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Most Common Reasons I See Endurance Athletes DNF

Dana Eshelman, MS, RDN, CSSD, METS I

You work SO hard to get the start line: hours of training, prioritizing recovery and sleep, time spent prepping extra nutrition, and simulating race day scenarios. A "DID NOT FINISH" (DNF) are three dreaded letters that no endurance athlete wants to carry! However, sometimes you have to make a tough decision to call your race.

Here are some top reasons I see athletes DNF:

  • Extreme hot or cold and/or unexpected change in environmental conditions

  • Inadequate nutrition intake on the course leading to hitting the wall/ bonking

  • Dehydration/ electrolyte imbalance

  • Nagging aches and pains -- bone stress injuries, tendonitis

  • Recurring illness -- upper respiratory infections, gastrointestinal (GI) issues/ pains

  • Mental fatigue

The environmental factors are not within your control. It is important to be prepared for all conditions, but if it becomes unsafe to continue on, you may need to take a DNF.

The last 5 factors of not finishing are things you can control and are something you can improve with practice.

Lack of Fueling

Fuel to perform, ladies and gents! This is a non-negotiable. Supporting and nourishing your body is a part of self care and being an athlete! Your training session does not start when you click "start" on your watch, nor does it finish when you hit "end". A part of your training is optimizing your gut to tolerate carbohydrate during training as well as providing your body with the resources it needs before and after your training. This allows you to reap the most benefit from the work you put in.

This is a huge miss I see in many athletes trying to scrape by with the least amount of fuel they can for their sport. By limiting your nutrition, you are putting yourself at higher risk of injury and illness which will prevent you from doing what you love to do! More on injury and illness below.


This is something you want to have a tried and true plan to stick to. Understanding your sweat rate is a great place to start! This allows you to see how much sweat you are losing per hour.

  1. Weigh before with minimal to no clothes and no shoes. Weigh after you have used the restroom, right before you are about to start your session to minimize variables.

  2. Run or cycle for 1 hour at race pace and in temps similar to how you would be racing to best simulate sweat losses. For simplicity, if you are able to have no fluids in or out during this hour so you do no have to weigh your urine, stool, or fluid consumed, that is best!

  3. After 1 hour run or cycle, undress to same condition you were in pre-run, towel off sweat, and weigh again.

  4. Calculate:

    • Pre run weight (kg) ___ (lbs/ 2.2 = kg)

    • Post run weight (kg) ___ (lbs/ 2.2 = kg)

    • Pre run (kg) - Post run (kg) = Change in body weight (grams) ___ (kg x 1000 = g)

    • Volume of fluid consumed (mL) ___ (oz x 30 = mL)

    • Change in weight (g) - Fluid consumed (mL) = Sweat losses (mL) ___ (1 g = 1 mL)

    • Exercise time (min or hr) 60 min

    • Sweat Rate = sweat losses/ exercise time

You may also consider having your sodium sweat rate tested to understand which electrolyte balance is best for you to consume. Sodium sweat losses in athletes vary from 250 mg/L up to 2000 mg/L with the average sodium concentration closer to 800 mg/L. Testing takes the guessing out of the equation so you can have a personalized plan based on your losses. Including more sodium is not necessarily better.

Aches and Pains

Bone stress injuries and tendonitis are common injuries seen with under feeding. Movement in repetitive motion (ie running, cycling, swimming) puts you at a higher risk for developing injury when the body does not have the resources it needs (food + hydration) to perform these motions. Exercise is a stressor on the body and when provided the right fuel to recover, the muscles respond positively with training adaptations of getting stronger, faster, more powerful, etc. However, without the appropriate fuel, the muscles, tendons, ligaments, bone and soft tissue do not have the resources they need to appropriately recover. Remember, fueling is a part of your training!


Chronically being bogged down with illness such as respiratory infections and/or GI issues are signs of under feeding in your day to day nutrition. If your body is consistently not getting enough nutrition, this add unnecessary stress on your system. Your body cannot thrive when it has increased stress from training + stress from working harder to find resources to recover, maintain normal physiological function, and perform. Provide your body with the nutrition, hydration, sleep and stress management techniques it needs to thrive in both life and sport.

Mental Fatigue

In endurance sports, a big part of training is overcoming mental barriers you may encounter on race day during training; developing a personal reason you are striving to complete this distance; and visualizing each step of the race and how you want to execute each step. This is developing your mental fitness. You can have ALL the physical endurance, by if your mental fitness is not up to speed, you are not setting yourself up for success on race day. Do the hard work in training by putting yourself in uncomfortable (but safe) situations, so you know your limits and where you need to put in a little extra work!

Nutrition plays a huge role in getting to the start line healthy, but also getting across the finish line successfully! You work SO hard as an athlete in your day to day training, especially for longer events. Commit yourself to also developing a bulletproof nutrition plan so you can continue to be a resilient athlete!

If you are confused where to start with your training and/or are needing help with developing a strategy around fueling, reach out to a certified sports dietitian (CSSD) to help you get where you would like to be! At DDN, this is exactly what we do with our athletes!

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