Protein requirement studies in elderly people: previously reviewed data

Protein requirement studies in elderly subjects, utilizing the N balance technique as the standard method of assessment, have been conducted by several research groups during the past 16 years (Table 1). Data that were available to and reviewed by the 1985 Consultation included the studies by Cheng et al, Uany et al, Zanni et al and Gersovitz et al. These studies did indeed provide conflicting results, with some studies (Cheng et al, 1978; Zanni et al, 1979) concluding that 0.75 g protein/kg/d is adequate, and others (Gersovitz et al, 1982; Uany et al, 1978) concluding that this protein intake is inadequate for N equilibrium in elderly individuals. Differences in results and conclusions among these four studies may be due, in part, to differences in physiologic and metabolic status of subjects within each study and between the studies, as well as differences in study design and the assumptions used in calculating N balance. Indeed, each study used a somewhat different formula to calculate N balance, none of which was exactly the formula recommended for use by the 1985 Consultation.

Cheng et al (1978) estimated that a mean protein intake of 0.77 g protein/kg/d was required for N equilibrium in seven elderly male nursing home residents. They concluded that 0.8 g protein/kg/d high-biological-value protein was adequate for these elderly men.

Table 1 Protein requirement studies in elderly people: subjects and protein intake information

M = male, F = female.

However, each subject's N balance was overestimated by not accounting for miscellaneous N losses, and no upward adjustment was made in the mean protein requirement to provide a safe level of protein intake for nearly all elderly people.

Uauy et al (1978) measured N balance in seven elderly women and seven elderly men provided weight maintenance diets for 10-day periods with various quantities of whole-egg as the protein source. Mean N equilibrium was calculated to be 0.83 g protein/kg/d in the elderly women, and estimated to range from 0.7 to 0.85 g protein/kg/d in elderly men. The authors concluded that these elderly women and men would obtain adequate protein with diets providing 12-14% of total energy intake as protein. This roughly corresponded to protein intakes of 0.95-1.10 g protein/kg/d for these elderly women and 0.830.97 g protein/kg/d for these elderly men, based on calculated mean energy intake data provided in the original publication. Uany et al did not specifically adjust the mean protein requirement upward to provide most elderly people with a safe level of protein intake.

Zanni et al (1979) estimated a mean protein requirement of 0.46 g protein/kg/d based on data from six elderly men, and concluded that the 1973 FAO/WHO safe intake of 0.57 g protein/kg/d was adequate for apparently healthy elderly men. Nitrogen balance was determined while each man consumed either a protein free diet for 17 days, or diets providing either about 1.44 or 1.73 times measured endogenous N losses (15 days each period). Several factors may have contributed to an underestimation of actual protein requirements in this study, including the measurement of differences in N balance over a small range of inadequate protein intakes (0.36-0.46 g protein/kg/d), including data from the protein-free balance period in the N balance calculations when it is generally recognized that this will lower the protein requirement estimate due to non-linearity of the N balance response in this submaintenance range (FAO/WHO/UNU, 1985), and increasing N retention by not providing a restabilization period between the 17-day protein-free balance period and the other balance periods.

Gersovitz et al (1982) provided weight-maintaining diets with ().8 g protein/kg/d to seven elderly men and eight elderly women continuously for 30 days. Nitrogen balance was assessed on study days 6-10, 16-20 and 26-30. During the initial 10-day period, all seven men and four of eight women were in negative balance. Mean N balance was improved (i.e. N equilibrium reached) for both men and women on days 26-30, but three men (43%) and four women (50%) were in negative N balance. These long-term N balance data indicated that 0.8 g protein/kg/d was not adequate for a majority of elderly men and women.

Table 2 Protein requirement assessments in elderly people

M = male, F == female.
^a Recalculated data presented by Campbell et al (1994).
^b Lower and upper limits of 95% confidence interval.
^c 1985 FAO/WHO/UNU nitrogen balance formula used for original calculation of protein requirement.