Introduction
β-hydroxy-β-methylbutyrate (HMB) is a metabolite of the branched-chain amino acid leucine, which has received increasing interest as an ergogenic aid for training-induced gains in strength and body composition. The mechanisms behind HMB supplementation are related to both anti-catabolic and anabolic effects, enhancing protein synthesis via a mammalian target of rapamycin (mTOR) dependent mechanism, and inhibiting the ubiquitin-proteasome proteolytic pathway responsible for the specific degradation of intracellular proteins [
1]. There is also evidence that HMB regulates adipose tissue function including fatty acid oxidation, lipolysis, and adipokine secretion, which may be mediated by mTOR and p-AMP-activated protein kinase α (AMPKα) [
2]. Combined HMB-supplementation and resistance training has been reported to positively affect health and physical performance by increasing the fat free mass (FFM) and decreasing fat mass (FM), reducing muscle soreness and improving strength in both previously untrained and trained individuals [
3‐
8]. Increased FFM has also been shown to increase metabolic rate [
9], maximal oxygen uptake, and ventilatory thresholds [
10,
11]. These adaptations have been attributed to the AMPKα-mTOR pathway and its effect on mitochondrial biogenesis, oxygen consumption and the efficiency of carbohydrate, glycogen and fat metabolism [
12]. However, various studies have also questioned the effectiveness of HMB to augment lean mass and performance gains with resistance training in healthy adults [
13‐
17]. Systematic reviews show that about one-third of the studies could not confirm any benefits on strength training-induced effects of HMB on body composition [
1,
18,
19]. Some of the conflicting results might be attributed to study designs and methodological issues. In particular, it should be noted that the majority of previous strength training studies on HMB were rather short, lasting eight weeks or less. There is also increasing interest whether HMB supplementation can augment strength training-induced gains in FFM when a high-protein diet is provided [
5,
20‐
23]. Recent studies suggest that protein balance is the critical driver for strength-training induced changes in muscle mass in resistance-trained men [
20,
22,
23]. Accordingly, it has been questioned whether HMB supplementation can provide any additional benefits over and above regular protein intake on body composition and performance. To investigate whether a combination of whey protein and calcium HMB exert additive effects on FFM and strength in untrained individuals, we conducted a randomized controlled trial in which healthy young men received a protein supplementation and HMB or protein supplementation and placebo during a periodized 12-week resistance training program.
Discussion
We investigated the effects of whey protein supplementation combined with HMB vs. whey protein supplementation and placebo on body composition, strength, and aerobic capacity in healthy, young men after a periodized 12-week resistance training. We found training-induced changes in whole-body and segmental FFM, and maximal strength in both groups. The intake of whey protein plus HMB resulted in larger increases in segmental FFM compared to ingestion of whey protein only.
Our results provide new insights into the discrepancy between previous studies showing that HMB supplementation enhances resistance training-induced gains in hypertrophy and those that could not confirm any additional ergogenic benefits of HMB on body composition. We found increases in whole-body FFM of 2.2 kg in WP + PLA and 3.4 kg in WP + HMB. These results are in agreement with previous studies, reporting gains of about 2 kg in whole-body FFM after twelve weeks of resistance training in young men not receiving any HMB supplementation [
4,
6,
8]. In contrast, we could not replicate the large gains in whole-body FFM reported for the HMB supplemented groups in these studies. Kraemer et al. [
4], Wilson et al. [
8], and Lowery et al. [
6] observed gains of about 7 to 8 kg FFM in subjects supplemented with 3 g of HMB per day. Our results are more in line with data recently published by Jakubowski et al. [
20], who reported 2.3 kg and 2.6 kg gains in whole-body FFM after 12-weeks of resistance training with protein plus HMB (3 g per day) and protein plus leucine (3 g per day) supplementation, respectively. Such moderate increases in FFM are also supported by a recent meta-analysis, showing that with total protein intake of at least 1.6 g·kg
− 1·day
− 1 training-induced gains in FFM can reach a maximum of about 4 kg with an average of about 1.5 kg [
38].
Since the difference in gains between WP + HMB and WP + PLA was considerably smaller compared to previous studies [
4,
6,
8], we failed to detect a significant treatment effect on whole-body FFM. To verify this, we performed a power analysis using the R package simr. Monte Carlo simulations were used to assess the power for the mixed model investigating the effects of Time and Treatment. One thousand simulations were run to assess power for samples between
n = 20 and 100. We found that a sample size of
n = 60 individuals per group would have been needed to detect a significant treatment effect of WP + HMB with > 80% power (α = 0.05, two-sided) if there is one. It is also possible that whole-body BIS might not have been accurate and precise enough to detect any differences in body composition between the groups. Whereas other studies also failed to detect changes in body composition using dual-energy x-ray absorptiometry (DXA) [
20], future studies should consider more sophisticated techniques such as DXA, air displacement plethysmography, or magnetic resonance imaging to examine the effects of HMB on whole-body composition.
Segmental FFM seemed to be more sensitive in detecting the effect of HMB. We observed larger effect sizes for gains in segmental FFM compared to gains in whole-body FFM (
d = 1.12 vs. 0.60 for leg FFM and whole-body FFM, respectively). Increases in leg FFM were significantly larger in WP + HMB (+ 14.2%) compared to WP + PLA (+ 7.0%), whereas no difference in gains was detected for whole-body FFM. We also investigated changes in segmental CSA
FFM using MRI measurements. Neither arm nor leg CSA
FFM revealed any hypertrophic effect. We attribute this to methodological differences between the approaches for assessing segmental FFM. In line with previous studies, CSA
FFM was assessed from a single MRI scan of the arm and leg, respectively. Training-induced changes in muscle volume are not identical along with muscles, and a single slice may not be representative of changes in total limb composition [
39]. This is in line with a recent validation study comparing muscle thickness and volume determined by ultrasound and MRI [
40]. The authors of the study concluded that a muscle thickness measurement at 50% of vastus lateralis length is a reliable index of muscle volume at a single time point, but poorly tracks changes in muscle volume. There was also no significant correlation between changes in muscle volume and changes in muscle thickness, highlighting the impact of resistance training on regional hypertrophy [
40].
In the present study, we also used segmental BIS to determine arm and leg FFM volume. We have previously shown that segmental BIS can provide accurate estimates of arm and leg muscle volume compared to limb volume measurements using MRI [
41]. Because segmental measurements for estimating FFM reflect more closely the underlying biophysical model of BIS to whole-body BIS (assumption of isotropic conductor with homogeneous cross-sectional area and consistent proportions specific tissues), we expected segmental measurements to be more accurate and precise than whole-body FFM estimations [
27,
28,
31]. The methodological difference between single slice recordings and total limb estimations in tracking changes in segmental hypertrophy may also explain the lack of detecting an ergogenic effect associated with HMB supplementation in two very recent studies, employing similar study designs [
20,
22]. However, it should also be noted that the subjects in the present study were untrained men, whereas the afore-mentioned study were performed in resistance-trained men. It is possible that the mTOR and ubiquitin-proteasome proteolytic pathways are already maximized in resistance-trained athletes due to high training loads and the time course of long-term physiological adaptations and explain why muscle protein synthesis may not be augmented in this population. This may be particularly true, when sufficient protein and energy intake is provided [
22,
23]. The additional effects of HMB on body composition over and above regular protein intake might therefore only be observed in untrained participants, the elderly population, and bedridden people, but not in resistance-trained athletes [
22].
HMB did not affect any of the performance measures that were assessed in the present study. Maximal oxygen uptake remained unchanged. The changes in 1-RM were in a typical range reported in previous 12-week resistance training studies for bench press [
3,
8,
20] and leg press [
3], but considerably lower than those reported by Kraemer et al. [
4] and Lowery et al. [
6]. Some of these studies reported no [
20] or a slight effect [
3] of HMB on 1-RM gains, whereas others found a strong effect [
4,
6,
8]. Our findings were in line with a study by Gallagher et al. [
42] that examined the effects of HMB, administered in doses of 0 g, 3 g or 6 g per day on FFM and strength during eight weeks of resistance training in 37 untrained men [
42]. HMB supplementation resulted in greater increases in FFM compared to a placebo group, but there were no differences in gains of 1-RM between groups. This supports the finding of the present study that longer training programs with combined HMB supplementation could lead to an increase in FFM without any significant changes in 1-RM. It is also possible that the discrepancy between changes in FFM and 1-RM is related to methodologies issues in measuring body composition, i.e., that the increases in FFM are due to an increase of extracellular water and not contractile tissue. Classical bioelectrical impedance analysis performed at 50 kHz is particularly prone to changes between extra- and intracellular because at this frequency the extracellular space dominates the resistance [
27,
31]. In the present study we employed bioimpedance spectroscopy that allows to distinguish between intra- and extracellular water based on a biophysical model. Since we did not observe any significant changes in ECW/ICW ratios between pre- and post-testing, suggesting that the changes in FFM are unlikely to be the result of an expansion of extracellular water.
Moreover, whereas Gallagher et al. [
42] found no changes in 1-RM, they reported group differences between HMB and placebo supplemented groups in peak isometric and isokinetic torque, stressing the importance of testing specificity for interpreting performance measures of strength. In the present study, care was taken to minimize the effect of non-specific testing, i.e., 1-RM was determined using exercises that were also specifically part of the intervention program (barbell bench press, dumbbell bench press, and leg press). It is possible that testing exercises unrelated to the training program would have revealed larger increases for WP + HMB compared to WP + PLA because of the greater increases observed in segmental FFM, and reducing the effects associated with neurological adaptations on maximal strength for specific exercises.
\( Relative\dot{V}{\mathrm{O}}_{2\mathrm{max}} \) decreased slightly as a result of higher body mass, but absolute
\( \dot{V}{\mathrm{O}}_{2\mathrm{max}} \) was unchanged. This is in contrast to several studies that reported an increase in
\( \dot{V}{\mathrm{O}}_{2\mathrm{max}} \) after HMB supplementation [
10‐
12,
43]. All of these studies investigated athletes in sports where aerobic capacity and endurance play a key role and/or participants who were exposed to a specific endurance training program during the HMB supplementation. In contrast, in the present study, participants were asked to refrain from any additional exercise other than the strength training program. We suggest that the differences between previously reported effects of HMB on
\( \dot{V}{\mathrm{O}}_{2\mathrm{max}} \) and our findings could be explained by the lack of intense cardiovascular training in the present study.
The majority of previous studies assessing the effects of HMB on body composition comprised a time frame of eight weeks or less, which may be inadequate to identify the adaptations unique to HMB supplementation [
44]. The present study comprised twelve weeks, and according to the authors’ knowledge, this is the longest placebo-controlled study combining HMB and whey protein supplementation to assess strength training-induced effects on body composition. Notably, Jakubowski et al. [
20] and Tritto et al. [
23] also investigated the effects of HMB supplementation during a 12-week resistance training program. However, the study by Jakubowski et al. [
20] was not placebo-controlled, and Tritto et al. [
23] did not assess the combined effects of HMB and protein supplementation [
20,
23]. Moreover, the training program in the present study was carefully designed and periodized into two mesocycles (weeks 1 to 6 and weeks 8 to 12), which were separated by a 1-week taper phase.
Our findings on whole-body composition confirm recent studies revealing no additional benefit of HMB supplementation over and above regular protein intake on body composition and strength [
20,
22,
23]. However, HMB supplementation was associated with significantly greater gains in leg FFM, suggesting that measurements of segmental limb composition are more sensitive to detect smaller changes in FFM. It is possible that a more elaborate timing of the supplementation schedule might have further increased these effects. Exercise stimulates a delayed increase in ubiquitin conjugating activity, which induces a late-phase rise in protein degradation that is critical for muscle adaptation to exercise [
45]. Hence, it is possible that the timing of HMB administration in the study was suboptimal. All subjects received protein supplementation immediately before exercise and about 1 h after completion of the exercise, but ingested HMB capsules at rather fixed points in time (in the mornings, at lunch and in the evenings) and not necessarily aligned with the training sessions. However, Wilson et al. [
46] showed that HMB consumed 1 hour prior to exercise decreased muscle soreness and reduced the accumulation of lactate dehydrogenase compared to a placebo or when the supplement was administered post-exercise [
46]. In line with that, the position stand on HMB as a supplement by the International Society of Sports Nutrition (ISSN) conclude that HMB’s acute effects are likely to be maximized by administering an HMB dose before exercise [
1]. However, the authors also recommend that the chronic anabolic effects are optimized by consuming three daily servings of 1 g each.
A critical limitation of the present study is the lack of a non-supplemented control group, which would have provided a better understanding of the resistance-training induced effects per se, irrespective of any protein and HMB supplementation. We also acknowledge that dietary habits were not assessed in the present study, which presents a limitation of the current results because it is possible that the placebo group might have ingested a sub-optimal amount of dietary protein to maximize adaptations to the resistance training program. Finally, it should be noted that the whey protein included 7.3% of leucine. Under normal conditions, approximately 5% (2 to 10%) of leucine is converted to HMB [
47]. Based on 60 g and 30 g of protein supplementation on training and non-training days, this corresponds to an additional availability of 0.2 g and 0.1 g (max. 0.4 g and 0.2 g), respectively. We do not expect these doses critically confounded our results by minimizing the effects attributed to the WP + HMB relative to WP + PLA.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.