There are 20 amino acids that humans need. 12 of which your body can produce or “synthesize” on their own from the foods that you ingest. The other 8 are known as the “essential Amino Acids” because your body is not able to make them on its own, and thus you must ingest them through your diet.
It was once believed that you needed to obtain all 8 of those essential amino acids within one food. Foods that contained all 8 essential amino acids were called “complete proteins” and those that contained only a few of the 8 essential amino acids were deemed “incomplete.” Meat and dairy were deemed essential parts of our diet during this time, because they were complete proteins. It was believed that plant foods were inferior, because they often lacked one or more of the essential amino acids.
Now we know that if you take in all the essential amino acids over the course of a week, your body is able to synthesize all the protein strands that you need. You do not need to take in all at essential amino acids in the same meal, or even in the same day. Amino acids are the exact same, whether they are ingested from a plant source of from an animal source. Also, no matter which source we take them in from, our bodies still have to break down the protein chains (the chains that connect the amino acids) and build them back up together to suit the human form.
One condition that may lead to a deficiency in protein in an individual who is eating enough food, are those who have absorption issues. Individuals who have damaged intestinal tracts, or other malfunctions of the digestive system can be low in protein, as well as other nutrients. This is not the foods fault, but a symptom that the body is damaged and needs some healing. Usually, a Naturopathic Doctor will work with someone who has absorption issues to repair the gut, and often they will prescribe a probiotic regimen, as well as recommend that the person eat more fermented foods.
So, in summary, as long as you are eating enough calories overall, from a wide range of sources, and you do not have an underlying absorption issue, you should never have a problem meeting your protein requirements.