What is a complete protein?
Protein is a crucial macronutrient made up of smaller units called amino acids. Of the 20 amino acids your body uses, nine are considered "essential" because your body cannot produce them on its own. These nine essential amino acids must be obtained through your diet. A "complete protein" is a food that contains all nine essential amino acids in sufficient quantities. Animal products, such as meat, fish, and eggs, are typically considered complete proteins.
Incomplete proteins and the limiting amino acid
Most plant-based protein sources, including brown rice and lentils, are considered "incomplete" because they are too low in, or lack, one or more of the essential amino acids. The essential amino acid that is the least abundant is called the "limiting amino acid". To create a complete protein profile on a plant-based diet, you must consume a variety of foods that complement each other's amino acid profiles.
The classic pairing: Brown rice and lentils
Traditional food cultures around the world have long paired grains with legumes, and for good reason. In this case, brown rice and lentils are a perfect example of two incomplete proteins forming a complete protein source when combined.
- Brown Rice: This whole grain is a good source of protein but is low in the essential amino acid lysine.
- Lentils: This legume is rich in lysine but is low in the sulfur-containing amino acid methionine.
When you combine brown rice and lentils, the high lysine content in the lentils compensates for the deficiency in the rice, and the higher methionine content in the rice makes up for the lower amount in the lentils. The result is a nutritionally balanced protein source containing all nine essential amino acids.
The debunked myth of mealtime combining
Decades ago, it was a common belief that vegetarians and vegans had to eat complementary proteins in the same meal to get all essential amino acids. This idea, which emerged in the 1970s, has since been debunked by modern nutritional science.
Your body does not require you to consume complementary proteins at the exact same time. Instead, your liver maintains a pool of essential amino acids throughout the day. As long as you eat a varied and balanced diet, your body can pull from this pool to create the complete proteins it needs for tissue repair and other functions. Therefore, you could have brown rice for lunch and lentils for dinner, and your body will still effectively form the proteins it needs.
More plant-based complete protein sources and combinations
While combining grains and legumes is a reliable method, it is not the only way to get a complete protein on a plant-based diet. A number of plant foods are complete proteins on their own.
- Single-source plant proteins:
- Quinoa
- Soy products (tofu, tempeh, edamame)
- Buckwheat
- Hemp seeds
- Chia seeds
- Other complementary protein combinations:
- Grains and legumes: Pita bread and hummus, whole-wheat toast with peanut butter.
- Legumes and nuts/seeds: A mixed bean salad with walnuts, or a trail mix with peanuts and sunflower seeds.
A varied diet is key
Regardless of whether you are combining plant proteins or eating animal proteins, the most important takeaway is to eat a wide variety of foods. A diverse diet ensures you receive a full spectrum of nutrients, including all the essential amino acids, fiber, vitamins, and minerals your body needs for optimal health. You can read more about plant-based protein options and combinations on sites like Healthline to ensure you meet your nutritional needs.
Comparison: Plant vs. Animal Protein
| Feature | Plant-Based Protein (e.g., Brown Rice & Lentils) | Animal-Based Protein (e.g., Chicken & Eggs) |
|---|---|---|
| Amino Acid Profile | Usually incomplete individually but can be combined for a complete profile. | Typically complete proteins in a single source. |
| Saturated Fat & Cholesterol | Generally low to none. | Often higher. |
| Fiber | High. | None. |
| Micronutrients | Rich in iron, magnesium, folate, and B vitamins. | Varies by source, can be rich in B vitamins and iron. |
| Sustainability | Lower environmental impact. | Higher environmental impact. |
| Preparation | May require soaking or longer cooking times. | Generally cooks faster and is ready to eat in some cases. |
Conclusion
In summary, the classic combination of brown rice and lentils does indeed create a complete protein, supplying all nine essential amino acids. However, the old advice about having to consume them in the same meal has been proven to be a myth. Your body efficiently stores and manages amino acids from various food sources throughout the day, so the emphasis should be on a balanced and diverse diet rather than stressing over every single meal. By incorporating a mix of grains, legumes, nuts, seeds, and other plant-based sources, you can easily meet all your protein requirements and enjoy the many health benefits of whole foods.
