How Much Water Should You Drink a Day? A Functional Medicine Answer

By Dr. John Bartemus, DC, CFMP, Functional Medicine Charlotte, PC. Last updated June 14, 2026.

A patient story: when the problem was water

One of my patients, Desiree, came in with lab evidence of dehydration. Her blood albumin was high and her urine specific gravity was elevated. Both are markers that point toward a body running short on water. She was not collapsing from heat stroke. She was mildly, chronically underhydrated, and it was showing up in how she felt and functioned.

This is more common than most people assume. Research shows that healthy women often do not drink the recommended daily amount of water. That matters because the consequences are not limited to thirst.

What the research shows: dehydration changes how your brain works

In a 2018 study published in Physiology & Behavior, researchers tested cognitive performance in healthy young women under two conditions: well hydrated and mildly dehydrated. The finding was direct. Mild dehydration caused deficits in visual and working memory and executive function.

Translated into plain English: not drinking enough water caused otherwise healthy adult women to experience memory lapses and a reduced ability to focus, plan, and follow through. When the women were dehydrated, they made roughly 12 percent more total errors on cognitive testing. Performance improved once they were properly hydrated again.

Stachenfeld, Nina S et al. “Water intake reverses dehydration associated impaired executive function in healthy young women.” Physiology & Behavior, vol. 185 (2018): 103-111. doi:10.1016/j.physbeh.2017.12.028

Here is the honest limitation: this was a small study of 12 healthy young women, so we should not overstate it. But here is the reasonable question it raises. If suboptimal hydration causes brain fog and memory loss in healthy women, is it likely to cause the same problems in women who are not healthy? In men? I would be willing to bet on the affirmative. The physiology of water and the brain does not stop at the edges of one study population. Treat that as a clinical inference, not a settled fact, and the practical takeaway holds either way: hydration is worth getting right.

How much water is too little? The 1.5 percent rule

You do not have to be visibly parched to be impaired. Losing just 1.5 percent of your body mass through water loss is enough to cause mild dehydration. That can happen by exercising in a warm environment or simply failing to keep up with fluid losses across a normal 24-hour day.

As the researchers put it, this kind of dehydration “may be more insidious because humans are likely to be unaware of these small consistent water losses, and thus do little to prevent them.” In other words, the deficit is small enough that you never notice the cause, only the symptoms.

And the symptoms are familiar: fatigue, headache, dizziness, and poor performance. Before you reach for a stronger coffee or assume something is medically wrong, it is worth asking a simpler question. Could this be as basic as not drinking enough water? It is quite possible.

How much water should you actually drink per day?

The 2018 study gives a usable number. The cognitive deficits “were reversed by drinking water to the requirements of 2.5 liters (about 85 ounces) per day for adult women.” That is the fluid target that restored normal memory and executive function.

For broader context, the National Academies of Sciences set adequate total water intake at about 2.7 liters (91 ounces) per day for women and about 3.7 liters (125 ounces) per day for men. One important nuance: those totals include water from food, which supplies roughly 20 percent of daily intake. The 2.5-liter figure from the study refers specifically to fluid you drink.

So how do you set your own number? A few reference points:

• Women: aim for at least 2.5 liters (about 85 ounces) of fluid per day.
• Men: aim higher, in the range of 3 to 3.7 liters, since men carry more lean mass and lose more water.
• Everyone: a practical starting target is half your body weight in ounces. A 180-pound person would aim for about 90 ounces.
• Adjust upward for heat, exercise, illness, high altitude, and intake of caffeine or alcohol, all of which increase fluid needs or losses.

These are starting points, not rigid prescriptions. People with certain heart, kidney, or liver conditions may need to limit fluids. If you have a diagnosed condition, set your target with your clinician.

Desiree’s “easiest hard” assignment was simple: drink at least 85 ounces of water a day. I tell patients that drinking enough water is the hardest easy thing, or the easiest hard thing, you can do, whichever way you prefer to look at it. It is not complicated. It is just easy to skip. But it is worth doing.

Drinking enough water is only half the battle

The other half is drinking the cleanest water you can. Quantity and quality both matter, and this is where filtration comes in.

Water filtration ranges across a wide scale: from the Brita pitcher in your refrigerator, to a Berkey countertop system, to a LifeSource Water Systems whole-home filtration system. My advice is straightforward. Get the highest-level system you can afford.

The best move I ever made for my family was installing a whole-home filtration system from LifeSource. I have had it for over eight years without a single problem. There is no filter you have to swap out on a schedule, because it is a self-cleaning system. Before installation, I could smell the chlorine in my municipal water while showering. Five minutes after installation, I took a shower and have not smelled or tasted chlorine since.

My kids are spoiled by the quality of our water. When they drink water at a restaurant or refill their bottles from another source, they notice immediately and comment that it tastes like metal or like a pool. I do not want my family consuming chlorine, fluoride, metals, or microbes from our water supply, and LifeSource removes all of that.

I made an in-depth video on the benefits of a whole-house water system, which you can watch here: “Is Toxic Water Causing Your Chronic Disease?”

You can learn more at LifeSource Water Systems. If you purchase a unit, tell them John Bartemus sent you and you will save at least $100.

Full disclosure: I do earn a commission if you purchase a unit through that link. I share it because I use the product myself and believe in it. Helping you and your family drink cleaner water is the point.

Want to go deeper on hydration and heart health?

Hydration is not only a brain issue. It directly affects blood volume, blood pressure regulation, and cardiovascular load. If you want a deeper look at why hydration matters for your heart, I made this video for you: Hydration and Heart Health.

The bottom line

If you regularly feel foggy, tired, or headachy and cannot pin down why, do not overlook the simplest variable. Mild dehydration is common, easy to miss, and measurably affects memory and focus. It is also just one of several root causes of brain fog worth ruling out. Set a clear daily water target, around 85 ounces for women and more for men, drink the cleanest water you can, and give it two weeks. It is the easiest hard thing you can do for your health, and the return on a glass of water is hard to beat.

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Frequently asked questions

How much water should you drink per day?

Research supports about 2.5 liters (roughly 85 ounces) of fluid per day for adult women. The National Academies set adequate total water intake at about 2.7 liters (91 ounces) for women and 3.7 liters (125 ounces) for men, including water from food. A practical target for most adults is half your body weight in ounces, adjusted up for heat, exercise, caffeine, and alcohol.

Can dehydration cause brain fog and memory loss?

Yes. In a 2018 study in Physiology & Behavior, mild dehydration in healthy young women caused measurable deficits in visual and working memory and executive function. Drinking water to the recommended 2.5 liters per day reversed those deficits. A fluid loss of just 1.5 percent of body mass is enough to impair concentration, planning, and follow-through.

What are the symptoms of mild dehydration?

Common symptoms include fatigue, headache, dizziness, poor concentration, memory lapses, and reduced physical and mental performance. Because these losses are small and gradual, most people do not connect them to hydration, which is what makes mild dehydration easy to miss.

Does the quality of the water matter, not just the amount?

Yes. Drinking enough water is half the goal. The other half is drinking the cleanest water you can. Filtration ranges from a pitcher filter, to a countertop system, to a whole-home system that reduces chlorine, fluoride, metals, and microbes from municipal water. Choose the highest level system you can afford.

How do I know if I am drinking enough water?

Two simple markers help. Urine color should be pale yellow rather than dark. On lab work, an elevated blood albumin and a high urine specific gravity can signal dehydration. If you regularly feel foggy or tired by mid-afternoon, test the simplest variable first by meeting your daily water target for two weeks.

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About the author: Dr. John Bartemus, DC, CFMP, is a functional medicine practitioner, educator, speaker, and Amazon international number one best-selling author. He specializes in optimizing health through Functional Medicine Charlotte, PC.

This article is for educational purposes and is not a substitute for individualized medical advice. Consult your healthcare provider before making changes to fluid intake, especially if you have a heart, kidney, or liver condition.