Steady Athlete

Featuring Annie Behrend, MS, RD

Dehydration can be sneaky. You might struggle with cramping and fatigue mid-race, or be hit with a headache, low energy, and brain fog 20 minutes after a long run.

It’s a familiar yet frustrating experience that seems to worsen over time as your thirst cues become less reliable.

For athletes 35 and older, dialing in hydration is essential. Beyond recovery and protein needs, age-related physiological changes affect how your body regulates fluid, signals thirst, and handles hard, prolonged efforts in the heat.

We talked with Annie Behrend, MS, RD, and endurance athlete, about what smart hydration looks like for masters athletes and how to nail it to boost your performance.

We may earn a commission through our affiliate links at no extra cost to you.

Why Hydration Gets More Complicated After 35

Basic hydration principles don’t change with age: drink ample fluids, consume water-rich produce, and aim for pale yellow urine. 

But how much water should you drink per day?

For healthy adults, National Academies of Sciences, Engineering, and Medicine guidelines suggest roughly 2.7 liters for women and 3.7 liters for men, including water from food and drinks.

Several physiological shifts that start in your 30s and 40s can gradually make you more susceptible to dehydration, including:

Less reliable thirst signals

It’s normal to feel less thirsty with age. But why? One theory is that older adults have a higher baseline plasma osmolality, which can increase dehydration risk.

You can be meaningfully dehydrated before your body tells you to drink. That’s more of an issue during a marathon or long endurance event on a warm morning than a 45-minute recovery run.

Kidneys become less efficient at conserving water

Around age 30, the kidneys’ ability to concentrate urine, or retain water, starts to decline. The impact is usually modest in your 40s, Behrend notes, but becomes more noticeable after 50. It means your body can’t compensate for fluid deficits as well.

You may have less total body water

“Older athletes may have less total body water (TBW) than younger athletes and therefore a smaller physiological reserve against dehydration,” Behrend explains, adding that individual impacts vary widely. “Because adipose tissue contains about 10–20% water vs. 70–80% in lean tissue, athletes who maintain a higher proportion of lean mass may experience less age-related decline in TBW.” This can lead to a faster onset of dehydration.

How to Avoid a Dangerous Dehydration Spiral

Think of dehydration as a chain reaction. Once it starts, it quickly escalates.

When fluid losses accumulate, blood volume drops. Lower blood volume means less blood flow to the skin, which reduces your body’s ability to sweat and cool itself. Core temperature rises, causing your muscles to burn through glycogen faster. 

The result? You fatigue earlier. GI distress becomes more likely. The risk of heat illness rises.

Dehydration also reduces your ability to absorb nutrients, causing a vicious cycle, Behrend says. “Dehydration accelerates glycogen depletion and increases reliance on exogenous carbohydrate sources at the same time that GI function and fuel tolerance may begin to deteriorate.”

Recognizing the dehydration cascade •  Decreased blood volume •  Decreased skin blood flow → decreased sweat rate •  Decreased cooling capacity → increased core temperature •  Increased muscle glycogen use → premature fatigue •  Increased risk of heat illness, nausea, GI distress

The 3 Biggest Hydration Mistakes Masters Athletes Make (And How to Avoid Them)

1. Relying on thirst to guide race-day drinking

Early in a race, when your gut function is best, fluids and electrolytes are easier to absorb. Waiting until you’re thirsty means trying to catch up when your GI system is already stressed.

Behrend learned this through her own training. 

During a long run preparing for a 100-miler, she measured her body weight before and after a prolonged effort. Despite feeling fine and not especially thirsty, she’d already lost 2% of her body weight — drinking roughly half of what she’d lost through sweat. 

“My drive to drink was not notably heightened even at a 2% loss,” she notes. “With a modest sweat rate/cool conditions, I can usually get away with drinking to thirst during a 3-hour marathon. But over the course of an ultramarathon, these deficits quickly compound to something unsustainable.”

To avoid this, have a hydration plan before you start. Measure your sweat rate during training and aim to drink consistently rather than reactively.

2. Copying another athlete’s strategy

The electrolyte mix your training partner loves might cause problems or just not work for you. Everyone’s hydration needs are unique in endurance performance. Generic strategies are a starting point, not a prescription.

Average sweat rates range from 0.3 to 2.4 liters per hour. Sweat sodium concentrations vary from 300 to 2,400 mg per liter. Every body is different, and your needs might vary significantly.

3. Arriving dehydrated and panic-drinking before the start

Starting slightly dehydrated happens more than most athletes admit. Poor sleep, long travel days, and not prioritizing fluids in the days before the race can cause a hydration deficit. Anxiously chugging water 30 minutes before the start isn’t a fix. It just means you’ll be looking for a restroom by mile two.

How & When to Hydrate Properly Before a Race

 As with a proper pre-race fueling strategy, the ideal hydration window is the days before your race.

“Start hydrating early in the day,” Behrend reminds us. “Water before coffee. Water before sparkling water. It’s easy to sip coffee, tea, or carbonated beverages throughout the day and feel as though you’re drinking plenty — only to arrive mildly dehydrated.”

As you taper training and increase carbohydrate intake, your body stores glycogen. For every gram of glycogen stored, it retains roughly 2.7 grams of water. A slight increase in body weight during the taper period can signal that energy and hydration are being optimized. This is a good sign.

Race morning hydration protocol

Assuming you’ve eaten and hydrated reasonably well in the days before, Behrend recommends consuming 5–10 mL per kg of body weight (roughly 2–4 mL per pound) of fluid about 2–4 hours before the start.

For a 135-pound athlete, that’s approximately 9–18 ounces. Pairing that fluid with sodium improves retention and helps trigger thirst at an appropriate time.

If your urine is still dark closer to the start, consume an additional 3–5 mL/kg about 2 hours out. Pale yellow is your target. It’s not a perfect measure, but it’s reliable enough for most athletes in most conditions.

Electrolytes: What You Need, When You Need It

“As a practical starting point: if moderate sweat loss is expected, plan to consume 500–700 mg sodium per hour. If heavy sweat loss is expected, plan for around 1,000 mg per hour. These are broad recommendations, not individualized prescriptions. Sweat rates and sweat sodium concentrations vary enormously from athlete to athlete.”— Annie Behrend, MS, RD

For most healthy adults, electrolyte needs are relatively modest. Baseline sodium recommendations are around 1.5 grams per day from NASEM — a threshold most people on a varied diet easily meet.

But during extended exercise in the heat, the question becomes how much and what kind of electrolytes you need.

Why prioritizing sodium matters

Sodium, the primary electrolyte lost through sweat, helps regulate fluid balance and maintain blood volume. During prolonged exercise, it’s normal for sodium levels to drop slightly as you lose salt and fluids through sweat.

However, balancing fluid loss and electrolyte intake is critical. Failing to replace sodium while drinking large amounts of plain water can cause a potentially dangerous imbalance called hyponatremia. Drinking large volumes of plain water can also stimulate urine production, reduce thirst, and slow rehydration.

After exercise, Behrend recommends replacing approximately 125–150% of fluid losses through beverages containing electrolytes, a pre-mixed electrolyte packet, or water consumed with salty food. Aim to replace roughly 20–24 ounces of fluid per pound lost during exercise.

Avoiding painful cramps

Even experienced athletes can be hobbled by cramps. Research also suggests that exercise-induced cramping rarely has one root cause.

One is hypohydration — the state of having too little water in your body.  A mild deficit can cause muscle tightness and elevated heart rate, while cramping, dizziness, and GI distress can indicate a more significant deficit.

However, the relationship between sodium levels and cramping is less direct than marketing suggests. Hypohydration contributes but is rarely the only factor. Simply adding more electrolytes doesn’t always resolve it.

To prevent hypohydration, consistently replace electrolytes during extended efforts. For races and long sessions, Behrend suggests:

• Moderate sweat loss expected: 500–700 mg sodium per hour
• Heavy sweat loss expected: ~1,000 mg sodium per hour

Individual sweat rates, sweat sodium concentrations, environmental conditions, and fueling strategies can alter what your body actually needs.

Athletes managing hypertension, renal disease, or other conditions affecting fluid balance should discuss electrolyte intake with their provider before applying these targets. 

Why Drinking Too Much Can Be Just as Dangerous

Most athletes know the impacts of dehydration. But drinking too much water can also be dangerous, especially in longer events.

Hyponatremia, a condition caused by abnormally low sodium levels in the blood, is often triggered by drinking too much plain water without replacing the sodium lost in sweat. Symptoms include nausea, headache, confusion, and in severe cases, seizure or death. 

It’s a documented risk in marathon and ultramarathon racing, particularly among slower athletes who are on course longer and drinking heavily. 

Behrend has seen both ends of the spectrum.

A friend who raced ultras decades ago — when hydrating with plain water was standard practice — developed symptoms mid-race and reached for salt tablets. “She proceeded to diurese so continuously that she didn’t even bother to stop running after a while,” Behrend recalls. A mild case that was caught early and resolved simply. 

“I have also worked with folks who have desperately sought out hydration plans after more severe EAH experiences that have landed them in the ER.” 

Ironically, many cases of hyponatremia mimic dehydration at first. Athletes feel unwell and assume they need to drink more water, which worsens the problem. If you’re racing long, drinking consistently, and still feel off, pay attention to electrolytes, not just volume. Drinking more water isn’t always the answer.

Do Energy Gels Require Water? 

Most athletes take gels with water out of habit or palatability. But skipping water can cause disruptive GI issues.

Most energy gels are hyperosmolar, with higher concentrations of sugar and molecules than your blood. To digest properly, they draw water from surrounding tissue into the gut. Without adequate hydration you may experience bloating, cramping, nausea, or diarrhea, often at the worst time.

To avoid these issues, Behrend recommends approximately 150–250 mL (5–9 oz.) of water per serving with conventional gels.

If GI issues persist, consider isotonic gels, which can be taken without water. However, they typically contain less carbohydrates per serving, so you may need to dose more frequently.

The stakes are even higher in the heat. You’re losing more fluid through sweat, and a poorly timed gel without water can trigger the kind of GI issues that end races. Hot conditions make the water-with-gels rule even more important.

Why Dehydration Spikes Heart Rate (And What to Do About It)

Some workouts feel harder than the numbers justify. You’re on pace, but your heart rate monitor shows elevated numbers, legs are heavy, and efforts seem disproportionate.

Dehydration is often the culprit. When fluid losses exceed roughly 3% of body weight, the drop in blood volume creates cardiovascular strain. Your heart has to beat faster to move the same volume of oxygen-carrying blood to working muscles. That’s why the same effort feels more challenging.

Poor sleep, heat, and long travel days compound this, causing an elevated heart rate even before your workout begins.

The best solution, Behrend says, is prevention. That means a proactive hydration strategy with consistent daily intake, pre-exercise fluid loading, and pre-race-day hydration.

Training Your Gut to Handle Fluids

Gut training is gaining traction in endurance sports. The idea is to better absorb and tolerate higher volumes of fuel, especially carbohydrates, during workouts.

Behrend notes the same principle applies to fluids. And for ultra-endurance athletes especially, it may matter more than carbohydrate tolerance.

“Just as we train our muscles, cardiovascular system, and fueling strategies, we can also train our ability to absorb and tolerate fluids,” she says. “For many athletes, that can make a meaningful difference in minimizing dehydration and maintaining performance during long events.”

In practice, this means using longer training sessions to try drinking at rates that more closely match your actual sweat losses, rather than sipping when you feel like it.

Start by understanding your sweat rate (see below), then train your gut to handle those volumes comfortably before race day.

How to Calculate Your Sweat Rate

Understanding how much fluid you lose during exercise is the foundation of any individualized hydration plan. You can start here:

1. Weigh yourself nude before your workout.
2. Exercise for a set period — 60 minutes works well for a baseline.
3. Track how much you drink during the workout.
4. Weigh yourself nude again after (don’t use the bathroom first).
5. Sweat rate = (pre-weight − post-weight) + fluid consumed.

Every pound of weight lost equals roughly 16 oz. (about 500 mL) of fluid. That number is your hourly sweat rate under those specific conditions. Recalculate in hot and cold temperatures, at race pace, and when running and cycling. The numbers will differ.

The goal during exercise is to limit body weight loss from sweat to less than approximately 2% of your starting weight. For a 160-pound athlete, that’s about 3.2 pounds (roughly 50 oz.) of fluid.

For a more precise calculation — including how to account for urine output and warm-up protocol — Annie put together a free printable worksheet with a sample calculation and blank rows to track your sweat rate across conditions.

Get the Sweat Rate Worksheet here.

Subscriber Bonus: Annie’s Favorite Recovery Smoothie

For a post-workout recovery boost, Annie loves this adaptation of a Run Fast Eat Slow recipe. Vibrant and flavorful, it features real food: orange and frozen banana for quick carbohydrates, Greek yogurt for protein and probiotics, and a tasty creamsicle flavor.

Subscribers get the recipe for free, including her optional add-in notes for creatine and whey protein to boost muscle repair. It’s quick and easy to make and can help accelerate recovery.

Get Annie’s Recovery Smoothie Recipe.

Meet Our Contributor

Annie Behrend, MS, RD is an endurance athlete specializing in sports nutrition. She completed her undergraduate degree in food and nutrition science and an MS in clinical nutrition. She helps endurance athletes at all levels develop evidence-based, individualized approaches to fueling and hydration — including the unique demands that come with competing and training seriously as a masters athlete.

Have a question for Annie or want to connect?