Fueling the Run

Not every run needs a gel. Not every run needs a sports drink. But some runs demand a fueling strategy as deliberate as your training plan. Your goal is to give your body the fuel and electrolytes it needs to perform and recover at its best.

When Does Fueling Matter?

Your muscles and liver store carbohydrate as glycogen, roughly 400–500 grams total in a well-fueled athlete. At moderate to high intensity, that supply runs out after 90–120 minutes. When it does, performance collapses. Runners know this as hitting the wall.

Research by Asker Jeukendrup and colleagues at the University of Birmingham established a duration-based framework for carbohydrate intake during exercise, since adopted by the American College of Sports Medicine (ACSM).

These thresholds are calibrated to moderate-to-high intensity. At an easy recovery pace, glycogen depletion is slower and fat oxidation carries more of the load. You have more margin than at tempo effort.

Hydration and Electrolytes

Dehydration and overhydration both degrade performance. The ACSM cites measurable performance decline at fluid loss of 2% of body weight, with heat strain rising sharply above 4%. Overhydration carries its own risk: exercise-associated hyponatremia, a dangerous drop in blood sodium caused by excessive plain water consumption. Target 600–1,200 ml/hr during exercise, scaled to conditions and individual sweat rate.

Sodium

Sodium is the electrolyte that matters most, and the one most concentrated in sweat. Sweat sodium averages around 800–1,000 mg per liter, and a practical target of 1,000 mg per liter of drink replaces losses without overcorrecting. In hot and humid conditions above 77°F (25°C), sweat rate can double and sodium needs scale toward 1,500 mg per liter.

Watch for swollen hands or fingers mid-run. It can signal either sodium depletion or overhydration from plain water, and the two problems have different responses. If you have been drinking heavily without electrolytes, ease back on fluid intake first. If your electrolyte intake has been low, increase it.

Potassium and other electrolytes

Sweat contains roughly four to six times more sodium than potassium, so potassium is a secondary concern during exercise. Most runners cover baseline potassium needs through diet, and omitting it from your drink mix is entirely reasonable for runs under three hours. Commercial products like Skratch, Tailwind, and Liquid IV contain only 88–176 mg of potassium per serving, which reflects this hierarchy. The adequate intake for potassium is 2,600–3,400 mg per day, and a single liter adding 700 mg on top of a normal diet is well within that range. Commercial products keep potassium low primarily for taste, and you may adjust according to your preferences.

For efforts beyond three hours, potassium chloride (sold as No Salt or Nu-Salt, approximately 350 mg per 1/8 teaspoon) provides useful insurance against cumulative depletion. For shorter efforts, omit it from your drink. Dietary potassium covers the baseline and sweat losses are too small to outpace it.

Magnesium and calcium are better handled through daily supplementation than a run drink. Magnesium glycinate at 200–400 mg taken nightly supports sleep and muscle recovery. Calcium is best split across two meals, with absorption capped at around 500 mg at a time. Including either in a run drink introduces GI risk without meaningful benefit in acute performance.

A practical electrolyte mix

Per liter of water:

• 1/2 tsp fine-grain table salt (approximately 1,150 mg sodium)
• 1/8 tsp No Salt or Nu-Salt, optional (approximately 350 mg potassium)

Scale sodium toward 3/4 tsp per liter in heat above 77°F (25°C) or if you are a salty sweater.

The Science of Carbohydrate Absorption

Your intestine absorbs carbohydrates through specific transporter proteins. Glucose and glucose polymers like maltodextrin use SGLT1, a sodium-dependent active transporter. Fructose uses a separate one, GLUT5. When you consume only glucose-based carbohydrates, SGLT1 saturates at around 60 g/hr. More glucose does not increase absorption. It sits in your gut, pulls in water, and causes bloating and GI distress.

Jeukendrup's group at Birmingham demonstrated that combining glucose and fructose uses both transporters simultaneously, pushing oxidation rates to 1.75 g/min under laboratory conditions, a 50% improvement over single-source carbohydrate. With both transporters working, more carbohydrate clears the intestine and reaches the muscle. Less accumulates in the gut, which is why GI distress drops at high intake rates when you combine sources. The ACSM caps practical recommendations at 90 g/hr: the laboratory peak assumes optimal conditions, trained absorptive capacity, and carbohydrate sources dialed to the right ratio. For most athletes, 90 g/hr is the realistic ceiling and the appropriate target for gut training.

This principle underlies three practical formulation approaches, each with solid scientific support.

The 2:1 ratio (glucose:fructose)

The original Jeukendrup formulation, and still the most widely cited. Two parts glucose or maltodextrin to one part fructose saturates SGLT1 while engaging GLUT5, pushing absorption well past the 60 g/hr single-source ceiling. It is the basis of most commercial endurance products and has the largest body of research behind it. Maltodextrin is nearly tasteless, making high-carb drinks palatable at concentrations that would otherwise be excessively sweet.

The 1:0.8 ratio (glucose:fructose)

More recent research suggests that increasing the fructose proportion further improves oxidation efficiency and reduces GI distress at high intake rates. GLUT5 has more capacity than the 2:1 ratio exploits. Studies comparing the two ratios at intakes above 80 g/hr favor 1:0.8, with equal or better performance and comparable gut tolerance in trained athletes. The practical difference is modest at 60 g/hr and more meaningful at the upper ceiling.

Table sugar (sucrose)

Sucrose is a disaccharide that the enzyme sucrase cleaves in the intestine into one glucose molecule and one fructose molecule, producing a 1:1 ratio that engages both SGLT1 and GLUT5. At the 30–60 g/hr range typical of half marathon and marathon efforts, the difference between 1:1 and the optimized ratios above is negligible. Table sugar costs a fraction of bulk maltodextrin or fructose, is available in any grocery store, dissolves cleanly in water, and works exactly as the science predicts.

Choosing your ratio

The practical difference between the three approaches comes down to sweetness and intake ceiling. Fructose is sweeter than glucose, so the more fructose in your formula, the sweeter it tastes at equivalent concentrations. Below 80 g/hr this rarely matters and all three approaches perform comparably. Above it, a highly concentrated sweet solution over several hours becomes a palatability problem, and flavor fatigue can suppress drinking at exactly the moment you need it most.

For most runners, table sugar is the right starting point. If sweetness becomes a problem at higher intakes, move to 2:1 maltodextrin/fructose. If you are pushing toward 90 g/hr and have already gut-trained at lower intakes, the 1:0.8 ratio is worth experimenting with.

Gut Training

The gut is trainable. With repeated exposure to carbohydrate during exercise, the intestine gets better at absorbing it. Athletes who practice fueling during training tolerate significantly more carbohydrate per hour than those who don't.

90 g/hr requires months of deliberate practice. Starting there on race day is a reliable path to GI distress, regardless of how well-formulated your gel is. Begin around 30–45 g/hr on long runs and build gradually over weeks and months. Whatever you plan to race with, train with it first.

In practice, add roughly 10g every two to three weeks, not every week. Test on efforts that approximate race conditions: similar pace, duration, and heat. A training run at easy effort on a cool morning tells you almost nothing about how your gut will respond on race day. If you experience bloating or distress at a given intake level, hold there for another two or three long runs before increasing. The gut adapts, but it adapts on its own schedule.

The Homemade Advantage

The core ingredients of elite sports nutrition are bulk commodities. The difference in cost over a full training block is real. More importantly, making your own puts you in control of concentration, flavor, and format in a way no packaged product can match.

The carbohydrate gel

All three formulas work as a gel. The process is the same regardless of carbohydrate source: mix dry ingredients, add boiled water gradually to your preferred consistency, allow to cool, and portion into reusable gel flasks. Boiling helps the powders dissolve fully without clumping. Refrigerate for up to 5–7 days, or freeze in flasks for up to 3 months. Take one flask every 30–45 minutes depending on your target intake rate.

For a standard batch covering roughly 3.75 hours at 60 g/hr, each formula yields 225g of carbohydrate and 375–425ml of gel, filling 3–4 standard flasks. For a marathon at 60 g/hr over 4.5 hours, scale any formula to 270g total carbohydrate:

All in One

Combine carbohydrate and electrolytes into a single bottle at roughly 6% concentration (about 60g carbohydrate per liter) with salt mixed in. Omit pectin; you want a drink, not a gel. This suits runners using a vest or anyone who prefers not to carry separate flasks. The tradeoff is that once mixed, hydration and carbohydrate are locked together: drinking more means taking in more carbohydrate, and you can't change the ratio mid-run.

Per liter, a starting point: 5 tbsp table sugar (60g carbohydrate) and 1/2 tsp salt. Scale carbohydrate, sodium, and potassium to your target rate and conditions.

Owning the Formula

Your training runs are the laboratory. Experiment with concentration, timing, and formulation. Arrive at race day with a strategy already proven on your own body.