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.
How does the average intake in the United States measure up against the Recommended Dietary Allowance (RDA) for protein?
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.
What if you exercise?
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.
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?
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.
Do vegetarians and people on macrobiotic diets get into problems with protein malnutrition?
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.
Can one take in too much protein?
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.
Do you recommend that an athlete consuming a high amount of protein should take in additional calcium and vitamin B6?
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.
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?
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.
In health food stores and drug stores, there are many protein supplements. Should people take them?
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.