted to know if hydration status plays a role in stress levels. They hypothesized that individuals with habitual low fluid intake and suboptimal hydration status would exhibit greater cortisol reactivity (the body’s cortisol response to a stressor) to acute psychosocial stress. In other words, do chronically dehydrated people have a greater stress response compared to those who are well-hydrated when exposed to the same stressful event? They published their findings in the Journal of Applied Physiology. Let’s break down what they found. 

How Was This Study Conducted?

Researchers recruited healthy, recreationally active men and women between the ages of 18 and 35. Before beginning the official study period, all potential participants recorded fluid intake from all sources, including water, milk, juice, coffee, tea, soda, alcohol and any other beverages for seven days. In addition, they were given containers to collect all urine produced between 4 p.m. and 8 p.m. on two different days, one weekday and one weekend day.

For this study, researchers only included those with low (less than 1.6 liters per day for males and less than 1.5 L/day for females) and high (at least 2.9 L/day for males and at least 2.5 L/day for females) fluid intake. To be categorized in either group, participants also had to meet criteria based on urine samples that suggested suboptimal (low) and optimal (high) hydration statuses.

Once participants entered into the study, they visited the lab and completed questionnaires about stress, anxiety and sleep. They were also informed about the mental performance task they would have to complete in the lab at the end of the study period.  

For the next seven days, participants were given a smart water bottle to make sure they met their recommended daily fluid intake, which was based on their baseline hydration numbers. On days five and six, participants submitted all of the urine they produced between 4 p.m. and 8 p.m. On days seven and eight, they fasted and took questionnaires at the lab, where they also gave another urine sample and blood sample. Also on day eight, the participants did their mental performance task, called the Trier Social Stress Test (TSST), which lasted about two hours. They were instructed to avoid eating, brushing their teeth and drinking anything with caffeine or alcohol two hours before they arrived. 

The Trier Social Stress Test involved a 30-minute acclimation period in a reception area, where participants could wind down from any stress they experienced before arriving at the lab. After acclimating, participants went through three elements while standing in front of two observers and a video camera: five minutes of prep, a five-minute mock job interview and a five-minute mental arithmetic task.

Saliva samples were collected at various intervals—pre-TSST and several post-TSST—to test cortisol levels. Participants’ heart rates were also continually monitored throughout the TSST using a chest strap heart monitor. 

What Did This Study Show?

After all data was collected and statistical analyses were run, researchers found that those in the low fluid group consistently had higher markers of stress in their saliva, urine and blood. Specifically:

• Despite similar increases in state anxiety and heart rate during the test, saliva cortisol increased significantly in the low hydration group but not in the high hydration group. 
• Pre-test hydration status was associated with the magnitude of saliva cortisol reactivity, with saliva cortisol reactivity being greater in the low hydration group.
• Darker urine pre-test (indicating low hydration status) was associated with greater saliva cortisol reactivity.

Researchers note that these results may be one possible explanation for why habitual low fluid intake and sub-optimal hydration status are related to poor long-term health outcomes, as water regulation and the body’s stress-response system share overlapping physiological pathways. 

One limitation of this study design is that it cannot infer causation, just correlation. In other words, it can only suggest that there is a connection between poor hydration status and cortisol levels. Conclusions about specific long-term health implications also cannot be drawn. Researchers suggest that larger, longer-term studies be performed to connect the dots between hydration, cortisol and health outcomes.

How Does This Apply to Real Life?

Cortisol is the body’s primary stress hormone. It’s released in response to both immediate and long-term threats, whether they’re real or perceived—like a jump scare in a horror movie. Cortisol is necessary when we need to take action against a perceived threat, but when it’s continuously released due to long-term stress, it can cause problems—physically, mentally and emotionally. After all, being in fight-or-flight mode can be pretty hard on you over time.

This study is interesting because it suggests that hydration status may partly determine how well you handle stress. Who knew that your water bottle could actually be a stress management tool?

If you’re one of the under-hydrated, it’s time to step up your hydration game. There are a few things to note, though, before you dive in. First, when you’ve been chronically dehydrated and start giving your body the hydration it needs, you might become bloated and feel puffy at first. This will pass if you just stay the course and continue hydrating—and yes, you will then be visiting the bathroom more often. (Just consider it an opportunity to get more physical activity.)  

Also, if you’re not used to hydrating well, drinking a lot more water each day may be overwhelming. As we’ve mentioned, experts recommend about 15.5 cups of water each day for men and 11.5 cups for women. (In this study, men saw benefits when drinking about 12.25 cups of water each day, while women drank about 10.5 cups.) 

If those numbers seem impossible, remember that it’s okay to break it down and start small. If you don’t think you could drink about 1 cup of water for each waking hour, try working your way up. Even starting with ½ cup of water each waking hour could help you change your habits and slowly get used to drinking more fluids. You could also track your water intake for a few days, take the average for your baseline and start gradually increasing your intake from there.

In addition to becoming and staying well-hydrated, other ways to combat stress include practicing deep (diaphragmatic) breathing and visualization, getting out into nature, going for a walk and doing other activities you enjoy. It’s also important to get to the root of chronic stress and determine what actions you can take to help resolve it. A therapist may be a good choice for you if you’re struggling with stress.

Our Expert Take

This study suggests that people with low hydration status and low fluid intake may have higher levels of cortisol, the stress hormone, when exposed to stress-inducing events. Higher cortisol may result in higher levels of inflammation and an increased risk of disease, including heart disease, cancer, arthritis, diabetes and autoimmune diseases. While most fluids can contribute toward your hydration status, it’s important to make sure you’re drinking enough plain water. If you don’t like plain water—or even if you do—consider switching things up with sparkling water or flavor-infused water using fruit and herbs. You’ll love our Lemon, Cucumber & Mint Infused Water and our Watermelon-Basil Agua Fresca.