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How to Fuel Your Long Run: Part Two - GI Issues

Who can't relate to this scenario? You start off a run feeling great - but a few miles in, that sinking feeling hits your stomach. All of a sudden, finding a port-a-potty becomes a matter of serious urgency.

Gut issues are extremely common amongst endurance athletes. I hear from so many runners who choose not to fuel their long runs because they also have GI troubles mid-run, and that's an understandable response. Nobody wants to spend the whole run looking for a bathroom! It feels much easier to just skip fueling on the run altogether. After all, you probably feel okay on most of your runs without fuel, so it can seem unnecessary. But as we discussed in Part 1 of this series, fueling actually is a necessary part of great training, so it's important to find a solution that works for your stomach rather than giving up.

There are a lot of reasons why GI distress can occur. The first step to solving your gut issues is to understand the underlying cause, so let's unpack a few of the main triggers I see in athletes.


Dehydrated runners often find themselves with tummy troubles. That has to do with the way your body manages blood flow. When you're exercising, your blood flow is diverted away from your gut (which is part of the "rest and digest" nervous system) so it can adequately meet the needs of the areas that are working hard during a run, like your heart, your lungs, and the muscles in your legs and core. Consuming fuel while you don't have good blood flow to the stomach can cause GI issues while food or liquid sits in your gut and isn't efficiently absorbed, and dehydration makes this effect worse. The only solution is to start the workout hydrated and continue to hydrate throughout the effort to avoid your stomach rejecting your fueling attempts.

In addition, the high concentration of sugar in most sports fuel can be a disaster if you're poorly hydrated. It's not that the products are poorly designed - it actually makes sense for these types of fuel to have a lot of sugar, because it's a simple carb that is ideal for a quick source of fuel. However, when an athlete consumes a highly concentrated substance like a gel or a pack of gummies, they have to be taken with water to promote absorption. Otherwise, that sugar pulls water into your gut from your cells, which not only dehydrates you, but results in a lot of liquid in your GI tract. And - you guessed it - that liquid's probably coming out in an undesirable way.

So how to fix this? Your urine should be a pale yellow to clear when you start your workout - this indicates that you're already hydrated. Make it a point to drink fluids frequently throughout the day (half your body weight in ounces is a good baseline goal), especially during hot and humid weather. Then, be sure you are hydrating throughout your workout, and drink water when you take a gel or gummies. For the most effective hydration strategy, I recommend a solution that contains electrolytes and a small amount of carbs for faster gastric emptying. Sugar-free hydration products can be okay for shorter efforts, but during longer runs or rides the solutions with some carbohydrates work best.

Food Choices

Sometimes the distress you feel is a result of what you're eating right before or during a run. A few common culprits include fiber, fat, caffeine, and artificial sweeteners.

Dietary fiber and dietary fat are difficult for your body to break down and digest, which makes both a bad choice right before a workout. If you're eating a whole apple, avocado toast, or a sprouted grain bread five minutes before you hit the road, you're likely to have a bad time. A simple starch that can be easily broken down is a better choice before you leave for your workout. Examples would be things like crackers, white bread, low fiber cereal, or a banana.

Caffeine is a known ergogenic aid, but it's not for everyone. It can be associated with an urge to use the bathroom, too. If you plan to drink caffeine before your run, allow yourself at least an hour to let it absorb if you notice it causes GI problems. Make sure you practice with any caffeinated products (coffee, gels, hydration) well ahead of any races to feel confident that your body can tolerate it. Artificial sweeteners can also cause runner's trots, particularly products that contain sugar alcohols like sorbitol or mannitol. Check out the label on your product to be aware of what you're consuming. Before a run, real sugar is usually more helpful for fueling, anyway.

Now when it comes to what you select to eat on the run, everyone is different - some athletes tolerate certain types of fuel better than others. If a gel isn't cutting it for you (and you're sure you're hydrated!), consider trying some sports chews or even a regular food product. Dates, fruit squeeze pouches, gummy bears, and granola bars are all foods I've had athletes fuel with. There are also a wide variety of sports products on the market, so if one doesn't work, don't give up. Test out something else and find what works for you.


One of the great things about your body is that it's very adaptable. Your gut is no exception! But if you don't practice fueling during a run, it won't have a chance to get used to fuel. If you're avoiding fueling because you've gotten sick in the past - you need to train that gut!

The best way to do this is to start small, with something like a piece of toast and a banana. If Ideally, allow at least an hour to digest your food before you leave for your run. If you don't have time for this, keep your intake very simple (about 15-30 gm carbs before leaving) and eat 5-30 minutes before you leave. If you can't tolerate a banana, try half a banana, or drink some Gatorade. When you adjust to that small amount, you can start gradually increasing what you're able to eat ahead of your run.

What about fueling on the run itself? Consider taking fuel with you on shorter runs to start practicing. You can try consuming something every 45 minutes or so to start, and over time work your way up to eating more.

Energy Imbalance

Generally speaking, if you're not eating enough food relative to the amount you're exercising, you're prone to GI issues. There are a lot of reasons for this that have to do with how the body manages stress and how your gut functions. When we don't eat enough, our gut doesn't function the way it should, and this can make you feel overly full even when you've eaten very little.

Frequently when I talk to athletes who are focused on "clean eating" and experience frequent GI issues on the run, it turns out they're also loading up on fibrous veggies, whole grains, and beans. Okay, yes - fiber is definitely good for you. However, if you chronically eat much more than the recommended daily intake of 25-35 g/day, you might experience ongoing GI issues - and you might not be getting enough fuel. Fiber is quite filling, so you may eat too few calories even though it feels like you're eating huge amounts of food. You can balance your fiber intake by incorporating more cooked vegetables, lower fiber fruits, and food sources that are mainly fats or proteins (tofu, tempeh, meat, fish, nut butter, olive oil, etc) to provide a better mix of nutrients and ease your stomach distress.

As you can see, there is a lot of nuance involved in discussing GI issues for athletes. We haven't even covered other issues like chronic diseases that affect absorption, stress and anxiety, sensitivity to certain compounds like lactose or fructose, or how your menstrual cycle can affect your GI tract! If you've tried changing up your fueling and training your gut and aren't seeing success, consider getting a dietitian involved to help you troubleshoot and fix the issue. In many cases, though, if you take the time to train your gut and fuel properly for your runs, you'll find you can perform better and solve the port-a-potty issue for good!

What's made a difference for you in managing your GI issues? Let me know in the comments!

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