We already know that our protein needs include a requirement for quantity, which is how many
total grams of protein we need to eat, but also a requirement for quality, which means
we want to make sure that we get enough of all the essential amino acids in the right
proportions.
Another parameter to be considered is the bioavailability of the proteins from a particular
source, basically how much of that protein is digested and absorbed.
This largely depends on inter individual variability: stomach acidity, enzymatic activity, carriers
and the co presence of some minerals and vitamins.
But it also depends on how foods are associated: as you can easily understand, if we eat a
yogurt alone we will absorb more of its proteins than if we eat it together with a high fiber
meal.
In some foods, The presence of inhibitors of proteolytic enzymes can decrease protein
availability.
Cooking also has an effect.
In general, cooking increases protein availability from most food both animal and vegetables,
because it starts denaturation, softens tissues, and may inactivate inhibitors.
On the other hand, excessive exposure to high temperatures or extended cooking times can
degrade proteins and create toxic compounds.
As a general rule of thumb, however, proteins from animal sources are absorbed for over
90%, while absorption from vegetal sources ranges between 70 and 90%.
So how much protein do we need?
We already know that our protein turnover is very high, about 250 grams per day, but
that doesn't mean we have to get all of those proteins from food, because most amino
acids from protein breakdown are recycled, so protein dietary needs are much lower than
protein turnover.
So we cannot use protein protein turnover to determine how much protein we need to eat,
and the way we do it instead is we just want to match the protein we lose with the protein
we eat.
We want to make sure that the amount of protein we lose or catabolize, we bring back in, so
that we maintain nitrogen balance.
How do we lose protein?
Our primary route of excretion is the urine.
Careful, we do not normally excrete protein themselves in the urine, but we excrete the
catabolic products of protein metabolism, such as urea.
By quantifying nitrogen in the urine we can back calculate how many grams of protein were
broken down to get that amount of urinary nitrogen.
And then on top of that we also lose some proteins directly with our feces, skin, hair,
nails, sperm, although for a healthy person that's normally a very small amount.
So based on this calculation, which is not easy by the way but we're not discussing
this here, we find out that our protein requirement is generally low but also highly variable.
The average adult needs 0.5 grams of protein per kilo of body weight per day.
But since most people are not average, some individuals have needs as low as 0.2 grams
per kilo per day, and some other have needs as high as 0.8 grams per kilo per day.
But because the individual person normally doesn't know if he needs 0.2, 0.5 or 0.8,
and because protein is so important, and it is considered that it is much better to have
more than you need rather than having less, the recommended daily allowance for protein
is 0.8 grams per kilo of body weight per day for everyone.
In reality only about 2% of the population really needs 0.8 grams per kilo per day, everybody
else needs less, but not knowing, and to be sure that everybody's covered, we recommend
the maximum for everybody.
So to have an idea, if we do this calculation for an average 70 kg adult, we get
70*0.8 = 56 g of protein per day.
When we express protein requirement like this, we also want to make sure that energy intake
is adequate, so we want to make sure that we get enough carbs and fats for energy use,
because otherwise we will have to divert part of that protein for energy production, and
then it's not enough anymore.
So this recommendation is for an efficient protein utilization.
Of course this calculation is based on a healthy body weight.
If you are obese, you don't make this calculation based on your actual weight, because that
would overestimate your protein requirements.
An alternative rule of thumb is to express protein needs as percent of total energy intake,
and make sure that proteins account for 10 to 35% of the total calories, which is the
acceptable distribution range set for protein by the DRI guidelines.
There are some situations in which protein needs are increased.
Growth is one.
Here you need more proteins because you are building new tissues, not just maintaining
them.
For this reason, infants, adolescents and pregnant women all have higher than normal
protein needs.
Another situation is resistance training, such as weight training or body building.
Here you want to build some new muscle, so again you are building new tissue, not just
maintaining it, and you'll need about 10 to 20 grams of extra proteins for it.
Also, if you are ill, injured or recovering from illness or injury, your protein need
will also be higher because you'll have a lot more repairing to do and because you
need to build acute phase proteins to fight infections.
Stress of illness can dramatically increase protein requirements, and some particularly
hyper catabolic states, such as burns or trauma, require twice if not three times the average
need for a healthy individual.
Meeting protein requirements with a normal diet is easy, and this is also true for athletes
that turn to expensive protein isolates, high protein bars and protein shakes.
Most of us have already more than covered our protein requirements before we even get
to dinner.
To get a sense of the amounts of proteins that are provided by regular servings of what
we normally eat, I will now make you a few examples of food servings, all providing high
quality proteins.
There are 21 to 25 grams of protein in a 3 ounce serving of meat, poultry or fish.
There are 16 grams of proteins in 2 eggs.
There are 11 grams of protein in a 6 ounces container of plain yogurt.
There are 14 grams of proteins in a 3 ounce serving of tofu.
A serving of brown rice with lentils provides 19 grams of high quality proteins.
A slice of whole wheat bread with three tablespoons of chickpeas hummus provides 8 grams of high
quality proteins.
There are 3 grams of proteins in 5 walnuts.
There are 4 grams of proteins in a tablespoon of peanut butter.
If we put two tablespoons of peanut butter on a slice of whole wheat bread, we get 12
grams of proteins, and if we drink an 8 ounces glass of soy milk together with it, which
provides 7 more grams of proteins, we get a total of 19 grams of high quality proteins.
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