elite fueling table Houston Marathon Trials
photo: 101 Degrees West

Five Questions About Fuel

Experts explain why it is worthwhile for runners to focus on fat-burning metabolism and blood sugar management.

elite fueling table Houston Marathon Trials

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Why should I care about fat-burning if I’m running great on carbs and they are easier to burn?

First, if you’re running a marathon (or longer), you simply can’t store enough carbs to make it through the race. Mark Cucuzzella, MD, Professor at West Virginia University School of Medicine, author and consultant, explains that you need over 3000 calories of carbohydrate for a marathon, and your total stores add up to around 2000—for the best-trained runners. You have, however, around 50,000 calories of fat.

“If you’re burning more fat,” says Bob Seebohar, exercise physiologist and sports dietician, “You’re preserving more of your very limited carbohydrates until you really need them, which typically, if you’re managing pacing well, is later in the marathon, the last 10–15K.”

strong male marathoner
photo: 101 Degrees West

To improve the body’s ability to burn more fat you need to reduce the insulin response, which comes when you ingest excess sugary “fast” carbs, signaling the body to burn and store the carbs and significantly reducing the delivery of fat to be burned.

Improving fat-burning requires more than race-day changes for most. Endurance training under the right conditions is key for the muscle to build capacity for fat burning. In our society, where we’re often constantly slurping a sugary drink, we  limit our ability to burn fat. “If we are always exercising under conditions of an excess of simple carbohydrates, we really haven’t adapted at all to burn fat,” says Catherine Yeckel, exercise physiologist, Assistant Professor of Clinical Public Health at Yale School of Medicine. “We don’t even have much of a muscle glycogen storage pool—we haven’t adapted to that.”

Beyond the marathon, Seebohar says that those who start burning more fat find that they are, “Feeling better, recovering faster from runs, sleeping better. The afternoon crash will be completely eliminated, energy will be sustained.”

As far as your daily running, Seebohar says, “You will be able to go out any time of the day—you won’t have GI distress, will be able go into a run without worrying about your gut, or worrying about, ‘Do I have enough energy.’” Plus, he adds, “Once body adapts to using fat as energy, your body simply requires fewer calories during runs.”

Why can’t I just keep replacing carbs during the race?

For one, most runners can’t absorb carbs fast enough to replace muscle stores. And trying to ingest enough can compromise your performance.

stomach cramping in marathon
photo: 101 Degrees West

“Getting simple carbs in is limited in part because blood supply to your intestines is going to decline,” says Yeckel. “You’re likely to experience stomach and GI cramping. Blood needs to go to active muscles and skin; there’s a direct competition for blood supply.”

Should I eat a big, high-carb breakfast before a race and take a gel before the start?

Contrary to conventional wisdom, experts say a sugary, high-carb breakfast can sabotage your race. “This will increase your insulin levels and lock out the ability to burn fat,” says Cucuzzella. You can fill your energy stores by not running and by eating adequate amounts of fats, protein, and some healthy carbohydrates the days prior to the race. “A light breakfast of mix carb/fat/protein is a good thing,” Cucuzzella says.

sweet pancakes
photo: Shutterstock

Similarly, be careful with loading sugary carbs before you start running. Research shows that ingesting simple carbohydrates at the start of exercise and throughout keeps insulin elevated and fat delivery low. “If you drink a typical sports beverage with simple sugars and eat a gel 20 minutes before the start, you’ll get an insulin response,” says Yeckel. “The sugar hangs out in the blood longer, at a higher level. The muscle has to use it—really it is being forced to use more carbohydrate.”

Ideal is a fasted state, where the body isn’t reacting to new fuel. “It allows the active skeletal muscle to decide what fuel mix it wants to use,” Yeckel says. “But people generally want to grab something before they exercise, so the challenge is usually, ‘What should I grab?’”

This is where slow-carbs shine, as they don’t produce as much of an insulin response. “It’s getting as close to fasted as you can while supporting your blood sugar,” Yeckel says. “You can keep fatty acid delivery high to promote fat oxidation.” A supplement drink like UCAN, with more complex, slower carbohydrates, makes it easier to ingest before exercise—when eating something like lentils would not be ideal.

Doesn’t the body burn carbs immediately while you’re exercising?

“During a run, insulin is suppressed because of exercise— you won’t necessarily have a big surge of insulin,” says Seebohar. So if you take in fast-carbs while running, you likely will not have as much of a surge and subsequent drop in blood glucose. Yeckel points out that research shows, however, that even though there is a smaller spike, with continued carbohydrate ingestion during exercise insulin stays higher—enough to decrease fat delivery and fat oxidation.

In addition, Seebohar warns, “If you’re dumping a lot of sugar in your body—doesn’t matter what mile it is—you’re reactivating your carbohydrate stores to be burned. If we’re putting in all these sugary gels we’re exacerbating carbohydrate utilization, and emptying carbohydrate reserve tank—in addition to GI distress. We’re exacerbating the already diminishing gas tank of carbs.”

Yeckel agrees. “If you drink a lot of fast-acting carbohydrates, it makes you have to burn carbohydrates,” she says. “The main issue of taking in simple, fast acting carbohydratates before you run or even while you run is that you are forcing a metabolic shift toward the muscle having to burn off carbohydrate even if it would have preferred to burn fat.”

late-race marathon fueling
photo: 101 Degrees West

When in later stages of a marathon or longer race you become depleted, you may need fast carbs, Yeckel says. “As time goes on, and muscle glycogen becomes depleted, you are forced to slow down, or really pull in a lot of glucose from the blood to burn,” she explains. “No question: Simple carbs have been shown to benefit in those types of long endurance events, when muscle needs to rely on another source of carb to burn.”

But Yeckel says, “We are setting runners up for this inefficient problem more often than is necessary. When we always train with simple carbs, we likely diminish muscle adaptation to store more glycogen.”

“For most other exercise scenarios,” she adds, “It is efficient to have blood sugar stable so you don’t have central fatigue, and its more about muscle trying to keep a mix of fat burning as high as possible and the glycogen reserve for when it needs it.” Taken before a race, or during—before you get depleted and need a rescue—slow-carbs will help keep your blood sugar stable.

Within a race, Yeckel says, “It is a balancing act, allowing muscle to be as efficient as possible, allowing it to use its hard-won training adaptations, not forcing its hand and making it use simple carbohydrate.”

Cucuzzella says, “If you can add a little gas along the way, this helps both glucose and fat burning. Adding just a little is best. Just a little sugar every 30 minutes is easy to digest, can be used directly by the muscles even in the absence of insulin, and can help keep you fat burning till the final miles.”

All agree individual response varies greatly, due to genetics, and dietary and training differences. It is important to learn how your body responds, and worth experimenting with a variety of fueling options and training protocols.

If I want to become more fat-burning, does this mean I need to have a special, high-maintenance diet?

“Getting people to start behavior change, that’s the hard part,” Seebohar says, “Easy part is food. Eat a bowl of cereal in the morning? Why not have some eggs. Or, let’s have a smoothie that has some protein in it.”

eggs and bacon
photo: Shutterstock

The key is to balance carbohydrates with protein and fat. “Athletes are so focused on carbohydrates,” he says. “I focus on protein and fat—knowing you’re going to get carbohydrate. But let’s not miss the protein and fat. I try to find every opportunity to sneak in a protein/fat into a meal plan.”

You don’t have to restrict major food groups and do anything drastic.

Is it worth trying a slow-release carb even if I don’t want to overhaul my diet or my training?

Seebohar says yes. “If you’re not willing to change training or daily nutrition, you can start the process by using a [slow-carb] sport-nutrition supplement,” he says. “It could be a great introduction.”

Yeckel agrees. “The beauty of the UCAN SuperStarch is that it is slowly-digested and essentially time-released. It helps to keep blood glucose stable and sustained,” she says. “For example, it showed in a study, under a prolonged, moderate-intensity 3 hour running protocol (ingested once, 15 minutes before exercise) that not only is blood glucose stable throughout, but fat oxidation stays high.”

“Everyone is individual. Diet matters, training matters,” Yeckel says. “It’s best to allow muscle to do what it wants to do. Rather than dictating muscle metabolism, SuperStarch allows it to play that intermediate game, which works for all types of athletes.”

From PodiumRunner