How Your Drink Mix Strategy Should Change During A 4+ Hour Race

There's a theory that does the rounds among longer-distance athletes: as a race stretches past 4 hours, your blood becomes more concentrated, so your drink mix should become weaker to keep pace with it and stay "isotonic" with your blood. It's a tidy idea. It's also not quite how physiology works.

What actually happens to your blood over a long race is more nuanced, and the real risk sits in the opposite direction to what that theory assumes. This article walks through what the research actually shows, why the practical advice around diluting carbs and holding sodium steady still stands, and what that means for your bottle in hour 4 onward.

Table Of Contents

• Should Your Drink Mix Get Weaker Over A Long Race?

• What Actually Happens To Your Blood Over A Long Race

• What Actually Happens To Your Blood Over A Long Race

• The Real Risk Past 4 Hours Isn't Rising Osmolality, It's Hyponatremia

• Why Carbohydrate Concentration Should Come Down Late In A Long Race

• Why Sodium Should Hold Steady Or Increase, Not Decrease

• A Practical Drink Mix Protocol For Races Over 4 Hours

Key Takeaways

Should Your Drink Mix Get Weaker Over A Long Race?

Not exactly. In well-hydrated, sodium-replete athletes, blood osmolality tends to rise slightly over a long race, so diluting your drink to chase it down doesn't hold up physiologically. The real risk past 4 hours is exercise-associated hyponatremia (EAH), a drop in blood sodium from drinking too much fluid relative to sweat losses¹.

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Reduce your drink's carbohydrate concentration later in a long race for gut comfort and better fluid delivery, and hold your sodium intake steady or increase it, managed separately from your carbs if needed, to guard against dilutional hyponatremia.

What Actually Happens To Your Blood Over A Long Race

In athletes who stay adequately fuelled and hydrated, blood osmolality typically rises a little over a long event. One study of ultra-endurance trail runners found plasma osmolality climbed from around 290 to 295 mOsm/kg in runners who kept their sodium levels normal, driven partly by rising urea concentrations from prolonged metabolic work, not simple dehydration².

If your blood is getting slightly more concentrated, matching it would mean drinking something stronger, not weaker. That's the opposite of the "dilute as you go" theory. The rise is small, and your body's own thirst and hormonal regulation handle it within a normal range without you needing to titrate your bottle against a number you can't measure mid-race anyway.

The Real Risk Past 4 Hours Isn't Rising Osmolality, It's Hyponatremia

The well-documented danger in races over 4 hours runs the other way: exercise-associated hyponatremia, where blood sodium and osmolality fall, sometimes dangerously. The consensus among sports medicine researchers is clear on the cause: it's the volume of fluid consumed relative to sweat losses that drives the drop, particularly when that fluid is low in sodium, not the concentration of any single drink¹. Sodium in your drink can help buffer the fall, but it can't prevent EAH if you're simply drinking more than you're losing.

This is why back-of-pack athletes in longer events, who are often out on course for many hours and have more opportunities to over-drink at aid stations, show up disproportionately in EAH case studies. 

Why Carbohydrate Concentration Should Come Down Late In A Long Race

There's still a real, evidence-backed reason to dilute your drink mix late in a long race, just not the one the original theory proposes. Carbohydrate concentrations above roughly 6-8% slow gastric emptying and fluid delivery compared with more dilute solutions³. Add fatigue, heat, and hours of sweet flavours, and most athletes develop a genuine aversion to strong, sugary drink mix as an event wears on, a pattern well documented in ultra-marathon nutrition research, where runners increasingly crave savoury, salty food over sweet gels and drinks the longer they're out there⁴.

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Keep a stronger carb concentration early in the race when your gut can handle it and carbohydrate delivery is the priority, then dilute the drink mix side from around hour 3-4 onward, leaning more on isotonic mixes or plain fluid, and topping up carbs through gels or solid food if your targets still call for it. Our How to Hit 90g Carbs Per Hour Using Drink Mix guide covers how to structure that carb load in the first place, and Isotonic vs Hypertonic Drink Mix Explained covers the concentration ranges referenced here.

Why Sodium Should Hold Steady Or Increase, Not Decrease

This is where the original theory gets the direction backwards. If you're drinking more total fluid volume as a race stretches on, and you dilute your electrolytes along with your carbs, you're doing exactly what drives dilutional hyponatremia. Sports nutrition guidance recommends maintaining adequate sodium intake throughout prolonged exercise, and this becomes more important, not less, as duration and fluid volume increase⁵, particularly in hot conditions where sweat losses climb further ⁶.

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After 4+ hours think about using a standalone electrolyte source like a low carb electrolyte drink mix/capsules or salt capsules rather than a high carb drink mix. That way you can back off the carb concentration without accidentally cutting your sodium right when you need it most.

A Practical Drink Mix Protocol For Races Over 4 Hours

Hours 0-3→ run a stronger carb concentration in your bottle if you're targeting a high carb intake, alongside your usual sodium strategy. This is when gut tolerance is highest and carbohydrate delivery matters most.

Hours 3 to finish→ dilute the carb side of your drink mix, shifting toward isotonic concentrations or plain fluid with electrolytes, and fill any remaining carb gap with gels or solid food rather than a stronger bottle. Keep your sodium intake steady or slightly increased through capsules or tablets, independent of how diluted your drink mix has become.

NOTE→ A longer race doesn’t automatically mean more fluid. If GI symptoms or bloating show up, our guide to avoiding GI issues during a marathon has the troubleshooting steps.

Key Takeaways

• Blood osmolality doesn't reliably rise in a way that means you should dilute your drink to "keep matching it". In well-fuelled athletes it creeps up slightly; in over-drinkers it drops.

• The real, well-documented risk past 4 hours is exercise-associated hyponatremia (EAH), a fall in blood sodium caused by drinking too much fluid relative to sweat losses, not by drink concentration alone.

• Carbohydrate concentration is still worth reducing late in a long race, but for gut comfort and fluid delivery, not to chase your blood's osmolality.

• Sodium should hold steady or increase as the race goes on, precisely because rising fluid volumes are what put you at risk of dilutional hyponatremia.

• Practical fix: go stronger on carbs early, dilute the carb side from hour 3-4 onward, and manage sodium separately with capsules or tablets so it doesn't get diluted along with your carbs

Ash Miller
Dietitian and Nutritionist (Masters)
Bachelor of Physical and Health Education
Instagram: @ashthomo_nutrition

References

1. Hew-Butler T, Rosner MH, Fowkes-Godek S, Dugas JP, Hoffman MD, Lewis DP, et al. Statement of the third international exercise-associated hyponatremia consensus development conference, Carlsbad, California, 2015. Clin J Sport Med. 2015;25(4):303-20.

2. Arnaoutis G, Anastasiou CA, Suh H, Maraki M, Tsekouras Y, Dimitroulis E, et al. Exercise-associated hyponatremia during the Olympus Marathon ultra-endurance trail run. Nutrients. 2020;12(4):997.

3. Vist GE, Maughan RJ. The effect of osmolality and carbohydrate content on the rate of gastric emptying of liquids in man. J Physiol. 1995;486(Pt 2):523-31.

4. Costa RJS, Hoffman MD, Stellingwerff T. Nutrition for ultramarathon running: trail, track, and road. Int J Sport Nutr Exerc Metab. 2019;29(2):130-40.

5. Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377-90.

6. McCubbin AJ, Allanson BA, Caldwell Odgers JN, Cort MM, Costa RJS, Cox GR, et al. Sports Dietitians Australia position statement: nutrition for exercise in hot environments. Int J Sport Nutr Exerc Metab. 2020;30(1):83-98.

Disclaimer:

The content in this blog is for general information only and is not a substitute for professional medical advice, diagnosis, or treatment. Always speak with your doctor or allied health team before changing your diet, exercise, or taking supplements, especially if you have a health condition or take medication. Please use this information as a guide only. Aid Station doesn't take responsibility for individual outcomes.