Protein needs vary with age, body-weight, health, lifestyle, and stress levels, as well as diet.
Our bodies compensate for these differences, in maintaining constant protein levels. That's true for vegetarians as well as non-vegetarians.
Nitrogen recycling makes this possible. Excess protein can't be stored; it's either broken down and burned for energy, or stored as fat (extra calories from any source are stored as fat).
When excess protein is burned or stored, the nitrogen is excreted. If you're consuming enough calories, and you don't get enough protein in your diet, your body retains nitrogen to make protein.
How much protein do we need? Let's ask Dr. Science: The nitrogen balance test compares normal daily nitrogen losses (such as sweat, urine, feces, shed skin, lost hair etc.) with intake.
A positive balance means more nitrogen coming in than going out, which means there is tissue growth. A negative balance means that more nitrogen is going out than coming into the body, or more protein is being lost than made by the body. This represents a net loss of body protein.
Here's the unscientific answer to how much protein we need: Do we look good, feel good, maintain optimum weight, and have good muscle tone? Do our hair and nails grow quickly? Do our wounds heal well? Are we generally healthy, and recover quickly from illness? If so, then we must be getting enough protein!
Our protein needs can change dramatically when we're injured or sick. They may double in the short term. The body gets this extra supply mostly from muscle, and it may take a month or more to restore the nutrients used up in the body while it fights off an infection or recovers from injury.
Increased protein needs during pregnancy and breast-feeding are usually met by the extra calories from eating more food.
Because infants and children are growing, they need more protein than adults in proportion to their body weight. Children on a varied, balanced diet usually get enough protein as long as they are getting enough calories. That's an important consideration for vegan children.
Athletes and other physically active people don't need a diet higher in protein than the recommended ratio with fats and carbohydrates. Eating more calories usually supplies the extra protein for building muscle mass. Carbohydrates are better for supplying extra energy for strenuous activity.
Protein needs are related to the amount of carbohydrate in the diet. One of the functions of carbohydrates is to "spare" protein. Non-essential amino acids can be made from glucose, for example. The liver maintains minimum blood sugar levels, and it will break down protein to supply glucose if needed.
For example, brain tissue and red blood cells use 140 to 150 grams of glucose a day. The liver makes sure they get it, either from food, or from converting internal protein to sugar. If there are enough carbohydrates in the diet, there is less need for the liver to make glucose from proteins.
What's the daily protein requirement? The body loses about 0.34 g of protein per kg body weight per day. With a safety margin added, apparently we need .45 g/kg per day of "ideal" protein to replace what's lost.
With safety margins for digestion and protein quality, our protein requirement is about .75g/kg. That's 56 grams of protein for a 70 kg man (160 lb, or an average sized guy). This figure varies widely, depending on where you look.
The variations are not that important since we get enough protein anyway, often way more than enough. So don't worry too much about scientific determinations of adequate protein intake. That way you can avoid having to translate pounds of body weight into kilograms.
There ARE ways to become protein deficient, but it's difficult. One way is not getting enough to eat. In parts of the world where famine is real, we can see people with bloated bellies who are obviously protein deficient. But they don't just lack protein - they lack calories, iron, calcium, vitamins - everything. In other words, they're starving to death.
Another way to become protein deficient is to get almost all of your calories from alcohol and/or sugar. Sugar contains no protein! Hard liquor contains almost no protein. Beer contains very small amounts. If you consume more alcohol or sugar than food, you could become protein deficient.
Another possible cause of protein deficiency is that babies are given foods they can't digest, like cows milk, or inadequate substitutes for breast milk, such as watered down formula, or rice milk.
Breast milk, the ideal food for human babies since humans began, provides 6% of calories as protein - far less than cow's milk, which has 22% of calories as protein.