Nutrition
Gut Training Calculator — Build Carb Tolerance Before Race Day
Turn your current fuelling rate and your race target into a week-by-week progression, with the glucose and fructose split each step needs.
g of carbs per hour you tolerate now.
g per hour you want on race day.
until your key race.
Week by week
| Week | Target | Glucose | Fructose |
|---|---|---|---|
| 1 | 51 g/h | 51 g | — |
| 2 | 56 g/h | 56 g | — |
| 3 | 62 g/h | 60 g | 2 g |
| 4 | 67 g/h | 60 g | 7 g |
| 5 | 73 g/h | 60 g | 13 g |
| 6 | 79 g/h | 60 g | 19 g |
| 7 | 84 g/h | 60 g | 24 g |
| 8 | 90 g/h | 60 g | 30 g |
Glucose fills first because it saturates its own transporter, SGLT1, at roughly 60 g/h; fructose covers anything above that through a separate route, which is why products above 60 g/h are formulated as blends. Practise at these rates in the sessions that resemble your race — long efforts at race intensity — rather than on easy runs, since gastric emptying and symptoms both change with intensity. Do not introduce a new product or a new rate on race day.
The ceiling is your gut, not your willpower
How many carbohydrates you can use per hour is limited by intestinal transport. Glucose crosses the gut wall via SGLT1, which saturates at roughly 60 g per hour no matter how much more you swallow. Anything beyond that sits in the gut, draws water in, and becomes the bloating and nausea most athletes recognise.
Fructose uses a different transporter, GLUT5. Because the two routes work in parallel, a glucose–fructose blend raises the usable ceiling to around 90 g per hour, and up to about 120 g per hour in athletes who have trained for it. This is why modern gels and drink mixes are formulated as blends rather than pure glucose — the ratio is doing real physiological work.
Tolerance is trainable
Repeated carbohydrate exposure during exercise increases transporter capacity and reduces gastrointestinal symptoms. Cox and colleagues (2010) reported measurable increases in exogenous carbohydrate oxidation after about four weeks of consistent gut training.
The practical version is unglamorous: add roughly 10 g per hour every one to two weeks, and hold a step for an extra week if it does not settle. This calculator turns your gap and your timeline into that progression, and tells you when the timeline you have chosen is faster than the usual guidance.
Practise where it counts
Gastric emptying and symptom tolerance both change with intensity, so tolerance built on easy runs does not fully transfer to race pace. Do the work in the sessions that resemble the race — long efforts at or near race intensity — with the products you will actually use.
Two rules that cost nothing: never introduce a new product on race day, and never introduce a new rate on race day.
How this fits with the other fuelling tools
This one builds the capacity in the weeks beforehand. For the race itself, the race day nutrition planner and the marathon gel calculator turn that capacity into a schedule.
Frequently Asked Questions
Why can I only absorb 60g of glucose per hour?
Glucose is transported across the intestinal wall by SGLT1, which saturates at roughly 60 g per hour. Above that the transporter is the bottleneck rather than the amount you have eaten, so extra glucose stays in the gut, draws water in osmotically, and produces bloating, cramping or nausea. Fructose uses a separate transporter called GLUT5, which is why blends get past the single-transporter limit.
What glucose to fructose ratio should I use?
In practice you fill glucose up to its ceiling and cover the rest with fructose, which is what this calculator shows for each step. At 90 g per hour that works out near a 2:1 glucose to fructose split, the ratio most commercial blends are built around. Commercial products already balance this for you, so the useful thing is checking that your chosen product actually contains both rather than glucose alone.
How long does gut training take?
Measurable adaptation has been reported after about four weeks of consistent exposure, and a sensible build adds around 10 g per hour every one to two weeks. If you are going from 45 to 90 g per hour, that is roughly five steps, so allow eight to twelve weeks before a key race rather than trying to compress it into the taper.
Can I really absorb 120g of carbs per hour?
Some trained, gut-adapted athletes can, and that figure is the upper end of what has been studied. It is not a target for everyone, and it is not somewhere to start. Reaching it takes a deliberate build, and going beyond it puts you ahead of the evidence rather than following it.
Should I gut train on easy runs?
Not only on easy runs. Gastric emptying slows and symptoms become more likely as intensity rises, so tolerance built at easy pace does not fully carry over to race pace. Do the bulk of the practice in long sessions at or near race intensity, using the exact products and concentrations you plan to race with.
Data sources & references
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