Aging is associated with a gradual and progressive loss of muscle mass and function [1
]. In addition, muscle protein synthesis is attenuated in older individuals in the postprandial period, characterizing a process called anabolic resistance [3
]. Middle-aged adults have similar negative metabolic consequences of aging [5
], and postmenopausal women are included in this context. This population deserves special attention because a decrease in serum estrogen levels is observed during this period [6
], which seems to have direct and indirect effects on skeletal muscle [8
], such as inhibiting the production of catabolic cytokines, including interleukins-1 and -6 and tumor necrosis factor-α [9
]. These effects may lead to greater muscle and strength loss, concomitantly with greater body fat accumulation [10
]. Resistance training and adequate protein intake are known strategies to increase lean body mass (LBM) [11
]. A greater intake of high-quality proteins seems to promote additional muscle gain in resistance exercise protocol [13
]. Thus, the association of both interventions seems to be an effective strategy to promote LBM gain [14
In this context, the protein intake proposed by the Recommended Dietary Allowance (RDA) for older adults, including postmenopausal women, is 0.8 g·kg−1
]. However, current studies have shown that a higher amount of protein, ~1.2 g·kg−1
, seems to be more effective in stimulating muscle protein synthesis (MPS) or muscle maintenance [16
]. This implies that the RDA recommendation seems to be insufficient for older individuals, although this is not a consensus [22
]. A recent study compared the intake of 1.2 g·kg−1
to 0.8 g·kg−1
in older men, over the course of 3 days, and no difference in acute MPS stimulation after exercise was observed between groups [22
]. However, considering that acute measurements of MPS do not seem to be directly associated with longer-term changes in muscular hypertrophy [23
], long-term studies are needed to evaluate the effect of this protein intake on LBM gain to confirm these results and include postmenopausal women in the target group.
Therefore, the aim of our study was to compare the effect of higher protein intake (~1.2 g·kg−1·day−1) with RDA recommendations (~0.8 g·kg−1·day−1) on LBM gain in postmenopausal women following resistance exercise protocol. We hypothesized that higher protein intake would enhance LBM gain associated with resistance exercise.
The main finding of the present study was that, contrary to our initial hypothesis, a higher protein intake (1.18 ± 0.3 g·kg−1·day−1) did not result in higher LBM gain when compared with an intake similar to that proposed by the RDA (0.87 ± 0.29 g·kg−1·day−1) after a 10-week resistance training protocol in postmenopausal women.
Recent studies have shown that protein intake recommended by the RDA seems to be insufficient to enhance the maximum MPS in elderly women and to maintain muscle mass during aging, therefore higher doses have been proposed [16
]. This difference in protein dose recommendation is possibly due to the methodologies applied. Protein intake proposed by the RDA uses the nitrogen balance method, which has several limitations [38
]. Currently, a new proposal for the recommended protein intake bases its methodology on the amino acid oxidation technique, which is an advanced, independent tracer-based method that circumvents many nitrogen balance limitations [18
]. However, in our study, we did not find a greater LBM gain in the HP group, suggesting that the higher protein intake, which is close to new protein proposal/recommendations seems to have no additional benefits in long-term changes of LBM in older women.
The protein intake values compared in our study were obtained through the means of each group, and some individuals ingested higher or lower doses than the group average. However, this does not appear to be a limitation in our study because when the LBM gain of the participants that only ingested an amount equal to or less than 0.8 g·kg−1
was compared to those who ingested equal to or greater than 1.2 g·kg−1
the results remained the same (data not shown). In addition, we did not find an association between Δ protein intake (g) and Δ lean mass (kg), when evaluating all individuals together, which showed that the increase in protein intake had little to no effect on lean mass gain. Furthermore, the recommendations of protein intake for maximum muscle hypertrophy proposed by the American College of Sports Medicine position is 1.2–2.0 g·kg−1
]; therefore, we suggest that active postmenopausal women may need an even higher quantity of protein to promote higher LBM gains than offered in the present study. A study conducted by Tieland et al. evaluated the LBM gain in elderly men and women performing resistance training 2 times/week over 24 weeks [13
]. The researchers observed a gain of ~1.3 kg of LBM in the group with consumption of ~1.3 g·kg−1
of protein (with protein supplementation), whereas in placebo (with consumption of ~0.9 g·kg−1
of protein), no LBM change was observed. In the present study, the postmenopausal women increased ~1.3 kg of LBM regardless of protein consumption. The differences between the studies for LBM gain can be explained by our younger sample age and the fact that the participants performed resistance exercise 3 times/week (6 sets of 8–12 repetitions), which contained exercises that recruited large muscle groups of upper and lower limbs. However, it is possible to suggest that if our volunteers were older, the intake of ~0.8 g·kg−1
of protein could promote lower LBM gain than higher protein intake, but new studies are needed to confirm it.
Besides total protein intake, the amount of protein intake per meal seems to be important for MPS and/or LBM gain, although this is not a consensus [35
]. A balanced distribution of total protein intake with meals containing at least 30–40 g of high quality protein seems to be more effective in stimulating rates of MPS throughout the day in older individuals [40
]. However, to the best of our knowledge, there are no chronic studies assessing the effect of protein distribution during the day on LBM gain in postmenopausal women, which is an important knowledge gap [41
]. In our study, only the HP group reached this threshold at lunch (32.41 ± 7.08 g protein), whereas the NP group did not reach the recommendation at any meal. A research conducted by Farsijani et al. [42
] demonstrated that over 2 years, people who consumed larger amounts of protein and more distributed throughout the day presented a greater amount of LBM. Another study found that more frequent consumption of meals containing 30–45 g of protein presents higher association with LLM and strength [43
]. Therefore, only 1 meal reaching the recommended protein intake (as found in our study) may not be able to produce higher changes in LBM, and possibly, more meals reaching that threshold are needed, which increase the amount of protein intake per day.
Moreover, controlled experiments have elucidated that the amounts of protein containing 2–3 g of leucine can stimulate maximum MPS [44
]. In our research, both the NP and HP groups reached the recommended amount of leucine per meal at lunch (2.05 ± 0.44 and 2.57 ± 0.61 g, respectively). In addition, the HP group increased the intake of leucine to ~2 g per day but this increase was distributed throughout the day, not just 1 meal. This did not appear to be sufficient to induce maximum MPS. McDonald et al. [16
] demonstrated in a longitudinal study that older individuals (>65 years) ingesting 1.26 g protein·kg−1
and 7.10 g/day of leucine presented higher LBM maintenance over 6 years, whereas lower intake was associated with LBM loss. In our study, the HP group presented similar intake of both protein and leucine, but a higher increase in LBM was not observed when compared to lower protein intake group. Although an interventional study (present study) cannot be compared to a longitudinal study, it is possible to suggest that the length of time that ~1.2 g·kg−1
vs. ~0.8 g·kg−1
of protein is ingested may be important to promote greater gains and/or maintenance in LBM. Therefore, new interventional studies lasting more than 10 weeks that evaluate the effect of the RDA versus higher protein intake plus resistance exercise in LBM gain in postmenopausal women, are needed.
In our study, both groups increased LBM (~1.3 kg) similarly after the intervention, demonstrating that the resistance training was able to promote muscle hypertrophy in postmenopausal women after 10-weeks of training. This gain is in accordance with the literature, which shows that resistance training was effective in leading to muscular hypertrophy [47
], including in postmenopausal women [8
]. We found improvements in bench press RM and leg extension RM only in the HP group (independently of lean mass gain), but there were no differences in relation to the NP group when evaluating time-treatment interaction and Δ values. The same strength gains might have contributed similar LBM gains between groups.
We also found that higher protein intake did not result in changes of body fat. As we offered a normocaloric eating plan, the absence of fat loss was already expected. A normocaloric diet was prescribed because our main aim was to promote muscle hypertrophy and it is known that caloric restriction can attenuate this process [48
]. In addition, there was no fat mass gain in the present study, which also seems important to promote maximum hypertrophy because the increase of trunk adiposity is correlated with lower LBM gain in postmenopausal women performing resistance training protocol [26
The present study has limitations. An underreporting of dietary intake was observed in both groups, since the individuals related an energy intake similar to resting energy expenditure and no fat mass loss was observed. The postmenopausal women evaluated in the present study had high adiposity (~40% of body fat) and it was already shown that overweight and/or obese women underreport their energy intake [49
]. This can be a limitation of the present study because the underreporting could be related to all macronutrients, including protein, which was the main dietary variable. However, it is possible to suggest that the protein intake was not affected by this underreporting because all dietary recalls were specifically focused on protein evaluation (daily and per meal), and the individuals reported a similar protein intake at baseline (~0.8 g/kg/day) than those found in other studies that evaluated postmenopausal women or older adults [13
]. Furthermore, Lafay et al. [52
] evaluated if energy intake underreporting occurs for specific food groups and found that certain food items rich in fat and carbohydrates such as butter, French fries, sugars, cakes, and biscuits were qualitatively underestimated. On the other hand, no difference was found for foods rich in protein when comparing underreported versus non-underreported individuals [52
]; therefore, all these factors strengthen the proposition that protein intake was not underreported in our dietary data. However, even if an underreporting in protein intake had occurred, it would likely have occurred in both groups and we would continue to show that higher amounts of protein did not promote higher LBM gain. Another limitation was the large drop-out rate (~50%) in our sample; however, both groups presented similar losses during the study and no differences were observed between groups at baseline, which demonstrates the reliability of our data.
As for strengths of the present study, the protein intake before and after exercise was controlled between groups, since it seems that the timing of protein intake might be important for LBM gain [35
]. In addition, indirect calorimetry was used to measure the resting energy expenditure of individuals for diet prescription. Additionally, our dietary intervention was based on increasing protein intake by a variety of protein sources, which represents a more realistic nutritional management in clinical practice.