How does hydration affect metabolic health?

Staying hydrated is vital for overall health and optimal functioning of the human body. Whether you drink too little, too much, or just the right amount of water can impact everything from the health of your organs to your ability to concentrate to how efficiently your body delivers nutrients and clears waste from your cells.

Your hydration status has implications for metabolic health, too. While research is mixed on the long-term metabolic effects of hydration (and dehydration), we know that water seems to play an important role in glucose processing. At the same time, when you drink water—and how much you drink at a time—could impact the likelihood of a blood glucose spike.

Below, we explore the perks of staying hydrated, highlighting emerging research on its metabolic effects. We also discuss what it means to be hydrated and how to optimally time your water intake for the biggest potential benefits.

What does it mean to be hydrated?

In simple terms, hydration means that your body contains the right amount of water to operate properly—which is quite a bit. Our bodies are around 55 to 60 percent water. Certain vital organs are composed of even more (for example, the brain and muscle are around 75 percent water), and all of our cells require fluids to maintain their structure and function.

But hydration is about more than drinking enough water—it’s also a matter of that water being distributed to the right places. When the water we drink flows through our digestive system, it’s absorbed by the intestines into blood circulation, where it can travel to tissues and organs. Some of this water enters our cells via semi-permeable cellular membranes, contributing to cell volume (i.e., cell size) and function, while the rest stays in circulation or becomes part of the fluids surrounding cells. The body needs to maintain relatively stable levels of water in these three areas in order to, for example, lubricate your joints, support cell signaling, enable enzymes to carry out metabolic reactions, and transport oxygen, nutrients, and hormones through the bloodstream to the body’s various tissues.

Electrolytes such as sodium and potassium, along with other solutes, help maintain fluid balance by creating what are called osmotic concentration gradients—this is when the concentration of solutes is higher on one side of a semi-permeable cell membrane than another, which helps shift fluids where they’re needed most. For example, when you’re dehydrated and the concentration of solutes is higher in the blood than in the cells (known as hyperosmolarity), water is drawn from the inside of cells to the extracellular space and then into the bloodstream, essentially shrinking the cell. Among other things, this can cause your tissues to contract—for example, brain tissue can essentially shrink and pull away from your skull, putting pressure on nerves and contributing to dehydration headaches. The body can also actively move electrolytes into or out of cells to optimally adjust fluid levels. This is why drinking enough water and maintaining proper electrolyte balance are both important for hydration. (More on how much to drink and when to supplement electrolytes below.)

Why hydration matters for overall health

Before we dive into water’s impact on metabolic health specifically, let’s look at the multitude of other ways staying hydrated can support health and help you feel your best:

• Healthy digestion: Water in the small intestine (along with digestive juices, which are composed of water mixed with enzymes) is used to help break down food into its individual nutrients, which supports optimal nutrient absorption. For dehydrated individuals, increasing water intake may also help alleviate constipation by softening stool and making it easier to pass.
• Nutrient delivery + waste elimination: Water is important for maintaining healthy blood volume and circulation (around half of your blood volume is water). Circulating blood is the transport system that delivers nutrients, hormones, and oxygen to cells, tissues, and organs. It also carries metabolic waste products to the kidneys for filtration and elimination via urine. So when you’re dehydrated, this essentially means there’s less blood available to efficiently transport these substances.
• Body temperature regulation: Being hydrated helps keep core body temperature within a healthy narrow range. Thanks to its thermal conductivity, water can absorb and rapidly transfer body heat to the skin as sweat, which has a cooling effect as it evaporates. With inadequate fluid intake, however, sweat output is insufficient to offset increases in core body temperature.
• Cognitive function and mood: Some studies suggest mild dehydration can negatively impact cognitive performance, alertness, and mood—but these symptoms typically improve upon drinking water. The exact mechanism is unclear, but dehydration may act as a physiological stressor that pulls attention away from cognitive processes.
• Headache relief or prevention: Dehydration headaches are well documented and thought to occur due to intracranial dehydration, or a lack of sufficient water within the skull cavity. The good news: They typically resolve within 30 minutes to three hours of drinking water. Dehydration may also be a trigger for people with chronic migraines.
• Healthy blood pressure: The decreased water content and elevated sodium and solute concentration of the blood when you’re dehydrated prompts your body to release arginine vasopressin (AVP), also known as antidiuretic hormone, in order to reduce the water in urine and conserve water. The hormone also constricts blood vessels, causing a temporary rise in blood pressure, which falls once you rehydrate. Research suggests regular bouts of dehydration may alter blood vessel function over time and potentially increase risk for hypertension and other cardiovascular issues.
• Physical performance and comfort: Water helps keep things well-lubricated. It’s a component of tears, saliva, and synovial fluid in the joints, and it adds elasticity to tissues, which can help prevent injury. A higher ratio of intracellular water per unit of lean mass has also been associated with strength and functional capacity.

Overall hydration status and metabolic health: What science says

A number of epidemiological and observational studies suggest that being properly hydrated is associated with having fewer metabolic risk factors.

A 2023 study analyzing data from 11,255 older adults over a 25-year period found that people with higher circulating sodium levels (which indicates poor hydration in otherwise healthy people) were more likely to experience advanced biological aging, a measure based on 15 factors, including blood sugar, cholesterol, and blood pressure. They were also more likely to develop a chronic disease like diabetes or heart failure.

Additionally, a 2020 Korean study analyzing data from 14,344 adults found that increased urine specific gravity (an indicator of low hydration status and high AVP levels) was associated with insulin resistance and increased abdominal fat. (Abdominal fat is highly correlated with metabolic syndrome.) And a 2021 meta-analysis found that low water intake was correlated with increased Type 2 diabetes risk.

Several interventional studies suggest a connection, too. It’s long been known that people with existing metabolic disorders such as Type 2 diabetes struggle to maintain fluid balance (high levels of glucose in the blood actually draws water out of cells and into the bloodstream in an attempt to balance the excess glucose in your system, triggering whole body dehydration. More recent research suggests that when these individuals don’t consume enough water, it’s even harder for them to control their blood glucose levels. In a 2017 study, three days of low total water intake (considered “acute hypohydration”) among men with Type 2 diabetes worsened response to an oral glucose tolerance test (OGTT), reduced disposal of glucose from the body during the test, and elevated cortisol levels. This suggests that short-term dehydration may impair blood glucose control in people with Type 2 diabetes, and cortisol may play a role (more on cortisol and other mechanisms below).

On the other hand, among people without diabetes, the effect of short-term dehydration on metabolic health appears to be less detrimental. In a 2019 study, healthy men and women spent an hour in a heat tent to stimulate acute hypohydration, which was followed by either fluid restriction or fluid replacement, and an OGTT. While the fluid restriction group experienced clear markers of dehydration (such as decreased cell volume, decreased muscle water, and increased copeptin, which is a marker for elevated AVP), both groups experienced similar glucose and insulin levels after the test. This appears to suggest that an acute bout of low fluid intake among otherwise healthy people may not negatively impact metabolic health—potentially because these people have a greater capacity to maintain homeostasis, or a state of physiological balance, in the face of metabolic challenges.

The real problem for generally healthy people may be sustained or chronic low water intake. While there’s an absence of interventional studies in this area, some researchers speculate that chronic low-grade hypohydration is a risk factor for Type 2 diabetes, obesity, and metabolic syndrome. Additionally, subtle elevations in copeptin are associated with increased risk of metabolic syndrome, diabetes, cardiovascular disease, and death. And it’s not uncommon for habitual low-volume water drinkers who may not realize they’re dehydrated to reach these problematic levels, some researchers say.

Overall hydration status and metabolic health: The why

There are several mechanisms at play to potentially explain the metabolic risks of subpar hydration, and these mechanisms may all be interrelated. Here are three processes often cited in the research:

• The cell volume connection: The presence of water within cells is important for a variety of reasons, including that water acts as a metabolic signal, facilitating all biochemical reactions and supporting normal enzyme activity. Studies on cell cultures have shown that adequate hydration leads to increased cell volume, which boosts cellular signaling in response to insulin, suggesting improved insulin sensitivity. Additionally, human research from 2003 suggests that increased cell volume from increased fluid intake leads to increased cell volume and may help reduce the breakdown of liver glycogen into blood glucose and increase lipolysis and subsequent fat oxidation (the breakdown of fats for energy). Similarly, a 2016 research review on animals suggests that an increase in water intake may increase lipolysis, potentially due to enhanced mitochondrial function in adipocytes.
• The vasopressin connection: Arginine vasopressin (AVP) is released during dehydration in response to low water and high sodium/solute concentration in the blood (i.e. hyperosmolarity). Research suggests elevated AVP levels activate V1a receptors on the liver, stimulating the release of stored glucose and the production of glucose from non-carb substrates, like amino acids—both of which increase blood glucose. AVP also triggers cortisol secretion, which further ramps up glucose production. It may operate through other mechanisms as well.
• The uric acid connection: Dehydration also activates an enzyme called aldose reductase. This enzyme fires up the polyol pathway, which prompts your body to convert circulating blood glucose into fructose. The metabolism of fructose by the liver, in turn, stimulates the production of an enzyme called AMP deaminase and creates uric acid as a byproduct. Both AMP deaminase and uric acid appear to block fat oxidation and promote fat accumulation. Interestingly, an animal study from 2019 suggests that uric acid can simultaneously activate the polyol pathway to create more fructose from glucose and promote fructose metabolism to create more uric acid—essentially driving a vicious cycle that contributes to the development of serious metabolic problems like fatty liver (which is associated with insulin resistance). Additionally, too much uric acid can produce unhealthy levels of reactive oxygen species, which drive oxidative stress and insulin resistance. It can also counteract nitric oxide, a molecule that promotes healthy blood flow, and this can prevent insulin from reaching target tissues.

This is an emerging area of research, and these studies don’t prove causation. More robust research is needed, particularly on the effects of rehydration (or increasing water intake) on improving metabolic health.

The metabolic impact of water intake at individual meals

Staying hydrated is important for metabolic health, but be mindful about how much you’re tipping back when you’re eating a meal or snack. Preliminary research suggests that pairing a big glass of water with a meal, particularly one containing carb-rich foods, may exacerbate post-meal blood sugar spikes. On the other hand, spacing significant water intake from meals by about 30 minutes may promote metabolic benefits.

In a small 2018 study on healthy participants without diabetes, those who drank a bottle of water with a jelly donut experienced significantly higher elevations in post-meal blood glucose compared to people who didn’t drink water or who drank water 30 minutes before or after eating the donut. This supports older research showing that drinking 10 ounces of water with a meat and potatoes meal increased post-meal blood glucose levels in healthy individuals and people with well-controlled diabetes. The likely reason: The more water you drink, the faster your gastric emptying rate (the rate at which food passes from the stomach to the intestines), which can speed glucose absorption into the bloodstream.

While taking small sips of water during meals to wash down your food is probably fine, consider spacing larger amounts of water about a half hour from your meals. In a 2021 study, participants with Type 2 diabetes who drank water 30 minutes before each main meal—for a total of one liter—experienced reductions in fasting blood sugar, triglycerides, LDL cholesterol, and copeptin after eight weeks.

Metabolic researcher Dr. Rick Johnson says that drinking plenty of water before salty meals or snacks may be particularly beneficial. High salt intake contributes to elevated blood osmolarity (just like dehydration) and the metabolic consequences outlined above, but pregaming with plenty of water can dilute those negative effects.

The best hydration plan

Staying hydrated doesn’t have to be overly complicated. Your best bet:

• Keep a refillable water bottle with you and drink throughout the day.
• Avoid chugging water at meals (take small sips instead).
• Space larger quantities of water 30 minutes before or after meals.
• Always drink before salty meals.
• Don’t let yourself get too thirsty. (If you’re thirsty, you’re already mildly dehydrated.)

The exact amount of water you need can vary from day to day and depends on factors like where you live and your BMI, activity levels, age, and health status. If you want a starting point, the National Academies of Medicine says most people’s needs can be met with a total water intake (from foods and beverages) of around 15.5 cups per day for men and 11.5 cups per day for women. Because we tend to get around 20 percent of our water from food, men should aim for around 13 cups (3 liters), and women should aim for 9 cups (2.2 liters) of water from beverages per day.

Caffeinated beverages like coffee and tea can contribute to your total daily fluid intake. Concerns have been raised about their diuretic effect, but research suggests this effect is short-lived, and no convincing evidence links caffeinated beverages to cumulative total body water loss over the course of a day. The water content of these beverages may also balance out the diuretic effect of typical levels of caffeine. (The FDA recommends capping your caffeine intake at around 400 mg per day, and a 12-ounce “tall” Starbucks dark roast has 195 mg).

Signs you may be dehydrated include:

• Headaches
• Fatigue
• Dizziness
• Dry mouth
• Elevated heart rate
• Muscle cramps
• Swollen feet
• Constipation
• Dark urine

Your urine is a great indicator of hydration status: Pale to light yellow urine indicates you’re well hydrated, while darker yellow is a sign of dehydration and brown urine warrants medical attention, per the U.S. Army Public Health Command.

Wondering if you need electrolytes? The average person who engages in moderate physical activity (even when exposed to higher-than-usual temperatures) probably gets plenty of sodium and other electrolytes from their diet to support proper hydration. On the other hand, endurance athletes or highly active people could potentially benefit from electrolyte beverages or supplements, as they lose far more electrolytes via sweat. Additionally, ketogenic diets have a diuretic effect that contributes to electrolyte loss via urine, and it’s important for anyone on these diets to prioritize adequate water and electrolyte intake and pay attention to how they feel.