Should I be worried about getting enough protein on a vegetarian/vegan diet?
No, not as long as you're taking in enough calories. Official recommendations suggest that eating 8% of our daily energy as protein will provide an adequate amount. National and international recommendations for protein intake are based on animal sources of protein such as meat, cow's milk and eggs. Plant proteins may be less digestible because of intrinsic differences in the nature of the protein and the presence of other factors such as fibre, which may reduce protein digestibility by as much as 10%. Nevertheless, dietary studies show the adequacy of plant foods, as sole sources of protein as does the experience of healthy vegans of all ages.
The main protein foods in a vegan diet are the pulses (peas, beans and lentils), nuts, seeds and grains, all of which are relatively energy dense. As the average protein level in pulses is 27% of calories; in nuts and seeds 13%; and in grains 12%, it is easy to see that plant foods can supply the recommended amount of protein as long as the energy requirements are met.
The short answer is: "No, sufficient protein can be obtained by eating a variety of foods", but here is a longer explanation:
Protein is synthesized by the human body out of individual amino acids. The body breaks down food into individual amino acids and then reassembles the proteins it requires.
All amino acids must be present in the body to make proteins. Those that can be synthesized from other amino acids are called "unessential" amino acids. You can live on a diet deficient of these if you eat enough extra of the other amino acids to synthesize these. Those that cannot be synthesized from other amino acids are called "essential" amino acids and must be present in the diet.
Protein that contains all essential amino acids is called "complete" protein. Protein that contains some, but not all essential amino acids is called "incomplete" protein. It used to be believed that all amino acids must be eaten at the same time to form complete proteins. We now know that incomplete proteins can be stored in the body for many days to be combined with other incomplete proteins. As long as all essential amino acids are in the diet, it does not matter if the proteins are complete or incomplete.
The amount of protein recorded on food labels only lists the complete proteins. A product may contain much higher amounts of incomplete protein that is not listed. Combining such products may increase the total amount of protein beyond the levels expected.
The 1989 revision of the FDA's RDA suggests a protein intake of 44-63 grams. Many scientists think this number is too high. Most scientists agree with this number. Do I need to combine proteins on a vegetarian/vegan diet?
Frances Moore Lappe popularised the idea of protein combining in her book "Diet for a Small Planet" in the '70s, however in her revised edition: "Diet for a Small Planet 10th Anniversary Revised Edition" she has since renounced it.
The 1988 position paper of the American Dietetic Association emphasized that, because amino acids obtained from food can combine with amino acids made in the body it is not necessary to combine protein foods at each meal. Adequate amounts of amino acids will be obtained if a varied vegan diet - containing unrefined grains, legumes, seeds, nuts and vegetables - is eaten on a daily basis.
"Food combining" is another term for the Hay diet and has nothing to do with the concept of protein combining.