“Running on empty” is a common idiom that can be used to describe a number of scenarios where someone is worn out or unmotivated. For runners that are underfueling, it can be quite literal. Whether you are knowingly or unknowingly under-fueling, there are important factors to know about your caloric and nutritional needs.

Despite the prevailing myth that weight loss boils down to a simple calories in, calories out formula, a variety of lifestyle factors and their ensuing hormonal responses affect the ways our bodies respond to exercise and food. In an effort to maximize weight loss, many people (particularly women) eat as little as possible. “High-performing female athletes are at a high risk of underfueling,” says Yasi Ansari, MS, RDN, CSSD, and national media spokesperson for the Academy of Nutrition and Dietetics. “This can take place without them even knowing.” There can also be such a focus on creating this calorie deficit that it can be easy to forget what calories actually do: fuel our bodies.

Your body responds to extreme caloric restriction by doing whatever it can to ensure your survival, mostly by conserving energy and putting calories toward its most basic functions. To do this, the body resorts to burning fewer calories. While in this survival mode your body produces more of the stress hormone cortisol, according to this 2010 study. High levels of cortisol can also cause changes to your sex drive, and might even be linked to anxiety and depression.

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If you’re not eating enough food, it’s also likely that you have some sort of micronutrient deficiency. According to the Micronutrient Information Center of Oregon State University, vitamin D, calcium, potassium, dietary fiber, and iron are commonly under-consumed in the U.S. Like eating too few calories, symptoms of micronutrient deficiency include fatigue, reduced immunity, lack of concentration, memory troubles, and mood fluctuations.

This impacts your training in several ways. When the body feels it must prioritize essential functions (like regulating breathing, body temperature, and blood pressure), it doesn’t feel that it’s safe to put resources toward things like rebuilding muscle tissue, which is the process that enables it to build strength. Training sessions therefore become harder when we’re underfed. Though you may feel like you’re performing with all you’ve got, you’re actually working at a severe energy disadvantage.

Without enough fuel, you can’t perform at your best. “When energy and intake is too low, it really cannot support the demands of health and high-level performance,” says Ansari. For endurance athletes, it means running out of gas more quickly while racing or out on training runs. Even if you manage to push through a workout made difficult by a lack of fuel, your muscles can’t rebuild, and your body may even resort to using the protein from your muscles themselves.

Runners that chronically under-fuel are at risk of developing Relative Energy Deficiency in Sport (RED-S). The syndrome is characterized by impaired metabolic rate, menstrual function, bone health, immunity, protein synthesis, and cardiovascular health.

So how do you know if you’re eating enough for your activity level? The list below of common symptoms should give you a better idea.

Signs you aren’t eating enough

1. You’re constantly tired.

Food is energy. As mentioned above, if you’re not eating enough calories, your body is going to use the ones it does have to support vital functions. This means there aren’t any left to do the things you love. If you’re dragging your feet at the gym every day or while out on a run, chances are you could benefit from more food.

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2. You’ve hit a training plateau.

Have you been working out with greater intensity but aren’t seeing any results? Do you see, as Ansari describes, “decreased performance in the weight room, out on the track or trails,” or feeling like you’re bonking or hitting the wall? Your body could be in starvation mode, fighting to preserve as many calories as it can. If you’ve hit a ceiling in your weight training and haven’t seen an increase in months, it’s likely that you need to eat more, both to fuel your training and to repair your muscles.

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3. You aren’t regular.

Only about five percent of Americans consume enough fiber each day, according to national consumption surveys. If you are under-eating, the chances of your body getting enough fiber grow slimmer, which can easily lead to constipation. Another factor to consider is dehydration, which also contributes to slower bowels.

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4. Your brain feels foggy.

Ansari who works mostly with runners in high school and college mentions that poor concentration in school is a symptom of under-fueling. You may also feel that at work or even have trouble staying mentally alert during a workout. Your brain relies on proper nutrition to function, as well as the rest of your body.

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5. You can’t sleep, even though you’re tired.

Appropriate food intake allows for improved blood sugar control. The combination of consuming too few calories and over-exercising leaves your liver depleted of the glycogen stores it needs to keep your blood sugar stable, forcing your body to release stress hormones that eventually lead to the production of new glucose. When stress hormones are high, we have trouble falling–and staying–asleep.

“Someone who is limiting carbs and/or overall calories may experience some sleep challenges,” says Ansari. “Carbohydrates help to make tryptophan, an amino acid that increases sleepiness, more available to the brain.”

Adding Fuel to Your Tank

Unfortunately, it’s impossible to give a one-size fits all on how many calories your body needs, particularly since your energy expenditure varies every day. The USDA Dietary Guidelines estimate that adult women need anywhere from 1,600 to 2,400 calories a day, and adult men 2,000 to 3,000 calories per day. However, this baseline estimate doesn’t include the additional calories needed for intense exercise, as their definition of an active lifestyle for their purpose is “physical activity equivalent to walking more than 3 miles per day…in addition to the activities of independent living.”

While there are many bodyweight calculators available that can tell you what your ideal weight (and thus ideal calorie intake) should be for your age, gender, and height, both fail to consider things like frame size and muscle mass. Ansari recommends connecting with a registered dietitian to really figure out how to meet your needs.

There are other simple steps you can take to start benefiting your nutrition, like adding more whole foods, especially fruits and vegetables, to your diet. This is especially important if you suspect you have a nutrient deficiency. A Registered Dietitian can help you determine if you need additional interventions like supplements or vitamins.

You can use these rules to get started, but listening to your body and looking for the above clues–hunger, fatigue, weight loss, fitness plateaus, etc.—will serve as much more reliable indicators of your needs.

“I have worked with athletes who had been under-fueling and experienced symptoms of RED-S,” says Ansari. “After focusing on an eating pattern that stressed the importance of nutrition adequacy and timing nutrition appropriately around training, we were able to improve their pace, energy, and the athlete was able to say that they felt stronger than ever before.”