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Fueling series part #2: Fueling during a workout

This is the second post in a series of three focusing on how to fuel around a workout. Last week I talked about pre-workout fueling – this week I’ll tackle how to fuel during a workout. Now despite discussing it as pre/during/post-workout fueling, for an endurance athlete eating and drinking throughout the day is sort of a ripple effect – how you fuel before impacts how you fuel during and after and so on. So you always have to take into account how you fueled the rest of the day (or even the previous days) when developing a plan.

This information is tailored towards endurance type activities, such as triathlons, running, cycling, and swimming, but beyond the type of activity another big determinant in fueling decisions is the length and intensity of the workout. I’ll be using this as the outline for today’s discussion but will also touch on other factors to consider, such as temperature and digestion.

WORKOUT LENGTH: Less than 60 minutes

Workouts of less than 60 minutes in length may not need much in the way of fueling, in particular if you did well with your pre-workout nutrition. But let’s consider a few scenarios where it may be necessary or beneficial:

  • First thing in the morning. You like your early AM training but can’t fathom eating or drinking before the start. If it’s a low to moderate intensity workout you may only need to worry about hydration. Many folks wake up dehydrated and sweating through a workout will only exacerbate the issue, so try getting accustomed to sipping a low carbohydrate sports drink or at least water. Since many endurance athletes are doing multiple workouts a day this becomes even more important – it’s tough to catch up on hydration. If you go with just water be certain to include electrolytes in your recovery meal, in particular sodium.

If the workout is high intensity, experiment with adding carbohydrates into the mix. Although the shorter duration of the workout limits the actual muscular use of ingested carbohydrates, their functions extend beyond this. Research has indicated athletes completing a “swish and spit” protocol with carbohydrate beverages have improved performance WITHOUT ever swallowing the beverage, which indicates there is a central nervous system impact and psychological role of carbohydrate ingestion.

  • It’s hot as h*ll or you sweat like a champ. High temps can send even your average Joe’s sweat rate through the roof, so if it’s extra hot outside or you already know you are a heavy sweater bring some fluids. Aim for having electrolytes as well, so either a low-carbohydrate sports drink or water with electrolyte tablet will work. What’s “hot”? It depends on the individual – genetics do have an impact on our sweat rate. Also recognize high humidity reduces the cooling effect of sweat and contributes to difficulties in regulating body temperature.

WORKOUT LENGTH: Roughly 75 minutes to 2.5 hours

Now we’re getting into a training area more common for half/full marathons, open water races, Olympic distance tri’s and half/full Ironman races. Calories (predominantly in the form of carbohydrates), fluids, and electrolytes will all be key here.

First, let’s take a look at carbohydrates. While the body will use a combination of carbohydrates and fat stores for energy, consuming high fat foods during a workout will slow digestion and contribute to gut distress. Plus they do not contribute directly to blood glucose stabilization. Also keep in mind fat is burned aerobically, so any high intensity (aka anaerobic) bursts or efforts will require carbohydrates in the tank.

The general recommendation for workouts of this length is 30-60 grams of carbohydrate per hour (smaller athletes may be on the lower end while larger athletes may be on the higher end). A few examples of roughly 15-20 grams: 1 small banana, 8oz sports drink, 1 gel. You can choose any form you desire – sports drinks, gels, or foods. It really depends on your gut but just make sure your overall plan meets all your fueling needs (carbs, fluids and electrolytes).

At this point it’s also a good idea to know your sweat rate, which you can estimate by completing a sweat test. This involves weighing yourself pre- and post-workout and accounting for any fluids consumed during the workout (keep in mind, your sweat rate can vary with environmentals). Aiming to replace roughly 75-100% of your sweat lost per hour will give you an hourly fluid intake recommendation. Avoid overconsuming fluids which can contribute to a condition called hyponatremia.

But fluids aren’t the only thing lost in sweat – electrolytes are lost, too. Sodium is the greatest electrolyte lost – on average anywhere from 500 to 2,000mg in 32 oz of sweat. Electrolytes can be replaced with foods, sports drinks, or electrolyte tablets/pills. If you are a salty sweater, you’ll need more than just a standard sports drink (rings on your workouts clothes or salt flaking off your skin are an indication you are a salty sweater.)

Fluids, electrolytes and carbohydrates all work together to aid digestion and absorption – if you are dehydrated you will have a lot of difficulty digesting carbohydrates; if you don’t consume electrolytes you’ll have trouble absorbing and retaining fluids. They are all important!

WORKOUT LENGTH: Greater than 2.5 hours

These workouts are more prevalent for marathon, Ironman, or century rides training as well as ultra-endurance events. While the recommendations for fluids and electrolytes don’t change, hitting those marks becomes even more important. Getting behind by small amounts over the length of a long workout can add up to big problems – dizziness, fatigue, difficulty regulating body temperature, and digestion problems to name a few. It’s not just about performance, it’s about safety!

The recommendation for carbohydrates changes slightly with an increase to 60-90 grams per hour. In order for the human digestive tract to be able to consume and digest this amount of carbohydrate per hour it requires consuming a variety of types of carbohydrates. Examples of different carbohydrates might be glucose and fructose – each are absorbed slightly different with their own unique transporters in the gut. By consuming a mix of these types you allow the gut to absorb more carbohydrate per hour than if you had consumed only one form. It may also help prevent some pesky gastrointestinal troubles. Most sports products are using this approach, but check your ingredients list to look for multiple sources or consider working with a dietitian if you thinks this could be an issue for you.

It’s also appropriate to have some fat and protein during these longer workouts. Again, this may come in the form of a sports or foods (think PB&J sandwiches or granola bars).


Start slow. If you haven’t been consuming much while working out don’t jump right to the recommended intakes – give your digestive tract time to adapt. If you ultimately need 3 gels an hour try starting with 1. Also, experiment with various combinations of drinks and foods. You may find you really need something solid by hour 3 and can’t fathom only sports drinks, or you may prefer to only consume sports foods.

And as always, practice what you plan to do on race.

Stay tuned next week for part #3: Recovery Fueling After a Workout!

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