Protein is vital to the human body. Among other things, it helps build and repair tissue, supports the immune system, balances body fluids, and helps transport oxygen through the body. Protein is made from amino acids, of which there are more than 20. However, the human body isn’t capable of manufacturing nine of those amino acids, which are known as essential amino acids. To keep your body running like a well-oiled machine, your only real choice is to eat foods containing these essential amino acids. And that, of course, means eating protein.

The recommended daily requirement for protein is 7 grams for every 20 pounds of body weight, according to the National Academy of Medicine (via The Nutrition Source). Since protein is found in many foods, both animal-based and plant-based, it is generally not difficult to meet this goal. However, certain weight-loss plans can inadvertently lead to restricting protein intake below healthy levels. Additionally, a number of health conditions may make it harder to maintain adequate protein levels. These include gluten intolerance and inflammatory bowel syndrome. Here’s a look at just what can happen to you if you’re not getting enough protein.

If you’re feeling hungry all the time, it could mean you’re not getting enough protein

One of the more obvious signs that you’re not getting enough protein is an appetite that just won’t quit, Dr. Nicole Avena, research neuroscientist and nutrition expert, told Health Digest. One reason is that eating protein is associated with feelings of satiety. A 2011 study published in the journal, Obesity, revealed that overweight men who were fed diets high in protein reported experiencing greater feelings of fullness throughout the day and less of a desire to eat at night than their peers who were fed lower-protein diets.

According to Dr. Avena, the reason for this may be that protein has a more gentle effect on blood sugar than some other macronutrients, thus avoiding the sharp rises and dips in blood sugar that can lead to feelings of hunger. In fact, a 2018 study published in the Journal of Dairy Science demonstrated that a high-protein breakfast was linked to lower blood sugar levels and reduced appetite later in the day.

If you’re getting less muscular, you might not be getting enough protein

If you’re regularly performing resistance exercise (which is known to promote muscle growth) but are finding your results to be less than optimal or you’re actually losing muscle mass, it could be because you’re not including enough protein in your diet, a study published in the University of Stirling Online Research Repository suggested.

Indeed, protein is vital to building and maintaining muscles, according to Alicia Galvin, resident dietitian for Sovereign Laboratories. Galvin’s view is supported by numerous studies demonstrating that diets higher in protein are associated with greater muscle mass, whereas diets lower in protein are associated with muscle loss.

As to why that may be, registered dietitian Kristin Gillespie explained to Health Digest that protein is so critical to the human body that when protein needs are not met through dietary sources, the body will start seeking protein from the muscles, with the effect being decreased muscle mass.

If you’re putting on fat, especially around the belly, you might not be getting enough protein

If you’re on a low-protein weight-loss plan, and you’re frustrated because the scale refuses to budge, or if you’ve noticed you’re actually gaining weight — especially in your belly — a shortage of dietary protein could be to blame. According to registered dietitian and health coach Cassie Christopher, greater fluctuations in blood sugar levels have been associated with weight gain and difficulty in losing weight, and eating enough protein is important for stabilizing blood sugar levels.

In addition, several studies cited by Healthline demonstrate that higher protein intake can increase levels of hunger-suppressing hormones while reducing levels of hunger-inducing hormones, and appetite suppression is associated with improved body weight management. Finally, your body expends more calories to digest protein than it does when you consume other macronutrients such as carbohydrates, and that "thermic" effect can continue long after you’ve digested any one particular high-protein meal. In fact, various studies, including one published in The Journal of the American College of Nutrition, indicate that higher protein intake can boost the metabolism, including the number of calories burned per day overall.

Not eating enough protein can lead to noticeable fatigue

If you’ve been feeling exhausted as of late, you might want to consider taking a look at your diet. It could be that you’re not eating sufficient protein, Alicia Galvin, resident dietitian for Sovereign Laboratories, told Health Digest. Although a single low-protein meal is unlikely to make a significant difference, not getting enough protein over time can lead to a loss of muscle mass. And a decrease in muscle mass can, in turn, leave you feeling sluggish. It can even make even the most basic of physical tasks, such as maintaining your balance, feel more difficult, Leslie Bonci, nutritionist and nutrition consultant for the Kansas City Chiefs, told Health.

Preserving muscle mass becomes increasingly important as you get older because muscle loss is a normal part of aging (this is known as sarcopenia). It may also be important to those already experiencing fatigue from certain conditions, like cancer.

Your tiredness could be related to anemia, which can be caused by not getting enough protein

Not getting enough protein can result in mild to moderate anemia. Anemia is a condition in which the blood does not have enough healthy red blood cells. Healthy red blood cells are needed for many reasons, not the least of which is carrying oxygen to the body’s tissues and organs. "As a result, it’s common to feel cold and symptoms of tiredness or weakness," the Cleveland Clinic explained.

Anemia caused by protein deficiency is most frequently seen in vegans, vegetarians, and others who tend to be prone to lower protein intake and may be complicated by other deficiencies associated with restricting animal protein, including vitamin B12, iron, and folic acid. The marginal lack of oxygen caused by anemia related to protein deficiency can leave you not only feeling fatigued and weak but also short of breath, Leslie Bonci, nutritionist and nutrition consultant for the Kansas City Chiefs, confirmed to Health.

You might want to increase your protein intake if you’re having trouble sleeping

How much protein you consume can have a significant effect on how well you sleep. A 2013 study published in the Journal of Epidemiology demonstrated that lower protein intake (which was considered to be less than 16 percent of total calories) was associated with poorer quality sleep. To a lesser extent, this same study also demonstrated that low protein intake can be associated with greater difficulty in falling asleep. Conversely, other studies have shown a clear positive relationship between levels of dietary protein and the ability to fall asleep more quickly.

It’s been suggested that what is behind protein’s ability to support sleep is tryptophan, an essential amino acid that the body cannot manufacture itself and therefore must be consumed in food or supplements. Tryptophan is essential to creating the neurotransmitter serotonin, which is needed to create melatonin. This hormone, which helps the body to make sense of environmental lighting, helps regulate the sleep/wake cycle, according to Jennifer Henry, director of the Counseling Center at Maryville University, who told Health Digest that reductions in melatonin lead to difficulty falling and staying asleep.

Depression can be a sign of protein deficiency

Another potential consequence of not getting enough protein is depression, Alicia Galvin, resident dietitian for Sovereign Laboratories, explained in an interview with Health Digest. According to a 2008 study published in the Indian Journal of Psychiatry, protein intake is critical to brain function and mental health because many of the brain’s neurotransmitters are made from amino acids that the body does not produce on its own and therefore must be supplied through diet. Lower levels of some of these neurotransmitters, including serotonin, dopamine, noradrenaline, and gamma-Aminobutyric acid (GABA) have been linked to depression.

The amino acid tryptophan, is essential to the body’s production of seratonin, which is often referred to as the "feel-good hormone" — and for good reason. A 2007 report published in the Journal of Psychiatry & Neuroscience revealed that lower blood levels of serotonin are also associated with "lower mood" whereas higher blood serotonin levels are associated with "better mood." In addition, not getting enough protein can lead to sleep disturbances, and sleep deprivation is associated with an increase in depressive symptoms.

You might not be getting enough protein if you’ve been feeling unusually anxious lately

If you’re not getting enough protein, you may actually experience increased anxiety levels, according to certified holistic nutritionist and hypnotherapist Melissa Kathryn. One reason is that the neurotransmitter dopamine, which has been associated with decreased anxiety levels, is made from essential amino acids that cannot be manufactured by the body and must be supplied through diet (via The Nutrition Source).

Getting adequate protein can not only supply you with dopamine, but it can also help keep blood sugar levels stable, which can also be important in managing anxiety. A drop in blood sugar ups production of the fight-or-flight hormone, adrenaline, which can lead to feelings of anxiety, according to WebMD. And when your adrenaline levels rise, you will likely experience some of the physical symptoms that are frequently associated with anxiety, including sweaty palms and heart palpitations, which can, in turn, exacerbate anxiety you were already experiencing — a truly vicious cycle. You may also misattribute the source of those physical symptoms to anxiety, as opposed to blood sugar fluctuations.

If you’ve been having trouble focusing, it could mean you’re not getting enough protein

"If you’ve been having trouble staying focused," certified holistic nutritionist Melissa Kathryn told Health Digest, "adding more lean protein to your meals could make a difference." Just as protein intake can have an impact on your mood, it can also have an impact on your ability to focus. The reason is that many of the brain’s neurotransmitters are made from essential amino acids that you need to obtain through food sources, as a report in the Indian Journal of Psychiatry explained.

Those neurotransmitters include serotonin and dopamine, both of which the body cannot manufacture without the amino acid, tryptophan. In fact, a 2011 study published in Behavioral and Brain Functions found that children with attention-deficit/hyperactivity disorder (ADHD) have nearly 50 percent lower blood levels of tryptophan. Another study from the same year revealed that out of 85 children (ages 4-18) with ADHD who were treated with a drug protocol consisting of "amino acid precursors of serotonin and dopamine," 67 percent achieved "significant improvement" to their ADHD symptoms.

If you’re not getting enough protein, your hair won’t be able to hide it

Inadequate protein intake can lead to hair loss. The reason? Alicia Galvin, resident dietitian for Sovereign Laboratories, explained to Health Digest that hair follicles are made mostly of protein. However, protein is needed to support virtually every aspect of the human body, including vital functions such as respiration. So when protein is in short supply, the body’ diverts whatever protein sources it may have away from supporting lush hair and toward supporting more vital bodily functions.

According to research neuroscientist and nutrition expert Nicole Avena, low protein intake is also associated with lower blood levels of the B vitamin, biotin. This is yet another reason that hair loss is associated with inadequate protein intake. This vitamin, which is found in meats (including organ meats), eggs, fish, meat, seeds, nuts, and certain vegetables, supports hair growth. In fact, it’s been given to both men and women hoping to reduce hair loss, dermatologist Wilma Bergfeld told the Cleveland Clinic.

Your nails and skin may also suffer if you’re not getting enough protein

Our fingernails and toenails are made largely from a protein called keratin. And just as protein deficiency can lead to hair loss, it can also lead to nail issues, including weakness, brittleness, and deformities such as deep ridges, registered dietitian Kristin Gillespie confirmed in an interview with Health Digest.

Our skin is also made largely of protein. Collagen is the most plentiful, followed by elastin and keratin. Since the skin is the largest of the body’s organs, and since the entire skin regenerates itself once every approximately 27 days, healthy skin depends upon adequate protein intake (via Cleveland Clinic).

Protein not only supports building and repairing the skin, but it also supports the skin’s ability to fight infection. In fact, one of the more serious problems that may arise when you’re not getting enough protein is that wound healing becomes more difficult (via University of Michigan).

Swollen ankles? It’s time to eat more protein

Adequate protein intake is vital to maintaining proper fluid balance in the body, according to certified holistic nutritionist Melissa Kathryn. The blood contains a number of different forms of protein, but when it comes to regulating fluid balance, the most important is albumin. Albumin works as a "water magnet," holding water and other fluids in the blood vessels. When blood levels of albumen become low, as can happen with inadequate protein intake, those fluids can leak out of the blood vessels. With nowhere else to go, they collect in other tissues.

The presence of fluid in tissues where it does not belong is known clinically as edema although you might recognize it as "swelling." Edema has many causes, of which protein deficiency is just one. However, it is actually one of the most common symptoms of protein deficiency, registered dietitian Kristin Gillespie told Health Digest.

If you’ve recently broken a bone, consider your protein intake

Our bones are made up of about 50 percent protein, and consuming enough dietary protein is critical to keeping them healthy, registered dietitian Cassie Christopher told Health Digest. Conversely, if you’re not getting enough protein in your diet, you may be at higher risk for bone breaks and fractures. This may seem surprising if your mind goes right to calcium when you think of bone health.

Calcium is, of course, critical to bone health. But for the bones to enjoy the benefits of dietary calcium, the intestines have to be able to properly absorb it. Getting adequate dietary protein is critical to intestinal absorption of calcium, according to a 2003 study published in the Journal of Nutrition, which demonstrated that inadequate dietary protein intake is associated with reduced calcium absorption. Over time, this can result in decreased bone density and higher rates of bone loss.

If you’ve been getting sick often, a protein deficiency may be to blame

Food proteins are known to play a critical role in the development of a healthy immune system. This is because antibodies that make up your immune system (aka immunoglobulins) are made up of proteins. And to be manufactured by the body effectively, they require dietary protein, Alicia Galvin, resident dietitian for Sovereign Laboratories, told Health Digest. Conversely, when you don’t take in enough dietary protein, your body has a harder time making manufacturing immunoglobulins, with the result being a less effective immune system.

That is why when you’re not getting enough protein in your diet, you might find yourself coming down with more colds, according to registered dietitian Cassie Christopher, who also told Health Digest that you might find it takes longer for your respiratory infections to clear up. In addition, you might find that cuts and minor infections take longer to heal. In addition to protein, other nutrients that have been found to be important to optimal immune function include zinc, selenium, iron, vitamin C, and vitamin D.