What Supplements Should Be Taken When Training for a Marathon

In the NURMI study of European endurance runners, 43% of marathon participants reported regular vitamin supplement use, 34% reported mineral supplement use, and 19% reported carbohydrate or protein supplement use. Most of those products did not change finish times. A small subset did. The aim here is to identify which categories sit on solid evidence for a runner training across a 12 to 20 week block.

Carbohydrates as the Primary Fuel

Carbohydrate availability is the rate-limiting factor in marathon performance after the 90-minute mark. Liver and muscle glycogen stores hold roughly 1,800 to 2,200 calories in a trained runner. Most runners deplete those stores between mile 18 and 22 if they take in nothing during the race. Recommendations for runs longer than 90 minutes call for 40 to 80 grams of carbohydrate per hour, typically as a mixture of glucose and fructose, since the small intestine has separate transporters for the two sugars and a 2:1 glucose-to-fructose ratio raises maximum oxidation rate above the glucose-only ceiling.

Training day carbohydrate intake follows a different logic. Most coaches recommend 6 to 10 grams per kilogram of body weight on heavier mileage days, with intake clustered around the long run. Carbohydrate-only products are useful as race-day fuel and for very long training runs. Outside of those windows, food typically does the work better. Trained athletes who target 90 grams per hour during peak race blocks usually train the gut by progressively raising intake during long runs across the build, since untrained guts often reject high-carbohydrate loads with cramping and nausea.

Electrolytes During Long Sessions

Sweat loss runs 500 to 1,500 milligrams of sodium per hour during sustained running, with substantial person-to-person variation. Salty sweaters, defined as athletes with sweat sodium concentrations above 60 millimoles per liter, can lose more than 2 grams of sodium per hour in heat. Replacing some fraction of that loss matters during runs over 90 minutes and during racing in warm conditions. Female runners on average produce sweat with lower sodium concentration than male runners, though variation within each group is wider than the gap between groups.

A trained runner using electrolyte powders during a long workout typically targets 300 to 700 milligrams of sodium per hour, alongside fluid intake of 400 to 800 milliliters per hour. Potassium, magnesium, and calcium losses through sweat are smaller, and most products include them at fixed ratios. Plain water without sodium replacement during multi-hour sessions raises the risk of dilutional hyponatremia, which is the condition the next section addresses.

Hyponatremia Risk in Endurance Events

A 2002 study of Boston Marathon finishers found that 13% had post-race blood sodium below 135 millimoles per liter, the threshold for hyponatremia. The strongest predictor was body weight gain across the race, which correlates with overdrinking. Hyponatremia symptoms range from nausea and headache to confusion and seizures in the most severe cases. Salt tablets, electrolyte powders, and many sports drinks reduce the risk by replacing sodium during the race instead of relying on water alone. Slower runners face higher risk because they spend more total hours on the course and more total time drinking water at aid stations.

Iron for Endurance Runners

Iron supports hemoglobin synthesis and oxygen transport. Endurance runners lose iron through sweat, gastrointestinal microbleeding, foot strike hemolysis, and menstrual losses in female runners. Studies of endurance running cohorts have documented iron deficiency in runners at rates above 50% in some samples, with female runners affected at higher rates than male runners.

Serum ferritin below 25 nanograms per milliliter is the threshold most sports medicine clinics use as a flag for low stores. Iron supplementation is appropriate only after a blood test confirms low levels. Casual iron use in non-deficient adults can produce gastrointestinal side effects and, in rare cases, iron overload. The dose used in clinical correction is 100 to 200 milligrams of elemental iron daily, taken with vitamin C and away from coffee, tea, and dairy, which limit absorption. Most clinicians retest after eight to twelve weeks to confirm the dose is restoring stores.

Vitamin D and Bone Stress

Bone stress injuries account for roughly 20% of running injuries seen in sports medicine clinics. Calcium and vitamin D are required for bone mineralization. Trials in collegiate athletes have reported reduced stress fracture rates among groups receiving vitamin D supplementation when baseline levels were low. Vitamin D deficiency is common in winter months at higher latitudes, and runners who train mostly indoors or in heavy clothing may not produce enough through sun exposure.

A typical maintenance dose is 1,000 to 2,000 IU of vitamin D3 daily, with higher doses used for correction in deficient athletes. Routine testing once or twice a year is enough for most runners and removes the guesswork. Calcium intake from dairy, fortified plant milks, leafy greens, and tofu supports the same pathway, with most adults needing 1,000 to 1,200 milligrams daily.

Caffeine as a Dose-Dependent Aid

Across meta-analyses of endurance time-trial studies, caffeine in the 3 to 6 milligram per kilogram range produces a 2 to 3% improvement in finishing time compared with placebo, taken roughly 60 minutes before the start. The mechanism is a reduction in perceived effort plus a small effect on fat oxidation. The effect sits at the high end for runners who normally drink little coffee and at the low end for daily heavy users. Side effects include disrupted sleep when caffeine is taken later in the afternoon and gastrointestinal distress when stacked with other stimulants. Studies of coffee before workout timing report similar dose-response patterns, with minimal benefit below 2 milligrams per kilogram.

Race-day topping up with smaller doses every 30 to 45 minutes during the second half of a marathon helps maintain plasma caffeine and may extend the perceived-effort benefit. Total intake on race day usually peaks around 6 milligrams per kilogram, since higher doses produce diminishing returns and a higher rate of side effects without further improvement in finishing time.

Protein for Training-Day Recovery

Endurance athletes need 1.2 to 1.7 grams of protein per kilogram of body weight per day, higher than the sedentary baseline of 0.8 grams per kilogram. Most runners hit this through normal meals if they eat protein at every meal and a snack after the long run. A scoop of whey or plant protein after hard sessions can help elite athletes who struggle to eat solid food in the immediate post-run window. Beyond that, the marginal value of protein products drops fast, and additional intake does not produce additional adaptation. Older runners, runners on calorie-restricted diets, and runners returning from injury can sit at the upper end of the range to support muscle preservation.

Categories With Strong Evidence

Across the published literature, four nutrient categories are well-supported for marathon training. Carbohydrates during runs over 90 minutes. Sodium and other electrolytes during long, hot, or salty-sweat sessions. Iron when blood tests show low ferritin. Vitamin D when serum levels run low and during winter at higher latitudes. Caffeine sits as an optional ergogenic aid with consistent but modest effect sizes. Protein products are a convenience choice rather than a performance lever. Anything outside these categories belongs to a much smaller evidence base, and most of it does not change marathon finish times. A simple test: if a category is not on this list, the runner should expect no measurable benefit from spending money on it.

By Nathan Spears

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