There is extensive research that shows that too much protein is a major problem with the standard American diet. I would add that protein deficiency is very rare. When is the last time anyone observed a case of protein deficiency? Our problem is too much protein, not too little.
Follow this link to on Dr. McDougall's website to see lots of good information about protein.
The meat and dairy industries (and the makers of sports bars and protein powders) would love to have us think that we need to eat lots of protein. They make athletes a prime target of their advertising. Although endurance and strength athletes might need a little more protein, the amounts of protein advertised by the protein marketers is way too much.
The China Study by T. Colin Campbell provides powerful evidence that populations that limit their intake of protein, especially animal and dairy protein, have far fewer incidences of heart disease and cancer. Populations that work hard, and live on mostly a plant based diet live on average longer that populations living on diets rich in animal proteins.
I, and a number of people I know, who are dedicated to a plant based diet, consume roughly in the range of 40 - 80 grams of protein per day. I am only an experiment of one. I average about 7-8,000 miles per year on my bike. Since converting to a plant based diet, my energy level and endurance has never been higher. I get all my protein from vegetables, fruits, whole grains, beans/legumes, raw nuts/seeds.
I periodically use a software program, CRON-o-Meter to measure my nutrient intake. In October of this year, my daily average protein intake from these sources was 63 grams. That's .38 grams per pound of body weight. The guidelines given to athletes preach protein intakes of from 1-2 grams of protein per pound of body weight. I know other people on a similar diet who have roughly the same protein intake as mine. There is no protein deficiency, no weak muscles, no lack of endurance or strength. There are many successful vegan bodybuilders and ultra distance athletes.
Sometimes people who transition to a lower protein diet initially feel weaker as their bodies adjust.
I feel pretty comfortable in saying that most of us can thrive on 40-80 grams of protein per day. I also believe that loading up with protein, especially protein from animals and from processed foods (bars, powders, etc.) is not good for our health.
One of the myths (fears) that keeps many people from adopting a healthy plant based diet is that they will become protein deficient. Not only do we need lots less protein we are told by the food marketers, but eating a variety of plant foods give us all the healthy protein we need.