Understanding the Demands of Ultra Running

Ultra running—defined as any race longer than a standard marathon (26.2 miles)—places extraordinary stress on every physiological system. These events often last many hours or even days, pushing athletes to their absolute limits. Among the most critical challenges runners face are maintaining proper fluid balance and stable blood glucose levels. Dehydration and hyperglycemia are two common yet preventable issues that can derail performance, cause serious medical complications, and force early retirement from a race. This article presents evidence-based strategies to help ultra runners avoid these hazards and safely complete their events.

Successful management of hydration and blood sugar requires a personalized plan that accounts for individual sweat rates, environmental conditions, and metabolic responses. No single approach works for everyone, but understanding the underlying physiology allows runners to make informed decisions on the fly. Below we explore the mechanisms of dehydration and hyperglycemia, then provide actionable tactics for prevention before, during, and after an ultra.

The Physiology of Dehydration in Endurance Athletes

Dehydration occurs when net fluid loss (through sweat, respiration, and urine) exceeds fluid intake. Even mild dehydration—a loss of 1–2% of body mass—can impair thermoregulation, increase heart rate, reduce blood volume, and diminish endurance performance. Symptoms progress from thirst and dry mouth to dizziness, fatigue, muscle cramps, headache, and in severe cases, heat exhaustion or heat stroke.

During an ultra, sweat rates can range from 0.5 to 2.5 liters per hour depending on intensity, temperature, humidity, and individual genetics. The key is to match fluid intake to these losses without overdrinking. Overhydration, or hyponatremia, occurs when excessive plain water dilutes blood sodium levels, leading to nausea, confusion, seizures, and even death. Therefore, ultra runners must replace not only water but also electrolytes, especially sodium, which is lost in significant amounts through sweat.

Recognizing Dehydration Early

Runners can use simple monitoring techniques to catch dehydration before it becomes dangerous. Checking urine color is a practical field indicator: pale yellow suggests adequate hydration, while dark amber signals a need for fluids. Thirst is a late sign and not a reliable trigger. Weighing yourself before and after training runs provides a more accurate measure of sweat losses. For every pound lost during a run, aim to replace it with 16–24 ounces of fluid during recovery.

Hyperglycemia: The Overlooked Danger of Excessive Carbohydrate Intake

Hyperglycemia (high blood sugar) is often associated with diabetes, but it can occur in healthy endurance athletes who consume too many simple carbohydrates without sufficient insulin response or who experience significant stress. During an ultra, runners commonly rely on gels, sports drinks, and chews that are high in sugar. When intake exceeds the body’s ability to clear glucose from the bloodstream, blood sugar can spike. Symptoms include increased thirst, frequent urination (which further exacerbates fluid loss), blurred vision, fatigue, and impaired cognitive function.

Chronically elevated blood glucose also promotes systemic inflammation and may contribute to gastrointestinal distress—a common complaint during long races. Runners with underlying insulin resistance or pre-diabetes are especially vulnerable, but even elite athletes can experience transient hyperglycemia if they “hit the aid stations too hard” or use fueling products that are poorly balanced.

The Role of Stress Hormones

Physical and emotional stress during an ultra triggers the release of cortisol and epinephrine, which raise blood glucose by stimulating glycogen breakdown and gluconeogenesis. When combined with high carbohydrate intake, this hormonal response can create a perfect storm for hyperglycemia. Runners should be aware that pushing through a rough patch can exacerbate blood sugar swings, making it essential to both fuel strategically and manage effort.

Pre-Race Preparation: Setting the Foundation

Success in preventing dehydration and hyperglycemia begins days before the start line. Pre-race hydration should focus on gradual fluid loading rather than chugging water right before the race. Consuming beverages with electrolytes (especially sodium) in the 24–48 hours before the event helps maintain plasma volume. Aim for urine that remains light yellow throughout the day.

Carbohydrate loading is a common pre-race strategy, but it must be done with balance. Loading with whole-food carbohydrates (e.g., oatmeal, sweet potatoes, brown rice) over 3–4 days is safer than loading with sugary drinks and gels, which can trigger rapid fluctuations in blood glucose. A diet that includes moderate protein and some healthy fats will help stabilize blood sugar and provide sustained energy.

Practice Your Nutrition in Training

The gastrointestinal tract can be trained, but it requires consistent exposure. Use long training runs to test different hydration and fueling products. Determine which brands, flavors, and textures work best for your stomach, and identify the timing that prevents both hunger and blood sugar dips. Also practice drinking on the move and eating while breathing deeply. A well-trained gut reduces the risk of nausea and vomiting — both of which can accelerate dehydration.

Hydration Strategies During the Race

During an ultra, fluid needs change continuously due to terrain, temperature, altitude, and pace. The following guidelines provide a framework, but individual adjustments are critical.

Establish a Baseline Intake Schedule

A typical recommendation is to consume 7–10 ounces of fluid every 20 minutes during moderate conditions. In hotter climates, increase to 12–16 ounces every 20 minutes. However, this is only a starting point. A more precise method is to drink based on thirst but to stay slightly ahead of it—never allowing yourself to become parched. Many experienced runners set a timer on their watch to remind them to take a sip or chew an electrolyte tablet every 15–20 minutes.

Use Multiple Fluid Sources

Relying solely on water is a mistake. Sports drinks with 4–8% carbohydrate concentration provide both fluid and fuel, but the sugar content can contribute to hyperglycemia if consumed in large volumes. Therefore, alternate between water and an electrolyte drink. Another option is to carry concentrated electrolyte capsules or powder and add them to plain water at aid stations. This allows you to adjust sodium intake based on how much you are sweating.

Electrolyte Replacement: Not Just Sodium

Sodium is the primary electrolyte lost in sweat, but potassium, magnesium, and calcium also play roles in muscle function and preventing cramps. Many commercial electrolyte products include these minerals. A good rule is to consume 300–600 mg of sodium per hour during an ultra, with adjustments for heavy sweaters (those with visible salt crusts on skin or clothes). Pay attention to cramping: if your calves or hamstrings begin to seize, increase sodium intake immediately.

Managing Carbohydrate Intake to Prevent Hyperglycemia

Carbohydrates are essential for performance, but timing and type matter greatly. The goal is to maintain blood glucose within a normal range (typically 70–140 mg/dL for most athletes) without spiking into hyperglycemic territory.

Choose Complex Carbohydrates When Possible

While simple sugars (glucose, fructose, maltodextrin) are absorbed quickly and provide rapid energy, they can cause sharp rises in blood glucose. Combining simple sugars with a small amount of protein or fat in each fueling session helps blunt the spike. Some bars and “real food” options (e.g., peanut butter sandwiches, dates with nuts, rice balls) provide more sustained release. During the latter stages of an ultra, when digestion slows, many runners prefer liquid carbs and easy-to-digest gels, but those should be spaced out rather than consumed in a single large dose.

Stick to a Schedule Based on Your Needs

Most ultra runners need 30–60 grams of carbohydrates per hour for events lasting 2–3 hours, and up to 90 grams per hour for longer events. However, the upper end increases the risk of hyperglycemia, especially if you are not accustomed to that intake. Start on the lower end and increase only if you feel your energy flagging without causing stomach issues or a racing heart. Listen to your body: if you are feeling jittery or excessively thirsty after a gel, cut back on the next one.

Monitor Blood Glucose if Possible

Continuous glucose monitors (CGMs) are becoming popular among endurance athletes as a training tool. Even non-diabetic runners can benefit from seeing how their blood sugar responds to different fuels and intensities. If you have a history of hypoglycemia or hyperglycemia during runs, wearing a CGM for a few events can teach you exactly how many grams of carbs to take and when. If you have diabetes, working with your healthcare team to set upper and lower glucose thresholds is essential before attempting an ultra.

Additional Tips for Preventing Both Conditions

  • Train in the Heat: Heat acclimatization improves sweat rate and sodium conservation, which reduces the risk of both dehydration and electrolyte imbalances. Spend 10–14 days training in warm conditions before a hot race.
  • Use a Hydration Pack or Vest: Carrying your own fluids allows you to stick to your schedule even between aid stations. A hands-free system also encourages more consistent sipping.
  • Practice Belly Breathing: Deep, rhythmic breathing helps regulate the autonomic nervous system and can blunt stress-induced hyperglycemia. It also reduces the likelihood of side stitches and gastrointestinal distress.
  • Seek Shade and Cool Down: When possible, pour water over your head and neck at aid stations to lower core temperature. A lower body temperature reduces sweat rate and conserves fluids.
  • Account for Altitude: Higher altitudes increase respiratory water loss and can suppress appetite, making it harder to both hydrate and fuel. Increase fluid intake by 10–20% and choose calorie-dense, easy-to-eat foods like nut butters or avocado.

Special Considerations for Runners with Diabetes

For runners with type 1 or type 2 diabetes, ultra running presents unique challenges because the interplay between exercise, insulin, and carbohydrate intake is complex. Hyperglycemia can occur from insufficient insulin or from stress hormones, while hypoglycemia (dangerously low blood sugar) can result from too much insulin or too little fuel. The best approach is to work with a sports endocrinologist to develop a race-day plan that includes:

  • Basal insulin adjustments prior to and during the event.
  • Frequent blood glucose checks (every 20–30 minutes) using a glucometer or CGM.
  • Fast-acting glucose sources (e.g., glucose tablets or gel) for impending lows.
  • Written guidance for aid station volunteers so they can help in an emergency.

Non-diabetic runners should also be aware that severe caloric restriction or an illness can trigger diabetic-like episodes, so carrying a backup sugar source is always wise.

Post-Race Recovery: Rehydration and Blood Sugar Rebalancing

Replacing fluids and stabilizing blood glucose after crossing the finish line is just as important as during the race. Within 30–60 minutes, consume a meal or drink that contains both carbohydrates and protein (a 3:1 or 4:1 ratio is ideal) to replenish glycogen stores and promote muscle repair. Continue drinking fluids containing electrolytes until your urine is clear or pale yellow. If you experienced hyperglycemia during the race, avoid large amounts of simple sugars for the first few hours; instead, opt for whole foods like fruit, yogurt, or a sandwich.

Weigh yourself before and after the race to determine how much fluid you lost. For every kilogram (2.2 pounds) lost, drink 1.5 liters of fluid over the next few hours. This ensures proper rehydration without overwhelming the kidneys. If you feel unusually fatigued, dizzy, or confused after an ultra, seek medical attention immediately—these could be signs of serious electrolyte imbalance or heat injury.

Conclusion

Ultra running tests every aspect of an athlete’s preparation and resilience. Dehydration and hyperglycemia are preventable with careful planning, consistent monitoring, and flexible adjustments. The most successful runners are those who treat hydration and fueling as active processes throughout the entire race, not just something to worry about when symptoms appear. By incorporating the strategies outlined here—regular small sips, balanced carbohydrate intake, electrolyte replacement, and awareness of individual limits—you can reduce your risk, improve your performance, and enjoy the remarkable journey of an ultra run.

For further reading, the Mayo Clinic provides guidance on hyponatremia and fluid balance, while the American College of Sports Medicine offers detailed position stands on exercise and fluid replacement. Runners interested in continuous glucose monitoring can explore evidence-based reviews from the Diabetes in Sport organization. Lastly, the International Journal of Sport Nutrition and Exercise Metabolism publishes research on carbohydrate fueling strategies for ultra endurance events.