What are 5 foods that would be an incomplete protein source?

November 9, 2025 •

4 min read

Over 60% of the world's population relies on a diet primarily based on plant sources, many of which contain what are 5 foods that would be an incomplete protein source. While these foods are highly nutritious, they lack one or more of the nine essential amino acids that our bodies cannot produce. Understanding these sources is key to planning a balanced and healthy plant-based diet.

Quick Summary

This guide details five common incomplete protein sources from plant-based foods, explaining why they are incomplete. It also provides practical advice on how to combine these foods to ensure a complete intake of essential amino acids for optimal health and wellness.

Key Points

Incomplete Protein Defined: An incomplete protein source lacks or is low in one or more of the nine essential amino acids required by the body.
Legumes are Incomplete: Foods like beans, lentils, and peas are incomplete proteins, often lacking sufficient methionine.
Grains are Incomplete: Whole grains such as rice and wheat are incomplete and typically low in the essential amino acid lysine.
Nuts and Seeds are Incomplete: Most nuts and seeds are incomplete proteins, though they are excellent sources of other nutrients.
Combining for Completeness: It is not necessary to combine incomplete proteins at a single meal. Consuming a variety of protein sources throughout the day provides all essential amino acids.
Collagen Lacks Tryptophan: The animal-derived protein collagen is an incomplete protein because it is missing the essential amino acid tryptophan.
Focus on Variety: A diverse diet that includes different plant-based foods is the most effective strategy for obtaining all essential amino acids.
Incomplete Doesn't Mean Inferior: Incomplete protein sources are often rich in fiber, vitamins, and minerals, providing significant nutritional benefits beyond their protein content alone.

Understanding Complete vs. Incomplete Proteins

Proteins are crucial macronutrients composed of amino acids, often called the body's building blocks. There are 20 different amino acids, and while our bodies can produce 11 of these, the remaining nine are 'essential' and must be obtained through our diet. A complete protein source contains all nine essential amino acids in sufficient quantities, such as meat, dairy, and soy. An incomplete protein lacks or is low in one or more of these essential amino acids, and these sources are predominantly plant-based.

Many plant-based foods, including vegetables, grains, legumes, nuts, and seeds, fall into the incomplete protein category. However, this doesn't mean they are inferior. Incomplete protein sources are often rich in fiber, vitamins, minerals, and antioxidants, offering numerous health benefits beyond their protein content. The key to a nutritionally sound vegetarian or vegan diet is to consume a variety of these sources throughout the day, allowing the body to form a complete amino acid profile from its internal amino acid pool.

5 Common Incomplete Protein Sources

1. Nuts and Seeds

While nuts and seeds are excellent sources of healthy fats, fiber, vitamins, and minerals, most are considered incomplete proteins. For example, sunflower seeds are high in protein but low in lysine, an essential amino acid. Similarly, almonds and other nuts are also incomplete protein sources. It is a myth that you need to combine them with another food at the same meal to get a complete protein. Consuming a variety of foods over the course of the day is sufficient.

2. Legumes (Beans, Lentils, and Peas)

Legumes are a cornerstone of many plant-based diets, providing substantial protein, fiber, and iron. However, most legumes, including black beans, kidney beans, and lentils, are low in the essential amino acid methionine. This is why the traditional pairing of rice and beans is so popular and effective, as rice contains higher levels of methionine.

3. Whole Grains (e.g., Rice, Wheat, and Corn)

Whole grains are a vital source of energy and fiber, but similar to legumes, they are incomplete proteins. Many grains, including brown rice and wheat, are low in lysine. This is why combinations like hummus (made from chickpeas, a legume high in lysine) with whole-grain pita bread form a complete protein meal.

4. Vegetables

Most vegetables contain some protein, but in varying and typically incomplete amounts. They are generally lacking in several essential amino acids. While vegetables offer a wealth of micronutrients, relying on them as a sole protein source would be ineffective. However, including them as part of a varied diet with other protein sources is highly beneficial.

5. Collagen

Though derived from animals, collagen is an incomplete protein because it lacks the essential amino acid tryptophan. It is primarily a structural protein found in connective tissues, skin, and bones. As a protein supplement, collagen is beneficial for joint health but should not be relied upon as the sole source for muscle building or complete protein needs.

Combining Incomplete Proteins for a Complete Profile

The once-common belief that you must combine specific incomplete proteins in a single meal to get all essential amino acids is now outdated. The body can draw from its internal 'amino acid pool' over the course of a day. The best strategy is to consume a variety of protein sources throughout your daily meals. For instance, having oatmeal (a grain) for breakfast and a lentil (a legume) soup for lunch provides the necessary mix of amino acids.

Comparison Table: Incomplete vs. Complete Protein Sources


Feature	Incomplete Protein Sources	Complete Protein Sources
Origin	Mostly plant-based (legumes, grains, nuts, seeds)	Mostly animal-based (meat, dairy, eggs) and some plants (soy, quinoa)
Essential Amino Acids	Lacks or is low in one or more essential amino acids	Contains all nine essential amino acids in sufficient quantities
Key Benefit	Rich in fiber, vitamins, minerals, and antioxidants	Highly bioavailable and supports muscle synthesis effectively
Dietary Strategy	Combine a variety of sources throughout the day	Can be consumed as a standalone source for complete amino acids
Example Pairings	Rice and beans, hummus and pita	N/A (Already complete)

Conclusion: The Bigger Picture of Protein

Understanding what are 5 foods that would be an incomplete protein source is a vital part of a balanced diet, especially for those on plant-based regimens. While no single one of these foods offers all the essential amino acids, their collective nutritional power is immense. The traditional stress on combining proteins in one meal has been replaced by the simpler, more flexible approach of ensuring variety over the course of the day. Incorporating a wide array of legumes, nuts, seeds, and grains ensures that your body has access to all the amino acids it needs for health, growth, and repair. Ultimately, a balanced diet rich in both complete and incomplete protein sources offers the most comprehensive nutritional benefits.

For more in-depth information on nutrition and diet, visit the Academy of Nutrition and Dietetics.

Authoritative Citations

Difference Between Complete and Incomplete Proteins - JSHealth Vitamins Understanding Complete and Incomplete Proteins - Ask Nestle The Difference Between Complete and Incomplete Proteins - WebMD What's the Difference Between Complete & Incomplete ... - Kroger Protein: Sources, deficiency, and requirements - Medical News Today Complete Vs. Incomplete Proteins: What's The Difference? - Swolverine Incomplete vs. Complete Proteins & Top Food Sources of Each - Peloton Understanding Incomplete Proteins: A Plant-Based Guide - GetLabTest.com Complete protein combinations for vegetarians - Karuna Wellness Studio What are some reliable sources of information for nutrition ... - Quora

Frequently Asked Questions

A complete protein contains all nine essential amino acids in sufficient quantities, while an incomplete protein lacks one or more of these essential amino acids.

No, it is not a major concern as long as they consume a varied diet. The body maintains an amino acid pool that can be used to form complete proteins over the course of a day by eating a mix of plant-based foods.

A classic example is rice and beans. Grains like rice are often low in lysine, while legumes like beans are low in methionine; together, they form a complete amino acid profile.

Soy, including foods like tofu and edamame, is one of the few plant-based protein sources that is considered complete, containing all nine essential amino acids.

No, incomplete proteins are not unhealthy. Many are packed with fiber, vitamins, and minerals that contribute to a healthy diet. The label only refers to their essential amino acid profile, not their overall nutritional value.

Yes, as long as they consume a sufficient variety of incomplete protein sources throughout the day, including grains, legumes, nuts, and seeds. Focusing on a varied diet ensures all essential amino acids are consumed.

No, this is an outdated recommendation. The body stores amino acids in a pool and can draw from them as needed. The most important factor is consuming a variety of protein sources over the entire day.

Yes, one notable example is collagen, an animal-derived protein found in bone broth and supplements. It is considered an incomplete protein because it lacks the essential amino acid tryptophan.