I know a lot of runners, especially the early morning crew, have a tendency to not eat a lot before getting out the door for our workouts. Sometimes this isn’t a big deal. If we’re just going to get a couple of easy miles in, or we’re part of the small minority of runners that needs to be doing fasted long runs, then it makes sense. But if we’re trying to get in a hard or long effort, or its cold out, we’re probably better off to get something inside us before we go.
First, a basic primer on metabolism while running. For the vast majority of the time we spend working out as endurance athletes we’re using fat as our primary fuel source. That’s why we spend a lot of effort making ourselves better at utilizing that fuel source. Even when we’re operating at fairly high intensities (while still being aerobic), the largest chunk of the metabolic power comes from fat. Which is great because we get orders of magnitude more energy from metabolizing fat than from sugar. But here’s the rub: we can only metabolize it so fast, and having some sugar laying around makes it easier to initiate the whole process. Fat may contain more total energy, but it’s a lot easier to extract it from sugar. This is why people hit the wall at mile 20 of a marathon. They still have plenty of fat to keep going, but they are out of glycogen (i.e. sugar) to jump start the breakdown of the available fat reserves. So all of a sudden the amount of energy we can produce per unit time drops drastically. This is why consuming some kind of sports beverage during the race leads to faster finish times. We can keep burning large amounts of fat for fuel for longer.
The hard workouts typically require burning some amount of glucose to get to those higher gears where we need to be to get the stimulus we’re looking for. And that means that we need to have some sugar available. Not just for jump starting the metabolism of fat, but to burn as fuel. It’s just not physically possible to operate at our highest gears if we’re not supplying our bodies with the rocket fuel those high intensity efforts requires. For most of us, this is going to be anything north of 80-85% of our maximal heart rate, or our lactate threshold. Even working just under that lactate threshold is easier to do if we have some blood sugar to help jump start the metabolism of our primary fuel source, fat.
For long runs it’s generally a good idea to get something in you before you go. Yes, we’re operating at lower intensities, but if we’re going to be out there for more than 90 minutes, chances are we’re going to burn through all of our stored glycogen in that time. Even if we’re onboarding our calories as we go (i.e. with the use of GU’s, drinks, etc.), the odds of being able to consume and then assimilate enough calories to meet our demands is pretty low. There’s a reason the wall is a thing when racing a marathon. If you’re out there for long enough, you’re going to run out of glycogen. And when that happens you’re probably not going to get anything else out of the workout besides making it take longer to recover before you can go again.
Cooler weather is normally appreciated this time of the year as we finally get summer behind us. But when it gets cool enough that we need to burn extra energy simply to stay warm, we once again are increasing the caloric demand of our workout. Even if we’re warm when we start, by the time we get done with an extended bout outside in the cold, we’ve burned through a lot of calories. By eating before we go, our bodies aren’t going to be sacrificing our fingers and toes in an effort to keep the core warm because we’ll have more fast burning calories available to burn. If we don’t get something in before we go, we may end up in the unenviable position of being cold for a long time after the run. Not to mention having more of our calorie usage being directed to staying warm than to producing movement when we have to start triaging where those calories are spent.
It doesn’t have to be much. Sometimes all you need is a banana, or a piece of toast with peanut butter. But getting something in allows us to jump start our metabolism and gets it revved up before we need it operating at high capacity. Fat metabolism goes a lot faster when we have some carbohydrates to prime the pump with. And our body is more likely to give us that dump of sugar (i.e. glycogen) if it’s not afraid that the lost calories aren’t going to be replaced.
*Caveat
Fat adapted/ketogenic athletes may be theoretically less susceptible to this because they have spent a lot of effort building the cellular machinery to burn fat fast enough to provide enough energy to keep going. But most endurance athletes aren’t in this category, and quite frankly, if you want faster finish times it means you need the rocket fuel. Yes, ketogenically adapted folks may be able to go for longer at their top end. But physics says that your top end is higher if you have carbohydrates to burn, which is why almost all of us utilize them. We want to get to the finish line faster.