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Weight Is Key to Protein Requirements

Study Shows Protein in Diet Should Be Based on Weight, Not Age
By Robynne Boyd
WebMD Health News
Reviewed by Louise Chang, MD

Nov. 7, 2008 -- The amount of protein an adult needs to stay healthy is based on weight, not age.

That's according to a new study published in the American Journal of Clinical Nutrition.

As people age, their metabolism and physiology usually change. And these changes can influence a person's nutritional needs. Although many researchers believe that older adults require more protein than younger adults, it's not reflected in the current Recommended Dietary Allowance (RDA) and Estimated Average Requirement (EAR), which are the same for all healthy men and women aged 19 and older.

The RDA and EAR for protein is 0.80 grams and 0.66 grams of protein per kilogram of body weight per day, respectively. That's an RDA of about 54 grams of protein a day for a 150-pound adult, or approximately 1.5 chicken breasts and a 7-ounce steak.

Wayne Campbell, PhD a researcher and professor of foods and nutrition at Purdue University, and colleagues developed a study to test whether protein requirements actually change with age.

The researchers recruited 42 people to participate in the study: 11 young men, 12 young women, eight older men, and 11 older women. The age range for the younger and older adults was 21 to 46 and 63 to 81.

Each participant underwent three 18-day study periods in which his or her diet was firmly restricted. During each 18-day trial, they were given 63%, 94%, or 125% of the Recommended Dietary Allowance of protein. They were also allowed to eat their usual diet for at least one week between the study periods.

During the 14th to 17th day of each trial, the researchers measured the participants' nitrogen balance. Nitrogen balance determines the difference between how much nitrogen (mainly from protein) a person is ingesting and excreting (as waste). Healthy adults eliminate the same amount of nitrogen as they consume.

Researchers say the study shows that younger and older adults don't require different amount of protein to be healthy. This means that the RDA of 0.8 grams of protein per kilogram of body weight should be adequate for virtually every older person.

However, in an editorial accompanying the study, Joe Millward, PhD, the head of nutrition and safety at the at the University of Surrey, England, writes that even though the study shows that an adult's daily protein requirement is likely to be independent of age, it should not "be classed as definitive in terms of the absolute magnitude of the requirement." He notes the difficulty in precisely assessing nitrogen balance.

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