How to Determine a Good Source of Protein

October 27, 2025 •

4 min read

According to the Dietary Guidelines for Americans, adult males need at least 56 grams of protein daily, while adult females need at least 46 grams. Knowing how to determine a good source of protein goes beyond just counting grams; it involves evaluating its quality, digestibility, and overall nutritional value.

Quick Summary

This article explains how to assess protein quality by understanding amino acid profiles, digestibility, and different scoring methods like PDCAAS and DIAAS. It outlines the differences between complete and incomplete proteins and provides a guide to evaluating both animal and plant-based protein sources for optimal health.

Key Points

Amino Acid Profile: A good protein source contains all nine essential amino acids, making it a 'complete' protein.
Protein Quality Scores (PDCAAS/DIAAS): Modern nutritional science uses advanced scoring methods like DIAAS to measure a protein's true digestibility and amino acid balance.
Complete vs. Incomplete Proteins: Animal-based proteins are typically complete, while most plant-based proteins are incomplete but can be combined to form a complete profile.
Consider the Protein Package: Look at the overall nutritional content, including fats, fiber, and vitamins, that come with the protein source.
Balance and Variety: For optimal health, consume a mix of protein from various animal and plant-based sources to ensure a wide range of nutrients.
Lean and Unprocessed: Choose leaner, less processed options like skinless poultry, fish, and legumes to avoid excess saturated fat and sodium.

Understanding Protein Quality: More Than Just the Numbers

When we think about protein, we often focus on the total grams per serving. However, the true value of a protein source depends heavily on its quality. Protein quality is determined by two main factors: its amino acid profile and its digestibility. A high-quality protein contains all nine essential amino acids (EAAs) in sufficient amounts that the body cannot produce on its own. It is also highly digestible, meaning the body can absorb and use it effectively.

The Amino Acid Profile: Complete vs. Incomplete

Proteins are made up of building blocks called amino acids. There are 20 amino acids in total, nine of which are essential and must be obtained from food. A protein source is considered 'complete' if it contains all nine EAAs in adequate proportions. Most animal-based proteins are complete, including meat, poultry, fish, eggs, and dairy products.

On the other hand, most plant-based proteins are 'incomplete,' meaning they are low in one or more EAAs. For example, legumes are often low in methionine, while grains are typically low in lysine. However, combining different incomplete plant proteins can create a complete protein meal. This practice, known as protein combining, ensures that those following vegetarian or vegan diets meet their EAA needs. For instance, combining rice (low in lysine) with beans (low in methionine) results in a complete protein profile.

Scientific Scoring Methods for Protein Quality

Nutrition experts use several scoring systems to quantify the quality of a protein source. While these methods can be complex, understanding their basics helps in making informed dietary choices.

Protein Digestibility-Corrected Amino Acid Score (PDCAAS)

The PDCAAS was the preferred method for many years and is still used by the FDA. It evaluates protein quality based on the amino acid requirements for humans and adjusts for digestibility.

How it works: Compares the amino acid composition of a food protein to a reference standard and corrects for digestibility. The score is truncated at a maximum of 1.0, meaning scores above this value are not reported as higher quality.
Limitations: The main limitation is the truncation of scores, which can underestimate the value of higher-quality proteins. Additionally, it uses fecal digestibility, which may not accurately reflect true absorption in humans.

Digestible Indispensable Amino Acid Score (DIAAS)

The DIAAS is the newer, more accurate method recommended by the Food and Agriculture Organization (FAO) of the United Nations.

How it works: Measures the digestibility of individual indispensable (essential) amino acids at the end of the small intestine (the ileum). Unlike PDCAAS, DIAAS does not truncate scores, allowing for a more accurate comparison of protein sources.
Benefits: Provides a more precise measure of the protein's true nutritional value and its ability to meet human amino acid needs.

The Protein Package: Considering the Whole Picture

While scoring systems provide a good metric for quality, it's crucial to consider the 'protein package'—the other nutrients and compounds that come with a protein source.

Animal proteins, like fatty cuts of beef, can come with high levels of saturated fat and sodium.
Plant proteins, such as legumes, offer the added benefit of fiber, antioxidants, and healthy fats, while being low in saturated fat.

Comparison of High-Quality Protein Sources

To illustrate the difference, here is a comparison of various high-quality protein sources, considering their amino acid completeness and overall nutritional profile.


Protein Source	Amino Acid Profile	Digestibility	Key Additional Nutrients	Notes
Eggs	Complete	Excellent	Vitamins (D, B12), choline	Considered the gold standard for protein quality due to excellent profile and digestibility.
Whey/Casein	Complete	Excellent	Calcium, easy absorption	Found in milk and dairy; excellent for muscle repair post-workout.
Soy (Tofu, Tempeh)	Complete	Very Good	Fiber, minerals, isoflavones	One of the few complete plant proteins, highly versatile.
Salmon	Complete	Excellent	Omega-3 fatty acids, vitamin D	Offers heart-healthy fats and is highly digestible.
Quinoa	Complete	Good	Fiber, magnesium, phosphorus	Considered a complete plant protein, excellent for vegetarians and vegans.
Lentils	Incomplete	Good	Fiber, iron, folate	Best combined with a grain to provide a complete amino acid profile.

The Best of Both Worlds: Combining Proteins

For most people, a diet that includes both animal and plant-based proteins is a balanced approach. For those on a plant-based diet, combining different sources throughout the day, like lentils with rice or peanut butter on whole-wheat bread, is a simple way to achieve a complete amino acid profile. This variety also ensures a wider intake of vitamins, minerals, and fiber.

Conclusion

Determining a good source of protein involves looking beyond the total grams and considering the quality, which is defined by the amino acid profile, digestibility, and the overall nutrient package. Animal proteins like eggs, dairy, fish, and lean meats are complete and highly digestible, but some can contain high levels of saturated fat. Plant-based options, while often incomplete on their own, offer a wealth of fiber and micronutrients and can be combined to create a complete protein source. For optimal health, focus on a varied diet rich in high-quality protein from both animal and plant-based sources, paying attention to the full nutritional context of each food.

Choosing the Right Protein: Key Considerations

Look beyond grams to understand the quality of the protein you consume.
Prioritize complete proteins, especially if you are plant-based, by combining various sources.
Pay attention to the 'protein package,' including accompanying nutrients like fat and fiber.
Utilize nutritional science from reputable organizations like the FAO, which promotes the DIAAS scoring method for better accuracy.
Choose leaner options and minimally processed forms for the most health benefits.

For more information on nutrition and healthy eating, Harvard's School of Public Health offers an excellent resource on protein sources: Choosing Healthy Protein - Harvard T.H. Chan School of Public Health.

Frequently Asked Questions

A complete protein contains all nine essential amino acids that the body cannot produce, while an incomplete protein is low in one or more of these essential amino acids. Animal products like meat and eggs are typically complete proteins, whereas most plant sources are incomplete.

Not necessarily. While most individual plant proteins are incomplete, they can be combined effectively to provide a complete amino acid profile. Many plant proteins also offer additional health benefits like fiber and lower saturated fat, which should be considered as part of the overall nutritional package.

Vegans and vegetarians can ensure they get enough high-quality protein by consuming a variety of plant-based protein sources daily. Combining legumes with grains (e.g., beans and rice) or including complete plant proteins like soy and quinoa in their diet is an effective strategy.

DIAAS stands for Digestible Indispensable Amino Acid Score. It is the most modern and accurate method for measuring protein quality, as it considers the digestibility of each essential amino acid separately and does not truncate high scores, providing a clearer picture of a protein's true value.

A diet with an adequate amount of high-quality protein is healthy and important for muscle maintenance, cellular repair, and satiety. However, consuming excessive amounts of processed or fatty protein sources can be unhealthy. Focusing on lean, unprocessed, and varied protein sources is key.

Cooking can affect protein digestibility. While excessive, high-heat cooking can potentially damage some amino acids, standard cooking methods like baking, grilling, and boiling generally do not significantly decrease the protein's overall quality or amino acid content.

Some of the best high-quality protein sources include eggs, lean poultry and meat, fish, dairy products like Greek yogurt, and plant-based options such as soy (tofu, tempeh) and quinoa.