Most Americans get more than enough protein in their diets.
And there's evidence that too much protein puts a strain on the kidneys because our bodies excrete the excess rather than store it as we do fat or carbohydrate.
But about 10 percent of European American women and 20 percent of African American women over age 55 consume less than 30 grams of protein daily, even though they get ample calories, according to the second National Health and Nutrition Examination Survey. That's half the Recommended Dietary Allowance (RDA).
"We're concerned about that group," says Marilyn C. Crim, a physician and nutrition scientist at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston.
"Elderly women who consume well below the RDA may be adapting by losing lean body mass and muscle function. This increases their risk for illness."
Dietary protein provides the raw material for muscle, bone, and internal organs, as well as for the enzymes that make our metabolic processes go.
Says Crim, "If we can prevent loss of muscle functionthe ability to get around and perform normal tasksthen I feel we can help them."
The issue has even broader international implications.
"In Guatemala and other developing countries, people get about three-quarters of the RDA on average. Many get only half the RDA," says Carmen Castaneda, a physician from Guatemala and now a nutrition scientist at the USDA center.
Her interest in the effects of low-protein diets led her to design a 9-week study with Crim and others for her doctoral thesis. Throughout the study, six women were given meals plus a beverage with enough milk proteins added to bring their protein intake a little above the RDA. Another six got the same meals and a low-protein beverage, providing about half the RDA for protein. The women were between 66 and 79 years old.
Few studies have assessed the amount of protein required by people over age 50. Until there's more data, the National Research Council has kept the RDA for elders the same as for adults under age 50 0.8 gram per kilogram of body weight per day. For a 140-pound person, that translates to 63 grams of protein each day. That's about the amount in two chicken breasts, three ground beef patties, or a cup of tuna salad.
Traditionally, protein requirements have been based on the amount people needed to stay in balance. That's when protein intake matches the amount being excreted from the body. When protein intake drops, people will excrete more than they eat for a while, explains Castaneda. Eventually, they reach a balance. "But you don't know what changes the body went through to reach that balance," she says.
Crim emphasizes that biochemical measurements "don't tell you much about how well the person functions in everyday life. We're in a new era of looking at physiological function as part of the parameters we measure."
To study the impact of adapting to low-protein intakes, the researchers measured the women's muscle function, their body composition, and their immune responsein addition to blood protein levels and nitrogen balanceto detect any changes.
Proteins are unique among the major nutrients in that they contain nitrogen. Researchers can follow protein intake and excretion rates by measuring nitrogen.
Only one of the women given half the protein RDA reached nitrogen balance by the end of 9 weeks, says Castaneda. The rest were almost in balance but still excreting more than they consumed. And the body's attempt to reach equilibrium took its toll on all of them.
"These women had significant losses in lean tissue, immune response, and muscle function," says Castaneda.
They lost an average 8 percent of lean tissue, most of which was muscle. One measure of immune responsea hypersensitivity skin testwas 50 percent lower by the end of the study. And the amount of weight they could push in a chest press exercise dropped by 12 percent.
By contrast, the six women who consumed a little more than the RDA stayed in nitrogen balance throughout the study without any changes in muscle mass. In fact, several measurements of muscle function and ermine response improved significantly, as did several blood protein measurements.
"Our findings suggest that this 'healthy' group of women may not have had an optimal diet prior to the study," the researchers conclude.
They also speculate that "the loss of lean tissue that has been attributed to aging may be due, in part, to chronic dietary protein deficiency."
"The women did very well on the higher protein dietwhich averaged 15 percent more than the current RDA," says Castaneda, noting that the study was not designed to determine a protein requirement.
The most striking impairment in the low-protein group may turn out to have an important implication. It occurred in a muscle function test that measures how long it takes the thumb's adductor pollicis muscle to contract and relax after a mild electrical stimulation.
This test has been used as a measure of early stages of starvation, says Castaneda. "We're looking for early indicators of chronic protein malnutritionwhen people are getting ample calories."
Just 3 weeks into the studybefore any measurable changes in body composition had occurred, the women on the low-protein diet had a 46-percent drop in normal muscle response to this test. By Judy McBride, ARS.
Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, 711 Washington St., Boston, MA 02111; phone (617) 556-3000; fax (617) 556-3344.
"Subtle Larceny: Too Little Protein in Elders" was published in the July 1995 issue of Agricultural Research magazine.