A protein-rich diet is not just for bodybuilders. The nutrient is critical for so many of your body's daily functions and can even help you maintain a healthy weight.
In fact, protein is considered the building block of life, according to the U.S. National Library of Medicine. It's essential for everything from building and repairing muscle tissues to strengthening your immune system.
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If you're getting at least the recommended dietary intake (RDI) of 46 to 56 grams each day, you can rest assured that you're probably not exhibiting signs of protein deficiency — barring any malabsorption issues.
This recommendation is based on estimated protein requirements of 0.8 grams per kilogram of body weight, and some experts say you may need more protein depending on your age, physical activity levels and other lifestyle factors.
Protein deficiency isn't very common — in fact, most Americans get more than enough protein, according to the Harvard T.H. Chan School of Public Health.
Still, there are certain people who are more susceptible to a protein deficiency, including those with eating disorders, undiagnosed Crohn's disease or poor appetites (more common in older people or those who have cancer), Cheryl Mussatto, RD, registered dietitian and author of The Nourished Brain, tells LIVESTRONG.com.
Even if you don't fall into one of these categories, there are some telltale signs that your protein intake may be lacking.
1. You're Having Skin, Hair and Nail Problems
If you've noticed negative changes in your skin, hair and nails, you might want to look at how much protein you're eating.
Dry and brittle hair can sometimes be attributed to low protein intake, registered dietitian nutritionist Elizabeth Gunner, RDN, says. "Since hair is primarily made up of protein, changes in hair texture or increased hair loss can be a sign that you are not eating enough protein in your current diet," she says.
Proteins like collagen, elastin and keratin exist in skin, hair and nails to keep them healthy. "If you're deficient in protein, you may notice brittle or thinning hair, dry and flaky skin and deep ridges on your fingernails," Mussatto says.
2. You're Retaining Fluid
Swelling, also known as edema, is caused by fluid retention in your body's tissues and can occur in your hands, feet, legs and abdomen. While there are several causes of edema, protein deficiency is one of them.
"Proteins like albumin circulate in your blood, helping keep fluid from building up in tissues," Mussatto says.
It's important to note that edema is a sign of a severe protein deficiency — and quite unlikely if you happen to skip protein at lunchtime. Health conditions like malnutrition and kidney and liver disease can lead to low protein levels that cause edema, per Harvard Health Publishing.
In its most severe form, protein deficiency results in a condition called kwashiorkor, which is characterized by a swollen belly and is extremely rare in developed countries.
3. You're Hungry After Meals
Have you ever felt hungry immediately after finishing a meal? If so, your diet may not be adequate in protein. "Not eating enough protein can lead to increased hunger between meals leading to snacking on not-so-healthy foods," Mussatto says.
Compared to carbohydrates and fat, protein has a greater effect on satiety and perceived fullness, according to a June 2015 study in the American Journal of Clinical Nutrition. "Protein helps us feel full since it takes longer for the body to break it down, creating satiety," Mussatto says. She recommends aiming for 25 to 30 grams of protein at each meal.
Increasing protein intake in your daily diet isn't difficult, but it's important that the protein comes from healthy sources. You could eat cheeseburgers and milkshakes to hit your goal, but you'll be eating a lot of calories, sugar, saturated fat and other unhealthy additions if you choose this route.
Fish, chicken, dairy, beans, nuts and seeds are some healthier choices for increasing your protein intake. In addition to protein, you'll keep your saturated fat intake low and increase your intake of healthy monounsaturated and polyunsaturated fats, fiber and other nutrients.
4. You're Losing Muscle Mass
One of the primary roles of protein is to build and repair muscle tissue. So if you're strength training, make sure you're getting your protein.
"If you are not eating enough protein, your body will be unable to rebuild muscle that is broken down during a workout or fully recover for your next training session," Maddi Osburn, RDN, says.
The same is true if you want to sustain and maintain your muscle mass. Sufficient protein intake is necessary in order to preserve muscle mass, especially as you age, Mussatto says.
Exactly how much protein should you be eating if you're active? Osburn recommends aiming for a goal of around 1.2 to 2 grams of protein per kilogram of body weight for active adults. So, if you weigh 150 pounds, that would come out to 82 to 136 grams of protein per day.
5. You Get Sick Often
Getting a cold every now and then is not that unusual, but if you find yourself getting sick often, you might want to check in with your protein intake. "A telltale sign that you are not consuming enough protein is that you find yourself becoming sick often," Gunner says.
As you may have gathered from your reading, protein plays many critical roles in the body. When it comes to your immune system, protein helps regulate antibodies and immune cell responses.
"Since protein plays an important role in immune function, a lack of protein can increase your risk of developing an infection," Gunner says.