Achieving and maintaining a healthy weight can be challenging, especially in a modern society where food is constantly available.
However, not eating enough calories can also be a concern, whether it's due to intentional food restriction, decreased appetite or other reasons.
In fact, under-eating on a regular basis can lead to a number of mental, physical and emotional health issues. Here are 9 signs that you're not eating enough.
Calories are units of energy your body uses to function.
When you don't eat enough calories, you're likely to feel tired most of the time.
The number of calories needed for these basic functions within a 24-hour period is referred to as your resting metabolic rate.
Most people have a resting metabolic rate higher than 1,000 calories per day. Adding physical activity can increase your daily needs by another 1,000 calories or more.
Although hormones also play a role in energy balance, generally if you take in more calories than needed, you will store most of the excess as fat. If you take in fewer calories than needed, you will lose weight.
Restricting intake to fewer than 1,000 calories daily can slow down your metabolic rate and lead to fatigue since you're not taking in enough calories to support even the basic functions that keep you alive.
Eating too little has particularly been linked to low energy levels in older people, whose food intake may decrease due to reduced appetite (1).
Other studies in female athletes have found that fatigue may occur when calorie intake is too low to support a high level of physical activity. This seems to be most common in sports that emphasize thinness, like gymnastics and figure skating (2, 3).
Yet even light physical activity like walking or taking the stairs may cause you to tire easily if your calorie intake is well below your needs.
Summary: Eating too few calories can lead to fatigue due to insufficient energy to exercise or perform movement beyond basic functions.
Losing hair can be very distressing.
It's normal to lose several strands of hair daily. However, if you're noticing an increased amount of hair accumulating in your hairbrush or shower drain, it may be a sign that you're not eating enough.
Many nutrients are needed to maintain normal, healthy hair growth.
Basically, when you don't take in enough calories and key nutrients, your body will prioritize the health of your heart, brain and other organs over hair growth.
Summary: Hair loss may occur as a result of inadequate intake of calories, protein and certain vitamins and minerals.
Being hungry all the time is one of the more obvious signs that you're not eating enough food.
One three-month study followed mice who were fed a diet containing 40% fewer calories than usual.
In humans, calorie restriction may cause hunger and food cravings in both normal-weight and overweight individuals.
In a study of 58 adults, consuming a 40%-calorie-restricted diet increased hunger levels by about 18% (10).
Essentially, if your calorie intake drops too much, your body will send signals that drive you to eat in order to avoid potential starvation.
Summary: Undereating can cause hormonal shifts that increase hunger in order to compensate for inadequate calorie and nutrient intake.
Undereating may interfere with a woman's ability to become pregnant.
The hypothalamus and pituitary gland located in your brain work together to maintain hormonal balance, including reproductive health.
The hypothalamus receives signals from your body that let it know when hormone levels need to be adjusted.
Based on the signals it receives, the hypothalamus produces hormones that either stimulate or inhibit production of estrogen, progesterone and other hormones by your pituitary gland.
Research has shown that this complex system is highly sensitive to changes in calorie intake and weight (12).
When your calorie intake or body fat percentage drops too low, signals may become impaired, leading to changes in the amount of hormones released.
Without the proper balance of reproductive hormones, pregnancy cannot take place. The first sign of this is hypothalamic amenorrhea, or having no menstrual period for three months or longer (15).
In an older study, when 36 underweight women with amenorrhea or infertility related to calorie restriction increased their calorie intake and achieved ideal body weight, 90% began menstruating and 73% became pregnant (16).
If you are trying to conceive, make sure to consume a well-balanced, adequate-calorie diet in order to ensure proper hormonal function and a healthy pregnancy.
Summary: Consuming too few calories can disrupt reproductive hormone signals, leading to difficulty getting pregnant.
Sleep deprivation has been found to lead to insulin resistance and weight gain in dozens of studies (17).
In addition, while overeating may cause sleeping difficulty, it appears that strict dieting can lead to sleep problems as well.
Animal and human research has shown that starvation-level calorie restriction leads to sleep interruptions and a reduction in slow-wave sleep, also known as deep sleep (18).
In one study of 381 college students, restrictive diets and other eating problems were linked to poor sleep quality and low mood (19).
In another small study of 10 young women, four weeks of dieting led to greater difficulty falling asleep and a decrease in the amount of time spent in deep sleep (20).
Feeling as though you are too hungry to fall asleep or waking up hungry are major signs that you're not getting enough to eat.
Summary: Undereating has been linked to poor quality sleep, including taking longer to fall asleep and spending less time in deep sleep.
If little things have begun to set you off, it could be related to not eating enough.
Indeed, irritability was one of several issues experienced by young men who underwent calorie restriction as part of the Minnesota Starvation Experiment during World War II (21).
These men developed moodiness and other symptoms while consuming an average of 1,800 calories per day, which was classified as "semi-starvation" for their own calorie needs. Your own needs may be lower, of course.
A more recent study of 413 college and high school students also found that irritability was associated with dieting and restrictive eating patterns (22).
To keep your mood on an even keel, don't let your calories drop too low.
Summary: Prolonged low calorie intake and restrictive eating patterns have been linked to irritability and moodiness.
If you constantly feel cold, not eating enough food could be the cause.
Your body needs to burn a certain number of calories in order to create heat and maintain a healthy, comfortable body temperature.
In fact, even mild calorie restriction has been shown to lower core body temperature.
In a six-year controlled study of 72 middle-aged adults, those who consumed an average of 1,769 calories daily had significantly lower body temperatures than the groups who consumed 2,300–2,900 calories, regardless of physical activity (23).
In a separate analysis of the same study, the calorie-restricted group experienced a decrease in T3 thyroid hormone levels, whereas the other groups did not. T3 is a hormone that helps maintain body temperature, among other functions (24).
In another study of 15 obese women, T3 levels decreased by as much as 66% during an eight-week period in which the women consumed only 400 calories per day (25).
Overall, the more severely you slash calories, the colder you're likely to feel.
Summary: Consuming too few calories can lead to a decrease in body temperature, which may be due in part to lower levels of T3 thyroid hormone.
Infrequent bowel movements may be related to inadequate calorie intake.
This isn't surprising, since consuming very little food will result in less waste in your digestive tract.
Constipation is typically described as having three or fewer bowel movements per week or having small, hard stools that are difficult to pass. This is very common in older people and can be worsened by poor diet.
One small study of 18 older adults found that constipation occurred most often in those who didn't consume enough calories. This was true even if they got plenty of fiber, often considered the most important factor for proper bowel function (26).
Dieting and eating too little food may also cause constipation in younger people due to a slowed metabolic rate.
In a study of 301 college-aged women, the strictest dieters were most likely to have constipation and other digestive problems (27).
If you're having problems with regularity, it's important to take a look at the amount of food that you're eating and evaluate whether you're getting enough.
Summary: Strict dieting and under-eating can lead to constipation, partly due to less waste product to form stool and slower movement of food through the digestive tract.
Although dieting itself may lead to moodiness, outright anxiety can occur in response to very low calorie intake.
In a large study of more than 2,500 Australian teens, 62% of those who were classified as "extreme dieters" reported high levels of depression and anxiety (28).
Anxiety has also been observed in overweight people who eat very low-calorie diets.
In a controlled study of 67 obese people who ate either 400 or 800 calories per day for one to three months, roughly 20% of people in both groups reported increased anxiety (29).
To minimize anxiety while trying to lose weight, make sure you're consuming enough calories and eating a healthy diet that includes plenty of fatty fish to ensure you're getting omega-3 fatty acids, which may help reduce anxiety (30).
Summary: Very low calorie intake may lead to moodiness, anxiety and depression in teens and adults.
Although overeating increases the risk of developing health problems, under-eating can also be problematic.
This is especially true with severe or chronic calorie restriction. Instead, to lose weight sustainably, make sure to eat at least 1,200 calories per day.
Additionally, be on the lookout for these 9 signs that you may need more food than you're currently taking in.