While all nutrients are essential to good health, marathon runners can have increased needs for certain nutrients to meet the physical demands of training and optimize performance and recovery. The nutrients marathon runners need are water, carbohydrates, protein, fat, electrolytes (sodium, potassium, magnesium, calcium, and chloride), vitamin D, iron, and antioxidants.
Often overlooked, water is an essential nutrient because the body cannot produce enough water itself or through the metabolism of food to fulfill its needs. The importance of hydration for distance runners cannot be stressed enough, as almost every biological process depends on a sufficient supply of water. Hydrating before, during, and after a marathon (or any long run) is essential to perform at your best. Even a 2% drop in body weight due to fluid loss can impair performance and leave you feeling fatigued and foggy during a run 
Hydration needs will vary from person to person but a good rule of thumb is to aim for 2 cups of fluid (16 fluid ounces) 2-3 hours before you run, and another cup of fluid (8 fluid ounces) 15-30 minutes before you head out the door. Water is generally the best option for staying hydrated;however, you may benefit from a sports drink for additional carbohydrates and electrolytes depending on the length of your run, the outside temperature, or type of sweat you produce.
Carbohydrates are the body’s preferred source of energy during a marathon and are an important part of your marathon nutrition plan. Like calorie intake, daily carbohydrate needs for runners fluctuate based on training load. Below are the reference ranges for carbohydrate intake per day, based on activity level:
Normal activity level (general fitness): 45% - 55% calories from carbohydrates [3–8 g/kg/day]
Endurance athlete training (2-3 hours of intense training, 5-6 days/week): >60% calories from carbohydrates [5–8 g/kg/day]
Essentially, the greater duration and intensity of your runs, the more carbohydrates you need.
Protein is essential for cell regulation, nerve function, and repairing and synthesizing new muscle after a workout 
. Many runners have increased protein needs to support muscle protein synthesis and glycogen repletion after a workout, which is especially true during heavy training/high mileage periods such as marathon training. But it’s not just about the amount of protein you’re consuming–protein quality and timing also have a big impact on performance, recovery, and training progression.
The recommended intake for protein ranges from 0.8-1.2 g/kg/day but marathon runners may need up to 2.2g/kg/day 
. Runners looking to optimize weight and body fat percentage and increase lean muscle mass may especially benefit from hitting this protein target.
Choosing high-quality fats (such as omega-3 fatty acids) can support hormonal health and inflammation, both of which are a concern during prolonged high-intensity exercise, such as a marathon 
Omega-3 fats block inflammation early in the inflammatory cascade and have been shown to be beneficial for muscle recovery post-exercise, heart health, liver function, metabolic health, and more [5
]. Some research suggests that up to 6g of fish oil (2,400 mg EPA, 1,800 mg DHA) a day may be helpful for reducing muscle soreness after a vigorous exercise session, although this is an area of ongoing investigation 
The recommended daily intake for fat is 0.5–1.5 g/kg/day. Therefore, if you are getting 55% - 60% of your calories from carbohydrates and 15% - 20% from protein, 20% - 30% of your calories should come from healthy fats 
Marathon runners tend to have higher micronutrient needs than non-athletes since marathon running requires higher rates of energy metabolism and puts intense physical demand on the body. The micronutrients that are particularly important for marathon runners are electrolytes (such as sodium, potassium, calcium, magnesium, and chloride), iron, vitamin D, and antioxidants (e.g. vitamin E, C, and beta carotene).
Electrolytes: Sodium, potassium, calcium, magnesium, and chloride
Electrolytes (like sodium, potassium, calcium, magnesium, and chloride) are electrically-charged minerals that are critical for muscle and nerve function and help regulate fluid balance. Calcium is also important for the growth, maintenance, and repair of bone tissue, which increases with marathon running.
Iron is incredibly important for runners as it is a key component of hemoglobin, a protein in red blood cells that transports oxygen to your working muscles when you run. Deficiency, with or without anemia, can impair muscle function and limit work capacity [9
]. Athletes undergoing intense training (such as marathon runners) need to take in an adequate amount of iron from the diet as losses from sweat, urine, and red blood cell turnover are typically greater [9
Like calcium, vitamin D also plays a key role in maintaining bone health as it regulates calcium and phosphorus absorption and metabolism [9
]. Growing evidence also suggests vitamin D has a role in mediating muscle metabolic function and may support athletic performance [9
]. Vitamin D can be challenging to obtain through the diet alone but can be synthesized from direct sun exposure. Depending on training location, the season, and amount of direct sun exposure while training, vitamin D supplementation may be necessary to support the demands of marathon runners.
Because exercise can increase oxygen consumption by 10- to 15-fold, it has been hypothesized that chronic training (such as that required for a marathon) increases oxidative stress on cells. Antioxidants, such as vitamin C, E, and beta-carotene, can play important roles in protecting cell membranes from oxidative damage. The best source for these nutrients is a diet rich in antioxidant foods (like fresh fruit and vegetables) since there is little evidence that antioxidant supplementation enhances athletic performance, and taking high-dose supplements can be pro-oxidative [9
Intense exercise, like running a marathon, increases the body’s need for certain nutrients. The nutrients that are especially important for marathon runners are water, carbohydrates, protein, fat, electrolytes (sodium, potassium, magnesium, calcium, and chloride), vitamin D, iron, and antioxidants.