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Daily Protein Requirements in Various Populations: Healthy, Sick, Physically Active
Robert M. Russell, M.D., and Carmen Castaneda Sceppa, M.D., Ph.D.

Dr. Castaneda reports no commercial conflict of interest. In the last three years, Dr. Russell has received grant/research support from BASF and Roche Vitamins. Dr. Russell has also served as a consultant for Whitehall Robbins-Quaker.


Release Date: 01/03/2003
Termination Date: 01/03/2006

Estimated time to complete: 1 hour(s).

Albert Einstein College of Medicine designates this enduring material for a maximum of 1.0 AMA PRA Category 1 Credit(s)™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

Albert Einstein College of Medicine is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.
 
Learning Objectives
Upon completion of this Cyberounds®, you should be able to:
  • To understand the importance of the Recommended Dietary Allowance (RDA) for protein as it refers to adequacy of dietary protein sources and protein needs at different age groups
  • To determine the reasons why particular sub-populations would be at risk for protein malnutrition
  • To learn about the importance of eating adequate amounts of protein and the risks of excessive protein in the diet.

 

For this conference, I had a conversation with Carmen Castaneda Sceppa, M.D, Ph.D.,a scientist working at the Jean Mayer USDA/Human Nutrition Research Center on Aging. Carmen's research emphasizes protein nutrition and physiological function of healthy older individuals, as well as older people with chronic illnesses.

Russell

My general feeling from reading the literature is that protein needs are pretty well met in the United States population. That is, we do not have protein deficiency, for the most part, being reported among members of any age group in our population. How does the average intake in the United States measure up against the RDA for protein?

Sceppa

The typical American diet provides enough protein, which is higher than the RDA in most instances. The RDA, defined as the Recommended Dietary Allowance, 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 per kg body weight per day. The typical mixed American diet provides once or twice the RDA for protein. For this reason, the assumption would be that protein deficiency is unlikely in the U.S. However, the RDA for protein has been derived from research studies performed on healthy individuals. The assumptions are that these individuals are consuming adequate energy in their diets, and they are mild-to-moderately physically active. We do know from various nutritional surveys that people in the U.S. in general do meet the RDA for protein. There are some sub-populations, however, that could be at risk of protein deficiency -- growing children, pregnant and lactating women, the elderly,and anyone undergoing severe stress (trauma, hospitalization, surgery), disease or disability. In these instances, protein homeostasis could be easily affected.

Russell

Yes, we will get into those sub-populations in a little bit. You know that protein requirements can increase greatly when a patient is under physical or environmental stress. For example, in a highly stressed surgical patient, or a patient who has sepsis, protein requirements can go up as high as 1.5 g/kg per day. You implied that exercise was another factor that could increase protein requirements. However, I note that the present edition of the Recommended Dietary Allowances does not make any increase in the protein recommendation for people doing exercise. Is there new information on that?

Sceppa

What we have seen from our work is that untrained subjects undergoing endurance training seem to increase their protein needs to about 1 to 1.2 g/kg/d, well above the RDA. In contrast, for subjects performing resistance exercises or weight lifting, the RDA for protein seems to be adequate. This is probably due to the fact that resistance exercise seems to exert an anabolic effect and allows for better protein utilization. Thus, the information is somewhat contradictory.

The main reservoir for protein is muscle mass. There is no storage for protein in the body, unlike the case of fat cells for fat and muscle or liver glycogen for glucose. We need to consume enough protein to allow our muscles to be healthy and perform work. At the same time, physical activity helps increase energy intake and favors the actions of anabolic hormones such as insulin and IgF-I, which, in turn, help maintain or increase muscle mass. A well balanced typical American diet, providing between one or two times the RDA for protein, is adequate for people starting any type of exercise training or for athletes. This is particularly true in the case of people who are not purposely losing weight or have no acute or chronic debilitating illnesses.

Russell

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

Sceppa

There are two major protein sources coming from animal and plant or vegetable foods. Animal protein foods include meat, poultry, fish, dairy products and eggs. Animal protein is known to be of high biological value. That is, it contains all 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. The best example of a complementary protein diet is a well-balanced, mixed diet, one made up of all the food groups of the food pyramid.

Russell

Now that we are talking about complementary proteins, let's talk about the issue of vegetarians and people on macrobiotic diets. Does this population get into problems with protein malnutrition?

Sceppa

There are two types of vegetarians. Lacto-ovo vegetarians and strict vegetarians or "vegans". The former group is fine because they get protein of high biological value by eating eggs and dairy products. The other type of vegetarian, vegans, are more limited as to their food sources. These would be the people more likely to take amino acids supplements to make up for a high biological protein diet. But, in fact, the way to meet their protein needs is by making sure they eat a variety of plant foods, that is, cereals, nuts, seeds, grains and legumes. 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.

Russell

Can one take in too much protein? Obviously, in patients with liver and/or kidney disease, protein restriction can be part of the treatment. In the normal individual, how often, if ever, does one get into the problem of protein toxicity and how does it manifest itself?

Sceppa

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.

There could be a toxicity risk when people start consuming too much protein (over 2.0 g/kg/d). An excessive load of protein represents a stressful stimulus for the kidney. This is even more of a concern as we get older. Very high protein diets will result in hyperperfusion of the nephrons, accompanied by high glomerular filtration rates and enlargement of the basement membrane of the nephron. This results in accumulation of protein-like compounds that effect the excretory function of the kidney. This phenomenon would be present regardless of age. But, certainly, due to the age-related physiological changes that occur in all organs, including the kidney, there is no need to add more stress to the already older and sometimes debilitated kidney.

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.

Russell

These responses are graded. That is, less hyperfiltration and calcium excretion at lower protein intakes, moderate amounts of loss of calcium or hyperfiltration at moderate protein intakes, and high filtration and calcium excretion at high protein intakes. Is that true and are there any sex or age factors that enter into this equation?

Sceppa

Yes, that is the case. The response of hyperfiltration and urinary calcium excretion would depend on the amount of protein consumed, assuming there is a healthy kidney. Gender may be not so much a factor to worry about as compared to age. The older one is, the more likelihood there is a risk of disease or disability, which could potentially effect kidney function and the kidney's ability to handle a protein load. We know that people with kidney problems tend to be older. If we add the age-related changes which occur in the kidney to the added insult that a very high protein diet could represent, the answer is to be more conservative and try to limit how much dietary protein is consumed.

Russell

I want to go back to the athlete who is consuming a large amount of protein. Do you recommend that an athlete consuming a high amount of protein should take in additional calcium and vitamin B6?

Sceppa

Athletes performing weight bearing type of exercises would not need to take calcium or B vitamin supplements, provided that they eat a well balanced diet. This type of exercise, in itself, helps prevent bone loss in addition to allowing for better protein utilization and muscle mass accretion. Vitamin B6 is also present in protein-rich foods. Thus, eating adequate protein sources results in adequate vitamin B6 levels as well. The concern for these nutrients come into play in the case of athletes trying to lose weight or to maintain very light weights, who are already consuming lower amounts of these nutrients.

Russell

Protein restriction can be an important part of the management of patients with severe liver disease and encephalopathy. Are there any strategies available, that is, are any types of protein better tolerated in these patients than others?

Sceppa

Liver disease certainly poses a problem as far as to how protein and amino acids are handled in the body. The liver represents the main organ for amino acid metabolism. When liver function is impaired, amino acids that usually are being metabolized and delivered to various organs build up and become toxic. This is particularly worrisome in the case of the so-called aromatic amino acids such as tryptophan and phenylalanine.

Branch chain amino acids (leucine,isoleucine, and valine), on the other hand, are not metabolized in the liver. These amino acids go directly into the peripheral tissues such as brain, skeletal muscle and kidney. Another amino acid that can be handled well in the body during liver disease is glutamine. This amino acid is metabolized in the gut and in the kidneys and does not tend to become toxic. Both branch chain amino acids and glutamine are used in liver disease as therapeutic measures to maintain protein nutritional status. There is also some evidence regarding the use of plant protein sources in liver disease. The evidence suggests that this type of protein may not increase amino acid toxicity of aromatic-type amino acids in particular. However, I am not sure as to the bioavailability and digestibility of these protein sources in liver disease. The goal for treatment of liver disease and encephalopathy is to reduce the load of amino acids that cannot be metabolized by the sick liver.

Russell

I notice in health food stores and drug stores that there are many protein supplements on the market. Do you want to talk about the advisability of people taking these? What is the purpose for them and who is the target consumer?

Sceppa

Amino acid supplements are widely used by athletes. The purpose is to have more substrate for skeletal muscle to get stronger and to 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. Aromatic amino acids are constituents of neurotransmitters in the brain. However, for them to act properly, there needs to be a fine balance between all of the various types of amino acids. Taking amino acid supplements freely may effect this balance. Furthermore, there is a clinical condition, known as eosinophylia-myalgia, that may result from using an excess of amino acid supplements. This condition is characterized by fever, skin rash, muscle and joint pains and edema of the legs. People taking amino acid supplements, who frequently go to health food stores and self-prescribe their supplements, should be aware of these problems. Doctors seeing such symptoms would do well to inquire as to the patient's diet and any supplements he or she might be taking.