The main finding of this study was that resistive-training-induced muscle hypertrophy did not differ significantly between older men who consumed a diet with beef as the predominant source of protein and similar subjects who consumed an LOV diet with soy as the predominant source of protein. This finding was contrary to our original hypothesis that muscle hypertrophy would be greater in the BC group than in the LOV group, and is different from previous research in our laboratory (8
Our previous research (8
) showed that older men who consumed an omnivorous diet (ie, including protein from beef, poultry, pork, and fish) experienced different whole-body and skeletal-muscle responses to a 12-wk resistive-training program compared with men who self-selected an LOV diet (ie, containing no striated tissue but including protein from dairy products and eggs). Differences between the previous and present studies included 1
) the degree of dietary control (completely self-selected versus self-selected with portioned quantities of protein, respectively); 2
) the types of striated-tissue protein tested (omnivorous versus beef only, respectively); 3
) the quantities of protein provided (≤ 1.0 g protein · kg−1
versus ≥ 1.0 g protein · kg−1
, respectively); 4
) the frequency of resistive exercise (2 times/wk versus 3 times/wk, respectively); 5
) the methods of whole-body-composition assessment (hydrostatic weighing versus plethysmography, respectively); and 6
) the methods of muscle size assessment (muscle fiber histochemistry versus computed tomography scanning, respectively).
This protocol can be generalized in that all the foods (supplemented and self-selected) are readily available to consumers and can be prepared in a self-selected manner. It appears that the amount of protein consumed (in g · kg−1 · d−1) may be crucial with regard to the potential differential effects of the protein source (omnivorous or LOV) during periods of resistive training in older men. Further studies are needed; these studies should include women, use a longer training duration, and vary the protein content to better evaluate the influence of the protein source on factors involved in muscle hypertrophy.
The average increases in midthigh cross-sectional muscle area from weeks 3 to 15 (4.2 ± 3.0% and 6.0 ± 2.6% for the LOV and BC diets, respectively) are comparable to the muscle hypertrophy results reported by others (17
) and support the effectiveness of the 12-wk resistive-training protocol for inducing muscle hypertrophy.
Regarding total protein intake, previous research (19
) suggests that older adults need a higher protein intake than the current recommended dietary allowance (RDA) of 0.8 g protein · kg−1
). Research also suggests that long-term consumption of the RDA for protein might result in skeletal muscle atrophy (21
) consistent with metabolic and physiologic accommodations to inadequate protein intake. If this is true, marginal protein intake might also explain the decrease in fat-free mass and the trend toward less hypertrophy of type II muscle fiber in a group consuming an LOV diet, as reported by Campbell et al (8
). The lack of a detectable difference in midthigh cross-sectional muscle area between the BC and LOV groups in the present study may indicate that total protein intakes were adequate to meet the demands of protein synthesis.
The resistive-training protocol used in the present study effectively increased muscle strength in both groups. The increases in strength are consistent with the results of other investigations of similar training regimens and subject populations (8
). This finding was expected given that in general, initial strength gains induced by resistive training result from enhanced neuromuscular recruitment of the myofibrils needed to perform the required exercise (24
) and do not necessarily result from muscle hypertrophy.
The lack of change in REE may be explained by the absence of change in fat-free mass. In accord with our results, Taaffe et al (25
) observed that strength increased by 30–40% as a result of resistive training in older women, without significant changes in fat-free mass or REE. Studies that showed an increase in REE after participation in a resistive-training program (8
) also showed an increase in fat-free mass.
As with many studies on resistive training, the relatively short duration of training (12 wk) and the methods used to assess body composition and muscle size are important factors, and potential limitations, of the present investigation. We chose to use 12 wk of resistive training on the basis of previous research showing that significant muscle hypertrophy can occur within this time period (8
) and that significant diet-related differences in muscle hypertrophy responses may also occur within this time frame (4
). Regarding body-composition assessment, not all methods are equally sensitive and accurate for detecting subtle changes in soft tissue. For the present study, we decided to use computed tomography scanning as the primary method for detecting changes in skeletal muscle on the basis of research by Nelson et al (27
). They reported that this technique was one of the most sensitive ways to detect changes in soft tissue after resistive training in older women. The significant increases in midthigh muscle area measured in the present study support the use of computed tomography scanning for this purpose. Given these factors, however, one might question whether muscle hypertrophy would have been greater in the BC group than in the LOV group if the resistive-training period had extended beyond 12 wk. Future studies should use a longer training duration and a larger sample size to insure adequate statistical power, as the statistical power for the present investigation was 0.50 on the basis of the cross-sectional-area data.
The resting muscle creatine values (creatine, phosphocreatine, and total creatine) of the present study are in agreement with those of Tarnopolsky and Parise (14
) but are higher than the phosphocreatine and total creatine values reported by Campbell et al (8
). In the current study, the observed muscle creatine values did not support our hypothesis that skeletal muscle creatine content would increase at week 15 in the BC group. Raw beef contains ≈30 mmol creatine/kg dry wt (28
), but exposing meat to high temperatures for an extended period of time catalyzes the conversion of creatine to creatinine, as observed when meat is rendered for processing as animal food. This can greatly diminish the amount of creatine available for consumption (≤ 3 mmol/kg dry wt) (28
). Therefore, depending on the cooking methods chosen, the subjects may have ingested more creatinine than creatine. The finding that there was no significant difference between groups for muscle creatine content appears to indicate that there is a feedback mechanism of exogenous creatine absorption on endogenous synthesis that maintains a constant myocellular content (29
Another interesting finding in the current study was the lower muscle total creatine content in this group of older men compared with values reported previously for middle-aged (mean: 49 y) sedentary men and women studied in the same laboratory with the same analytic methods (14
). The total creatine values in the current study were 11.5% lower, and the values reported by us previously in a similar group of men were 21% lower (8
) than those of the younger group studied by Tarnopolsky and Parise (14
). These findings, in combination with the observation that creatine supplementation did not have a differential effect on strength gains in the older men and women who performed resistive exercise (30
), could indicate a relative deficiency in the creatine transporter (31
) in older people.
In summary, the primary aim of this study was to determine whether consumption of striated tissue in the form of beef influenced muscle hypertrophy in older men during 12 wk of resistive training. These results indicate that the predominant source of dietary protein (beef or soy), when consumed by older men in amounts that averaged 1.1 g protein · kg−1 · d−1, did not influence the extent of muscle hypertrophy during 12 wk of resistive training.