The primary aim of this study was to explore the effects of chronic low-to-moderate dietary protein intakes on appetitive responses in younger and older men. A controlled feeding design was used to ensure dietary compliance. The protein intake-dependent differences in BUN and urinary total nitrogen excretion confirmed that subjects followed the protocol. Previous studies primarily focused on levels of protein intake above the current estimated average requirement (EAR) of 0.66 g protein · kg BW−1
and RDA of 0.80 g protein · kg BW−1
). However, because critical physiologic changes in muscle mass occur when the amount of protein consumed is at the RDA (22
) and below the EAR (24
), the current study examined protein intakes above, within, and below the normal range of adequacy, or 1.00, 0.75, and 0.50 g protein · kg BW−1
, respectively. Previous studies have shown that individuals who eat lower amounts of protein are at higher risk for weight gain (21
). This is potentially due to decreased satiety associated with lower protein intake, a lower thermogenesis rate observed with lower protein intake, and/or lower preservation of fat-free mass (20
). In our study, regardless of age, hunger and desire to eat were higher when the subjects consumed diets that contained 63 and 94% of the RDA for protein, intakes that might be considered inadequate and marginal, respectively, than when 125% of the RDA for protein was consumed. These results indicate that appetitive responses are affected by the amount of protein consumed within the range of adequacy. These results also indicate that it is possible to detect differences in appetite with relatively small (0.25 g protein · kg BW−1
) changes in protein intake.
A paucity of data exists on the effects of habituation at different levels of protein intake on appetite in humans. Long et al. (7
) evaluated whether a person’s habitual protein intake affected the appetitive response to ingesting high-protein test meals. Subjects were recruited based on their self-selected protein intakes, which were ~1.0 (125% RDA) and ~1.4 (175% RDA) g protein · kg BW−1
, in the lower and higher protein groups, respectively. On a testing day, each subject rated their hunger and fullness hourly over a 13-h period and consumed test meals at h 0, 5, and 9 that provided 2.2 g protein · kg BW−1
(275% RDA). For the next 13 d, the lower and higher protein groups consumed diets that contained 0.75 (94% RDA) and 1.96 (245% RDA) g protein · kg BW−1
, respectively, and the same testing protocol repeated. On the first testing day, the lower protein group reported lower hunger and higher fullness than the higher protein group when both groups consumed the same high-protein meals. A similar trend, that was not supported statistically, was observed after the 13-d period of dietary manipulation. Long et al. (7
) concluded that a person’s habitual protein intake will influence their appetitive response to a high-protein meal. This experiment did not evaluate the appetitive responses of the 2 groups consuming meals that contained the same amounts of protein that they were habituated to, as was done for the current study.
Providing the subjects with isoenergetic-controlled diets with 3 levels of protein provided us with a unique opportunity to document their appetitive responses without the confounding influence of changes in energy intake, because they were not allowed to act upon any appetitive changes. A shift in motivation to eat protein has occurred in individuals who are protein deficient (14
). Malnourished Mexican infants ingested ~23% more soup when it contained casein hydrolysate (18.4 ± 2.7 mL) than plain soup (14.1 ± 2.1 mL) (P < 0.01) (28
). The nonmalnourished control children ate similar amounts of both soups (28
). In another study measuring chemosensory perception, 26 persons tasted soup with 0, 1, 2, 3, 4, and 5% casein, w:v. Of 26 persons, 16 were elderly (aged 70–92 y) and 10 were younger (aged 18–26 y). The elderly persons had lower serum protein and albumin and higher BUN concentrations than the younger persons. The elderly persons rated the higher casein hydrolysate concentrations as more palatable than did the younger participants (14
). Among the younger and elderly persons combined, persons with lower concentrations of serum albumin and persons with elevated BUN expressed greater liking for a higher concentration of casein hydrolysate (1.5 vs. 0.4%, w:v) (14
). Consistent with the idea that people who consume inadequate protein show a preference for higher protein foods, Gibson et al. (29
) showed, in 18–49 y–old subjects, that consumption of a lower-protein (vs. higher-protein) breakfast resulted in a preference for higher-protein flavored foods at a midday meal. Further, this preference for higher-protein flavors occurred mainly at the end of the meal. The authors suggested that the preference for protein was most likely to occur after a person’s energy needs were met. Collectively, these findings support the hypothesis that chronic ingestion of a diet containing inadequate protein, but adequate energy, elevates the motivation to eat protein, affects protein flavor preference, and triggers appetitive changes consistent with a desire to consume food.
The gLMS subjectively assesses the indices of appetite in individuals who have unique and personalized definitions for the terms “hunger,” “fullness,” and “desire to eat.” Tools like the gLMS are difficult if not impossible to validate against an objective measure such as food intake; however, when used under conditions of fixed feeding and repeated measurements within subjects, like the current study, they might provide important information that likely would not be possible to obtain from measurements of voluntary food intake (30
). The gLMS is documented to be sensitive to a variety of experimental manipulations including changes in energy intake and diet composition [see (30
) for review]. The use of a quasi-logarithmic scale to space the intermediate identifiers results in the potential for a relatively small quantitative change at the lower end of the scale (i.e., toward “barely detectable”) to represent a larger magnitude difference in perception than a comparable quantitative change at the higher end (i.e., toward “strongest imaginable”). The shift from below to above one of the intermediate identifiers represents an order of magnitude change in rating. These issues might be important with regard to the findings from the current study. Although statistically significant, the quantitative changes in hunger and desire to eat documented using the gLMS were relatively small (increased from P125 to P63 of ≤0.8 cm for daily mean hunger and ≤1.1 cm for daily mean desire to eat, on a 10 cm full range gLMS). These changes, however, shifted the mean values from between the “weak” and “moderate” identifiers to between the “moderate” and “strong” identifiers. Likewise, peak hunger shifted from between the “moderate” and “strong” identifiers to between the “strong” and “very strong” identifiers.
Appetite changes with age, and the anorexia of aging may relate to numerous social, psychological, hormonal, and medical factors that collectively result in inadequate feeding (31
). The present finding, that the older men perceived greater fullness than the younger men despite consuming less total energy daily, supports observations that older men are generally more satiated (34
) and become satiated more quickly during a meal (35
). Some researchers have reported an age-related decrease in hunger (36
), although this was not observed in our study. It is important to note, however, that the use of a controlled feeding experimental design limits generalization of these findings to an ad libitum diet.
As protein was varied to specified amounts, fat and carbohydrate did not change. Thus, the consistency of these macronutrients (as a percentage of nonprotein energy) likely minimized macronutrient influences on appetite. Trial-dependent differences in fiber intake occurred and could have confounded the results. However, the finding that hunger was significantly different between the P125 and P94 trials when there were no differences in fiber intake, argue against fiber being the dominant dietary factor causing the differential appetite responses among trials. All fiber intakes were generally 25–35 g/d. Recent studies have demonstrated that fiber has a limited effect on appetite (37
). Overall, this indicates the results presented are likely due to the differences in dietary protein, not fiber. The results of this study indicate that inadequate dietary protein intake leads to increased hunger and desire to eat in men.