Source: Fatfree: The Low Fat Vegetarian Archive
Two of the most pervasive myths about vegetarian diets concern protein:
Both of these are false.
How much protein do we need? This has been studied extensively. The studies try to determine how much protein is used and absorbed before there is an excess. Usually this is done by determining nitrogen balance. Nitrogen is a component of protein, and measurements of nitrogen can be used to determine the amount of amino acid quantity. By comparing the amount of nitrogen excreted with the amount of nitrogen ingested, we can determine whether or not protein is accumulating in the body, remaining at the same level, or is decreasing. The nitrogen balance test compares normal daily nitrogen losses (such as sweat, urine, feces, shed skin, lost hair etc.) with intake. A postive balance means that more nitrogen is being ingested than excreted, which means there is tissue growth, such as that in a child. A negative balance means that more nitrogen is being excreted than ingested, or more protein is being degraded than synthesized in the body. This represents a net loss of body protein.
Most of us have heard by now that the average protein requirement is about 0.8 g dietary protein / kg body weight (about 56 grams of protein for a 70 kg man). How did they arrive at that figure and how much protein does the average diet provide? Note that this figure varies, depending on where you look. In the end, the variations are not that important since we get enough anyway.
The protein requirement is not really for protein per se, but for amino acids. We break down all protein we eat into its constituent amino acids and absorb them that way. That's why diabetics can't just take insulin orally -- the insulin is a peptide hormone, and it would be broken down into its amino acid constituents as it passes through the digestive tract. It doesn't matter what the source of the amino acid is, beef, egg, or soybean. We really need to get the 8 essential amino acids from diet and it doesn't matter where they come from. The often quoted protein quality has nothing to do with the amino acid quality, lysine from beans has the same chemical structure as lysine from eggs.
The protein requirement in adults is simply a replacement for obligatory losses. Basically it amounts to keeping a leaky bucket topped up. We lose protein through skin, sweat, hair, urine and feces. We turn over a lot of protein in our bodies. We don't rely on dietary protein to build all the proteins we need, instead, we are very efficient at recycling our own tissues. In fact, we recycle between 100 and 300 grams of our own protein every day. We have an amino acid pool in our body from which we take out amino acids to build new ones, and to which we add amino acids by breaking down protein (endogenous and exogenous). We have to eat regularly to keep adding to this pool, because we don't have a store for proteins like we do for fats or carbohydrate. So our protein requirement is just that which supplies enough amino acids to replace that which is lost.
In the theoretical average 62.5kg person, about 240 g of protein is synthesized and degraded daily. This process requires about 260g of amino acid, of this amount about 1/6 must come from diet. This means that only about 1/6 of the amino acids released by endogenous protein degradation are not recycled. Where does all this protein come from? A large part (maybe even more than half) of it comes from the digestive juices, from the instestinal mucosal cells, breakdown of muscle and other tissues, etc.
So what then is the requirement? The minimum requirements for the essential amino acids are variable, dependent on study. These numbers are more for academic interest, since the Western diet, vegetarian or not, almost always provides more than enough amino acids and sometimes far in excess of requirements.
The specifics are:
|Amino||Canadian diet mg/g||Adult mg/kg||Adult mg/g|
|Methionine & Cystine||34.1||13.0||16.7|
|Phenylalanine & Tyrosine||81.8||14.0||17.9|
Taken from Nutritional Recommendations, 1990, Health and Welfare Canada.
|Amino||Infant (4-6 mos)||Child (10-12 yrs)||Adult||(For a 70kg adult)|
|Methionine & Cystine||49||22||10||700|
|Phenylalanine & tyrosine||141||22||16||1120|
What affects these requirements? Are they always the same? Definitly not. Our protein requirements can change dramatically when we are injured or sick.
Now these are the minimum requirements for the amino acids themselves, and proteins in foods contain variable amounts of these amino acids. So requirements, expressed in terms of a protein requirement not an amino acid requirement, have to take into consideration things like variable quantities of amino acids, and variable amino acid requirements.
They may double in the short term. Where does the body get this extra supply when suffering from an illness -- especially if we don't eat? From muscle, mostly. It may take a month or more to restore the nutrients used up in the body while it fights off an infection.
The protein requirement can also be affected by the amount of carbohydrate in the diet. Nonessential amino acids can be made from glucose, for example. Sometimes a significant amount is made. Adequate carbohydrate supplies reduces the need for the liver to synthesize glucose from amino acids. The liver will make sure the blood sugar is at a minimum level, and it will break down proteins to supply glucose if the glycogen reserves aren't kept at an adequate level. Brain tissue and red blood cells use 140 to 150 grams of glucose over the period of a day for example. These two tissues require glucose and the liver will make sure they get it, either from food or from converting internal protein to sugar. Long duration exercise can also lead to the burning of BCAAs. One of the effects of carbohydrates, then, is to "spare" protein.
So what's the daily protein requirement? Well, the average theoretical losses amount to about 0.34 g of protein per kg body weight per day. Clearly a recommendation to replace this loss has to have an adequate safety margin. With 2 standard deviations added to this value, it comes to 0.45 g/kg per day of "ideal" protein. Adding safety margins for digestibility and protein quality, the requirement is thus in the region of 0.75g/kg.
Different countries have different recommendations. In Canada and the U.S., the recommendation is about 0.8g/kg, while in Denmark it is around 2g/kg and in Russia, it is even higher I think. I believe the British recommendation is around 1.2g/kg, but am not exactly sure.
Stating the requirement as a percentage of calories, the Canadian RNI for protein is about 10% protein by calorie. They recommend that protein make up 13 to 15 percent of the calories of the daily diet. But remember, they also recommend that fats be kept to 30% or less, leaving little room for carbohydrates. This is not what many consider to be the optimal recommendation. Health and Welfare Canada use what they view as a pragmatic approach I guess.
One of the most common questions vegetarians hear is "So where do you get your protein?". The answer is "from everything I eat". It is a commonly held belief that we need lots of protein in our diet and the foods that supply these proteins are meat, fish and dairy products. Any diet that excludes these foods must be deficient in protein. This is false. We've discussed that safe and adequate level of protein intake for the average male adult is about 56 grams. This can be just about met by eating nothing else but 1 cup of rice and 1 cup of beans three times a day. One package of Loblaw's tofu contains 53 grams of protein (not quite 1.5 cups). As I pointed out, it doesn't matter where the requisite amino acids come from -- tofu can supply it just as well as beef can, maybe even better.
An example of a study that compared protein intake is as follows:
|Phenylalanine and Tyrosine||10.3||8.8||7.0|
|Methionine and Cysteine||4.3||3.2||2.7|
|Total protein intake||121||97||82|
So we see that even the vegans in this study got more protein than they needed.
What about protein quality? Many people have been taught that animal proteins are superior to plant proteins because animals proteins have more essential amino acids per gram of protein, some animal proteins are better digested than plant proteins and that the amino acid profiles of animal proteins better match a theoretical ideal protein than plant proteins.
It is true that animal proteins tend to have more essential amino acids per gram of protein, in general, than plant protein. But so what? We get too much protein in our diets anyway. A 12 oz T-bone steak supplies a whopping 70 grams of protein. Pity our poor kidneys. It's worthwhile to note that excessive amounts of the sulphur containing amino-acids (methionine and cysteine) have having adverse health effects. Generally only animal proteins contain large quantities of this amino acid. Many studies indict excessive protein intake by linking chronic diseases to excess protein in the diet. There is also a limit to the amount of protein we asimilate at any one time. This limit is around 25 grams. Excess protein is broken down and stored as fat. So are animal proteins at an advantage? Maybe not. What do we get from a 12 oz T-Bone? Fat and probably sick.
The amount of amino acids per gram of protein is called the amino acid profile. There is an ideal protein that we use as a reference to determine the "quality" of a protein and a few foods for comparison.
This table should dispense once and for all the claim that plant proteins are inferior to animal proteins because they are missing or lacking some essential amino acids. All plant proteins contain all essential amino acids in varying amounts. Anyone who says that we have to eat a amino acid in particular ratios at each meal doesn't really understand how the body deals with protein. What we need to do is to replenish our amino acid pool, and the amino acids do not, repeat, do not have to come from the same protein. Not even from the same meal. Soy protein can supply the lysine at lunch, wheat protein can supply the methionine at dinner. Although we do not have a store of amino acids per se, we do have an amino acid pool (circulating amino acids in the blood). The half-life of amino acids in this pool vary, but the life of the essential amino acids is at least 4 to 6 hours after digestion. Protein synthesis goes on all the time and the source of amino acids is the amino acid pool, and not necessarily the proteins you absorb immediately following a meal. Eating replenishes the amino acid pool, and there is enough of a supply that you don't have worry about eating enough of a particular amino acid at each meal. It all works out over the day. In the Western world, it is quite difficult to eat a diet that results in a protein deficiency. In Third World countries, where there are food shortages, protein deficiency coupled with insufficient calories does occur with some frequency.
We're very good at digesting protein. We can digest and absorb 70% - 90% of plant protein and 85% - 100% of animal (or human) protein. Remember, we digest human protein all the time -- our own tissues. We're very efficient at reclaiming and recycling our own protein.
An example of the digestibility of foods is:
|Food||Digestibility of protein (%)|
|Meats, Poultry, Fish||85-100|
So our digestive system will not allow much protein to slip through the body undigested. And it doesn't matter a great deal what the source of the protein is. What does it matter if the digestibility of wheat is 91% and that of eggs is 97% since the amount of protein we eat is in excess of our minimum requirements anyway?
So what are good sources of protein if you decide to eliminate all animal products from your diet?
While you get protein from just about all the foods you eat, legumes and grains can supply significant quantities of protein in the diet. Foods like TVP, tofu, seitan are concentrated sources of protein. TVP is almost all soy protein, and seitan is basically almost all wheat protein (gluten). Legumes, in general, supply large quantities of protein (12 - 15 grams/cup). Nuts and seeds also supply significant quantities of protein (like almonds or pumpkin seeds). Grains like quinoa, amaranth, oats, wheat, spelt are also very good sources of protein. Vegetables like brocolli and potatoes supply significant amounts of protein. Even a banana supplies a gram of protein.
Protein deficiency is not very common in the Western world. Kwarshiorkor is fairly common in the Third World where it is often accompanied by a deficiency of calories as well as protein (marasmus). Protein deficiency as a result of diet, in our society, can basically only happen during a restricted calorie diet or by possibly following a very restricted diet such as a fruitarian diet.
What sorts of problems can longterm excess consumption of protein lead to? Doctors like McDougall point out that excess protein in diet can contribute (not cause) to osteoporosis, for example.
Diets high in protein are often also high in purines -- this can lead to gouty arthritis.
Diets high in the sulpher containing amino acids might put people at greater risk for cardiovascular disease.
Diets high in protein stress the kidneys, because the kidneys have to get rid of the protein breakdown by-products, which can be very toxic if left to accumulate.
How much is too much? Some experts feel that anything higher than double the daily requirement is too high.
The protein requirements of humans can be readily met by a vegetarian diet with no particular effort required to combine proteins or to carefully select foods for each particular meal.