The Economics: Commercial vs. DIY
A typical 16-week marathon training block calls for roughly 60–80 individual gels during long runs, track workouts, and race day.
Per 40g–50g carb packet. That’s roughly £160–£240 per marathon cycle, plus endless micro-plastic tear tabs.
Per 60g–90g carb dose in a reusable soft flask. Total season cost drops to under £25.
The Dual-Transporter Advantage (1:0.8 vs 2:1)
Your gut processes carbohydrates through specialized intestinal transporters. Glucose (and maltodextrin, which is simply glucose polymers chained together) utilizes the SGLT1 transporter, which saturates around 60g per hour. Attempting to ingest 80g or 90g of pure maltodextrin will leave excess carbs fermenting in your digestive tract, causing nausea, sloshing, and runner's trots.
Fructose, however, absorbs via the non-competing GLUT5 transporter. Pairing maltodextrin with pure fructose unlocks a secondary metabolic pathway, safely boosting total carbohydrate absorption to 90g to 120g per hour without gastrointestinal distress.
Soft Flask Batch Calculator
Calculate your exact powder and liquid measurements per flask
Your Recipe Measurements
Optional: Add 0.5g pectin powder if you prefer thick gel viscosity over concentrated liquid syrup.
What You Need (Raw Ingredients & Gear)
All of these ingredients are food-grade commodity items available in bulk on Amazon. A 2.5kg bag of maltodextrin will comfortably fuel an entire marathon training block.
Bulk Maltodextrin Powder (2.5kg)
Pure complex carbohydrate with a neutral flavor and rapid absorption without tooth-aching sweetness.
Pure Crystalline Fructose Powder (2kg)
Unlocks the independent GLUT5 intestinal transporter to safely push past 60g up to 90g+ carbs per hour.
Special Ingredients Sodium Citrate (100g)
Buffer salt that delivers essential sodium while being significantly gentler on the stomach and palate than table salt.
150ml Running Gel Soft Flasks (2-Pack)
Collapsible bite-valve flasks that slip flat into waistbands or half tights without bounce or sloshing.
Step-by-Step Batch Mixing Method
- Weigh powders into a small saucepan: Place your pan on a kitchen scale and measure out your maltodextrin, fructose, and sodium citrate according to the calculator above.
- Add warm water gradually: Pour in approximately half of the recommended water volume. Whisk vigorously over low heat. Do not boil—you only need warm liquid (~50°C) to fully dissolve the maltodextrin crystals without scorch marks.
- Flavor and acidify: Add 2–3 drops of natural citrus extract (orange or lemon/lime) and a pinch of citric acid to balance sweetness and preserve the mixture.
- Cool and funnel into flasks: Allow the gel liquid to cool to room temperature. Pour using a small silicone kitchen funnel directly into your 150 ml soft flask.
- Storage: Sealed flasks keep in the fridge for up to 7 days, or freeze indefinitely for upcoming weekend long runs.
Want to calculate your exact race fuel timing?
Use our free race pacing and carb demand calculator.