If you haven’t been eating enough, it could be because you’ve become more active and haven’t upped your calories to match, or perhaps you’re trying to lose weight but have restricted your caloric intake too much. Maybe you’re dealing with disordered eating, or skipping meals because you’ve been so busy or stressed out. Whatever the reason, there can be serious consequences for your health when the body doesn’t get enough of the nutrients it needs.

You’ve probably heard that you should never eat fewer than 1,200 calories a day. That’s deceptive advice, because it implies that a 1,200-calorie diet is enough to meet everyone’s basic nutritional needs, which just isn’t true. Even if you’re dieting, 1,200 calories likely isn’t enough, especially if you’re active. According to the 2015–2020 Dietary Guidelines for Americans, even sedentary adult women need 1,600–2,000 calories a day to maintain optimal health, while sedentary men need 2,000–2,600. Whether you’re intentionally trying to cut calories or just not paying attention to your caloric needs, there are a number of ways your body will let you know that you aren’t eating enough.

If you are struggling with an eating disorder, or know someone who is, help is available. Visit the National Eating Disorders Association (NEDA) website or contact NEDA’s Live Helpline at 1-800-931-2237. You can also receive 24/7 Crisis Support via text (text NEDA to 741741).

You’re probably not eating enough if you’re constantly hungry

One of the most obvious signs that you aren’t eating enough is constant, ravenous hunger. Being in a caloric deficit triggers a number of physiological responses that increase hunger. One 2010 study found that participants who ate a 1,200-calorie diet for three weeks had elevated levels of cortisol. Cortisol is a stress hormone that increases appetite.

Another study found that ghrelin levels change following exercise and calorie restriction. Ghrelin is considered the "hunger hormone," and individuals’ baseline ghrelin levels dictate how large or small a person’s appetite generally is. Ghrelin also fluctuates throughout the day, tending to peak around mealtimes and drop during the night. The researchers discovered that being in an energy deficit caused participants’ baseline ghrelin levels to rise. The mealtime peaks became more pronounced, and nighttime levels were also higher. From a survival standpoint it makes sense: When you aren’t getting enough calories, your body ups the hormone most likely to drive you to eat more.

Research also indicates that calorie deprivation leads to stronger food cravings, greater preoccupation with food, and increases in hedonic hunger (the desire to eat for pleasure rather than because of physical hunger).

You may not be eating enough if you feel like you have no energy

Everything you do — even reading this sentence — requires your body to expend energy. This energy comes from the calories in carbs, fat, and protein. So it’s not surprising that eating too few calories will leave you running on empty and feeling sluggish. According to the Mayo Clinic, "metabolism is the process by which your body converts what you eat and drink into energy. During this complex process, calories in food and beverages are combined with oxygen to release the energy your body needs to function."

Your metabolism is made up of four components. Basal metabolic rate (BMR) refers to the calories burned just keeping your body alive (breathing, controlling body temperature, repairing cells, et cetera). We also burn calories in the process of digesting and absorbing food. Incidental movements like walking around your house or picking up a book also require calories. And, of course, physical activities like hiking or playing tennis burn calories.

The calories needed for BMR and digestion are essential for survival, so that’s what your body prioritizes. If you aren’t eating enough, you simply won’t have enough fuel in the tank to spare as you go about your daily activities.

If you’re always cold, you may not be eating enough

If you’re piling on sweaters in July, it may mean you’re not getting enough calories. One 2011 study found that long-term calorie restriction with adequate nutrition reduced individuals’ core body temperature. Although a lower core body temperature may be one of the reasons calorie restriction could lead to greater longevity, "running cold" can make you miserable.

A caloric deficit lowers your body temperature because it decreases the amount of T3 hormone secreted by your thyroid. In addition to overseeing metabolism, thyroid hormones regulate body temperature. In a study published in the International Journal of Obesity, participants were divided into two groups. One group consumed 1,200 calories daily for the entire 18 weeks of the experiment, while the other group ate 1,200 calories daily for four weeks, then consumed only 400 calories a day for eight weeks, and finally slowly worked up to 1,200 calories over the last four weeks. For those in the first group, T3 levels dropped by as much as 40 percent. In the second group, T3 levels plummeted by as much as 66 percent during the 400-calorie phase of the study. Even during the final refeeding phase, T3 levels remained about 22 percent below baseline.

Getting sick all the time could be an indication that you’re not eating enough

When you drastically under-consume calories, you rob your immune system of the fuel it needs to do its job. And as a result, you may find yourself catching colds often. In a 2018 paper published in Geroscience, researchers noted that long-term calorie restriction, even with adequate nutrition, led to "markedly lower circulating levels of total leukocytes, neutrophils, lymphocytes, and monocytes." These are white blood cells that perform a variety of important tasks, including identifying and destroying invading organisms, managing inflammation, and clearing away dead or damaged cells. Calorie restriction also causes lymph tissue to temporarily atrophy.

According to the Cleveland Clinic, our lymphatic system is "a network of tissues, vessels and organs that work together to move a colorless, watery fluid called lymph back into your circulatory system." Considered part of the immune system, the lymphatic system produces lymphocytes (a type of white blood cell), helps clear damaged or abnormal cells, and maintains fluid balance. Research in mice has shown that long-term calorie restriction decreases lymphocyte numbers by as much as 75 percent, leaving the mice particularly vulnerable to bacterial infections.

You may not be eating enough if you’re having trouble sleeping

Sleep-related problems affect approximately 50 to 70 million Americans, with 11 percent of adults reporting they get insufficient sleep every night. Sleep issues can be caused by a number of factors, including physical and psychological health conditions, medication use, lifestyle factors, and how much we eat (or don’t eat).

One paper, published in Sleep Medicine Reviews, examined the sleeping patterns of individuals with eating disorders. They found that, especially among individuals with anorexia nervosa who heavily restricted their calorie intake, sleep became more fragmented throughout the night. Malnutrition as a result of extreme calorie restriction also led to a decrease in slow-wave sleep (the deepest and most restorative part of the sleep cycle). Additionally, a study published in the American Journal of Clinical Nutrition found that after only a month of restricted calorie intake, participants (who were all classified as overweight) had increased sleep problems. In addition to reduced slow-wave sleep, it also took them longer to fall asleep at night.

If you’re irritable all the time, you’re probably not eating enough

Do you constantly feel like you could practically kill someone for a donut? We often joke about being "hangry" (a clever mashup of "hungry" and "angry"), but it’s a real phenomenon. In an interview with the Cleveland Clinic, Dr. Christine Lee, a gastroenterologist, explained that when we haven’t eaten in a while, our blood sugar drops, which triggers the release of cortisol (a stress hormone). "The release of cortisol can cause aggression in some people," Lee revealed. "Also, low blood sugar may interfere with higher brain functions, such as those that help us control impulses and regulate our primitive drives and behavior."

In an interview with Shape magazine, Dr. Gary L. Wenk, a psychology and neuroscience professor at The Ohio State University, outlined how depriving yourself of calories can cause fluctuations in the neurotransmitter serotonin, which can lead to drastic mood swings. In addition to the problems that directly stem from not eating enough, even the act of saying "no" to tempting foods can have negative consequences for your mood. Along with exhausting your willpower, exerting self-control to keep yourself away from food can lower your blood sugar. And what’s one telltale sign of hypoglycemia (low blood sugar)? You guessed it: irritability.

Constantly anxious? You might not be eating enough

If you aren’t usually an anxious person but find yourself on edge while dieting, it’s probably a sign you’re taking calorie restriction too far. A study published in the Journal of Child Psychology and Psychiatry explored the link between dieting behavior and mental health in teens. Researchers found that 62 percent of those classified as "extreme dieters" had high levels of both depression and anxiety.

Another study, published in 2006 in the Journal of Endocrinological Investigation, placed obese individuals on extreme low-calorie diets for three months to evaluate their effectiveness and safety. One group ate only 458 calories a day, while another ate 800 calories. In addition to several individuals experiencing serious complications from the diet (ministroke and AFib), approximately 20 percent of participants across both groups experienced anxiety.

When it comes to anxiety, the degree of caloric deprivation may make a big difference. In a 2014 study published in Physiological Reports, rats fed at a 25 percent calorie reduction showed fewer signs of anxiety. The researchers concluded that mild calorie restriction has anti-anxiety properties and is not severe enough to negatively affect the body’s stress response. It’s still unclear, however, if these findings can be applied to humans.

Not eating enough can lead to brain fog

When people complain of brain fog, what exactly are they experiencing? According to a 2019 article in Patient, "Brain fog is a general term for a set of symptoms affecting the cognitive processes. It isn’t a medical condition in itself, but rather occurs as common feature of other conditions." And one of these underlying conditions is inadequate calorie intake. Brain fog can be frustrating as it causes problems with memory, information processing, concentration, higher-level thought, and speaking to or understanding others.

It’s not too surprising that undereating can cause mental fuzziness, given how calorie-intensive brainpower is. Although the brain makes up only about 2 percent of our body weight, it demands approximately 20 percent of our resting metabolic rate (the calories we burn just keeping our bodies alive), according to Scientific American. More advanced cognitive power, such as what’s needed to solve a tricky math problem, does require more calories, although not substantially more. Our decision-making abilities may be particularly susceptible to the effects of calorie deprivation. Research suggests that when we’re hungry, we’re more likely to prioritize instant gratification over long-term benefits.

Consipation can be a sign that you’re not eating enough

Almost everyone has an occasional bout of constipation, but if you’re regularly having difficulty passing stool or going three or more days between number twos, it may be a sign you’re overly restricting your calories. Although most discussions about constipation tend to center around the importance of fiber, the simple fact is that when you don’t eat a lot of food, there’s less waste and it moves more slowly through your digestive tract.

One study of college women, published in the Journal of American College Health, found that individuals who engaged in the most extreme dieting had the most number of gastrointestinal complaints, including constipation. In another study, which focused on constipation in older adults, researchers discovered that constipated subjects ate fewer meals and fewer calories than non-constipated seniors, even though their fiber and fluid intake was the same.

Speaking of fluids, dehydration is another common cause of constipation. And because approximately 37 percent of people regularly misinterpret thirst as hunger, dieters may be at greater risk of not getting enough liquids. If you’re regularly ignoring hunger pangs, you may be inadvertently ignoring your body’s demands for water as well.

You may not be eating enough if your period has disappeared

Menstrual cycles can be irregular or absent for a variety of reasons, including not eating enough calories. Undereating can cause a condition known as hypothalamic amenorrhea (HA), in which the hypothalamus "turns off" ovulation and menstruation in response to signals that the body doesn’t have enough calories to spare for reproduction. HA is commonly seen in women with eating disorders. In fact, until its removal in the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) in 2013, absence of three consecutive periods was a mandatory criteria for diagnosing a woman with anorexia nervosa.

A 2015 study published in the American Journal of Physiology, Endocrinology and Metabolism attempted to quantify how much of a caloric deficit would negatively affect menstruation. The menstrual anomalies tracked included luteal phase defects (problems with the growth of the uterine lining after ovulation), anovulation (lack of ovulation), and oligomenorrhea (a cycle lasting more than 35 days). The researchers found that frequency of menstrual issues was directly correlated to the degree of caloric deficit. Approximately 22 percent of participants with a daily deficit of 470 calories experienced menstrual issues, while 42 percent of those underconsuming by 810 calories had problems with their periods.

If you’re having trouble getting pregnant, you might not be eating enough

Although it’s a topic many feel embarrassed to talk about, infertility is very common. According to the National Institutes of Health, approximately 11 percent of women and 9 percent of men have experienced fertility issues. Infertility can have many causes, including undereating.

Carrying a fetus to term and breastfeeding for a number of months afterward require a lot of calories. As an article published in Ageing Research Reviews noted, "Reproduction is a physiologically costly process that consumes significant amounts of energy, and the mechanisms controlling energy balance are therefore closely linked to fertility. This close relationship ensures that pregnancy and lactation occur only in favorable conditions with respect to energy." The authors explained that when the pituitary gland and hypothalamus receive signals that the body doesn’t have enough calories to spare, they lower levels of important reproductive hormones to ensure conception and pregnancy don’t happen.

Severely undereating can impair men’s fertility as well. According to a 2019 paper published in Nutrients, "Calorie restriction can have adverse effects on male reproduction, including reductions in sperm production, testis size, diameter of seminiferous tubules and sperm quality."

Not eating enough can cause your hair to fall out

Although everyone wants a luxurious, flowing mane, hair is pretty superfluous, biologically speaking. Because it isn’t necessary for survival, hair only gets nutritional resources when we have some to spare. If you’re undereating to the point of nutritional deficiencies, your hair is likely to be one of the first places you’ll see the effects. As the authors of a 2013 article in Dermatologic Clinics explained, "A caloric deprivation or deficiency … can lead to structural abnormalities, pigmentation changes, or hair loss."

And there are many deficiencies that can wreak havoc on your hair. According to one 2017 paper, the most common deficiencies that lead to hair loss include protein, iron, zinc, niacin (vitamin B3), selenium, folic acid, biotin, and vitamins A, D, and E. The authors also noted that hair follicles are very metabolically active, meaning they require a lot of energy to nourish a growing hair shaft. If your body doesn’t have the calories to spare, both hair structure and growth may be affected.

If your nails are discolored or break easily, you’re probably not eating enough

"Both [hair and nails] are a barometer of how well (or how poorly) you’re feeding the body, as well as your overall health," dermatologist Jessica Wu told HuffPost. The lack of adequate amounts of certain micronutrients in a heavily calorie-restricted diet can lead to very specific changes in the fingernails.

In a 2015 article for Podiatry Today, Dr. Kristine Hoffman outlined a number of these. Biotin deficiency, for instance, can make your nails more prone to fungal infections. If your fingernails are a brown-gray color, this may be a sign that you’re not getting enough vitamin B12. Iron-deficiency anemia can lead to nails that are white, brittle, have a sunken-in spoon shape, or have a central groove or ridge. If you notice a central ridge on your nails, this can be caused by inadequate protein or folic acid intake. A lack of protein, especially sulfur-containing amino acids, can also cause spoon-shaped nails, whereas Beau’s lines, a series of horizontal depressions on the nail, may be the result of severe zinc deficiency.

If you’re not eating enough, your metabolism has probably slowed down

You may have heard that eating too few calories will put your body into starvation mode. According to Healthline, "what people generally refer to as ‘starvation mode’ (and sometimes ‘metabolic damage’) is your body’s natural response to long-term calorie restriction." The technical term for this response is "adaptive thermogenesis." When you consistently fail to meet your calorie needs, your body slows your metabolism as a protective measure to try to maintain your current weight.

A study published in the American Journal of Clinical Nutrition found that caloric restriction among obese participants led to a drop in 24-hour energy expenditure of approximately 1,500 kilojoules (358 calories). About half of this was due to a decrease in resting metabolic rate (the calories your body uses to fuel basic functions like breathing and tissue repair). The other half was attributed to a combination of a lower thermic effect of food (the calories your body burns digesting food) and a drop in the amount of calories participants burned during exercise as they lost weight. Unfortunately, research shows that this slowed metabolism can last long after calorie restriction ends, and it may be a contributing factor of post-diet weight gain.