Is festive sugar-loading a bad thing pre-run?

The holiday season is upon us, which means the sugar content in our diets will be on the rise. No complaints here; sugar is delicious! Plus, as runners, we can get away with eating a lot more than our non-running friends.

When we run, as most runners know, glycogen (chains of carbs stored in our muscles) is our primary source of fuel. Not only that, the percentage of our energy that comes from this source goes up as the intensity of our running goes up. After a run, the sugar we ingest first gotraining-and-performancees towards replenishing that depleted glycogen, and thus does not contribute to fat in the same magnitude when compared somebody who had NOT gone for a glycogen depleting run.
I have written about fueling with carbohydrates in the past HERE. If you go for a run, and you don’t have enough glycogen, your pace is at the mercy of the rate of fat metabolism (much slower than carb metabolism). When our glycogen is gone, our energy and pace plummets- otherwise known as bonking. So, when we forget to eat carbs, can we make up for it last minute by downing a box of cookies?

Carb Ingestion Before

One element of fueling I have always emphasized is ingesting carbs in large quantities no closer than a few hours before an important run or race. This is a safe oversimplification that I would like to elaborate on today.

The risk of gastrointestinal issues aside, the thinking behind this is that ingesting sugar results in an insulin spike, and therefore hypoglycemia (drop in sugar found in the blood). Your body goes into a fed state and food storage mode- not a good thing when you want that glucose readily available in your blood to run fast!

This relationship has been evaluated many, many times over the years. Take for instance THIS early study. Subjects were given 200g of carbs 50mins before they did a 4-hour bought of exercise. They showed that glucose and insulin where high at the very start leading to a 40-100% increase in glucose utilization and hypoglycemia. On top of that, they showed a suggested significant decrease in fat metabolism indicating that insulin was taking away another potential source of fuel for our run. This metabolic cascade has been reproducibly shown for decades.

There are even studies that look at how drastic the difference in insulin levels are when the sugar is consumed at varying time intervals. For instance, in THIS 2003 study  looked at the impact of ingesting insulin 15, 45 and 75mins before activity. To no surprise, insulin levels at the start of the activity were highest in the blood in the group who ingested carbs 15 minutes before, while it was lowest in those who ingested carbs 75 mins before. So what happened to the insulin once the subjected started cycling? Stay tuned for more on that. For now, the take home message is that it is quite reproducible and expected that insulin goes up, hypoglycemia is triggered, and a decrease in fat metabolism follows after acutely ingesting carbs pre-exercise.

This cascade of sugar intake, acute spike in insulin, acute hypoglycemia and acute drop in fat metabolism is a relationship that has been proven time and time again. That should mean it makes us go slower… but does it?

Insulin and Performance

As with anything in science, it is always important to not make assumptions, no matter how logical or tempting they are. Wouldn’t it make sense that downing sugar, causing the body to go into an acute “fed-state” would have an acute detrimental impact on performance? It sure does make sense. However, when we ignore these clear and consistent physiological markers and look strictly at performance, things start to get a little murky.

In fact, it is quite difficult to find a study that actually shows a decrease in performance resulting from acute ingestion of carbs immediately before the activity. There are a few examples, like this one from way back in 1979.  These researchers showed that the time to exhaustion performance declined when 75g glucose was ingested before the activity. Interestingly, they attributed this decline not from the hypoglycemia, but from the blunted fat mobilization.

While there are a few studies showing a somewhat detrimental impact of pre-exercise glucose ingestion, there are way more showing either no change or suggesting an enhanced performance effect with taking in some sugar before.

Pre-Exercise Sugar is OK?

Take for instance the above mentioned study which looked at the 15, 45 and 75min pre-exercise feedings. Another component of this study looked at what happens once the subjects start to work out. These researchers essentially showed that whatever changes they saw disappeared within 10 minutes of starting their activity, as the insulin and glucose levels returned to normal very quickly! They also showed that from a performance standpoint, there was no statistical significance between when or if they ingested carbs pre-exercise. Their average wattages for a 20min cycling test remained consistent in all trials!

Not only are there plenty of studies showing no impact on performance when ingesting carbs pre-exercise, there are also many showing that it might actually make us faster! Take for instance this study from 1991. In this study, subjects either consumed 1 or 2 volumes of carbs, or a placebo. To no surprise, they saw the expected spike in insulin when the carbs were ingested. However, contrary to what we might expect, the time trial results were significantly enhanced in the carb groups when compared to placebo.

SO, does carb intake cause a clear spike in insulin? Yes. Is it logical to expect this to decrease performance acutely since it sends the body into a “fed state”? Yes. Do we actually see that drop in performance? Probably not.

Practical Applications:

When applying these principles, it still is not wrong to avoid eating within a few hours of a big workout or race. The best bet for success is to replenish your glycogen the day before, and top it up with a light carb-rich meal ~3 hours before the start of your run. However, this research does give us more confidence when we are forced to go with a plan B:

  • To eat or not to eat: If you have not eaten well the day before or throughout the day, and your only chance to take anything in is within an hour of your big workout or race, this research shows it is a much better idea to ingest carbs within that hour. The very small and possibly non-existent drop in performance associated with an insulin spike is a much better choice then bonking and running exclusively on fat metabolism.
  • Safe Plan-B, pre-exercise carb intake: If you have always ingested carbs within close proximity to important workouts and don’t want to break that habit, it makes more sense to wait until 10-15 mins into your warm up or even after your warm up. As studies like the one above show, exercise blunts the fed-state metabolic cascade from kicking in as we ingest carbs. Even though ingesting carbs is likely safe either way, waiting to ingest them until your HR is elevated will further increase your odds of success.
  • Example pre-exercise carb intake: For my athletes, recommendations on pre-race or workout carb ingestion really depend on a number of factors (distance, goal of session, diet leading into the race etc.). To give you a general example: If it’s an A-race, 5-10K distance, typically a 20-30minute warmup that starts ~30-40minutes before race start is a common approach. I will often recommend taking in 30g of carbs near the end or just after the warmup depending on a few factors. By using this approach, I think an athlete increases the odds of being one of the individuals who reap the benefits of having an extra boost in blood sugar while also decreasing the already very unlikely odds of having any drop in performance due to an insulin spike.
  • Hidden risks: There is, believe it or not, a lot more that goes along with decisions about pre-exercise carb intake. For instance, there is a body of evidence showing that symptoms of hypoglycemia occur for some even in the absence of actual hypoglycemia.  Listen to your body and do some experiments with your own physiology before a big race!

I hope this article helps to steer you in the right direction about when and how to eat in close proximity to an important run!

Happy Holidays, and thanks for reading in 2016! Looking forward to more Training & Performance fun in 2017!


  • Jerry Huiskamp December 16, 2016 11:18 am

    Sean – Interested in your comment ” there is a body of evidence showing that symptoms of hypoglycemia occur for some even in the absence of actual hypoglycemia.” Would be interested in references and even a blog on this subject. Keep up the good work.

    • Dr. Sean Delanghe December 16, 2016 1:50 pm

      Hey Jerry,
      Thanks for reading! Most (or at least many) of the studies looking at the impact pre-exercise carb-loading has on performance will also make note of symptoms (i.e. do people feel slower or hypoglycemic). This is so that they can compare the athlete’s perception to if there is an actual performance decline. So for instance in this study:
      3 subjects reported extreme symptoms of low blood sugar DESPITE them having glucose levels of (3.7, 4.6 a1d 3.1 mmol/ only one was actually hypoglycemic).

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