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Mastering Marathon Swim Fueling: The Ultimate Guide

Mastering Marathon Swim Fueling by Erica Beine for Run Tri Bike Magazine

Mastering Marathon Swim Fueling

As athletes, we enjoy pushing the limits of our bodies to see what we can accomplish.  Running a marathon is absolutely no joke.  Anyone who has trained for and finished a marathon is a true champion in my book.  Personally, I have no desire to run a marathon at this point in my life.  I ended up in the water for a reason.

Many athletes I work with, just like myself, are most comfortable in water.  We find ourselves not high-level achievers on land and feel clumsy in anything that may require hand-eye-coordination.  Thus, swimming seems to be the ultimate “I can’t hurt myself or look foolish in the water” last effort at athletics.

Some of the athletes I work with swim open water races, including the “swimming marathon”, the 10k.  The world’s best male open-water swimmers can complete a 10k race in under two hours and the best women cross the finish line around the two hour mark.   

Mastering marathon swim fueling takes careful planning and preparation.  There are many aspects to consider including pre-race fueling, during-race fueling, and post-race recovery.

Training Day Fuel

During rigorous training and pre-race fueling, carbohydrates are of utmost importance to an ultra-endurance athlete.  Carbs not only provide energy, but they also help rapidly metabolize fats for energy.  If the liver and muscle are depleted of fuel, aka glycogen, the athlete will experience “bonking” or what I refer to with clients as “running out of gas.”  During training weeks, endurance athletes typically need between 6 to 10g of carbohydrates per kg of body weight daily.  To find your optimal range of carbs, simply take your body weight in pounds, divide it by 2.2, and multiply it by 6 and 10 to get your range of daily grams of carbohydrates.  In higher training days, the upper end of the scale will be more appropriate.  On lower volume days, the lower end will be adequate.  

Protein and fats are also very important to the training day fueling routine.  Protein intake can be calculated between 1.2g to 2g of protein per kg/body weight.  Many factors can affect the need for increased protein intake including injury recovery, fat loss goals, strength training, and weight gaining goals. 

With fats, I tell my athletes to focus on consuming a variety of healthy fats from food sources such as olive & avocado oils, walnuts, chia seeds, flax seeds, avocados, salmon, and considering an Omega-3 supplement.

Every person is unique, so it is important to play with your ideal daily macro intake during training to find your “sweet spot.”

36 to 48 Hours Pre-Race

In the 36 to 48 hours leading up to a marathon swim, it is important to increase carbs from 6 to 10g of carbohydrates per kg of body weight to 10 to 12g of carbohydrates per kg of body weight.  By increasing the amount of carbs in the days leading up to the race, this by default can increase the number of overall calories, resulting in fatigue, sluggishness, and digestive distress.  Since protein is still important for muscle support, it is not ideal to cut back protein intake significantly.  To prevent the overall total calorie intake significantly increasing, the simple solution is to cut back fats when increasing carbs in the 36 to 48 hours leading up to the race.  

During this time, also be mindful of your overall fiber intake.  Fruits and veggies with high fiber content can cause the body to spend energy processing and digesting. Examples of these fruit and veggies are apples, broccoli, cauliflower, kale, and cabbage. Smoothies with blended fruit and dairy can be fantastic carb sources that are easy to digest and carbohydrate rich.

Race Day Fueling

Research shows that ingesting carbs before a race is necessary for optimal performance, especially in an ultra-endurance type of event.  Ingesting carbs prior to the race increases blood glucose. This will spares the liver and muscle glycogen from being used quickly and extends time to fatigue or “bonking.”  

The number of carbs consumed before a race will vary based on the amount of time allowed between eating and racing.  In general, the greater the amount of carbs consumed, the greater amount of time that should be allowed before the race starts.  Consuming 1 to 4g of carbohydrates per kg of body weight in the 1 to 4 hours leading up to the race is optimal.  Once you know your optimal quantity of carbs to consume, you can play around with proper timing and amounts that make you feel in optimal racing mode.

During Race Fueling

As glycogen stores in the body become depleted, the body relies on blood glucose for fuel.  It is important to ingest carbs at feeding stations in a race or at planned fueling stops in a training swim.  Consuming carbs during activity can enhance performance, however, consuming too many or the wrong types may decrease performance by creating digestive distress.  Consuming 30 to 60g of carbohydrates per hour is recommended to continue to feed the body properly and perform optimally.  Some athletes can tolerate up to 90g of carbs per hour.  With this range being so wide, playing around with the optimal number is ideal.  

Some of our favorite recommendations for high-carb, low volume intake are:

Maurten Gel 100 – 25g Carbs

Maurten Drink Mix – 39g or 79g Carbs

Honey Stinger Gels – 24g Carbs

Gnarly Fuel2O – 50g Carbs

Having go-to products that you routinely use for training and racing will be key to success in your ultra-endurance swim.  So, try out some different options and find what works best for your digestive system and energy needs.

Post-Race Recovery

Many athletes have the mindset that carbs are only for pre-race fueling. Reality is that carbs are extremely important in the recovery process as well.  With glycogen stores severely depleted after an ultra-endurance swim, it is vitaly important to replenish those stores. This will help with your general feeling, future training sessions, and future races. 

Carbs should be consumed as soon as possible after the event so head over to the refueling tent (aka the “carb tent”) and grab some carbs.  Ideally, you should hit the “carb tent” within 15-30 minutes after the race for optimal glycogen replenishment.  Muscles are most receptive to glycogen storage from carb intake as soon as possible after exercise.  For the next four hours after the race, it is important to continue to consume 1 to 1.2g of carbohydrates per kg of body weight. This will allow you to optimally recover.  So don’t be afraid to continue to eat and drink carb rich foods regularly in the hours post-race.

Overall, carbs truly are the master fuel for training and performance.  Through experimentation from the recommended ranges, you’ll optimally find your best carb intake number. From there you will be able to find the products to help you with mastering marathon swim fueling.


Erica Beine Run Tri Bike Magazine Contributor

Erica Beine is a Board-Certified Holistic Practitioner and Certified Sports-Nutrition Consultant with thirteen years of collegiate coaching experience. Erica has worked with hundreds of athletes at all levels from age-group to international medalists helping each to learn how to eat, supplement, and recovery for their genetics.