What Is the Difference Between Complete and Incomplete Proteins?

Complete proteins contain all nine essential amino acids. An incomplete protein is missing one or more of the essential amino acids.

Proteins are a necessary part of your diet. But not all protein sources provide the same quality of nutrients. A complete protein contains substantial amounts of all protein-based molecules that your body needs. An incomplete protein is missing one or more of these necessary components.  

Proteins consist of molecular building blocks known as amino acids. Your body uses a total of 20 different amino acids to make all of the proteins that you need to survive. 

Your body can make 11 amino acids, but you need to get the other nine from your diet. These nine dietary amino acids are known as essential amino acids. 

Complete proteins contain all nine essential amino acids. The essential amino acids are: 

• Phenylalanine
• Isoleucine
• Leucine
• Histidine
• Lysine
• Methionine
• Tryptophan
• Valine
• Threonine 

Incomplete proteins are missing one or more of these amino acids. 

The only exception to this list of essential amino acids is in the case of certain medical conditions. One example is a condition called phenylketonuria (PKU). People born with this genetic disorder cannot convert the essential amino acid phenylalanine to tyrosine. 

If you have this condition, you will need supplemental tyrosine and other essential amino acids in your diet. You’ll also have to manage the side effects of having too much phenylalanine in your body, which turns into a toxic byproduct with severe developmental consequences if it accumulates over time. 

Most animal sources of protein count as complete proteins. Plants also contain proteins, but very few of them are considered complete. Plant and animal source of complete proteins includes: 

• Beef 
• Poultry
• Fish
• Pork
• Eggs
• Many dairy products
• Soy sources — including tofu, tempeh, miso, and edamame
• Quinoa 
• Blue-green algae
• Buckwheat 

In contrast, sources of incomplete proteins mostly come from plants and include: 

• Beans
• Peas
• Lentils
• Nuts 
• Nut butter — like peanut butter 
• Seeds 
• Whole grains
• Vegetables

The standard American diet contains plenty of protein. The problem is that much of the protein comes from relatively unhealthy sources. When choosing animal-based protein, your best bet is to select the leanest — or least fatty — options available. Use products like 93% ground beef and eat chicken without the skin. 

You’ll also be much healthier if you get your protein from various sources. You can get some of your daily protein from plant sources, even if you’re not a vegetarian or vegan. 

Most protein sources other than products like protein powders come packed with different nutrients. When you eat a diversity of proteins, you’re also eating a lot of additional nutrients necessary for your health. Additional nutrients found in diverse protein sources include: 

• Omega 3s — these are healthy fats found in fish
• Fiber — this helps maintain your digestive system
• Vitamin D — this is good for your bone health
• The vitamin B complex — these help build your tissues and form red blood cells
• Iron — you need this to prevent anemia
• Zinc — this helps support your immune system
• Magnesium — this helps your muscles function and is used to build your bones

At one time or another, you may have heard a rumor that every meal needs at least one source of every essential amino acid, as either a complete protein or a balanced mix of incomplete proteins. 

But current research indicates this is incorrect. You need to make sure you’re consuming all nine of the essential amino acids at some point throughout the day, not in every single meal. Luckily, if you’re getting your daily recommended amount of protein from a mix of different sources, you shouldn’t have to worry about each specific amino acid. 

Your body uses proteins for almost all of its functions and processes. Proteins are one of three sources of calories that your body uses for energy. The other two sources are fats and carbohydrates. 

Most of the time, fats and carbohydrates fuel your body’s energy needs. You switch to protein metabolism during aerobic exercise and when you’re starving. 

Your body also needs proteins to help with the following: 

• Build muscles
• Maintain your immune system
• Produce hormones 
• Produce enzymes — small molecular machines that perform necessary functions in all of your cells
• Improve your athletic performance — by transporting oxygen to your muscles

The amount of protein that you need each day depends on several different factors, including: 

• Age
• Sex 
• Height
• Weight
• Amount and type of daily physical activity 

You can talk to your doctor or a nutritionist about the best recommended daily intake (RDI) for your body. 

The U.S. Department of Agriculture recommends getting certain amounts of protein daily based on age and sex. They recommend: 

• Two-ounce-equivalents for children 12 to 23 months old
• Two to five ounce-equivalents for children two to four years old
• Three to five-and-a-half ounce-equivalents for children five to eight years old
• Four to six-ounce-equivalents for nine to 13-year-old females
• Five to six-ounce equivalents for 14 to 30-year-old females
• Five to six ounce-equivalents for females over the age of 30
• Five to six-and-a-half ounce-equivalents for nine to 13-year-old males
• Five-and-a-half to seven-ounce-equivalents for 14 to 18-year-old males
• Six-and-a-half to seven-ounce-equivalents for 19 to 30-year-old males
• Six to seven-ounce-equivalents for 31 to 59-year-old males
• Five-and-a-half to six-and-a-half ounce-equivalents for males 60 years and older

One ounce of most meat, poultry, and fish products is a one-ounce equivalent. But this can vary for other protein sources. For example, one-quarter cup of cooked beans is one-ounce equivalent, as is an egg, one tablespoon of nut butter, and one-half of an ounce of nuts or seeds. 

If you don’t get enough protein in your diet for an extended time, you can develop a serious health problem known as chronic protein deficiency. The exact condition depends on your total nutrient intake.

If you’re getting enough calories but lacking in dietary proteins, you could develop a condition called kwashiorkor. This name comes from Ghana, where this condition is common and characterized by swelling of your feet and abdomen, poor skin, and slowed growth. 

If you’re not getting enough proteins and calories, you develop a condition called marasmus or starvation. You become very thin and wasted-looking. This condition is common in areas with consistent poverty, famine, and civil unrest. In America, it primarily occurs because of severe diseases or the eating disorder anorexia nervosa.

In the U.S., so many protein sources are available that most people don’t have to worry about not getting enough protein. Instead, focus on fitting a variety of healthy sources into a complete, balanced diet that significantly benefits your overall health and nutrition.