The Challenge of Fueling an Ultra: Why Blood Glucose Matters

An ultramarathon—any footrace longer than the standard 26.2-mile marathon—pushes the human body to extraordinary limits. Over six, twelve, or even one hundred hours of continuous movement, your muscles burn through massive amounts of energy. While stored glycogen in your muscles and liver provides a critical reserve, it is the glucose circulating in your blood that becomes the immediate, fine-tuned fuel for your working muscles and, just as importantly, your brain. When blood glucose levels drop too low, a cascade of problems follows: the dreaded “bonk” (a sudden, overwhelming fatigue), cognitive fog, loss of coordination, and an increased risk of injury or DNF (did not finish). Mastering blood glucose management is, therefore, one of the most powerful tools an ultrarunner has. This guide provides a comprehensive, science-backed approach to keeping your blood sugar steady from the start line to the finish line and beyond.

Understanding Blood Glucose and Energy Metabolism in Ultramarathons

To develop an effective nutrition strategy, you need to understand how your body produces and uses glucose during prolonged, intense exercise. Your body stores roughly 300–400 grams of glycogen in your muscles and another 80–100 grams in your liver. During an ultra marathon, these stores are gradually depleted. As glycogen levels fall, your liver ramps up gluconeogenesis—producing new glucose from lactate, amino acids, and glycerol. At the same time, your exercising muscles dramatically increase their uptake of glucose from the blood. This creates a delicate balancing act.

If your carbohydrate intake from external sources (gels, drinks, real food) cannot keep pace with your muscles’ demand, blood glucose falls. Even moderate hypoglycemia (blood glucose below 70 mg/dL) can cause shakiness, dizziness, and impaired decision-making. Severe drops can lead to collapse, confusion, and a race-ending emergency. Conversely, consuming too many simple sugars too quickly can cause a rapid spike in blood glucose followed by a compensatory insulin surge, leading to a reactive hypoglycemic crash. The goal is to maintain a steady, moderate elevation in blood glucose—a state that provides sustained energy without triggering extreme hormonal swings.

Pre-Race Nutrition: Building Your Glycogen Stores

Your race performance is largely determined in the days and hours before the start. A common mistake among ultrarunners is to “carb-load” the night before, but effective glycogen supercompensation takes 24–48 hours. For most athletes, consuming 8–10 grams of carbohydrate per kilogram of body weight each day in the two days before the race is sufficient. Focus on low-fiber, easily digestible sources to avoid gastrointestinal distress. White rice, pasta, potatoes, bananas, and refined grains are excellent choices. Avoid high-fat or high-protein meals that slow digestion and may cause discomfort.

The Pre-Race Meal: Timing and Composition

Eat your final substantial meal 3–4 hours before the start. This meal should be high in carbohydrates (about 1–2 g/kg), moderate in protein (0.15–0.25 g/kg), and very low in fat and fiber. Examples: a bowl of oatmeal with a sliced banana and a drizzle of maple syrup; white rice with a small portion of grilled chicken; a bagel with jam. During the hour before the race, you can top off with a small snack (50–100 calories) if your stomach tolerates it, but avoid large amounts.

Hydration and Electrolyte Loading

Proper hydration begins 24 hours before the race. Drink water throughout the day, and include electrolyte sources (sodium, potassium, magnesium) in your meals. Avoid excessive water intake without electrolytes, which can dilute blood sodium levels. In the final hour before the start, sip water as needed but do not force fluids. A final 16–20 ounce bottle of an electrolyte drink 60–90 minutes before the gun is a good practice for many runners.

  • Carb load: 8–10 g/kg for 2 days pre-race.
  • Pre-race meal: 3–4 hours before, low fat/fiber, high carb.
  • Top-off snack: 50–100 calories 30–60 minutes before.
  • Hydration: Maintain clear urine color; include electrolytes.

During the Race: Strategies for Steady Glucose Levels

Race-day nutrition is a dynamic science. The simple “30–60 grams of carbs per hour” guideline is a useful starting point, but ultrarunners often find their optimal intake can vary from 40g/hour up to 90g/hour, depending on pace, ambient temperature, body weight, and individual gut tolerance. The key is to start fueling early—before you feel hungry or low—and to fuel consistently throughout the event. Waiting until you sense a drop in blood glucose is a recipe for under-fueling.

Carbohydrate Sources: Gels, Drinks, and Real Food

Most runners begin with commercial gels and sports drinks because they are convenient and easy to digest. However, many experienced ultrarunners shift to a mix of solids and liquids to prevent palate fatigue and to offer a broader range of nutrients. Good choices include:

  • Gels and chews: Provide 20–30g of carbohydrate per serving. Aim for those with multiple transportable sugars (glucose + fructose) to increase absorption without causing GI distress.
  • Sports drinks: A 6–8% carbohydrate solution provides both fluids and energy. Avoid hyperconcentrated drinks (above 10%) as they can delay gastric emptying and cause cramping.
  • Real food: Dates (4–5 dates = ~30g carbs), boiled potatoes dipped in salt, peanut butter and jelly sandwiches cut into small pieces, and energy bars that break down easily.

Experiment during training to identify what works for you. Many runners adopt a rotation—a gel every 30 minutes, a few sips of sports drink in between, and a bite of a potato or date at the 2-hour mark.

Timing and Frequency

The ideal fueling schedule is every 20–30 minutes, not based on hunger. Use your watch or set an alarm. For example: at the top of every half-hour, consume 20–30g of carbohydrates. If using a drink, sip continuously rather than chugging a full bottle. Spreading intake reduces the risk of a glucose spike-and-crash cycle and gives your digestive system a steady trickle rather than a flood.

Hydration and Electrolytes: The Critical Balance

Hyponatremia (low blood sodium) is a serious risk in ultras, especially when you drink large volumes of plain water. Your body loses sodium through sweat, and if you replace fluids without electrolytes, you dilute your blood sodium concentration. Symptoms include nausea, headache, confusion, and in severe cases, seizures. Balance your hydration by drinking to thirst—not to a predefined volume—and ensure every bottle or cup contains electrolytes. Many runners carry a separate electrolyte powder or salt tablets and add them to water at aid stations. A general recommendation is 500–1000 mg of sodium per hour, adjusted for sweat rate and heat.

Training the Gut

Your gut can be trained to absorb more carbohydrates efficiently. In the weeks and months before an ultra, practice your race-day nutrition during long runs. Start with lower amounts and gradually increase your carb intake per hour. This improves the transport capacity of your intestines and reduces the risk of nausea, bloating, and cramping on race day. A well-trained gut can handle 90–100g of carbs per hour in some athletes, but this requires consistent practice.

Using Technology: Continuous Glucose Monitors (CGM)

Many elite ultrarunners now wear a continuous glucose monitor (CGM) during training and racing. These devices provide real-time data on blood glucose trends, allowing you to see how different foods, paces, and environmental conditions affect your levels. A CGM can help you calibrate your fueling strategy and detect early signs of hypoglycemia before symptoms appear. However, CGMs are a tool, not a replacement for hands-on experience. Always combine data with how you feel. If you are diabetic or have a history of low blood sugar, consult a sports endocrinologist before relying solely on a CGM.

Addressing Common Issues: GI Distress, Hyponatremia, and Hypoglycemia

Gastrointestinal Problems

Nausea, cramping, and diarrhea are the most common reasons for dropping out of an ultra. The cause is often improper fueling: too much fat or fiber, too-concentrated drinks, or insufficient gut adaptation. Solutions include:

  • Choose products with multiple transportable carbohydrates (glucose + fructose).
  • Alternate between gels and water to dilute the concentration.
  • Include bland, low-fat real foods like white potatoes or plain bagels.
  • Stay hydrated—dehydration itself can cause GI stasis.
  • Do not introduce new foods on race day.

Hyponatremia Prevention

Again, the mantra is “drink to thirst.” If you find yourself urinating clear and excessively, reduce fluid intake and increase electrolytes. Additionally, ensure you consume salty foods or sodium supplements. A small packet of salted pretzels or a pickles can be a lifesaver in the later miles.

Managing Hypoglycemia During the Race

If you feel symptoms of low blood sugar—sudden weakness, dizziness, confusion, shaking—take immediate action. Stop briefly and consume a fast-acting source of carbohydrate: 4–5 gel blocks, a cup of regular soda, or a sugary sports drink. Wait 5–10 minutes for the glucose to enter your bloodstream. Do not resume running until you feel steady. After recovery, continue with your regular fueling schedule but consider increasing your intake slightly for the next hour to prevent a recurrence.

Post-Race Recovery: Replenishing and Repairing

The finish line is only half the battle. Proper recovery nutrition restores glycogen stores, repairs muscle damage, and rebalances fluid and electrolyte levels. The first 30–60 minutes after you stop running is the “metabolic window” when your muscles are most receptive to glucose uptake.

The Recovery Meal: Carbohydrates + Protein

Consume a meal or snack providing at least 1–1.5 g of carbohydrates per kg of body weight, along with 20–30 g of protein. A chocolate milk smoothie (milk, banana, protein powder) is a practical and effective choice. Other options: a turkey sandwich on white bread, a bowl of rice with chicken, or a recovery shake designed for endurance athletes. If you have no appetite, a liquid meal can be easier to tolerate.

Rehydration

Weigh yourself before and after the race (if possible). Drink 16–24 ounces of fluid for every pound lost, but do not overdrink. Include electrolytes—a recovery drink with sodium or a salty snack. Monitor urine color: it should be pale yellow within a few hours.

Long-Term Recovery: The Next 24–48 Hours

Continue to eat carbohydrate-rich meals with moderate protein for the next two days. Good choices include whole grains, fruits, lean meats, and dairy. Avoid alcohol immediately after the race as it can impair glycogen synthesis and increase inflammation. Rest, gentle movement (walking), and sleep are equally important for physiological recovery.

Special Considerations: Individual Variation and Medical Conditions

Not all runners respond identically to the same nutrition plan. Age, gender, training status, and genetics all play roles. Female athletes, for example, may have different carbohydrate utilization patterns during the menstrual cycle. Runners with type 1 or type 2 diabetes, or those with a history of reactive hypoglycemia, must work with a healthcare provider to adjust insulin or medication. The strategies in this guide apply to healthy individuals; if you have a medical condition, do not make changes without professional oversight. Additionally, extreme environments—hot, humid, high altitude—increase fluid and electrolyte needs, and you may need to reduce your pace to manage carbohydrate intake effectively.

Conclusion: Practice, Personalize, and Execute

Stable blood glucose during an ultramarathon is not an accident. It is the result of intentional pre-race loading, disciplined mid-race fueling, and smart recovery. Start with the established guidelines (30–90g carbs per hour, 500–1000 mg sodium per hour, drink to thirst), but use your training races to refine what works for your body. Keep a log of what you eat, how you felt, and how your blood glucose responded (using a monitor or simply symptoms). Over time, you will develop a personalized nutrition plan that supports peak performance. The right fuel can mean the difference between a strong finish and a DNF. Invest the time before race day, and your legs—and your brain—will thank you when the miles get tough.