By Tyler Fox

Simply put, endurance is the ability to sustain prolonged physical activity and fueling allows us to sustain a higher rate for longer. Proper hydration allows us to better tolerate the fuel while allowing for better thermoregulation (heat management) and better resource distribution during activity.

The information below isn’t an attempt to convince readers as to the importance of fueling and hydration for performance in endurance related activities. Instead, it’s a starting point for those interested in utilizing fueling and hydration methods to improve their fitness and performance.

For the purposes of this article, we’ll largely discuss fueling during training runs and races. I’ll include a few notes below on considerations for fueling around activity, but if you have more questions about day-to-day nutrition, I’d suggest reaching out to a RD or other qualified professional!

Fueling Around Training

If you’ve ever been around runners or other endurance athletes, often you’ll see pre training meals with a high volume of carbohydrate. Although recent research has suggested that the carbohydrate intake immediately before a run has no influence on glycogen stores, I doubt the high carbohydrate pre-run meals will go out of fashion.

Even still, I think it points out an important consideration. Fueling for a training session (or race) starts the day before. It starts during the previous day’s session or soon after. If we’re waiting to top off the tank 2 hours before a run, chances are it’s too late.

Each training session is different and as a result fueling for recovery is different. However, a good rule to shoot for would be:

Consume 1g of carbohydrate/kg of body weight sometime in the first hour after activity.
Additionally, aim for protein intake at the same time in a 3:1 or 4:1 carb:protein ratio.
For example, if a runner is 80kg, they should shoot for 80g of carbohydrate and roughly 20g of protein!
If the session is longer or contains higher intensity, increasing post-activity carbohydrate intake would improve glycogen recoveries and likely lead to a more effective training session the next day (or…for some, later that same day)

Fueling the Runs

This is the meat and potatoes (pun fully intended) of this article. The biggest mistake I have seen as a coach over the years is an athlete taking the fueling during training less seriously than the training itself. The second biggest mistake would be an athlete being less precise with fueling during training assuming that all will go smoothly to plan when it matters most.

Just as we adapt to the physical training, our GI systems adapt to the demands that we place on it. It is possible to show up and complete a race without practicing with fueling strategies in training, same as it’s possible to finish a steep course without having run up or down hills in training. However, in both situations, an athlete will have a much better time in the race with appropriate exposure in training.

Some ideas to be familiar with:

Rates of fueling are described in grams of carbohydrate/hr (g/hr). One gram of carbohydrate is the same as 4 kcal of carbohydrate.
Fueling doesn’t have to be expensive. There are many recipes floating around for homemade options that work just as well if you practice with them. It should be noted that most companies use a mix of sugar sources (maltodextrin and fructose). Often you’ll see either 2 parts maltodextrin to 1 part fructose or 1 part maltodextrin to .8 part fructose. If trying to create a DIY gel or drink mix product, that’s a good ratio to shoot for. If making more real food based options, disregard!
We’re leaving proteins and fats out of this conversation. It’s not that they are unimportant macronutrients, but for the sake of endurance fueling, carbohydrates are the topic of discussion.

What runs should we consider fueling:

Runs lasting longer than 90 minutes of any intensity.
Any run with time spent at a higher intensity or with intervals of higher intensity.
Any time we may be entering a run with lower energy available. For example, if we’re running first thing in the morning and don’t have time to eat beforehand. Or, if we haven’t eaten for a few hours before starting out.

When during the run we should fuel:

If we’re running for 90 minutes or 5 hours, we should start fueling at the same time - 20-30 minutes into the run.
At that point, it’s best to fuel at a predetermined rate. If we’re shooting for 60g carb/hr, we should consume 60g between minutes 20 and 80.
Don’t overthink it. Being able to stay flexible and adjust a plan as feels needed is an important skill to develop during this process as well.
Spreading out the fueling is often better than trying to get the same amount all at once. For example, 60 grams spread out to 20 grams every 20 minutes is favorable to 60 grams one time each hour.

How much to fuel:

This answer changes depending on the athlete, the level of fitness, the session being fueled, performance goals, and many other factors that are beyond the scope of this article. However, starting at a base rate of 60g/hr is a good introductory target.
On the first run fueled (anything longer than 90 minutes), start with 20g at 20-30 min into the run. Repeat every 20 minutes until the finish (yes, that includes at minute 80).
If 60g/hr is well tolerated, plan on trying to add 10g/hr to the next run that would warrant practice:

Runs where we extend the duration are opportunities to increase the rates of intake
Runs where we increase the intensity are opportunities to increase rates of intake
There is an upper limit. Some can tolerate 80g/hr, 90g/hr, 120g/hr before finding that limit. The gut adapts and will improve tolerance as we stress it and make subsequent adaptations.

6-8 weeks out from bigger goals, aim to consume carbs at, or above (preferably above), the rate planned for that event on any run 90+ minutes of a similar intensity.
The gut quickly trains and detrains to the demands that we put on it. The final 2 weeks leading into a race or big goal are some of the most important. At this point, it’s useful to fuel even shorter runs at the rate similar to the plan for the event.

There are so many caveats to the above information. We all respond differently and the best approach is the one that is well practiced and one that we gain confidence with over time!

Hydration

Fueling is only part of the equation to feeling our best during training and racing. Hydration is another big part of that equation and one that’s admittedly more challenging to dial in.

First, hydration comprises both the fluid intake and electrolyte intake - more specifically, sodium intake. There are other important electrolytes, but most products include these at the rates they should be consumed relative to the sodium of the product.

Unlike fueling, I would not suggest DIY’ing’ electrolytes. There are some great resources to do so, but after messing it up and convincing myself I was hyperkalemic (which even involved a call with poison control) I decided I’d pay full price for those fancy drinks and often suggest others do the same!

Oftentimes, there is sodium included in running specific nutrition. This is most common in drink mixes, but can be found in gels and chews. The sodium found in these products needs to be accounted for when creating a hydration plan for training runs and races. I generally suggest having a fueling plan that is separate from the hydration plan. If the day is especially warm or cold, it can be difficult to correct for both fluid needs and fueling needs at the same time if all of the calories are mixed in with the fluids. It can certainly be accomplished, but it’s a layer of complication that has derailed many races.

Finding fluid loss rates:

The best way to find fluid loss rates is to take a pre and post run body weight. If you are uncomfortable tracking body weight, I have found that holding something of an unknown weight that is heavy during that pre/post run weighing can give an accurate value to work with without knowing exactly how much you weigh (which I completely understand and don’t think tracking weight fluctuations is a productive practice in sport and longevity with sport).

Pre-run

Use the bathroom
Weigh yourself naked or in dry minimal clothing (with optional heavy object of unknown weight)
Record pre-run weight

2. Run for ~60 minutes

Run at a moderate / realistic training effort
Drink normally (don’t force or restrict, but do measure and record exactly how much you drink)
3. Post-run

Towel off sweat
Weigh yourself again (same clothing state as pre-run - dry clothes for more accurate results)
Record post-run weight (with optional heavy object of unknown weight)
4. Calculate

Subtract your post-run weight from your pre-run weight to find how much weight you lost.
Convert weight lost to fluid:

For every 1 pound lost, you lost about 16 ounces of fluid.

Add the amount of fluid you drank during the run.
This gives ‘roughly’ fluid loss in terms of oz/hr
5. Refine

It’s important to note that the fluid loss rates will change depending on conditions. For more accurate information, you could take measurements in different conditions and track these closely to have a better idea of how you respond in different conditions (i.e. in sunny vs. cloudy, 60° vs. 80°, at higher or lower altitudes).

Finding sodium loss rates:

Unfortunately, there aren’t great ways to do this from home, at least none that I’m aware of. There are some biosensors on the market that claim to give accurate sodium and fluid loss rates, but having never used these products and seeing mixed reviews, I’m not confident recommending them.

The best approach that I know of is to test sodium loss at a Precision Fueling and Hydration test center. There are likely other labs in the area that could also test for sodium loss, but that would depend on where you live.

Once you have a sodium loss rate (expressed in mg sodium lost/L of fluid lost), you can create a plan that replaces both sodium and fluid during a training run or race. Others give different advice, but I suggest thinking about sodium replacement in terms of mg/L of fluid consumed versus mg of sodium consumed/hr. The rate is the important aspect instead of total sodium consumed. If you lost 800 mg of sodium/L of fluid and aim to replace 800 mg sodium/hr that can work well if you drink around 1L of fluid during that same hour. It will be the wrong rate if you end up drinking more than/less than that liter, which will often be the case as conditions fluctuate during races, especially long ones!

Everything above should be viewed as a starting point. Practice around carbohydrate intake and fluid management seems to change year to year and we all have slightly different needs, which are likely different depending on the goal at hand. The good news is that we should often be fueling training and this gives plenty of opportunity to refine systems and find out what works best!

Why Fueling & Hydration Matters