With my wedding six months away, I decided it was time I shed some excess weight. For a short time, I attempted the OMAD approach—one meal a day—along with a daily four-mile run. This meant that every day around 3pm, I’d hit the pavement, come home, pace around the flat in a haze of hunger, and then finally eat around 5. I figured if my stomach didn’t have any food to burn, my body would be burning the fat. Makes sense, right?
Actually, it turns out, sometimes that fat fights back. I’d come home and feel lightheaded and weak.
In theory, running on an empty stomach helps you burn fat faster. But is it really that simple, or just a runner’s myth? Could this be the reason I feel ill every time I stand up after a run? I asked some running and diet experts to find out.
Simply put: Yes, running on an empty stomach means your body will resort to burning your fat storages. This is often referred to as “fasted cardio,” says Jonathan Jordan, a group instructor at the iconic Equinox gym in San Fransisco. “If your main goal is to burn fat and lose weight, then the research is pretty convincing [it will give] you an edge.”
But before you duct-tape your fridge shut and lace up your joggers, you should figure out if your body will actually respond to this routine. Yes, there are benefits, says Jason Fitzgerald, the head coach of StrengthRunning.com, but “it’s an advanced training strategy that’s not right for everyone.”
Calories In vs. Calories Out
First and foremost, running on empty is good only for short runs. In essence, by limiting the amount of energy you feed your body, you’re limiting how much you can exercise—and limiting the calories you can burn.
“It is actually true to some degree that you burn body fat instead of the food in your stomach [when running hungry], but it’s not a big enough impact to override how you eat the rest of the day, or after your workout, for that matter,” says dietitian Abby Sharp.
“Calories are calories, and if your run burns 500 calories, it’s the same if you are running on a full stomach or an empty one,” says Paul Ronto, marathon runner and purveyor of RunRepeat.com. Largely, he continues, it depends on your diet and whether you’ve “trained” your body to burn fat as energy—for example, by going keto. “If your body is not used to using fat as its fuel source, the transition from sugars (carbs) to your fat can be hard.”
Avoiding the Bonk
Transitioning your body from burning carbs to burning fat for energy is best left to fitness experts, Fitzgerald says. It’s not a question of whether you’re burning fat or carbs, he explains, but “simply a matter of shifting the balance slightly more toward fat, [which] can be helpful for marathoners and ultramarathoners who are teaching their body to better conserve fuel.”
For example, he says an ultramarathoner “may want to run a fasted 15-miler at an easy effort. Once those long runs get even longer, this runner is going to want to fuel during a run. Elites may want to experiment with fasted runs to better optimise their fat-burning capacity, but these improvements are small, subtle and have risks.”
“For the vast majority of runners,” he says, “90 percent of their results will come from focusing on fundamentally smart training decisions. Most runners are far better off focusing on performance, which requires adequate fuelling.”
In other words, unless you are fine-tuning the exact science of how and when your body burns or conserves energy, running hungry isn’t going to dramatically help you lose weight.
And if you do, there’s a good chance you’ll “bonk,” which is runner-speak for hitting a wall.
Going beyond a 5K on an empty stomach is “where most people bonk,” Ronto tells me. “They obviously have fat sources but their body doesn’t know how to best utilise them, so they run out of sugar (carbs) and then the body is confused as to what to do next.”
According to Fitzgerald, training your body to burn fat takes a while. “It probably takes two to three months to fully adapt to a very-low-carbohydrate diet. Expect mood swings, energy crashes and a general feeling of unwellness. It’s not fun!”
And even then, he says, you’ll never train your body to exclusively use fat as fuel. “It’s a backup fuel source ideally suited for low-intensity exercises like ultramarathons. The body’s preferred fuel source is sugar, and it will always prefer that source.”
So What Should You Eat Before a Run?
All four experts say this is largely a matter of preference.
“It is definitely a personal thing depending on physiological and psychological effects,” says Sharp, the dietitian. “Some people do perform better on an empty stomach, while others need fuel in the tank, and further variability may depend on the type of activity you’re doing.”
“If you feel better running on empty, do it,” says Jordan, the Equinox trainer. “If you find you can’t perform on an empty stomach, have something light with good carbs. Things like a banana, an energy bar — something that won’t make you so full you feel heavy, but something that will give you good carbs to keep from bonking.”
“I will say that running on a full stomach is hard,” Ronto says. “It can lead to cramps and other issues that can slow you down — like burping, indigestion and bowel pressure (feeling like you have to poop).”
“My advice would be to eat clean (no processed foods, no added sugars), get lots of liquids and have the proper fuel in your system for any run,” he adds. “If you eat clean and run often, you will burn fat and lose weight. If you don’t eat clean, running hungry isn’t going to help you burn fat or lose weight in the long run if that’s your goal.”
Sharp agrees: It’s better to eat a 105-calorie banana to fuel you through a good run than not to eat at all and bonk halfway through.
“If you’re not going to run fasted first thing in the morning, research suggests that having some carbs before a run — about an hour before — is ideal for providing you with some fuel for your workout,” she explains.
“If it’s going to be a longer run, then adding in some protein will help improve the satiety factor so you don’t burn out mid-jog. Go for easy digesting options like a banana-and-yogurt smoothie, yogurt and berries, oatmeal with hemp hearts, or toast with nut butter.”