Protein is an essential component of your diet, and complete proteins contain all nine essential amino acids. Most complete proteins are animal products; however, there are some plant-based alternatives.

Many whole grains contain a decent amount of protein, for example, oatmeal. But are they complete proteins?
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Is Oatmeal a Complete Protein?
Oats on their own aren’t a complete protein, and neither is oatmeal. It is, however, possible to prepare your oatmeal in such a way as to include all nine essential amino acids, making it a complete protein.
Oatmeal does contain protein and is an excellent source of minerals, vitamins, and other nutrients. So, adding plain, unflavored oatmeal to your diet will certainly benefit your health.
Why Is Oatmeal Not a Complete Protein?
Oatmeal prepared with water isn't a complete protein. However, that prepared with whole milk contains the amino acids necessary to be considered a complete protein.

Oatmeal contains eight of the nine essential amino acids that your body doesn't produce. It’s quite low in lysine, so include foods rich in this amino acid in your meals.
Although oatmeal isn't a complete protein, the protein it contains is of a higher quality than that from other whole grains.
How Can You Make Oatmeal a Complete Protein?
A one-cup serving of oatmeal cooked with water contains about 5.9 g of protein, around 12% of the recommended daily intake.
Cooking it with whole milk adds lysine and other amino acids, creating a complete protein. This method also increases the protein content to 7.9 mg per cup.
If you don’t eat dairy, you can still create a complete protein by adding other lysine-rich foods to your oatmeal, such as cranberries, mangoes, pumpkin seeds, or almonds. These don't have to be mixed with your oatmeal but should be eaten within 24 hours of your meal.

Is Oatmeal Good for You?
Like other whole grains, oatmeal is an excellent source of fiber, with one cup containing 16% of the recommended daily intake. Fiber feeds good gut bacteria and keeps you feeling full longer after eating.
Oatmeal also contains 68% of the recommended daily intake of manganese, which supports connective tissue formation and contributes to healthy bones, blood clotting factors, and sex hormones. It’s also crucial for carbohydrate metabolism, calcium absorption, and blood sugar regulation.
Oatmeal also contains good amounts of iron, magnesium, phosphorus, zinc, selenium, and antioxidants, which flush out free radicals and prevent cellular oxidative damage.

These antioxidants also help lower blood pressure by dilating blood vessels and may protect you against cardiovascular disease and cancer.
Oatmeal also contains anti-inflammatory plant compounds, which studies show can decrease the risk of childhood asthma.
Experts believe that the beta-glucan fiber in oatmeal relieves constipation and reduces levels of bad cholesterol, reducing your risk of heart attack and stroke. It also reduces arterial inflammation and prevents tissue damage.
Applied to the skin, oatmeal can relieve skin irritation and soothe the symptoms of eczema.
Conclusion
Although oatmeal isn't a complete protein, cooking it with milk or adding lysine-rich foods turns it into one.

Oatmeal is a great protein source for those following a plant-based diet.
Sources: Nutrition Data, PMC, and National Library of Medicine