by Nils Osmar

Ask most people how much water they should drink, and many will say “eight glasses a day.” It’s one of those health rules that has become so universal it feels like settled science. Others scoff and say that we don’t need that much; we should only drink when we’re thirsty.

What I found surprising when I researched the topic is that the recommendations from several leading health institutions in 2026 are actually significantly higher — and the story of how those recommendations were developed is in itself interesting.

What the Mayo Clinic Recommends

The Mayo Clinic, citing guidelines from the U.S. National Academies of Sciences, Engineering, and Medicine, recommends approximately 3.7 liters (about 15.5 cups) of total fluid per day for men, and 2.7 liters (about 11.5 cups) for women. ^[1]

When we do the math, it reveals that 3.7 liters is almost exactly one gallon.

If you’re a man drinking roughly a gallon of fluids daily — including your morning coffee — you’re right on target. Not excessive. Not extreme.

For women, the Mayo Clinic recommendation of drinking 2.7 liters of water per day breaks down to approximately 3 quarts — again, including beverages that include water, such as coffee or tea.

Crucially, these recommendations cover all fluid sources — drinking water, coffee, tea, milk, and even the water content of the foods you eat. The Mayo Clinic notes that food alone typically contributes about 20% of daily fluid intake, with water-rich foods like fruits and vegetables providing a meaningful contribution. ^[1]

If you eat a big slice of watermelon, for example, you’re getting a lot of water. If you eat some wild blueberries., you’re getting a much smaller amount.

How Did Mayo Clinic Arrive at That Number?

The underlying research comes from the Institute of Medicine (IOM), which published comprehensive dietary reference intakes for water in 2004.

The IOM concluded that it was not actually possible to set a precise universal requirement for water intake, due to extreme variability driven by body size, metabolism, climate, and activity level. Instead, they established “adequate intake” (AI) levels based on the median fluid consumption observed in healthy, adequately hydrated adults in national dietary surveys. ^[2] In other words: they looked at what well-hydrated, healthy people actually drink — and made that the benchmark.

How Other Organizations Compare

Not all health organizations agree on the same figure. Here’s a quick comparison:

Organization	Men	Women
IOM / Mayo Clinic (USA)	3.7 L (~1 gallon)	2.7 L
EFSA (European Food Safety Authority)	2.5 L	2.0 L
The “8 glasses a day” rule	~1.9 L	~1.9 L

The European Food Safety Authority (EFSA) arrived at its lower figures using a different methodology — basing recommendations on observed intakes in populations with healthy urine osmolality markers and appropriate water-per-calorie ratios. ^[2] The gap between the IOM and EFSA figures is genuine and reflects real scientific debate, not error on either side.

A point of agreement is that it’s important to stay hydrated. I’m not suggesting fixating on hydration in a fanatical way, but staying aware that serious health problems, including accelerated aging, can occur when we are not.

The “Eight Glasses” Myth

The IOM formally discredited the “eight glasses a day” guideline back in 2004. Their report stated that healthy adults can generally rely on thirst as a guide to hydration needs, and that fluid requirements are met through the combined intake of all beverages and water-containing foods — not plain water alone. ^[3]

The “8×8” rule (eight 8-ounce glasses) has persisted largely through repetition. At roughly 1.9 liters, it falls well below both the IOM and EFSA recommendations — and significantly below what research suggests healthy men actually consume.

Do Coffee and Tea Count?

Yes — with one caveat.

Moderate caffeine consumption does not cause dehydration. Research indicates that caffeinated beverages contribute to hydration on par with non-caffeinated options at normal intake levels. The IOM panel that authored the 2004 report specifically concluded that caffeinated beverages count toward daily fluid totals. ^[3]

The exception: intake above roughly 600mg of caffeine per day (around six cups of coffee) may require additional water to compensate for mild diuretic effects. ^[4] For most people drinking one to three cups a day, this is a non-issue.

Why Trust Mayo Clinic’s Recommendation?

At the risk of sounding like an ad for Mayo Clinic, they are considered a leading authority on health due to their non-profit model that combines expert clinical practice, extensive research, and education. They have no shareholders, no profit motive, distorting their recommendations. They are ranked among the top hospitals in the world, because of their focus on evidence-based medicine.

Why Hydration Matters to Healthy Anti-Aging

One of the most underappreciated consequences of inadequate hydration is its effect on blood glucose — with high glucose being a known trigger of accelerated aging.

When we are dehydrated, several things happen simultaneously that push blood sugar upward.

First, hemoconcentration: as the volume of water in your bloodstream decreases, glucose becomes more concentrated in the blood — even if the total amount of glucose hasn’t changed.

Second, dehydration triggers the release of vasopressin (also called antidiuretic hormone), a survival hormone that tells the kidneys to retain water. Vasopressin also signals the liver to produce additional glucose, and it reduces insulin sensitivity in peripheral cells — meaning the glucose already circulating has a harder time getting absorbed. ^[6]

On top of this, dehydration activates the stress response — releasing cortisol and adrenaline — which further stimulates liver glucose production while suppressing insulin function. The result is a feedback loop: dehydration raises blood sugar, and elevated blood sugar can cause osmotic fluid loss through the kidneys, deepening dehydration further. ^[7]

A landmark nine-year French cohort study following over 3,600 normoglycemic adults found that low water intake was independently associated with increased risk of developing high blood sugar over time — even after controlling for diet, exercise, and other metabolic risk factors. The researchers concluded that increasing water intake could represent a simple, low-cost intervention to reduce the risk of hyperglycemia and eventual diabetes. ^[8]

For anyone monitoring blood sugar — or interested in longevity more broadly — consistent hydration is not optional. It is a foundational metabolic practice.

What Chronic Dehydration Does to the Human Body

Most people associate dehydration with thirst and dry mouth. In reality, chronic mild dehydration — the kind that accumulates quietly over years — has been linked to a wide range of serious health consequences. The following are among the most well-documented:

• Elevated blood glucose and increased diabetes risk — via the vasopressin-gluconeogenesis pathway described above ^[6,8]
• Kidney damage and chronic kidney disease — recurrent dehydration contributes to progressive renal damage, particularly in those with diabetes or hypertension ^[9]
• Kidney stones — insufficient fluid concentration in the kidneys allows mineral crystals to form and aggregate ^[1]
• Cognitive impairment and accelerated dementia risk — a study of over 1,000 adults over 65 found that dehydrated individuals had significantly higher risk of dementia; dehydration may impair clearance of toxic proteins including beta-amyloid from the brain ^[10]
• Sustained attention deficits — Penn State research found that everyday dehydration (from normal activity, not exercise) measurably reduced the ability to sustain attention over time in middle-aged and older adults ^[11]
• Orthostatic hypotension and fall risk — reduced blood volume causes dizziness upon standing, significantly increasing fall and fracture risk in older adults ^[9]
• Cardiac arrhythmias — electrolyte imbalances caused by dehydration can disrupt normal heart rhythm ^[9]
• Urinary tract infections and constipation — both are closely associated with insufficient fluid intake ^[12]
• Accelerated biological aging — emerging research suggests chronic dehydration is associated with elevated serum osmolality, which has been linked to increased mortality risk and faster biological aging in longitudinal population studies ^[13]

The aging angle is important. The blood sugar–aging connection has been well-established through glycation research: elevated glucose accelerates the cross-linking of proteins (a process called glycation), contributing to tissue stiffening, inflammation, and cellular senescence.

Dehydration — by quietly nudging blood glucose upward — may therefore be an underappreciated contributor to accelerated aging that is almost entirely within our control.

Can You Drink Too Much?

Yes. Overhydration is also a very real condition — though for healthy adults drinking water steadily across the day, it is extremely rare. The primary risk is hyponatremia, in which sodium becomes diluted in the blood as water floods into cells, potentially causing nausea, confusion, seizures, and in severe cases, coma. ^[14] This concern is one of the reasons I take electrolytes when drinking filtered water.

Overhydration typically requires drinking large volumes rapidly — usually more than a liter per hour over extended periods. It is primarily seen in endurance athletes who overcorrect during long races by drinking far more than they sweat out without replacing electrolytes. ^[14] For most people drinking a gallon spread across a waking day, the kidneys — which can process roughly 20 liters per day — are not remotely stressed.

The practical signal: totally colorless urine (as opposed to pale yellow) can suggest you’re drinking more than your body needs in the moment. Pale yellow remains the target. Those with heart failure, advanced kidney disease, or certain electrolyte disorders should consult their physician about individual fluid limits.

A Note for Older Adults

One factor that doesn’t get nearly enough attention in mainstream hydration advice: the sense of thirst declines with age. Research shows that both thirst regulation and fluid intake drive diminish in older adults, meaning that relying on thirst alone becomes a less reliable guide over time. ^[5]

This makes conscious, consistent hydration — drinking on a schedule rather than waiting to feel thirsty — a genuinely smart longevity practice, not just a wellness trend. If you’re over 60, tracking your intake rather than relying on thirst signals is arguably one of the simplest, lowest-cost health interventions available.

The Practical Bottom Line

If you’re a man drinking roughly a gallon of fluid per day — including water, coffee, tea, and water-rich foods — you’re meeting the IOM’s recommended adequate intake, and doing what the Mayo Clinic suggests healthy, well-hydrated adults actually do. If you are a woman drinking 3 quarts of water a day, the same as true.

In the face of the somewhat conflicting recommendations that I mentioned above, a simple real-world check is urine color. Pale straw or lemonade-colored urine generally indicates good hydration. Apple juice color suggests you need more. ^[1] Urine that looks like water is an indication that you may be drinking too much.

Thirst and Other Factors

The Mayo Clinic recommendation is a good guideline from my perspective — I value their recommendations — but it of course doesn’t know us individually. Our level of activity, body weight, climate, and diet are also elements to be aware of.

Re: thirst, it of course matters. But it’s worth noting that the hypothalamus, the part of the brain that regulates both thirst and hunger, can mistake one for the other, causing dehydration to feel like hunger. This contributes to snacking and weight gain, which again can also accelerate pro-aging. If you find yourself reaching for snacks, with the result being weight gain and higher blood glucose levels, you might want to try water first — you may just be thirsty.

What I’m Doing

I live near a Seattle co-op (PCC) which sells a high quality purified water that has been through several types of filtration. I bring in 7 empty glass gallon bottles, and fill them once a week. I pop open a gallon in the morning when I’m making coffee, and aim to finish it by the end of the day.

I keep a tall glass of water next to my workstation, so I can sip on it without giving hydration a lot of thought during the day. I’m unlikely to think about getting up and going to the kitchen to fill a glass; but I find myself sipping water routinely and increasing my consumption significantly when I do this.

Now and then I’ll buy a case of Mountain Valley carbonated, water, which comes in a glass container, which I take with me when I’m teaching. I stop drinking water by around 5 or 6 PM, to minimize getting up in the middle of the night to urinate.

I keep take an electrolyte capsule in the morning to replace the electrolytes that have been filtered out.

References

[1] Mayo Clinic. Water: How much should you drink every day? mayoclinic.org

[2] Smedts & Mekkes. Water intake: validity of population assessment and recommendations. PMC / European Journal of Nutrition, 2015.

[3] National Academies of Sciences. Report Sets Dietary Intake Levels for Water, Salt, and Potassium. nationalacademies.org, 2004.

[4] Creme Global. How Much Water Do We Really Need? cremeglobal.com

[5] Harvard T.H. Chan School of Public Health. The Nutrition Source: Water. nutritionsource.hsph.harvard.edu

[6] Smiles Med Supply. Hydration and Diabetes: Why Drinking Water Is Crucial for Blood Sugar Control. smilesmed.com, 2025.

[7] Vital Health Facts. Does Dehydration Increase Blood Glucose? 2025.

[8] Roussel R, et al. Low Water Intake and Risk for New-Onset Hyperglycemia. PMC / Diabetes Care, 2011.

[9] StatPearls / NCBI Bookshelf. Adult Dehydration. ncbi.nlm.nih.gov, 2025.

[10] Alzheimer’s Drug Discovery Foundation. Can Dehydration Impair Cognitive Function? alzdiscovery.org

[11] Penn State University. Dehydration in Middle-Aged and Older Adults May Lead to Attention Challenges. psu.edu, 2024.

[12] GuideStar Eldercare. Dehydration, Cognitive Functioning, & ADLs. guidestareldercare.com

[13] Li S, et al. Hydration Status in Older Adults: Current Knowledge and Future Challenges. PMC / Nutrients, 2023.

[14] Cleveland Clinic. Water Intoxication. my.clevelandclinic.org, 2024.