What Happens When You Don’t Eat Enough

By Shoshana Pritzker, RD, CDN, CISSN, LDN

You’re doing all the right things — exercising, cutting calories, getting more sleep, and even trying to stress less — yet it feels like you’re running on a hamster wheel that never stops. The old mantra of “eat less, move more” isn’t working. Why? Because not eating enough can quietly sabotage your progress.

A calorie deficit is necessary for weight loss, but cutting too many calories not only stalls the scale, but it can throw your hormones out of balance, tank your energy, and make your body cling to fat instead of letting it go. That’s why understanding the science of undereating — and how to fuel your body the right way — is one of the most important steps you can take. Here’s what you need to know.

Why Women Undereat

Women undereat for emotional, psychological, physical, and social reasons. Some individuals don’t realize they’re consuming too few calories, while others feel constant pressure not to “overdo it.” Let’s look at the most common factors that lead women to underfuel.

Diet Culture & Confusion

For decades, diet culture has taught us to fear food and calories. What’s supposed to be nourishment is labeled the enemy. It’s also normal to hear women talk poorly about their bodies, your body, or beat themselves up over what they ate. It’s normal to share the bad things you hear about food and ingredients and ban them from your diet. 

This mindset doesn’t come from nowhere. Diet culture is fueled by a range of factors, including marketing and media, socioeconomic status, social networks, and even religious traditions. Add in today’s food environment, which often promotes opposing views surrounding food and ingredients — and it’s no wonder so many are confused about what they should or should not be eating. 

Related: Protein Myths Debunked

Stress

Stress and anxiety can take a toll on your appetite. They often trigger digestive issues like nausea, an upset stomach, or a general lack of hunger. That’s because the body’s fight-or-flight response is designed to keep you alert and energized, which isn’t conducive to sitting down to a meal. When hormones surge, your appetite naturally dips. 

For many people, this means eating less until the stressful situation passes. How quickly your appetite returns depends on you and your lifestyle. Factors like sleep, self-care habits, and overall stress load can all play a role. 

Poor Appetite

Remember when diet culture told us to chew gum or drink water whenever hunger pangs hit? Somewhere along the way, ignoring your body’s natural signals became “normal.” But hunger is your body’s fuel gauge. Think of it like your car’s gas light: if the light stopped working, or you kept ignoring it, you’d eventually run out of gas. 

The same goes for your body. Without enough nutrition, you don’t just lose energy, you lose the ability to recognize when you need to eat. Over time, constantly overriding hunger cues can blunt your appetite altogether. 

What Happens When You Undereat

Undereating isn’t just about feeling hungrier than usual — it affects nearly every system in your body. From slowing down your metabolism to disrupting hormones, depleting muscle, draining energy, and even shifting your mood, the consequences can be far-reaching.

Your Metabolism Slows Down

Your metabolism, or metabolic rate, is the energy your body burns to keep you alive. Even at rest, you need a baseline number of calories per day for things like breathing, circulating blood, and repairing cells. Everyday activities like brushing your teeth, walking to work, or doing the dishes add to that demand. Exercise raises it even further.

Consistently eating too few calories can have deleterious effects on your metabolic rate. Initially, you may see weight loss, but over time, your metabolism will slow down to match the calorie level you’re taking in, making it harder to lose weight on the same calories that once worked. 

Related: How to Fuel For a Workout

It Causes Hormonal Changes

Your body relies on a delicate balance of hormones to keep everything running smoothly. When you cut calories for too long, that balance gets disrupted.

Thyroid hormones, which play a big role in regulating metabolism, start to drop. At the same time, your body produces more of a “brake” hormone that slows things down even further. Stress hormones like norepinephrine also decline, reducing the amount of energy your body burns at rest.

On top of that, leptin — the hormone that helps you feel full — decreases as you lose weight. When leptin is low, your brain thinks you’re running on empty, so it slows your metabolism to conserve energy. The result? A system designed to protect you from starvation, but one that makes it harder to keep losing weight.

You Lose Muscle

When you don’t eat enough, your body looks for fuel elsewhere, often turning to muscle tissue, breaking it down into amino acids for energy. On top of that, calorie restriction raises the stress hormone cortisol, leading to further breakdown of your hard-earned muscle. 

You’re Fatigued

Calories are your body’s fuel. Without enough, it’s like trying to drive a car on empty. Combine that with the metabolic, hormonal, and muscular changes that happen on a low-calorie diet, and it’s no wonder you feel drained and exhausted.

You Get Mood Swings

Ever notice that when you haven’t eaten in a while, you get cranky? Not only does lack of food cause mood swings, but research suggests that chronic calorie restriction is associated with depressive symptoms. Studies have shown that long-term calorie restriction reduces 5-HT (a precursor to serotonin and melatonin) in the brain, which plays a significant role in regulating mood, sleep, and appetite.

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Signs You’re Not Eating Enough

Consistently eating too few calories doesn’t always look obvious — it can sneak up in the form of symptoms that are easy to brush off or blame on something else. But your body has ways of signaling that it isn’t getting enough fuel.

• Cravings
• Sleep disturbances
• Lack of satiety cues, or always feeling like you’re hungry or need food
• Irritability
• Mood swings or depression
• Inability to lose weight, or you start gaining weight
• Fatigue

If several of these feel familiar, it may be worth checking how much you’re really eating. A simple way to do this is by using a food tracking app to help you determine your daily intake. Compare your average calorie intake for the week with your suggested Basal Metabolic Rate (BMR) — the number of calories your body needs at rest just to function. 

You can calculate your BMR with the following formulas:

For men: (9.65 x weight in kg) + (573 x height in m) – (5.08 x age in years) + 260

For women: (7.38 x weight in kg) + (607 x height in m) – (2.31 x age in years) + 43

Related: What to Eat After Your Workout

Don’t Fear Fuel

It’s time to kick diet culture to the curb and change your mindset about calories. They’re not the enemy; in fact, they’re necessary in abundance for a healthy body and body weight. Instead of thinking about all of the foods you can’t eat, look at all the foods you can. An abundance mindset flips the script on villainizing calories and instead puts you back in control. You’ll get to eat more of the foods you love (because you should enjoy what you eat) and have the healthy body you’re striving for, too!