A model for 4-24 hour cycling events... a.k.a., don't run out of steam....

Ultra endurance events (e.g. 4+ hours in duration) present some unique fueling challenges.  Too few calories, and there is a high chance running out of stored glycogen and bonking, and too many calories and you significantly increase the chance of gastric distress.  In this article, I want to share some general concepts on fueling (and some associated links for further reading), as well as possible model for planning / reviewing fueling on cycling events lasting 4 to 24 hours in duration.  In a future article, I'll address some of the differences for long course triathlon fueling.... but the fundamental concepts are similar.

To provide a general background, there are few key things to understand when planning for race day fueling:

1. Calories consumed are only valuable if they can actually be absorbed by your gut. Over-consuming calories generally leads to excess food, fluid and gas in the gut, which can lead to cramping, nausea, stomach shutdown and an early end to your race. This article is one of the best summaries on what most people can generally actually absorb per hour, written by Asker Jeukendrup, a leading exercise physiologist and sports nutritionist.   In a nutshell, most people can absorb around 60 grams of a single type of carbohydrates per hour, which translates to around 240 cals/hr (4 cals/gram).  If you mix multiple sources (e.g. maltodexterin and fructose), absorption rates can often reach 90 grams of carbohydrates per hour (or around 360 cals/hr). Besides the quantity of carbohydrates, other factors impact absorption as well, such as:
   • Exercise intensity.  The higher your intensity level, the more your blood is diverted away from your stomach to working muscles. This slows the digestion process and can potentially reduce absorption rate of calories.  Ironically, this means that as you are going harder at the beginning of races and are burning calories at a higher rate, you are processing your food at slower rates, increasing the deficit between what you are consuming versus burning.
   • Carbohydrate concentration.  Your stomach processes carbohydrates more efficiently with water.  If you have too many carbs and not enough fluid, your stomach will process the carbohydrates more slowly.  The takeaway is if you have solid foods (or even gels/GU), it is important to consume them with water to assist absorption.  Drinking a sports drink on top of a GU may end up with high concentrations of carbs, slowing absorption.
   • Individual variance.  Absorption rates can be impacted by "training your gut" to a degree. This is simply practicing your race day fueling on your long workouts, so your body becomes more adapted to processing fuel while exercising.  Also worth noting is that body size is not necessarily correlated to absorption rates... meaning larger people don't inherently absorb carbohydrates faster than smaller individuals.
2. The amount of calories burned is a function of the work you are performing over time. For cycling, an estimate of your calories burned is approximately 3.6 calories per watt of average (not normalized) power... more details can be found here.  So, for a person averaging 100w per hour, they will burn around 360 cals/hr, while someone riding at 200w per hour will burn 720 cals/hr. The takeaway is that the harder your work (in absolute terms, watts... not relative terms, intensity), the more calories you need for a particular event.  The paradox is that larger individuals generally require more power to achieve the same speed, but may not process (absorb) calories any faster than a smaller person.
3. You don't actually have to consume as many calories as you burn.  Your body has stored glycogen in the muscle, which serves as energy stores for muscle contraction. In general, most people have around 375-500g of glycogen stored within the body, which translates to 1500 - 2000 calories.  One important note is that glycogen stored in one muscle cell cannot be transported to another... meaning unused glycogen stored in the bicep muscle cannot be moved to a quadriceps that is running out of energy.  The point is that not all 2000 calories stored in the body may be readily available to your working muscles.  The other key factor is your muscles also utilize fat for energy as well. Oxidizing fat for energy (e.g. burning fat) is a slower process than utilizing carbohydrates, and in general your ability to utilize fat as a fuel source is dependent on exercise intensity. The higher the intensity... the more your body relies on carbohydrates rather than fat.  As long as your calories burned is less than the sum of calories consumed + fat oxidation + stored glycogen, you avoid the bonk.
4. Hydration and Electrolytes can impact your performance and/or health.  There is a lot of debate about the proper amount of fluids required for longer events.  One one hand, there have been studies showing that performance can be impacted when weight loss from sweat exceeds 2% of body weight. Decreased hydration levels tend to increase heart rate, reduce sweat rate (increasing core temperature), and increase glycogen usage in the muscles.  On the other hand, consuming too much water (or fluids) and lead to hyponatremia, which is a potentially life threatening condition.  The American College of Sports Medicine has a very informative paper on both hydration and electrolytes here.  In general, they are recommending that you supplement water up to your sweat loss rate (but not more), and 0.5-0.7g of electrolytes (sodium) per liter of water.  Here is an online tool to help you calculate your sweat rates, based on your actual workouts.  Note that sweat rates are highly dependent on temperature (and also humidity), so it is a good idea to understand your sweat rates at varying conditions.  For example, under relatively normal riding conditions (e.g. 70 degrees) I lose around 24 oz per hour on the bike.  On cool days may lose 16 oz/hr or less, and on warm days I may lose up to 40 oz/hr (and have recorded over 50 oz/hr running on hot days).  Personally, I expect to lose some weight on the course.  I sweat quite a lot and my gut isn't always willing or able to absorb enough fluids to keep up on a really hot / humid day.  Likewise, as you burn your stored glycogen, you will lose the associated weight as well.
5. Not all types of foods are processed by your gut equally.  Protein, fat and fiber all slow the digestion process, so they generally should be avoided for race day supplements. Although protein can help reduce some muscle damage for long events, in general it's not needed for events lasting less than one day.  For multi-day events, protein is recommended to help minimize muscle loss over time. Protein is also great outside of race day for training recovery, and muscle building, as discussed here. So... on race day... focus primarily on carbs, and try to avoid foods that will sabotage your calorie absorption. There are a lot of prepackaged options for carbohydrates, including sports drinks, gels (GU), chomps, waffles, bars, etc. Often a mix of products helps to provide a taste overload of one type of product, and may help with absorption if they are from multiple types of carbohydrates. The key is to test different products during long training rides, to identify what your stomach will tolerate.  One product that seems to be tolerated by most people is Maltodextrin. It is the primary ingredient in CarboPro, as well as in the Hammer gel products.  The other consideration is the use of caffeine during exercise. Most studies show some benefit on performance, although it can create GI problems for some people. Again, using the product during some training sessions can help you identify how it impacts you individually.
6. "Winging it" generally isn't a good strategy... or really a strategy at all.  A few small things can really help keep your fueling on track. Of course you initially need to consider if you are carrying, buying, refilling along the way.  As mentioned previously, it's good to test your fueling while training.  This can help you determine if you need to carry most of your nutrition with you, or if your gut is tolerant of things you can pick up (at supported races) or buy at a convenience store for unsupported races.  As you consider options, it's important to review where you have opportunities to refill water and restock your nutrition (e.g. mix up sports drinks, move gels to convenient feed storage, etc).  This should be planned in advance, so you are not wasting time figuring it out during the race.  Finally, consider how you are going to stay on task.  Personally, I set my Garmin to give me an alert every 15 minutes, triggering me to eat or drink to stay on my planned schedule.

With a common frame of reference on the basics of fueling, the following spreadsheet allows you to model your fueling for events from 4 hours to 24 hours in duration.  You simply input five variables (Fat Oxidation Rate, Starting Average Power, Power Decay, Total Event Time and Target Calorie Intake) and it will show an approximation of your calorie deficit per hour (or surplus) as well as a running total of your deficit or surplus.

I read a quote some time ago that I find applicable... "All models are wrong, some are still useful" (George Box).  With that in mind, it's important to understand that there are a LOT of variables that will impact the accuracy and results of this model (e.g. your actual fat oxidation rate, your metabolic / cycling efficiency, your actual fatigue decay, etc.).  That being said, this model can provide a general framework that can be helpful for both reviewing past races as well as planning for future races.

The spreadsheet is shown below.  Note that there are two pages... the first is the model, and the second can be used for manual input of hourly average power and fueling, for reviewing past races.
 

For an example of how the model can be used, I'll use the some data from two gravel races I did this year, using the Actual Ride tab first:

The first is for a 125 mile gravel race, where I rode around 7 hours and 20 minutes, and consumed around 193 calories per hour.  I held wattage / intensity fairly well, but started feeling a bit low on energy and my power dropped around hour 7.  At this point, my calorie deficit had gotten below -1600 calories based on the model... and my wattage dropped after that point.

This second example was a 150 mile gravel race, over a little more than 8.5 hours of actual race time.  In this case, I held intensity though hour 6 again, but started dropping off again in hour 7. Although I consumed more calories in the second race, I also burned calories at a faster rate. Again, at around -1600 calorie deficit, my power started dropping off.  

Graph of actual power (blue line), modeled power decay (orange) and calorie deficit line (grey).  Note that around -1600 calories my power dropped off significantly, which was similar in the first example as well.

Is a 1600 calorie deficit the "magic" point to avoid for everyone?  No... this is simply two data points for a specific individual (me).  But, the point is that based on my race history, I can use this tool to help create a strategy for my next race in terms of wattage and the associate calorie intake. For example, if I want to make sure I can ride for 14 hours and stay out of the -1600 calorie deficit range, I'll need to cut back my power slightly at the start and consume more calories during the race.  Or, I can consider higher wattages, but I need to make sure my gut can handle the additional calorie load without ending up in GI distress.  This tool simply makes it easy to adjust the numbers, and see how it impacts your calorie deficit over time.

Often the biggest obstacle to success is not having a plan.  The next biggest challenge is following a plan at all costs.  It's important to create road maps for success, but also being willing to adapt as conditions change.

Again, all models are wrong... but I'm hopeful that you'll find this model useful.  The most important point is understanding the basics of fueling, and creating an appropriate plan for your specific needs.

Additional Notes:

1. I have notes included on the spreadsheet for more information on fat oxidation rates. There is a fairly wide range of individuality, based on your fitness, types of food you consume, sex, etc.  0.5 g/min is a reasonably conservative value for a typical endurance athlete, who is not utilizing a high fat low carb diet.
2. The tool works best if you know your actual power output.  Keep in mind, this is average power power per hour, not normalized power, for 1 hour periods.  I have some links on the spreadsheet under the notes section, for estimating your power output in terms of FTP (Functional Threshold Power).  In general, for a >4 hour event, most average power starting ranges (first hour) would be between 75% to 85% of FTP, with elite racers at the high end (or even slightly higher) and newer or more conservative racers on the lower end of the range.  The power decay rate allows you to modify how your power decreases with each hour of the race.  The larger the number input, the faster the decay of power. If you have an idea of how much your power drops by a certain point based on past races, you can adjust the power decay variable accordingly.
3. The total estimated time is your best estimate of the race duration.  Note that you can also be conservative and pick something an hour or more after your projected race finish, to see if that impacts your calorie deficit in case the race takes longer than expected.