Hydration

How hydration affects your health and aging

Water makes up about 60% of your body and is involved in virtually every metabolic process. Lose just 1-2% of your body water and your concentration drops, your mood worsens, and your physical performance declines measurably. Many people walk around mildly dehydrated without realizing it. The consequences go well beyond thirst: chronic underhydration contributes to headaches, constipation, fatigue, kidney stones, and urinary tract infections.

A 2023 NIH study that tracked 11,255 adults over 30 years found that those with serum sodium levels above 142 mmol/L (a marker of low fluid intake) were 50% more likely to show accelerated biological aging and 21% more likely to die prematurely ^[1]. Participants with serum sodium in the optimal range of 137-142 mmol/L had the fewest chronic diseases and the lowest mortality. That's a striking association for something as simple as drinking enough water.

Why water alone isn't enough: the electrolyte factor

Drinking plain water doesn't automatically hydrate your cells. Water crosses cell membranes by following electrolytes, primarily sodium, through osmotic gradients. Without enough electrolytes, water passes through your system without being absorbed effectively. This is why athletes who drink only plain water during prolonged exercise can develop hyponatremia, a dangerous drop in blood sodium that causes confusion, seizures, and in rare cases death ^[2].

The four electrolytes that matter most for hydration:

• Sodium (2,000-3,000 mg/day for active people): the main extracellular electrolyte that drives fluid retention and nerve signaling
• Potassium (3,500-4,700 mg/day): the main intracellular electrolyte, best obtained from food sources like bananas, potatoes, and leafy greens
• Magnesium (400-600 mg/day): involved in 300+ enzymatic reactions, supports muscle relaxation and sleep quality
• Chloride: works alongside sodium for fluid balance and stomach acid production

Research into the Beverage Hydration Index has shown that drinks containing electrolytes, protein, or fat (such as milk or oral rehydration solutions) maintain hydration status longer than plain water because they slow gastric emptying and reduce urine output ^[3].

How much water do you actually need?

The "8 glasses a day" rule has no real scientific basis. The National Academy of Medicine recommends about 3.7 liters of total fluid for men and 2.7 liters for women daily, but roughly 20% of that comes from food. Your actual needs depend on body size, activity level, climate, altitude, and diet. Someone doing intense exercise in heat may need 1-2 additional liters per hour.

The simplest monitoring tool is urine color. Pale straw yellow means you're well hydrated. Dark yellow or amber signals a deficit. Body weight changes before and after exercise also work well: every kilogram lost represents roughly one liter of fluid that needs replacing ^[4].

Hydration, cognition, and physical performance

A 2018 meta-analysis of 33 studies found that dehydration beyond 2% body mass loss impairs attention, executive function, and motor coordination ^[5]. The effects on complex cognitive tasks are more pronounced than on simple reaction time. For athletes, even mild dehydration reduces endurance, increases perceived effort, and impairs thermoregulation. Rehydration reverses these deficits, but it takes time: full recovery of cognitive and physical performance can require 45-60 minutes after drinking.

Hydration for kidney health

Kidneys filter about 180 liters of blood per day, and adequate water intake keeps this filtration system running efficiently. A study using NHANES data found that better hydration status was significantly associated with lower kidney stone risk ^[6]. For people who've already had a kidney stone, maintaining urine output above 2.5 liters per day cuts recurrence rates substantially ^[7]. Chronic dehydration also concentrates waste products in urine, increasing the risk of urinary tract infections and potentially accelerating decline in kidney function over time.

Populations with higher hydration needs

Older adults face a double challenge: their thirst sensation weakens with age while their kidney concentrating ability declines, making dehydration both more likely and harder to detect. A meta-analysis found that low-intake dehydration is common among non-hospitalized older adults and is associated with increased mortality and hospitalization ^[8]. Athletes in endurance sports need structured hydration plans that include both fluid and electrolytes. People on high-protein or high-fiber diets need more water because protein metabolism produces urea (which requires water for excretion) and fiber absorbs water in the gut. Pregnant and breastfeeding women also have significantly elevated fluid requirements.

Practical strategies that work

• Front-load your hydration: drink 500 ml within the first hour after waking to replace overnight losses
• Add a pinch of unrefined salt or a balanced electrolyte mix to your water, especially if you exercise or sweat heavily
• Eat water-rich foods like cucumber (96% water), watermelon (92%), celery, berries, and soups
• Don't rely on thirst alone, particularly if you're over 60
• Avoid drinking more than about 1 liter per hour to prevent diluting blood sodium