Q. How many grams of protein do we need each day?
A. I don't know!
But then again, neither does anyone else. :)
All nutrient recommendation are given in relation to the needs that will cover over 98% of the population and therefore include built in safety margins for most people. None of them ever represent the actual daily needs of any one person on any one day.
The only way to know the actual protein needs of any one person on any given day is to do a nitrogen balance study on that person on that day. But, realize that whatever your needs where today, they may be different tomorrow.
However, we can get a better idea of about how much we need when we look at national recommendations.
According to the National Academy of Sciences, Institute of Medicine
"The Recommended Dietary Allowance (RDA) for both men and women is 0.80 g of good quality protein/kg body weight/d and is based on careful analysis of available nitrogen balance studies."
For a 150 lb person, this would equate to about 55 grams
Recently (2/2012), as reported in their paper, "Scientific Opinion on Dietary Reference Values for Protein, the European Food Safety Association paper," set the Population Reference Intake (PRI) at .83g/kg body weight per day for both Adults and for Older Adults. Again, this would meet the needs of 98% of the population.
As a percentage of energy:
From the, "Dietary Reference Intakes (DRIs): Acceptable Macronutrient Distribution Ranges Food and Nutrition Board, Institute of Medicine, National Academies."
Protein 10–35% of calories.
On an 1800 calorie diet, 10% would equate to 45 grams
In grams, from:
"Dietary Reference Intakes (DRIs): Recommended Intakes for Individuals, Macronutrients Food and Nutrition Board, Institute of Medicine, National Academies"
Adult Men: 56 g/day
Adult Women: 46 g/day
"Protein requirements of adults, including older people, and women during pregnancy and lactation.(PROTEIN AND AMINO ACID REQUIREMENTS IN HUMAN NUTRITION) COPYRIGHT 2006 World Health Organization"
Furthermore, recent detailed balance and body composition studies have shown that with a suitable program of resistance exercise sarcopenia (muscle loss) can be reversed and muscle strength increased on a protein intake of 0.8 g/kg per day (68). This intake is similar to the 1985 safe allowance and lower than usual intakes in this population. (68) Campbell WW et al. Dietary protein adequacy and lower body versus whole body resistive training in older humans. Journal of Physiology, 2002, 542:631-642.
So, there you have it.
Now realize the .8 gram/kg is based on a persons healthy weight, and includes a fairly liberal safety margin. Nitrogen balance studies have shown adults can maintain nitrogen balance on as little as .5-.66 gm/kg though no one needs to shoot for the minimum. Patients who must limit their protein intake due to kidney issues are usually put on a diet that limits protein intake to around .5-.6 grams/kg and studies have been shown you can actually build strength and muscle at that level too with a proper resistance exercise program..
The simple answer is this, as long as you consume adequate calories to maintain a healthy weight from a variety of whole plant foods, (and not from junk foods and/or just fruit) you will get in all the protein and amino acids that you need.