Protein 101: A Complete Guide

Protein is an important part of a healthy diet as it plays many vital roles in the human body. Here is a complete guide to protein and the best sources to help increase your dietary protein intake.

What is protein?

Protein is a macronutrient required for essential functions in the human body. It is considered an essential nutrient because the body cannot biosynthesize enough to maintain health and survival.

Protein is also an energy-providing nutrient, as it supplies 4 calories per gram, along with carbohydrates and fat, which supply 4 and 9 calories per gram, respectively.

Protein is composed of amino acids. Of the 20 amino acids that make up protein, 9 are considered essential amino acids and must be obtained through food. Amino acids are classified into 3 groups ⁽¹⁾:

• Essential
• Non-essential
• Conditional

Essential amino acids cannot be made by the body and, therefore, must be consumed through food. These include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Non-essential amino acids can be made by the body from essential amino acids or in the normal breakdown of proteins. These include alanine, arginine, asparagine, aspartic acid, glutamic acid, glutamine, glycine, proline, serine, and tyrosine.

Conditional amino acids are those that the body cannot produce in sufficient amounts during periods of illness, stress, injury, or rapid growth, such as infancy. These include arginine, cysteine, glutamine, tyrosine, glycine, ornithine, proline, and serine.

Why do we need protein?

All cellular processes in the human body involve protein in some way. Proteins function as structural components, enzymes, hormones, immune factors, transporters, acid-base regulators, and neurotransmitters ⁽¹⁾.

Protein is also required for the development and maintenance of muscle mass. The single largest tissue store of bodily protein is muscle ⁽²⁾. Skeletal muscle makes up about 40% of the human body’s weight and contains 50-75% of its total protein ⁽¹⁾.

How much protein should you have per day?

Exactly how much protein you need will vary by age, height, weight, gender, activity level, and health goals.

Based on current dietary guidelines, the recommended dietary allowance (RDA) for protein is 0.8 grams per kilogram of body weight per day or 0.36 grams per pound of body weight per day ⁽³⁾. However, the RDA for protein is the recommended minimum intake to prevent a deficiency and was derived from studies on sedentary individuals.

A growing body of evidence has shown that this intake is suboptimal for healthy aging, appetite regulation, weight management, muscle protein synthesis, and optimizing athletic performance ^{(4, 5, 6, 7, 8)}. The current evidence suggests:

• For optimizing health, the recommended intake is 0.54 to 0.7 grams of protein per pound of body weight per day.
• For weight loss, the recommended intake is 0.7 grams of protein per pound of body weight per day.
• For weight gain, the recommended intake is 0.7 to 1.0 grams of protein per pound of body weight per day.

Risks of Protein Deficiency

Not consuming adequate dietary protein can lead to protein deficiency, which can cause wasting and shrinkage of muscle tissue, edema or swelling due to fluid retention, anemia, thin and brittle hair, skin lesions, and slow growth in children ⁽⁹⁾.

That said, protein deficiency is rare in the U.S. general population. Data shows that the protein intake of U.S. adults averages 88.2 grams per day, which amounts to 14% to 16% of total daily calories, and meets the RDA of 0.36 grams per pound of body weight per day ⁽¹⁰⁾.

All data are provided by Food Data Central.

What foods contain protein?

Protein is found in a wide range of animal and plant foods, such as:

• Meat – beef, pork, lamb, bison, etc.
• Fish and Seafood – tuna, salmon, shrimp, crab, mussels, etc.
• Poultry – chicken, turkey, duck, etc.
• Dairy – yogurt, skyr, cheese, etc.
• Eggs
• Legumes – beans, lentils, tofu, etc.
• Nuts – peanuts, almonds, pistachios, cashews, etc.
• Seeds – hemp seeds, pumpkin seeds, sunflower seeds, etc.
• Whole grains – wheat, amaranth, quinoa, etc.

While many foods contain protein, the nutritional value of protein is generally measured by its essential amino acid composition. Different foods contain different amounts of amino acids, which affects their bioavailability, digestibility, and overall protein content.

Animal protein sources contain all essential amino acids and, therefore, are considered “complete” proteins. Plant protein sources do not contain all essential amino acids, or do not contain them in optimal ratios, and, therefore, are considered “incomplete” proteins.

According to both the protein digestibility corrected amino acid score (PDCAAS) and digestible indispensable amino acid score (DIAAS) used to assess the ability of a dietary protein to meet the body’s amino acid requirements, animal protein sources have higher digestibility and bioavailability than plant protein sources ^{(11, 12)}.

Moreover, animal proteins are primarily sources of protein and fat, with little to know carbohydrates or fiber, outside of milk proteins, while plant proteins are sources of carbohydrates, fiber, and fat with moderate levels of protein. This is an important factor to consider when trying to increase protein intake while counting calories or macros, or eating a plant-based diet primarily focused on vegan or vegetarian protein.

How to Increase Protein Intake

Meeting your optimal dietary protein intake can easily be achieved through food sources and, if needed, supplement sources of protein. Some of the best ways to increase protein intake are:

1. Eat a source of protein at every meal,
2. Increase your current portion size of protein,
3. Combine plant and animal proteins in meals,
4. Eat protein-rich snacks,
5. Use protein powders.

By regularly creating balanced meals with a source of protein, carbohydrates, fiber, and fat, and including protein in snacks, especially post-workout snacks, you can help to ensure you are reaching your optimal daily protein needs.

Should you struggle to meet the recommended protein intake for your goals, you can use protein supplements, such as protein powder, collagen powder, and protein bars, to help support your intake.

Is too much protein dangerous?

High-protein diets, typically defined as more than 20–25% of total daily calories, have been touted as dangerous and harmful to health; however, the research does not fully support these claims.

While many people believe that high-protein diets are unhealthy and can lead to kidney damage, liver damage, cardiovascular disease, osteoporosis, and even an increased breast cancer risk, the evidence does not support it and, in some cases, demonstrates the opposite.

Several meta-analysis studies concluded that protein intakes have a trivial to non-existent effect on kidney function, while another study found that high protein intakes had no harmful effects on bone mineral content or bone density, and another showed that higher protein intakes resulted in significant decreases in hip fractures ^{(13, 14, 15, 16, 17, 18)}.

With the exception of pre-existing kidney disease, there does not appear to be any imminent risk with high protein intakes within the discussed guidelines. High-protein diets are generally considered safe for healthy individuals and, in many cases, beneficial.

Overall, the quality of the protein consumed is more influential than protein intake alone. While it’s important to consume adequate protein, the “package” which the protein comes with (i.e., fat, fiber, vitamins, minerals, phytonutrients, etc.) has a greater impact on health. Choosing a high-quality protein, such as salmon or lentils, that provides all the amino acids, beneficial fats, fiber, and micronutrients, is far healthier than consuming a low-quality protein, such as a burger or processed meat, that also contains excessive saturated fat or sodium.