What to Eat to Fuel a Trail Run

Ultrarunners eat handfuls of chips and M&Ms and carry hydration packs full of sports drinks, gels and chews. But what do regular folks need to ingest on, say, a 40-minute trail run? We asked Bryan Bergman, a trail runner, Ironman competitor, and associate professor at the University of Colorado, U.S., in the division of endocrinology.


RW: What you need to eat and drink on a 40-minute trail run?

BB: Honestly, you don’t need to do anything special. Pre-hydration will give you the biggest bang for your buck, so make sure you’re hydrated before your run.

Most people cannot hydrate enough while they run. So making sure your hydration stores are really topped off will increase your ability to run hard and train hard.


RW: How do you know if you’re well hydrated?

BB: The colour of your pee is a reasonable estimate of hydration status if you haven’t had any alcohol or coffee, both of which are a diuretic. Look at the colour of your pee – you want it to be clear. In addition to being a diuretic, alcohol’s ethanol inhibits your body’s ability to make the glucose you need for energy while you run. So, drinking the night before a big run is not the best way to pre-hydrate.


RW: Do people need to eat on runs shorter than two hours?

BB: That varies, based on a couple things. For instance, what kind of training did you do the day before? What kind of training will you do the next day?

Consider where you are in your day-to-day training. Hard training is fuelled mostly by glucose as the energy source. Your muscles and liver stores become depleted during days of back-to-back training. You need good recovery days and good nutrition to replete those stores. So, if you had a hard training day on Monday and are running on Tuesday, you should be fuelling sooner during your Tuesday run because your glycogen stores started at a lower level, and you are likely to run low on fuel for intense workouts more quickly. If it’s Monday and you’re going to have a hard workout on Tuesday, you should be fuelling earlier on Monday – even for a short, easy run – so you start your workout the following day with good glycogen stores.

Generally speaking, if you’re going over two hours, I’d say eat gels or chews during your run.


RW: Does this vary person-to-person?

BB: It kind of scales. As a general rule, a bigger person running at the same speed as a smaller person will burn more muscle glycogen because they are moving more weight and therefore doing more work, while a small person running burns a lot less.

However, this is a general rule that is not always true. World-class runners have a large capacity to work (large VO2 max) and can burn a ton of energy, regardless of their body weight.


RW: Ultrarunners often suffer stomach upset. What causes that?

BB: It’s often from dehydration. A runner’s muscles demand blood, so when you’re trying to digest nutrients while running, the blood flow going to your gut decreases dramatically.

Add thermal stress, when your body is trying to stay cool, and your skin is demanding all this blood flow, so no blood gets to your gut. Gut damage results in bacterial infiltration in your blood and causes people to get quite nauseous.


RW: What should someone do if that happens on a long run?

BB: Ratchet down the intensity and try to take in more fluids.


RW: Big question: How are chews different from gummy bears?

BB: Gummy bears and other candy is made of fructose and sucrose – simple sugars. Your intestines see this huge load of simple sugars that release in your body so quickly that they can cause water to rush into your gut. You feel jostling, and can get diarrhoea.

Energy chews made for endurance sports are made of maltodextrin. It’s a train of glucose molecules, so when you eat it, your enzymes hack them off one at a time. It’s not an enormous load of glucose and sucrose one at a time. Chews are designed to not spill a lot of simple sugars into your gut and cause GI distress.

Know that fructose in high amounts causes GI distress. But fructose is added to most sports products because, when added to maltodextrin, it can increase the amount of carbs that can be absorbed. It can actually increase the transportation of carbs by almost a third, and your body uses those carbs as fuel.

So, yes, energy chews are better for you than gummy bears.

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