The Everyday Ultra Newsletter

Ultra training tips to help you be a better endurance athlete, every day
by Joe Corcione

#ENDUREBETTER 🐺 

How to avoid stomach issues in your next ultra and nail your nutrition plan

The #1 cause of DNF’s in an ultra? Stomach issues.

If the title got you interested, you’ve probably been in a situation like this before:

You’re in the race that you’ve trained so hard for. Months and months of training, leading to this very moment on the trail.

Everything is going to plan: the pacing is great, your legs feel strong, and your splits are all on point.

When all of a sudden…

You feel an uncomfortable feeling in your stomach.

Every time you try to run to your normal pace, it gets worse. And now, you can’t eat anything because you can’t take anything else down. Help!

This is the most common reason why people don’t hit their ultramarathon goals. And it’s the #1 cause of DNF’s.

Why? Because stomach issues can easily cause a series of unfortunate events in an ultra that can spiral into issue after issue.

Think about it:

• Your stomach hurts, so now you stop eating

• At the same time, you’re running slower because going faster hurts your stomach

• Because you’re not eating, your muscles get more fatigued, and now you go slower

• Going slower can lead to the situation being mentally tougher, causing feelings of “damn, this isn’t going well”, which can lead us to go slower.

• And the cycle continues.

The reason why this happens so much in ultras is because there can be a plethora of reasons why the stomach issues are happening.

But, there are many things you can do in your training and races to prevent stomach issues from arising.

As an ultrarunning coach who has worked with over 100 ultra athletes and have raced over 30 ultras myself, here is the most impactful (and often missed) thing you can do to dramatically increase your chances of having no stomach issues in your race.

And here it is: before you look at what you’re eating, look at what you are DRINKING.

You read that right. The #1 reason why stomach issues happen actually isn’t because the nutrition plan is off.

It’s commonly because the hydration strategy is off.

Research shows that even mild dehydration reduces blood flow to the gut, which makes digestion harder and dramatically increases the likelihood of nausea, cramping, and vomiting during long ultras.

This is also why on really hot races, it is more common to get stomach issues, too.

So, you have to make sure your hydration is dialed to dramatically increase your chances to avoid your stomach issues.

The tricky part is that hydration is a very individual approach. But the good news is there is a proven process to dial it in, that I encourage all of my athletes to go through.

Here is the process to dial in your hydration:

1. Perform an at-home sweat test. It sounds fancy, but all you need is a scale to weigh yourself.

   1. On a day that you have a 1 hour, easy run, weigh yourself without clothes before your run and note the weight

   2. Go out for the run (with clothes on, of course). Try not to drink water out here, but if you do, make sure to note exactly how much you drink. Note the temperature and conditions you are running in (for example: 55 degrees and sunny).

   3. After the run, pat yourself down with a towel and weigh yourself again without clothes. The weight will be lower, as this is all water weight.

   4. From here, subtract the after weight from the before weight, to get your water weight loss during the run. Take that difference and multiply it by 15.5, as each pound lost is equal to 15.5 ounces of water. For example, if you lost 1 pound, that means you lose 15.5 ounces at that temperature, at that intensity you ran, in the conditions outside.

   5. Now, note that this sweat result is not for ALL runs. It’s just for the same temperature, same conditions, and same intensity. So, I suggest to do this test on similar conditions to your goal race, if possible.

   6. Then, take your sweat loss result and take 70-90% of it to find out how much you should drink. This is VERY important as you don’t want to replace 100% of the fluids you lose to avoid overhydration. The bigger the ounces lost, the closer to 70% you go (usually, I cap athletes out at 43 ounces since the stomach can only handle so much).

   7. And there you have it: your fluid intake!

2. Now, find your sodium loss amount. This one is a bit more tricky to dial, but also essential, as you need to balance fluid with sodium to have optimal hydration. Here are three ways you can find this number

   1. The first way is a trial and error method. On average, a person will lose 450-750mg of sodium per pound of sweat lost. You can try to test these numbers in relation to your sweat rate numbers and test things out. I’d start in the middle first, and then toggle between lower or higher depending on your symptoms

      1. Low sodium level signs can be swelling in the fingers, sloshing liquid in your stomach and frequent peeing

      2. High sodium level signs are nausea, feeling like you are SUPER thirsty even though you are drinking a ton, swelling in the fingers COMBINED with feeling very thirsty.

   2. The second way is getting a sweat/sodium concentration test. You can do this via Precision Hydration on their website to find a place where you can get your exact sodium concentration tested in-person. It is pricey, but you get the number right away and you avoid all the guess-work

   3. The third way is using a sweat tracking device like an hDrop or Nix sensor. These are sensors you can wear while running and it’ll tell you how much sodium you lose. Again, these are a little pricy, but you’re paying to avoid the guess work.

3. Correlate your sodium with research-backed suggestions on how much to take, and your sweat loss to put it all together. These recommendations are based on the latest 2025 review by Alan McCubbin on sodium balance during endurance exercise, which reinforces that sodium needs depend on sweat rate, fluid replacement, and sweat sodium concentration—not total losses

A) If your sweat sodium is 920–1380 mg/L:
Replace 20–50% of what you lose per hour.
Example:

• Sweat rate: 0.68 L/hr (23 oz/hr)

• Sodium loss: ~680 mg/hr

• Replace: ~135–340 mg/hr

B) If your sweat sodium is <920 mg/L:
→ “Season to taste.”
You likely don’t need a structured replacement plan — add sodium based on feel and cravings.

C) If your sweat sodium is >1380 mg/L:
Replace 30–65% of losses.
Example (same sweat rate):

• Sodium loss: ~940 mg/hr

• Replace: ~280–610 mg/hr

So, You don’t need to replace everything — just enough to keep your blood sodium balanced and avoid cramping, nausea, and energy dips.

An important note here is this:

Your fluid loss levels WILL change based on temperature, conditions, intensity, your overall fitness, and how acclimated you are to heat.

So, this is why it’s best to re-test your sweat rate at different temperatures at different points in your training to get the most accurate data.

BUT, your sodium concentration WILL NOT change much on these factors. For example, if you lose 1000mg/L at 55 degrees, it will be the same at 75 degrees.

So, your sodium needs will have to adjust based on that fixed number to the fluid (pun intended) change in your sweat loss.

Bottom line: find your sweat loss and sodium concentration to come up with your hydration plan at race conditions. Do this, and you’ll dramatically increase your chances of avoiding stomach issues.

Some other things to try out if hydration is well, but you’re still getting sick

If your hydration is already dialed, but you’re still having tummy troubles, here are a few things to look at and adjust next:

1. Are you training your stomach in your long runs?

   1. For each of your long runs, you should be executing the same nutrition and hydration plan that you plan to use during the race. That way, your stomach is used to intaking that many carbs and fluid out there

2. Are you eating enough in your day-to-day life outside of running? Research on low energy availability in endurance athletes shows that chronically under-eating slows gastric emptying, reduces gut motility, and makes the digestive system more sensitive; all of which increase the likelihood of GI distress once intensity or duration ramps up. So, eat up!

3. Is there an ingredient you are sensitive too? Some athletes can’t handle maltodextrin as well (the carb base that most gels are made of). And some athletes can’t handle cane sugar as well. Look at the ingredients you are eating when your stomach goes sour and see if there are any common culprits to try cutting them out.

4. Are you eating frequently during your runs? Try getting in your hourly carbs more frequently. Meaning, if you are fueling every 60 minutes, try fueling every 20-30 minutes instead. When you fuel frequently, you fuel in smaller amounts to make it easier for your digestive system to handle. Just keep the same hourly amount, and split it in smaller increments.

Since there are a lot of issues that can cause stomach issues, the most important thing is this:

Understand it is a process, and it takes time to learn what works best for you.

Keep trying different approaches. Note which nutrition plans work and which don’t (I always suggest tracking this via a training log in TrainingPeaks, which you can sign up for a free account here. That way, you can note what works and what doesn’t). And always be willing to try new approaches.

I had stomach issues constantly for years in my races when I first started ultrarunning. And I can say that I haven’t had a major stomach issue in over 3 years since I dialed this system down.

Be patient. Keep trying. And you’ll avoid the tummy issues when you get it all sorted.

What I’m loving in my current training

Practicing the hard thing: patience!

After my last race at Kodiak 100M, I took almost two months off of unstructured, low volume running, to take a proper offseason.

And it worked. I feel excited to train again for the next race (announcement coming soon!), my paces are back to normal, and my body feels great!

But, I’m feeling a very common feeling most people get when they are rebuilding: the feeling of wanting to get back to the same volume/intensity of training that you were at before, right away.

And this is the hardest part of the build back: executing patience.

When you take a decent amount of time off, whether due to an offseason, injury, personal time, or anything else, you can’t just jump right back into where your peak mileage was before.

And the longer the time you took off, the more this matters

It’s because your body (specifically, your muscles, tendons, joints, and bones) need to get used to the impact forces of running again.

When you run, about 2x your bodyweight shoots back into your body. And if you haven’t been running much, your body eases off a bit on its ability to handle all that load.

In general, people get injured because they ramp up training too fast. And the load on the body exceeds the capacity that our muscles, tendons, joints, and bones can handle.

This same principle applies when you jump back into higher volume and intensity of running too quick after some time off. I see people get injured A LOT this way.

So, even though your mind is STOKED on the training. Even though you may mentally feel like you can do more. And maybe even your lungs feel like they can go more…

You have to meet your body where it’s at.

I’m not saying you have to start from scratch. Just take it a bit conservative to see how your body feels. If you feel really good, you can try to build back quicker. If you feel any aches, niggles, or soreness, adjust the ramp up a bit slower. If you’ve built back before, you can use prior data on how quick you bounced back without issues (or, with issues) to inform where to start.

For me, I know I can usually bounce back quicker than most. But I’m being very conservative to start. Coming off of 100+ mile weeks in my peak to Kodiak, my first week back was only 31 miles. This week will be around 50. And I’ll be open to adjusting the plan if my body gives me other signals.

It’s really tough. But we do these things because we want it to be tough. And if you can be patient, you set yourself up for the best return ever because you’ll be strong, ready to go, and injury free. Let’s go!

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• A personalized training plan built around your goals, fitness, and schedule
So you always know exactly what to do each day — no confusion, no guesswork, and no forcing training into a life that’s already busy.

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• A fully dialed-in race-day nutrition and hydration plan
So you avoid bonking, prevent stomach blowups, and execute on race day with confidence.

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If you want structure, community, and a coach in your corner who actually understands the demands of real life and big goals, while making you feel like a champion, every day:

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Have questions about running, training, or anything else? Respond to this email and I’ll be happy to help.

Remember, endure better, every day.

- Joe 🐺