The Physiology of Marathon Fueling
Completing a marathon requires immense cardiovascular stamina and a robust metabolic fueling plan. When running at race pace, your body derives energy from two primary fuel sources: glycogen (stored carbohydrates in your muscles and liver) and fatty acids (body fat). While fat reserves are virtually unlimited, your body can only store about 1,500 to 2,000 calories of glycogen—typically enough to power 90 to 120 minutes of running.
To prevent running out of glycogen in the final stages of the race (the infamous "hitting the wall" or bonking), endurance athletes must consume exogenous carbohydrates during the run. These carbs, consumed in the form of energy gels or sports drinks, enter the bloodstream and are oxidized directly by active muscle fibers, sparing your finite muscle glycogen reserves.
Carbohydrate Absorption Ceilings: Glucose vs. Fructose
Simply eating as many gels as possible is not a viable strategy. The human digestive system has a physiological limit on how many carbohydrates it can absorb per hour. Consuming too much fuel leads to osmotic fluid shifts in the gut, causing severe bloating, nausea, and stomach cramps.
- The SGLT1 Transporter Limit: Standard glucose (and maltodextrin) relies on the SGLT1 active transporter in the intestinal wall. This transporter saturates at roughly 60 grams of glucose per hour.
- Dual-Source Carb Ratio: To absorb more than 60g/hr, gels introduce fructose, which uses a completely separate transporter (GLUT5). By combining glucose and fructose (in a 2:1 or 1:0.8 ratio), trained runners can increase their absorption rates to 90 or even 120 grams per hour.
For most recreational marathoners, target absorption rates between **40g and 60g of carbohydrates per hour** represent the sweet spot for performance without inducing stomach cramps.
Water and Gel Osmolarity
To absorb an energy gel, the stomach contents must be dilute enough to pass through the gastric pylorus. Gels have very high osmolarity (they are highly concentrated). If you swallow a gel without water, the body must pull water out of your bloodstream and into your gut to dilute the gel. This delays energy delivery and leads to dehydration-induced stomach distress.
Rule of thumb: Always consume 4 to 6 fluid ounces (120 to 180 ml) of plain water with every gel. Avoid washing gels down with high-carb sports drinks, as this further concentrates the stomach contents and halts absorption.