2 Factors That Determine the Quality of a Protein

Essential amino acids in a protein are an important factor you should look for.

Protein is made up of individual amino acids strung together into long chains.  Athletes, particularly bodybuilders, are concerned about getting enough of this macronutrient in their diet – for good reason. When you lift weights, you create tiny tears in the fabric of muscle fibers. Your body uses amino acids from protein to repair those tears. This helps the muscle grow stronger and larger in size.

Although athletes focus on the muscle-building aspects of amino acids and protein, humans also need protein to make enzymes, hormones, hemoglobin, and to regulate the fluid balance in your body. During long periods of exercise when your body runs low in glycogen, protein can serve as an alternative source of fuel. In other words, protein is not a “one trick pony.”  It has a variety of purposes.

You often hear the experts debate about how much protein an active person needs. Recommendations are that a sedentary person consumes around 0.8 grams of protein per kilogram of body weight.  In this case, protein would make up 10 to 35% of total calories. However, many fitness experts believe you need more protein if you resistance train or do any type of intense exercise on a frequent basis, although the exact quantity is debatable.

There’s another issue related to protein you hear less about – protein quality. Not all proteins are created equal. In fact, there are two main factors that determine the quality of a protein. These are the amino acid composition and how effectively you digest and absorb a particular protein. Let’s look at each one.

Essential Amino Acids

As mentioned, proteins are made up of long chains of amino acids, some of which your body can make and others that it can’t. The ones your body is unable to make are called essential amino acids. All total, there are 9 essential amino acids you must get through diet. The essential amino acids include histidine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine. The remaining 11 amino acids are called non-essential because your body can make them from essential amino acids.

If your body lacks one or more essential amino acids, it will break down existing muscle tissue in an attempt to get the amino acid it needs. Over time, this can lead to muscle loss and a variety of health issues.

Complete versus Incomplete Proteins

In terms of amino acid composition, the highest quality sources of protein are those that contain all of the essential amino acids. Such proteins are called “complete” proteins. Meat and dairy are the best examples of complete proteins.

Most plant-based sources of protein are “incomplete,” meaning they lack one or more of the essential amino acids your body needs but can’t make. These include whole grains, legumes, nuts, and seeds. The exception is soy-based foods like tofu and tempeh. Soy contains all the essential amino acids your body needs and can’t synthesize.

Just because most plant proteins aren’t complete doesn’t mean you can’t build muscle on a plant-based diet, but you’ll need to eat a variety of plant-based foods to get all the essential amino acids. For example, legumes are deficient in the amino acid methionine while whole grains lack lysine. Legumes supply what the essential amino grains lack and vice versa. Now you know why beans and brown rice are a popular dish among vegetarians.

You can argue that meat and dairy foods are a higher quality source of protein, in terms of amino acid composition than most plant foods, with the exception of soy, since you get all the essential amino acids in one place.

Digestibility and Absorption

The second factor that influences protein quality is how digestible a protein is. If you can’t completely digest and absorb a protein source, you won’t get the benefits of all the amino acids the food contains.

One way of assessing protein quality is to measure net protein utilization or NPU. Scientists determine NPU when you eat food by measuring the percentage of protein from that food that’s retained by your body.  A greater percentage indicates a higher quality protein. To determine the quantity retained, researchers measure the amount of nitrogen excreted in the urine. The less nitrogen excreted, the more protein your body hangs on to.

NPU is expressed as a percentage of how much protein is retained. When you look at the NPU of different protein sources, human breast milk tops the list with an NPU of 94, followed closely by eggs with an NPU of 87. Based on NPU, these are among the highest quality proteins.

Another closely related value that scientists use to rate the quality of a protein is called biological value. This is very similar to NPU except for NPU measures how much protein was CONSUMED whereas biological value tallies how much was ABSORBED.

Biological value (BV) is also expressed as a percentage of how much your body retains. For example, a BV of 99 indicates 99% of the protein in a food is preserved by your body. When you take into account biological value and NPU, whey protein, eggs, and beef have the highest values.

These aren’t the only ways to measure a protein’s quality. Another commonly used test is called the protein digestibility-corrected amino acid score (PDCAAS). This test compares a protein’s amino acid to a standard reference protein and takes into account the food’s digestibility. A high PDCAAS score means a protein provides your body with a high percentage of essential amino acids. Whey, casein, egg, and soy have the highest PDCAAS scores with meat and soybeans falling close behind. Other plant-based sources of protein have relatively low scores.

The Bottom Line

One whole food that stands out in terms of protein quality and nutrition are eggs. While whey protein scores very high on the protein quality scale, the way most people consume whey is in the form of processed protein powder. Eggs give you an exceptionally high-quality protein source in an unrefined form.

Although eggs win points for quality, it’s wise to diversify the types of protein you eat. Plant-based protein has advantages over animal-based foods because these foods contain fiber and other phytochemicals that are important for health and disease prevention. Plus, some studies show a link between red meat, especially processed meats, and colon cancer. Although protein is important for muscle growth, don’t make it your only focus. Aim for a diet balanced in all three macronutrients – protein, unprocessed carbohydrates, and healthy fats.



Am J Clin Nutr. 2011 Mar;93(3):525-34. Epub 2011 Jan 12.

Expert-Nutrition.com. “Protein Quality – Why Some Proteins are Better than Others”

Protein Quality Evaluation. Report of Joint FAO/WHO Expert Consultation.

Journal of Sports Science and Medicine (2004) 3, 118-130.

Live Science. “New Health Warning Explained: How Processed Meat Is Linked to Cancer”


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