How Much Protein Do You Need?

Robert M. Russell, M.D., and Carmen Castanada Sceppa, M.D., Ph.D.

Carmen Castanada Sceppa, M.D, Ph.D., is a scientist working at the Jean
Mayer USDA/Human Nutrition Research Center on Aging at Tufts University
School of Medicine. Carmen's research emphasizes protein nutrition and
physiological function of healthy older individuals and those with chronic
illnesses.

RMR
How does the average intake in the United States measure up against the
Recommended Dietary Allowance (RDA) for protein?

CCS
The typical American diet provides plenty of protein -- more than the RDA in
most instances. The RDA represents the minimum amount of protein needed to
fulfill protein needs in 97.5% of the population. This value is equal to 0.8
g of protein per kg body weight per day. The average mixed American diet
provides from one to two times the RDA for protein. You might think, then,
based on this that protein deficiency is unlikely in the U.S. . However, the
RDA for protein has been derived from research studies performed on healthy
individuals. Growing children, pregnant and lactating women, the elderly,
and anyone undergoing severe stress (trauma, hospitalization, surgery),
disease or disability need more protein.

RMR
What if you exercise?

CCS
We have seen in our lab that individuals undergoing endurance training
increase their protein needs to about 1 to 1.2 g per kg per day, well above
the RDA. In contrast, for subjects performing resistance exercises or weight
lifting, the RDA for protein seems to be adequate. In resistance training,
you are building up muscle and protein is used more efficiently.

Muscles are built from protein. Unlike fat cells for fat and muscle or liver
for glucose, there is no place in the body to store protein. We need to
consume enough protein to allow our muscles to be healthy and perform work.

RMR
Carmen, we hear a lot in the media about balancing different types of
proteins. What are the best sources of protein and what exactly is meant by
complementary proteins?

CCS
Animal and plant or vegetable foods are the two major protein sources.
Animal protein foods include meat, poultry, fish, dairy products and eggs
and are said to be of high biological value. That is, they contain all nine
essential amino acids that can not be synthesized in the body (histidine,
isoleucine, leucine, lysine, methionine, phenylalanine, threonine,
tryptophan and valine).

Plant protein sources, although good for certain essential amino acids, do
not always offer all nine essential amino acids in a single given food. For
example, legumes lack methionine, while grains lack lysine. What is needed
are complementary proteins, various protein food sources that, eaten
together, enable a person to meet the standards of a high biologic protein
diet.

RMR
Do vegetarians and people on macrobiotic diets get into problems with
protein malnutrition?

CCS
There are two types of vegetarians. Lacto-ovo vegetarians and strict
vegetarians or "vegans". Lacto vegetarians eat animal protein of high
biological value, eggs and dairy products. Vegans, however, eat a more
limited diet and often must take amino acids supplements to make up for
their not-so-high biological protein diet. If vegans eat a variety of plant
foods -- cereals, nuts, seeds, grains and legumes -- they'll be fine. They
don't have to eat all these food items at a given meal. However, they should
consume most or all of them during the course of the day to insure a well
balanced protein diet of high biological value.

RMR
Can one take in too much protein?

CCS
The typical American diet, as we said earlier, is already providing plenty
of protein. There is no value in adding even more protein to that amount,
since protein cannot be stored in the body and the excess is eliminated in
urine and feces.

When people start consuming too much protein (over 2.0 g/kg/d), the extra
protein can become a stressful stimulus for the kidney. This is even more of
a concern as we get older and our organs are less efficient and effective.

Very high levels of dietary protein have also been correlated with increased
urinary calcium excretion. The loss of calcium through urine could
potentially be harmful for bone turnover, with the added risk of
osteoporosis. Finally, protein requires vitamin B6 in order to be
metabolized and ultimately utilized in the body. Very high levels of dietary
protein increase the requirement for this B vitamin.

RMR
Do you recommend that an athlete consuming a high amount of protein should
take in additional calcium and vitamin B6?

CCS
As we said before, athletes performing weight bearing type of exercises
don't need extra protein and, therefore, won't need to take calcium or B
vitamin supplements, provided that they eat a well balanced diet. Indeed,
weight-bearing exercise, in itself, helps prevent bone loss.

Endurance training, on the other hand, demands extra dietary protein but,
fortunately, vitamin B6 is also present in protein-rich foods. If an athlete
is trying to lose weight or to maintain very light weights, and, thus, is
already consuming lower amounts of these nutrients, then I'd recommend
additional calcium and vitamin B6.

RMR
We often put patients with severe liver disease and brain disease on
protein-restricted diets. For these individuals, are there any types of
protein that are better tolerated?

CCS
Liver disease certainly poses a problem as far as how protein and amino
acids are handled in the body. The liver is the main organ for breaking down
amino acids, so when it is impaired, amino acids levels can build up and
become toxic. This is particularly worrisome in the case of the so-called
aromatic amino acids, such as tryptophan and phenylalanine, which are
processed by the liver.

But there are other amino acids, the so-called branched chain amino acids
(leucine, isoleucine and valine), which are not metabolized in the liver and
go directly into the tissues (brain, skeletal muscle and kidney). Another
amino acid, glutamine, is broken down in the gut and the kidneys and,
therefore, for a patient with liver disease, does not tend to become toxic.
Foods (e.g., legumes) rich in these amino acids help these patients maintain
their protein nutrition.

RMR
In health food stores and drug stores, there are many protein supplements.
Should people take them?

CCS
Amino acid supplements are widely used by athletes who believe that having
more of these basic building blocks available enables skeletal muscle to get
stronger and have better endurance without the added calories. Amino acid
supplements are not digested and absorbed in the body as readily as amino
acids coming from food sources. Moreover, amino acid supplements tend to
cause an imbalance of the amino acids already present in the body. Most of
these supplements contain aromatic-type amino acids, such as tryptophan,
which are constituents of the brain's neurotransmitters. Neurotransmitters
will not act properly if the fine balance among all of the various types of
amino acids is disturbed by too much supplementation. There are other
conditions, one characterized by fever, skin rash, muscle and joint pains
and edema of the legs, known as eosinophilia-myalgia, that may result from
excess amino acid supplements. People taking amino acid products, who
frequently go to health food stores and self-prescribe them, should be aware
of these potential problems.


References
1. Proteins and Amino Acids, 1989, In: Recommended Dietary Allowances.
National Research Council, 10th. ed., National Academy Press, Washington
52-77.

2. WHO/FAO/UNU. Energy and Protein Metabolism. Geneva: World Health
Organization, 1985.

3. Munro, H., 1989, Protein Nutriture and Requirements of the Elderly. In:
Human Nutrition a Comprehensive Treatise. Nutrition, Aging, and the Elderly,
H. Munro and D. Danford, eds., Volume 6, Plenum Press, New York, 153-181.

4. Crim MC, Munro H. Proteins and amino acids. In: Shils, Olson, and Shike,
eds., Modern Nutrition in Health and Disease. Lea and Febiger, 1994:1-30.