What are the stages of a nutrition intervention plan for a marathon participant to enhance performance and prevent dehydration and electrolyte imbalances?

Marathon Nutrition Intervention Stages

Marathon nutrition intervention should be structured into three distinct temporal stages: pre-race preparation (days before), race day execution (during the event), and post-race recovery, with carbohydrate loading and individualized hydration planning as the cornerstones to prevent morbidity from hyponatremia and dehydration.

Stage 1: Pre-Race Preparation (Days Before the Event)

Carbohydrate Loading Protocol

• Consume 10-12 g/kg/day of carbohydrates over the 36-48 hours prior to the marathon while tapering exercise to maximize muscle glycogen stores, which directly enhances performance by allowing optimal pace maintenance before fatigue 1, 2.
• Glycogen elevation improves performance specifically in exercises lasting longer than 1.5 hours 1.
• Avoid consuming elevated-carbohydrate meals less than 60 minutes before the race, as this can trigger an insulin surge leading to paradoxical hypoglycemia and impaired performance 1.

Pre-Race Hydration Strategy

• Start the race in a euhydrated state by drinking 6 mL of fluid per kg of body weight every 2-3 hours before running 3.
• Confirm euhydration through objective markers: daily body mass changes <1%, first morning urine specific gravity <1.020, and plasma osmolality <290 mmol/kg 3, 4.
• Measure baseline body weight and calculate individual sweat rate during training runs by weighing before and after typical runs 3.

Critical Pre-Race Education

• Athletes must understand that 64% of marathon runners are not concerned about hyponatremia risk, and over 80% have no hydration assessment method—this knowledge gap increases morbidity 1.
• Educate on risk factors for exercise-associated hyponatremia (EAH): excessive fluid consumption, race times >4 hours, female sex, low BMI, and weight gain during exercise 1.

Stage 2: Race Day Execution (During the Marathon)

Carbohydrate Intake During the Race

• Consume 30-60 g/hour of carbohydrates during the marathon to maintain exercise capacity and motor output 1, 3, 5.
• The American College of Sports Medicine specifically recommends this range for endurance events lasting longer than 1 hour 1.
• Carbohydrate mouth rinses may benefit athletes experiencing gastrointestinal discomfort from liquid consumption 1.

Hydration During the Race

• Consume 0.4-0.8 L/hour of fluid containing 0.5-0.7 g/L sodium (20-30 mmol/L sodium and 2-5 mmol/L potassium) 1, 3, 5.
• For intense exercise lasting longer than 1 hour, drink between 600-1200 mL/hour of a solution containing both carbohydrate and sodium 5.
• The critical safety principle: never lose more than 2-3% of pre-race body weight, but also never gain weight during the race 1, 3.

The "Drink to Thirst" Strategy

• Drinking when thirsty is the safest strategy for most athletes and prevents both dehydration and overhydration 1.
• This approach is particularly important given that EAH occurs when athletes consume free water far in excess of losses without electrolyte replenishment 1.

Race-Specific Risk Mitigation

• Athletes at highest risk for EAH (race duration >4 hours, female, low BMI, slower pace) should have specific hydration plans developed with sports dietitians 1.
• Medical providers must rapidly identify symptomatic hyponatremia presenting as bloating, nausea, vomiting, headache, or altered mental status 1.

Stage 3: Post-Race Recovery

Aggressive Rehydration Protocol

• Consume fluids and electrolytes to offset 100-150% of body mass losses if aggressive rehydration is needed 3, 4.
• Replace fluids at a modest rate in conjunction with sodium and potassium rather than rapid consumption 1.
• This can be achieved by drinking water supplemented with snacks or by consuming sports beverages 1.

Recovery Nutrition

• Continue carbohydrate intake with 0.2-0.4 g/kg/hour of protein to optimize recovery 4.
• Monitor urine specific gravity to confirm return to euhydration (<1.020) 4.

Critical Safety Considerations and Pitfalls

Exercise-Associated Hyponatremia Management

• Mild symptomatic EAH without confusion: provide oral hypertonic solutions (16 runners recovered within 30 minutes using this approach) 1.
• Severe EAH with confusion, seizures, or coma: administer up to three 100 mL boluses of 3% sodium chloride solution spaced at 10-minute intervals per current consensus guidelines 1.
• The absolute sodium value is not a reliable predictor of clinical severity—symptomatic presentation determines management 1.

Common Pitfalls to Avoid

• Never administer intravenous hydration to athletes with suspected EAH symptoms—this worsens dilutional hyponatremia and increases mortality risk 1.
• Avoid hyperhydration strategies, as overdrinking is the primary cause of EAH, not excessive sodium loss 1.
• Do not rely on fixed hydration schedules that ignore individual sweat rates and environmental conditions 1, 3.

Red Flags Requiring Medical Evaluation

• Persistent symptoms despite adequate hydration and electrolyte replacement 3.
• Confusion, non-fluent speech, extremity weakness, or postural hypotension indicating significant volume depletion 4.
• Any weight gain during the race, which indicates overhydration and EAH risk 1.