What Happens If You Don’t Eat Enough? 9 Ways Undereating Affects Your Body and Mind

I spent several years of my life living in an energy (calorie) deficit. At the time, I never stepped back to wonder what happens if you don’t eat enough. I didn’t understand how my entire being—mind, body, and emotions—were being affected by living that way. I’m not being dramatic when I say that every part of me was drastically and negatively impacted by undereating.

Today, as an eating disorder dietitian, I see how the effects of undereating touch every facet of my clients’ lives. Undereating is a main symptom of most eating disorders, and it can have a variety of different causes and triggers, from shame and guilt to body image distress, trauma, and beyond. People without eating disorders can also experience the negative effects of undereating. Recent research shows a high prevalence of undereating in certain populations, like athletes who don’t eat enough to fuel their training, resulting in a negative energy imbalance.

“Some people find themselves in chronic deficits unintentionally—their lives are busy, schedules are packed, perhaps their physical activity is high, or they’re struggling with a low appetite,” says Caroline Burkholder, MS, RD, CEDS. “Others may find their chronic deficit has deeper roots: body image concerns, pressure from others, or an internalized list of rigid food rules.”

Let’s look at what actually happens if you don’t eat enough, and how it can significantly impact you mentally, emotionally, and physically.

1. Emotional struggles

In my practice, I notice heightened anxiety is one of the main consequences of not eating enough. Undereating, which often comes with malnutrition and excessive weight loss, can also cause or worsen depression due to changes in brain chemistry.

Kristin Grimes, RDN, LD, explains that is due, in part, to the fact that not eating enough can lead to tissue damage in the brain, as well as a lack of nutrients—specifically vitamin B12, folate, and zinc—that may cause depression, cognitive decline, and irritability.

Also, lifestyle medicine physician Nneoma Oparaji, MD, DipABLM, DABOM, says that undereating disrupts mood-regulating neurotransmitters like serotonin and dopamine, which can worsen symptoms of anxiety and depression.

“Emotional resilience decreases and small stressors become overwhelming, [and there’s] reduced patience, mood swings, and difficulty regulating emotions,” she explains.

2. Food obsession and poor concentration

It’s one of the first concerns clients who are undereating share with me when we start working together: they believe they’re uniquely obsessed with food, when in fact their bodies are simply trying to survive.

“Your body doesn't understand that this ‘famine’ is self-inflicted, so it's going to be on high alert to increase its ability to survive and find food,” explains Equip Nutrition Director Erin Reeves, RD. “You might be thinking about food more or feel more out of control when you do eat because your body can't trust there won't be a famine again.”

In one famous landmark study (called the Minnesota Study on Human Starvation by researcher Ancel Keys), a group of men were intentionally underfed over an extended period and reported the following symptoms:

• Fixation on food
• Decreased concentration
• Slower rate of learning
• Poorer judgment
• Eating faster than normal
• Dreaming of food
• Watching others eat

These negative effects on thinking ability don’t have anything to do with intelligence or intellectual ability—they’re natural consequences of not giving your brain enough fuel.

“When our brain is underfed, it leads to excessive worry, restlessness, difficulty relaxing, decreased concentration and productivity, and brain fog,” says Oparaji.

According to a 2021 meta-analysis of adults with anorexia and those without, those with anorexia (which involves undereating as a core symptom) did a worse job on cognitive tasks, and showed significantly impaired memory.

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3. Personality changes

When you aren’t eating enough food, Burkholder says, “much of the ‘airtime’ in your mind is consumed by thoughts of food, leaving little room for other things.” And when you don’t have the space or energy for anything besides food or your body, it’s almost impossible to stay connected to your core values and authentic self.

In the landmark Minnesota Study on Human Starvation, the underfed men reported

In the landmark Minnesota Study on Human Starvation, the underfed men reported “dramatic personality changes,” like:

• Increased irritability
• Loss of sexual interest
• Decreased self-initiated activity
• Lethargy

Underfueling can also lead someone to become a more rigid thinker than they are when they’re eating enough, as well as to social withdrawal, which Equip peer mentor Jamie Drago experienced firsthand. “I was so focused on the amount I was eating—and deviating from what I thought was ‘right' brought about such anxiety—that I ended up isolating myself from family and friends,” Drago shares. “This took a toll on my emotional and mental health because I had fewer and fewer avenues for joy and community.”

4. Slowed metabolism

Other signs of undereating include a decreased basal metabolic rate (BMR), which is the energy your body needs to perform basic functioning. The body is focused on survival, so if you’re not adequately fueling it, it slows down all its processes to conserve energy and survive with less food.

“There are no tricks around this by eating a certain spicy food or doing a certain workout,” explains Reeves. “Your metabolism will slow down if you aren't giving it enough."

This is one of the reasons that so many people who spend years yo-yo dieting to lose weight often end up at higher weights than when they started: their bodies get thrown off by undereating and try to recalibrate by adjusting metabolic speed, only to get confused again, and the cycle continues.

Signs of a suppressed metabolism can also include:

• Fatigue
• Dry skin
• Feeling cold often

On a related note, when you don’t have enough blood sugar circulating in your blood, (which can occur when you’re regularly undereating and have depleted all the sugar stores in your liver and muscles), you can experience hypoglycemia, or low blood sugar. This can cause symptoms like sweating, fatigue, dizziness, and in severe cases, loss of consciousness.

5. Reproductive problems

Amenorrhea (losing your period for more than three months) is a common side effect of undereating. It can lead to a long list of serious health issues, according to The Journal of Clinical Endocrinology & Metabolism Clinical Practice Guidelines. These include:

• Infertility
• Osteoporosis
• Anxiety
• Depression
• Malnutrition

One 2022 research review of women showed that eating too little can disrupt hormone signals from the brain, causing decreased reproductive hormones and disrupted reproductive function. Essentially, when you don’t eat enough, your body prioritizes vital functions (like breathing and keeping your heart beating) and shuts down processes it deems non-essential (like reproduction).

“Your body downregulates to conserve as many calories as possible, and that means forfeiting reproductive function,” Burkholder explains.

6. Nutrient shortage and muscle loss

Reeves says another serious side effect of undereating is nutrient deficiencies. Being deficient in certain nutrients can lead to a wide variety of health conditions, like:

• Anemia
• Cold intolerance
• Hair loss
• Skin problems
• Insomnia
• Bone issues

Undereating can also cause catabolism, or when the body breaks down lean tissue like muscle for fuel, which can lead to loss of muscle mass and muscle wasting. This may be particularly noticeable for athletes who are undereating.

“Your muscles will get broken down faster because they have neither the energy nor the building blocks—amino acids and protein—available to maintain themselves,” Reeves explains. Plus, if you’re engaging in physical activity while undereating, your muscles will have a harder time recovering and you’re likely to experience decreased athletic performance, according to Burkholder.

Since the heart is largely made of muscular tissue, it will also start to weaken, and vital signs like blood pressure and heart rate can fall dangerously low.

“This weakening of the heart reduces the ability to pump blood throughout the body, which can lead to fatigue and shortness of breath,” Grimes explains. “Malnutrition also disrupts the electrical activity of the heart, the power behind the heart’s contractions, which can be fatal.”

7. Digestive issues

Malnutrition can also break down tissue in the digestive tract, and over time, this can weaken muscles in the stomach and intestines. In fact, research shows that malnutrition and weight loss from undereating can slow down the gastrointestinal (GI) tract, which can cause issues like:

• Gastroparesis (slowed movement or motility of stomach muscles)
• Constipation
• Reflux
• Heartburn
• Bloating
• Diarrhea

Reeves explains that undereating will also decrease the number of enzymes your body makes to break down food, leading to further GI symptoms. In my practice, I’ve observed that undereating-related GI issues cause significant disruptions in clients’ day-to-day functioning and overall quality of life.

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8. Feeling out of control

In the eye-opening Minnesota Study on Human Starvation, the men subjected to undereating reported that once the restriction period ended, they felt an initial loss of control around food. Some of them said they wanted to eat continuously, while others said they binge ate (and never had before the experiment). More than half of them reported overeating.

In my work, I’ve noticed that most of my clients are surprised when they realize their bingeing or overeating is often rooted in undereating. The thing is, when we’re underfed, our bodies will do everything possible to get us to reclaim a state of equilibrium, and that can sometimes manifest in out-of-control eating, binge eating, or an inability to listen to and honor your hunger and fullness cues. Undereating followed by binge eating can become a vicious cycle (known as the binge-restrict cycle), and is often accompanied and enforced by feelings of guilt, shame, and frustration.

9. Weakened immune system

Eating enough is like putting on a strong suit of armor each day, and when that armor disappears or is weakened, it compromises every system in your body, including your immune system.

“When the body is underfueled, the immune system is one of the first systems to suffer,” Oparaji explains. “A person may notice more frequent colds or infections, and slower wound healing.”

According to a 2022 research review, good nutritional status is strongly linked to optimal immune function. This is likely due to the fact that undereating can cause deficiencies in key immune-supporting nutrients, like:

• Protein
• Fat
• Vitamins like A, C, and D
• Minerals like zinc and selenium

How to stop underfueling your body

If you’re experiencing any of the signs and symptoms discussed above, you may be undereating. To better understand if you’re not giving your body enough food, ask yourself these questions:

Do I…

• Feel heightened anxiety and depression?
• Feel more irritable than usual?
• Have obsessive food thoughts?
• Isolate myself socially because of food?
• Feel out of control around food?
• Have a missing period?
• Experience significant digestive discomfort?
• Get cold or sick often and easily?

Regardless of your symptoms, once you’ve found yourself in a pattern of chronically undereating, it’s tough to restore balance on your own—whether or not you have an eating disorder. That’s why re-establishing healthy eating habits and reversing the effects of undereating usually takes professional guidance from a team of providers (generally at least a therapist and a dietitian).

“Learn from a non-diet dietitian how much your body really needs. It's difficult to use online calculators or providers that endorse weight loss, because you will likely get a calorie goal that is far too low,” Reeves says. “Stay in your lane: don't compare yourself to others or ask your friends how many calories they eat. Your body is its own unique machine, and looking at a snapshot from a peer or someone's 'what I eat in a day' video is not going to give you the personal guidance you need to thrive.”

With your dietitian, you’ll learn how to nourish yourself to meet your specific energy and nutrient needs, which usually means eating balanced meals and snacks every few hours throughout the day. You may also learn other practices as necessary, like intuitive and mindful eating and flexible meal planning. Check out more skills and insights on how to stop undereating here.

Undereating effects are widespread and have serious impacts on mental, physical, and emotional well-being. But there’s good news: most can be reversed with adequate and balanced nourishment. While the road there can be challenging, trust that your hard work and commitment will pay off in powerful ways for the rest of your life.

Frequently Asked Questions (FAQs)

What are the side effects of not eating enough?

There are several symptoms of not eating enough, including food obsession, loss of control around food, amenorrhea (loss of period for more than three months), personality changes like increased irritability, lethargy, and digestive issues like constipation and bloating.

What are the long-term effects of not eating enough?

Chronic undereating can lead to a variety of negative health consequences, including reproductive problems, musculoskeletal issues like osteoporosis, and digestive problems. It can also have long-term effects on your emotional health, relationship, and quality of life.

How does undereating affect metabolism?

Undereating can suppress your metabolism by decreasing your basal metabolic rate (BMR), which is the energy your body needs to perform basic functioning. Without enough calories, your body will slow all its processes to try and save energy and survive with less food.

Can undereating cause digestive issues?

Yes, undereating can include digestive issues, like general GI dysfunction, gastroparesis (or slowed movement or motility of stomach muscles), constipation, reflux, heartburn, bloating, and diarrhea. Such digestive issues can significantly impact overall health and quality of life.