Intra-Workout Fueling 3-hr Example

by Dr. Alex Harrison | Jul 14, 2020

A common question asked is “is it okay to eat some of my intra-workout carbs?”  Yes, absolutely.  The only caveat is that when you’re consuming gels, chews, carb-rich cereal bars, jelly sandwiches or otherwise, you stand to dramatically increase the effective concentration of your consumption on a grams of carbs, per liter of fluid, basis.  For most scenarios, limiting concentration of consumption to 12-14% is wise.  If you’re hypoglycemic and well hydrated, temporarily increasing to 18% (180g carbs per liter of fluid) is acceptable.  To calculate concentration: divide total grams of carbohydrate consumed by total liters of water consumed and multiply by 100.  

Example: 3-hour ride where you know you’ll burn about 1500 kcal because that’s what your power meter indicated for previous similar ride.  

You decide to target 90g carbs per hour and 800 mL per hour, but you really don’t like the taste of, or the sticky-mouth created by, your favorite endurance carb/electrolyte beverage, so you’d like to consume 8% solution, rather than 12.5% as would be needed by consuming all carbs and fluid together.  Further, you’d like to have some plain water, too, to sip after each chewable carb consumption, and you plan to consume 600mL fluid as plain water over the duration of the 3 hours.  First figure out the total needs for the ride, then break it down per hour, then make an hourly consumption plan so that you don’t get behind on your consumption rates on the ride.  If you’re savvy, plan consumption points at the beginning of any known lull’s in effort to take advantage of ease of consumption and slightly increased gastric emptying when exercise intensity is lower.  Here’s how the math shakes out.

3-hr ride * 800mL fluid per hour = 2400 mL total fluid consumption. 

3-hr ride * 90g carbs per hour = 270 g total carb consumption.

2400 mL total fluid – 600 mL plain water = 1800 mL fluid available to make 8% solution with powdered carb source.

1800 mL fluid with carbs * 8% (8%=.08) = 144 g carbs from powdered beverage mix.  We’ll round to 145 g for simplicity’s sake.  Actual concentration will be ~8.05%, or, ~80 g carbs per liter.

270 g total carbs – 144 g carbs in beverage = 126 g total carbs from chewable food or gels, and we’ll round to 125 g.

125 g total carbs not in fluid = 5 gels at 25g carbs each.

Now, figuring out the consumption timing and per hour.  Start consumption at onset of exercise or even 10 minutes before.  Since the gels each have 50mg caffeine in them and you know the peak blood concentration of caffeine is roughly an hour after consumption, you’ll start with one of those and consume at even intervals throughout the first 2 hours.  One gel at 0, 30, 60, 90, and 120 minutes into activity.  That puts you at 50g/hr carb consumption, and 100mg per hour caffeine consumption for the first 2 hours and 25g/hr carb consumption for the final hour, if considering just the gels.  The remainder needs to come from the carbs mixed with fluid.  You’ll consume 40 g in each of the first two hours and 65 g in the final hour, from your powdered carb source in water.  Shown below is how I can most easily visualize the math to figure out how much of each beverage to consume each hour.

1800 ml * 1000ml/L = 1.8 L

145 g carbs / 1.8 L = 80.5 g of carbs per liter of fluid, and we’ll round to 80 g for simplicity.

40 g carbs needed from fluid source / 80 g carbs per liter of fluid = .5 L, or 500 mL of carb beverage per hour for each of the first two hours.

800 mL/hr desired fluid consumption rate – 500 mL/hr fluid from carb beverage = 300 mL plain water per hour.  

For the final hour, you’ll consume the remainder of your carb beverage: 1800 mL – (2 * 500 mL) = 800 mL which will meet your needs for ~65 g carbs from fluid sources and for desired hourly fluid consumption rate.  But, you’d like to have some water to chase the carb beverage with, so you plan to add 100 mL to your plain water allotment, raising it to 700 mL plain water needed onboard.  This sort of addition is quite common as you’re weighing tradeoffs for your intra-workout fueling and hydration strategy.

Finally, you can dial in your consumption strategy within each hour of the event by breaking down hourly consumption into planned consumption points at regular intervals.

To maintain the minimum degree of hypertonicity of solution within your stomach at any given time, so that gastric emptying can be maximized during hard exercise and likelihood of gastrointestinal cramping is minimized, it is wise to always consume water immediately after any solid food or gels.  This can also serve to prevent breathing or swallowing issues created by food or gels remaining in your mouth or throat during intense exercise.

Hour 1 and 2 of this 3-hr ride are the same, so we’ll treat them similarly.

We’ll target no greater solution concentration within the stomach at any given time.

25 g carbs from first gel * .18 maximum solution concentration acceptable = 138.89 mL and we’ll round to 140 mL, for plain water consumption with each gel.

140 mL plain water with each gel * 2 gels per hour = 280 mL

300 mL per hour plain water – 280 mL = 20 mL plain water remaining for a splash of water as a mouth rinse after each drink of the carb beverage.

We’ll split the 500 mL/hr carb beverage consumption into 4 drinking points in each hour: 10, 20, 40, and 50 minutes into the hour.

500 mL/hr / 4 drinking points = 125 mL per consumption, each followed by ~5 mL plain water, if desired.  In this very real-world example, you may have noticed that 5 mL water might not be enough to actually rinse your mouth, so you might decided to just increase your hourly consumption of fluid by 100 mL and add it all to your plain water allotment, OR you might decide that you’re okay with slightly more concentrated carb beverage in exchange for slightly more water to chase it with.  We’ll take the 100 mL/hr plain water consumption increase because we’re already sitting at the edge of acceptable hypertonicity and if the workout gets extended beyond 3 hours, it would be great not to be flirting with that 2% body weight lost dehydration line.  These are the kind of decisions you can make, on the fly, now that you understand the various tradeoffs at play.

Adding 100 mL per hour plain water consumption split over 4 drinking points adds 25 mL to each of the 5 mL consumptions, leading to 30 mL consumption after each 125 mL drink of the 8% carb beverage.  Now you’re up to carrying 900 mL plain water, instead of the originally planned 600 mL (remember we added 100 mL to the final hour for the same reason).

For the final hour, you’ll plan consumption every 10 minutes after the 5th and final gel at 120 minutes into activity.  You have 800 mL carb beverage and 100 mL plain water left onboard.

800 mL carb beverage / 5 fueling points = 160 mL carb beverage consumed at each fuel point.  But, you decide to bias some of the carb beverage consumption slightly earlier in the final hour because you know that it takes time to absorb the carbs and for them to hit your blood stream.  Not all of the carbs consumed in the final 10-30 minutes of any exercise bout are going to be absorbed and so it might be worth biasing them a bit earlier in the final hour.  And, you know that volume of sweet-flavored carbohydrate used in mouth rinses isn’t critical for the purpose of increased cognitive drive and power output, and since that’s where much of the benefit in these final fueling points is going to come from it doesn't matter that they're lower volume (you’re not hypoglycemic because you nailed your fueling plan so increased rate of absorption of carbs probably isn’t present).  So, you decide to consume 225, 225, 200, 100, and 50 mL of carb beverage over the 5 consumption points, every 10 minutes, in the last hour to take maximum advantage of the fuel available.   You’ve got 100 mL of plain water left, so:

100 mL / 5 consumption points: 20 mL plain water as a chaser to each carb beverage consumption in the final hour.

So, your final consumption strategy might be as follows:

  • 0 Minutes: 1 gel providing 25g carbs, 140 mL plain water
  • 10 Minutes: 125 mL 8% carb beverage providing 10 g carbs, 30 mL plain water
  • 20 Minutes: 125 mL 8% carb beverage providing 10 g carbs, 30 mL plain water
  • 30 Minutes: 1 gel providing 25g carbs, 140 mL plain water
  • 40 Minutes: 125 mL 8% carb beverage providing 10 g carbs, 30 mL plain water
  • 50 Minutes: 125 mL 8% carb beverage providing 10 g carbs, 30 mL plain water
  • 60 Minutes: 1 gel providing 25g carbs, 140 mL plain water
  • 70 Minutes: 125 mL 8% carb beverage providing 10 g carbs, 30 mL plain water
  • 80 Minutes: 125 mL 8% carb beverage providing 10 g carbs, 30 mL plain water
  • 90 Minutes: 1 gel providing 25g carbs, 140 mL plain water
  • 100 Minutes: 125 mL 8% carb beverage providing 10 g carbs, 30 mL plain water
  • 110 Minutes: 125 mL 8% carb beverage providing 10 g carbs, 30 mL plain water
  • 120 Minutes: 1 gel providing 25g carbs, 140 mL plain water
  • 130 Minutes: 225 mL 8% carb beverage providing 18 g carbs, 20 mL plain water
  • 140 Minutes: 225 mL 8% carb beverage providing 18 g carbs, 20 mL plain water
  • 150 Minutes: 200 mL 8% carb beverage providing 16 g carbs, 20 mL plain water
  • 160 Minutes: 100 mL 8% carb beverage providing 8 g carbs, 20 mL plain water
  • 170 Minutes: 50 mL 8% carb beverage providing 4 g carbs, 20 mL plain water

One additional benefit of the biasing of carb beverage consumption to the beginning of the final hour of the ride is that the effective concentration of the fluid consumed diminishes to just below 6% throughout the hour, speeding the gastric emptying rate when it’s needed most at the end of exercise.

In the real world of intra-workout nutrition, you should lay out a plan as above and do your best to stick to it.  You won’t.  There will be hiccups.  You’ll completely forget to drink while you’re in a groove, or in a particularly technical section of the event.  You’ll decide you don’t want as much carb beverage because you just hammered a portion of the ride and your gastric emptying rate slowed temporarily (you know because your gut is cramping) to below 600 mL per hour, and you know there’s a long descent coming up where you can bump it back up to 1.2 L per hour and catch up on your hydration and nutrition.  Having a well-laid plan and knowing all the applied science you used to formulate the plan, will be one of your best weapons when adjusting on the fly, when it counts.

You may have noticed that electrolyte consumption was neglected in this calculation example.  Since most sport beverages contain electrolytes, it’s quite common to be able to make these sorts of carbohydrate and fluid concentration calculations and end up “in the ballpark” for sodium and potassium needs.  The athletes that become hyponatremic during endurance activity are almost always dramatically under-consuming carbohydrate as well.  But, since it might be 90 degrees and somewhat humid out during the ride, let’s double check to ensure sodium concentration is on point.

We’ll assume you’re a moderately heavy-sweating (1.2 L/hr) athlete with average sodium content of 800 mg/L of sweat.  As a quick cross-check, 1.2 L/hr sweat means that only slightly over 1% of body weight will have been lost due to sweating (3.6 L/hr water lost sweating, 2.7 L consumed, resulting in net fluid loss of .9 L or .9 kg, so between 1-2% weight lost for athletes 50-90kg) and since respiratory hydration losses during exercise make up much less than 10% of water loss, hydration needs have been met.  

800 mg sodium per liter of sweat * 1.2 L sweat per hour = 960 mg sodium lost via sweat.

960 mg sodium per hour * 3 hours = 2880 mg sodium. 

Gels generally have 50-100mg sodium per gel (be sure to check your gels if you think you might be on the short end of sodium consumption).

5 gels * 75mg sodium per gel = 375 mg sodium from gels for the ride, total.

Popular endurance carb and electrolyte beverage mixes often have 200-400mg sodium per 20g of carbs, which is a typical serving size.

145 grams of carbs from beverage powder / ~20 g serving of carbs = ~7 servings.

7 carb and electrolyte beverage powder servings * 300 mg = 2100 mg sodium

2100 mg sodium from beverage + 375 mg sodium from gels = 2475 mg sodium, or within 20% of the sodium lost during training.  If you’re eager to ensure there is no chance of even mild hyponatremia, feel free to add 1/8th of a teaspoon table salt (~300mg sodium) to your beverage mix for the day.  

For events under 3-4 hours, if you’re within a 1000mg of your sodium losses during training, you’re doing just fine.  If your event is longer, then targeting sodium needs more exactly is probably a safe bet.

Now that you’ve planned your next competition’s fueling strategy, go try it.  Try it again and again.  Return to the drawing board with logistics adjustments and make contingency plans.  It will be daunting and time-consuming at first, but once you have your system down, it will be second nature. 

To calculate your daily carb needs with precision and ease, as an endurance athlete, check out the RP Endurance Macro Calculator (EMC)
To learn more about just how to decide how much carbs, electrolyte, and fluid to consume during your training, check out The RP Diet for Endurancereplete with comprehensive tables and figures to make your intra-workout nutrition perfectly tailored to your needs.