Cutting calories can be tempting if your goal is weight loss, but logging miles at a calorie deficit can have serious implications in the moment and in the long run.


You don’t have to understand much about exercise science or nutrition to grasp the concept that calories = energy. You consume calories so your body has fuel to burn and therefore convert into energy. Boom. That’s science.

It’s a common adage that weight loss is all about calories in versus calories out. And while it’s not quite that simple, it is true that when you have a surplus of calories, they get stored as fat. But when you have a calorie deficit, you don’t just lose weight. In fact, when you’re consistently operating at a calorie deficit—even unintentionally—the cumulative effects can wreak havoc on your performance.

What You Need to Know About Calories

Let’s start with the basics: Active men should be consuming up to 3,000 calories a day, while women should be consuming up to 2,400 calories a day, according to the 2015-2020 edition of the Dietary Guidelines for Americans. Keep in mind that these are just general guidelines and they can be scaled up or down depending on your personal needs, goals, and activity levels.

Here’s what happens to those calories: Metabolism are the processes by which your body converts what you eat and drink into energy. “When we talk about calorie burning, we are including calories burned for basal metabolic rate—those calories we need just to maintain our temperature and breathing, etc.—plus the extra calories burned in physical activity,” explains Natasha Trentacosta, M.D., a sports medicine specialist and orthopedic surgeon at Cedars-Sinai Kerlan-Jobe Institute in Los Angeles. “The extra activities we do in life burn calories beyond the requirement for basal metabolic rate, and to burn these calories, your body first looks toward readily-available sources, and then subsequently stored energy sources.”

What Running on a Calorie Deficit Does to Your Body

Carbohydrates are our bodies’ preferred source of energy as they are the most-readily available and easiest to convert. When we eat carbs, most get converted to glucose, which is our main source of energy during activities, says Trentacosta. If we take in carbs but don’t use them readily, they may get stored as glycogen in the liver and muscles. When you’re working out for a long time (…or you haven’t eaten) and there’s no readily available glucose, your body breaks down this glycogen back into glucose. “Eventually, longer duration exercise will deplete the readily available glucose and the glycogens stores, requiring the body to get energy from fat or even muscle protein,” she explains. “Your body literally breaks down muscle to provide energy for your run, decreasing your overall lean muscle mass.” This, of course, is not a good thing for athletes.

Protein is actually a relatively inefficient energy source because it’s harder to break down compared to the other macronutrients—it’s your body’s last resort during prolonged exercise (or starvation). And in addition to the fact that it literally wastes away your muscles, “it produces nitrogen as a byproduct of metabolism, which must be removed by the body and requires energy to do such,” says Trentacosta.

Here’s an interesting aside: Runners who follow low-carb diets may actually circumvent this, because those diets (like the keto diet) decrease the availability of glucose, says Volek. “They force the body to become very efficient at breaking down and utilizing fat for fuel. The more adapted you are at burning fat, the more efficiently you can, if you’re not eating enough calories, use your fat stores. That spares breakdown of lean tissue and some of the negative effects.” Even the leanest among us have ample amounts of fat to use as fuel.

But logging serious miles without consuming enough fuel or priming your body to burn more fat actually tricks your body and its hormones into starvation mode.

“Your metabolism will slow down,” says Volek. “Your thyroid, which produces the hormones that regulate the body’s metabolic rate, levels will fall. You’re going to feel more sluggish because your body isn’t in the fight or flight mode, it’s conserving energy because it senses that caloric deficit. If you’re trying to train hard, you’re going to feel quite awful.”

“A slower metabolic rate could also slow down your recovery, because you’re not able to provide the substrates [the substance on which an enzyme acts] for restoring cells that are damaged as a result of the exercise,” he adds. “And that’s not ideal in terms of adapting positively to training.”

Not to mention, calorie restriction often leads to poor intake of essential vitamins and minerals such as iron, folate, or vitamin B12, which can lead to anemia and fatigue, says Trentcosta. “It may be associated with poor calcium and vitamin D intake along with drops in estrogen and testosterone secondary to calorie restriction,” she says. “This can lead to weaker bones that may more easily fracture in the face of intense workouts.” Not ideal for race prep, right?

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This is also where you start to run into the risk of more serious conditions like relative energy deficiency in sport (RED-S), a broader term for what was formerly known as female athlete triad (because, guess what, it happens to men, too).

But What Does Running On a Calorie Deficit Feel like?

Kind of like bonking or hitting the wall at mile 20 of a marathon. “When you bonk or hit the wall after exercising for more than a couple of hours, that’s really an energy crisis,” explains Volek. Essentially, your brain is running off the energy cliff because you don’t have enough glycogen, and you can’t absorb it fast enough on the go. “If your brain doesn’t have a sustained source of glucose, it shuts the body down, and that’s why you basically experience this catastrophic fatigue,” he says. It’s not exactly running out of fuel (you still have body fat, after all); it’s more about not being able to use the fuel you have (fat) efficiently.

Imagine you’ve been eating at a deficit for a couple hundred calories for a week, and you’re in the middle of a training plan. Instead of feeling that fatigue hit at mile 15 or 16, you might start to feel it at mile nine or earlier. Basically, your body has “no quick energy available for the run, and requires ingestion of calories quickly to overcome that wall,” says Trentacosta.

That makes sense in a marathon, but shouldn’t be something you’re feeling in an easy 10-miler or even at half marathon distance. “Part of that feeling is the lack of metabolic fuel, and part of that is central drive,” explains Volek. “You’re fatiguing sooner because the brain may be perceiving some type of metabolic deficit and be sending signals to the muscles to not be active because that’s burning precious fuel.”

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At the end of the day, your calorie intake should support your healthy and active lifestyle. While it’s fine to want to lose weight and we know that running is one of the best and most accessible ways to do so, you never want to sacrifice your performance. Remember, it’s more complex than simply calories in versus calories out, and the number on the scale doesn’t represent body composition, so think of food as fuel and eat in a way that supports your training and personal goals.

A true love for sports


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