What Happens If You Don’t Eat Enough? 8 Ways Undereating Affects Your Body and Mind
A woman in a tank top sits on a couch, holding a bowl of cereal, looking sadly into the distance

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 dramatizing when I say that every part of me was drastically and negatively impacted by undereating. I suffered serious consequences, some of which I’ll share with you today. Fortunately, I committed to healing and recovery over a decade ago, which meant first moving out of an energy imbalance by eating more food, exercising less, and learning how to manage stress.

Today, as an eating disorder dietitian, I see the side effects of not eating enough in my clients and how it infiltrates every facet of their lives every day. Sadly, undereating and setting goals to undereat are normalized and supported across mainstream and social media, without much (if any) regard for the negative impact it might have.

Let’s look at the different ways that undereating can significantly impact you mentally, emotionally, and physically.

1. Emotional dysregulation

In my practice, I notice heightened anxiety is one of the main side effects of undereating, and, personally, I know now that spending years feeling on edge and suffering panic attacks was a direct result of not eating enough food.

Undereating, which often comes with malnutrition and excessive weight loss, can also cause or worsen depression as well as anxiety due to changes in brain chemistry. Kristin Grimes, RDN, LD, explains that inadequate nutrition 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,” says Kristin Grimes, RDN, LD. She also notes that malnutrition may also lead to a decrease in production of dopamine and serotonin, two mood-regulating hormones, which can worsen symptoms of anxiety and depression.

On top of that, undereating makes it more difficult to deal with tough emotions and hard life experiences and relate to other people. “People who frequently underfuel often describe an increase in irritability, a decrease in patience, and a tendency to have diminished capacity for emotional regulation,” explains Caroline Burkholder, MS, RD, CEDS. “They tend to experience mood swings, with less tolerance for ‘riding emotional waves’ throughout the day.”

2. Food preoccupation

When I was undereating, I remember feeling like I was abnormal because most of my mental space was taken up by thoughts of food. It was hard for me to stay present in conversations or to pay attention to my work and studies. Now, it’s one of the first concerns clients who are undereating share with me when we start work 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. “Therefore, 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. They reported the earliest and most present symptom of their starvation experience was a fixation on and thoughts about food. All the men said they experienced “intellectual inefficiency,” including decreased concentration and rate of learning, or poor judgment. Additionally, some men responded by gathering recipes and cookbooks, eating faster than normal, and dreaming of food, and some said they wanted to watch others eat often while others said they couldn’t bear doing so.

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. This is one of the non-physical effects of not eating enough that I find scariest: without room or energy for life outside food and body, it’s impossible to live in alignment with your true core values. In other words, it’s impossible to be yourself.

In the landmark study I mentioned, the men reported “dramatic” personality changes alongside emotional issues like increased irritability. These personality changes included social introversion, loss of sexual interest, decrease in self-initiated activity, and lethargy. Underfueling can also lead someone to become a more rigid thinker than they are when they’re eating enough.

In my work with clients who aren’t eating enough, the changes that I observe over our time together can be mind-blowing. As they work toward fueling their body adequately, it’s as if they’re re-emerging from a dark haze that was keeping them disconnected from themselves, their lives, and the people around them. Their sense of humor and mental flexibility returns, their voices become more authentic, hobbies are more interesting, their ability to access life’s pleasures come back, boundaries become easier to set and relationships richer, and their genuine paths in life often become clearer.

4. Suppressed metabolism

Another common side effect of undereating is a decreased basal metabolic rate (BMR), or the amount of energy your body needs to perform basic functioning. The simplest way to think about this is that your body is focused on survival, so if you’re not adequately fueling your body, it will slow down all of its processes in order to conserve energy and survive with less energy. “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.

5. Reproductive disturbances

Amenorrhea (or loss of period for more than three months) was my body’s main indicator of not yet being fully recovered from my own eating disorder. It’s a common side effect of undereating and can lead to a myriad of issues, including infertility and osteoporosis. Food restriction, anxiety, depression, emotional stress, excessive exercise, energy imbalance, and malnutrition are associated with development of amenorrhea, according to The Journal of Clinical Endocrinology & Metabolism.

One 2022 research review concluded that low energy availability from undereating can suppress all reproductive hormones. “In females, levels of estradiol and progesterone—two important reproductive hormones—decrease. Estradiol is responsible for bone growth and remodeling, so low estradiol levels can quickly diminish bone density,” Burkholder explains. “In males, low energy availability disturbs testosterone levels and leads to fatigue, low sex drive, sexual dysfunction, and low bone mineral density.” Essentially, when you don’t eat enough, your body prioritizes vital functions, like heartbeat and lung expansion, and shuts down processes it deems non-essential. “Your body downregulates to conserve as many calories as possible, and that means forfeiting reproductive function,” Burkholder adds.

6. Nutrient deficiencies & muscle breakdown

Another serious side effect of undereating is the development of nutrient deficiencies, which can lead to health conditions like anemia (often a result of iron deficiency), cold intolerance, hair loss, skin problems, insomnia, bone issues, and a weakened immune system. “Your immune system will be depressed, causing an increase in infection, cold and flu or other illnesses,” says Reeves. “Your heart rate slows, there’s less oxygen flow, dizziness and fatigue.”

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. “Your muscles will get broken down faster because there are neither the energy nor the building blocks—amino acids and protein—available to maintain itself,” Reeves explains. Since the heart is largely made of muscular tissue, it will also start to weaken with chronic underfueling, 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.” And without enough blood sugar circulating in your blood, which can occur when you’re regularly undereating and have depleted all the glucose stores in your liver and muscles, you can experience hypoglycemia, which can also be deadly. 

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.

7. Digestive issues

Grimes explains that in addition to affecting heart tissue, malnutrition can result in the breakdown of tissue in the digestive tract, which can, over time, lead to a weakening of muscles in the stomach and intestines. In fact, research shows malnutrition and weight loss from undereating can slow down the gastrointestinal (GI) tract, which can cause general GI dysfunction, as well as issues like gastroparesis (or slowed movement or motility of stomach muscles), constipation, reflux, heartburn, bloating, and diarrhea. Reeves explains that undereating will also decrease the amount of enzymes your body makes that help break down food, leading to further GI symptoms. In my practice, I’ve observed GI disturbances from undereating that cause significant disruptions in clients’ day-to-day functioning and overall quality of life.

8. Feeling out of control

Circling back to the landmark study I mentioned, the men reported that once the restriction period ended, they felt an initial loss of control around food during the refeeding period. Some of them reported wanting 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. Additionally, undereating and restriction can drive the development of a full-blown eating disorder.

“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,” Burkholder says. “Others may find their chronic deficit has deeper roots: body image concerns, pressure from family members or coaches to maintain a low weight, or an internalized list of rigid food rules.” Also, several of my clients presented with undereating that stems from trauma, and they’re using restriction to cope, numb, disassociate, and indirectly express hard feelings.

How to stop underfueling your body

The effects of not eating enough are widespread and serious. But there’s good news: most can be reversed with adequate nourishment. Since I stopped undereating and now live within energy balance, my anxiety is manageable, I feel generally more at ease, my focus and ability to stay present with others is good, I have ample brain space for life outside of food, and my reproductive system is fully functioning.

However, it’s tough to get there on your own. Establishing or re-establishing energy balance and reversing the effects of undereating usually takes professional guidance from a team of providers (usually 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' is not going to give you the personal guidance you need to thrive.”

If you’re concerned that you’re not eating enough, we’re here to help. Reach out to our team for a free consultation.

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Caroline Young
Contributing Writer, RD
Clinically reviewed by:
Erin Reeves, RD
Director of Nutrition at Equip
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