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Fatigue is a sign of low protein intake.
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Protein deficiency is rare in developed countries, where there is broad access to high-quality protein sources. In fact, according to the Harvard T.H. Chan School of Public Health, most Americans get more than enough protein. However, certain populations may benefit from greater protein intakes.

Protein Deficiency Symptoms and Causes

In its most severe form, protein deficiency results in a condition called kwashiorkor, which is extremely rare in developed countries. According to MedlinePlus, kwashiorkor is most common in places where there is famine, short food supply and low education levels, and thus a lack of knowledge about proper nutrition. In very poor countries, it is often concurrent with natural disasters, drought and political unrest.

In the United States, protein malnutrition in children is typically a sign of abuse or neglect. Protein deficiency may also occur among the elderly in nursing homes, an estimated 20 percent of whom experience undernutrition, according to a review in Current Opinion in Clinical Nutrition and Metabolic Care in January 2015. Last, individuals with conditions that cause malabsorption of nutrients, such as celiac disease, Crohn's disease and tropical sprue, may become deficient in protein.

According to MedlinePlus, early, low-protein symptoms include fatigue, lethargy and irritability. As the deficiency progresses, more serious symptoms may occur, including:

  • Changes in skin pigment
  • Decreased muscle mass
  • Diarrhea
  • Failure to gain weight and grow
  • Changes in hair color or texture
  • Immune system damage resulting in increased frequency and severity of infections
  • Protruding belly
  • Loss of muscle mass
  • Rash
  • Swelling or edema

Severe, low-protein levels can result in shock or coma, and may also cause permanent mental and physical impairment.

Read more: 7 Popular Protein Myths Totally Busted by Science

How Much Protein You Need

If you're in the U.S. and fairly healthy, it is more than likely that you are at least meeting your minimum protein requirements. Still, if you're feeling tired, lethargic and irritable, you might be wondering if it's due to low-protein levels.

Only your doctor can confirm whether or not you have a protein deficiency. However, you can use the intake guidelines set by the Food and Nutrition Board of the National Academies of Medicine as a rough measure of your protein status.

If you're getting at least the recommended dietary intake (RDI) of 46 grams for women or 56 grams for men each day, you can rest assured that you're probably not exhibiting signs of lack of protein — barring any malabsorption issues. However, this recommendation is based on estimated protein requirements of 0.8 grams per kilogram of body weight. If you're quite heavy, you may need more protein than the RDI. For example, if you weigh 200 pounds, you would need 72 grams of protein.

Surpassing the RDI

There are many experts who say the RDI isn't enough. At the Protein Summit 2.0 held in Washington, D.C. in October 2013, more than 60 health experts, nutrition educators and nutrition scientists convened to discuss optimal protein intake for human health.

In opposition to the common belief that Americans exceed their daily protein needs, participants at the Protein Summit argued that this is a misconception, and that higher protein intakes can improve diet quality, weight management, body composition and increased lean muscle mass, according to a summary published in the American Journal of Clinical Nutrition in April 2015.

Protein's role in weight management has been extensively researched, and results show that protein affects appetite and metabolic function in positive ways. Although total calorie intake ultimately determines weight loss or gain, protein may help people control calorie intake by promoting satiety through a combination of sensory, cognitive and physiological signals, according to an article in the journal Trends in Food Science & Technology in February 2015.

Protein digestion can also temporarily boost metabolism via the thermic effect of food, or diet-induced thermogenesis. Nutrient digestion, absorption, transport, metabolism and storage requires energy, and the amount of energy required depends on the nutrient being processed.

According to authors of a review article in Nutrition & Metabolism in November 2014, protein processing increases base metabolic rate by 15 to 30 percent, whereas carbohydrates provide a 5 to 10 percent boost, and fats, 0 to 3 percent.

Other populations likely need more protein, too. Because of medications, illness, dental problems, isolation, depression and decreased mobility, many older adults do not get enough protein because they don't get enough total calories. Resistance-trained athletes also have higher protein requirements due to increased demands of exercise recovery and muscle growth.

Read more: 12 Tips to Succeed at Your Weight Loss Resolutions

Getting More Protein

If you think you need extra protein in your diet, have a discussion with your doctor or a nutritionist about your current intake and your goals. For weight loss, a study published in Obesity Facts in June 2017 found that adults who ate a high-protein diet providing 1.34 grams of protein per kilogram of body weight lost significantly more weight than those who ate a standard protein diet with 0.8 grams per kilogram.

Resistance-trained athletes' needs may be even higher. According to the International Society of Sports Nutrition Position Stand: Protein and Exercise, intakes of 1.4 to 2 grams of protein per kilogram of body weight is appropriate. Additionally, there is some evidence that even higher intakes could positively affect body composition.

Increasing protein intake in your daily diet isn't difficult, but it's important that the protein comes from healthy sources. You can eat cheeseburgers and milkshakes to get enough protein, but you will also get a lot of calories, sugar, saturated fat and other unhealthy additions.

Fish, chicken, low-fat dairy, beans, nuts and seeds are your healthiest 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.

Animal foods provide "complete" protein with a high biological value. This means the protein comes packaged with all the essential amino acids your body can't make but needs to support muscle growth and maintenance and a healthy immune response.

But plant foods offer protein that is just as valuable when eaten as part of a balanced diet. Plant-based protein is often referred to as being "incomplete" or having low biological value because some plants are missing or low in one or more essential amino acids. However it's not necessary to get all the amino acids at the same time. As long as you get adequate supplies from all the foods you eat, your body has everything it needs to synthesize proteins.

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