Protein supplementation might increase muscle anabolism and, if combined with exercise training, could also improve physical performance [1
]. The anabolic response to protein intake can be affected not only by factors such as individual nutritional state, digestive/absorption capacity, or sensitivity of muscle anabolic pathways, but also by the source of protein [4
]. In this regard, numerous protein sources have been investigated for their effect on muscle anabolism, including milk [5
], eggs [6
], soy [7
], rice [8
] or bovine colostrum [9
]. Although supplements containing whey protein (WP) have received the most attention due to their high content in leucine and potential anabolic effect [10
], beef protein (BP) has gained popularity in recent years [12
Acute ingestion of BP can increase muscle protein synthesis in both young and older individuals [13
], and these benefits seem to be maximized when combined with exercise training (particularly resistance training) [16
]. There is thus biological rationale to support BP supplementation as a potentially effective strategy for improving lean body mass (LBM) and physical performance (notably, muscle strength). However, the evidence on the effectiveness of BP supplementation for increasing muscle mass or performance is mixed, with some studies reporting benefits compared to no protein supplementation [17
] but others finding no such benefits [19
]. Moreover, the effectiveness of BP for improving body composition or performance compared to WP remains unclear [18
On the other hand, owing to its high content in heme iron, BP supplementation could also increase total iron intake and thus theoretically benefit hematological parameters, and indeed some benefits have been reported [20
]. However, to our knowledge, there is not yet meta-analytical evidence on the potential benefits of BP supplementation.
Given the purported benefits of BP supplementation on body composition and performance, and the lack of consensus on its actual effectiveness, the main aim of this systematic review and meta-analysis was to compare the effects of BP, WP or no protein supplementation (NP) combined with exercise training on body composition and exercise performance. Changes in nutritional intake and hematological parameters were also analyzed as secondary endpoints.
Protein supplementation might be a potentially beneficial strategy for maximizing exercise training-related gains in body composition and performance [2
], with WP receiving the most attention to date. This systematic review and meta-analysis showed that, when combined with exercise training, BP supplementation provides benefits on protein intake and LBM that are similar to those elicited by WP. Our results also show that BP supplementation might be an effective means of increasing total daily protein intake compared to NP, and suggest it might be also useful for improving LBM and lower-limb muscle strength, although these results should be corroborated in further studies.
The popularity of WP compared to other protein sources is mainly based on a higher digestibility of the former together with a greater content of essential amino acids such as leucine [27
]. Indeed, WP has proven to stimulate muscle protein synthesis to a greater extent than other popular protein sources such as casein or soy [28
]. The ingestion of BP has also demonstrated promising anabolic effects, with the ingestion of 30 g (equivalent to 113 g of lean beef) increasing muscle protein synthesis by ~50% compared with fasting conditions [13
]. Moreover, the combination of BP and exercise training might provide additional benefits, as the increase in muscle protein synthesis observed after BP intake followed by a resistance training session is higher than that observed with the former alone [15
]. However, WP has been reported to have a higher digestibility (as reflected by a higher Protein Digestibility Corrected Amino Acid Score, 1.00 vs. 0.92) as well as a greater content of essential amino acids than BP (52% vs. 44% of total protein, respectively), including more leucine (13.6% vs. 8.8%) and lysine (10.6% vs. 8.9%) and a similar methionine content (2.5% vs. 2.3%) [27
]. Of note, in human muscle, essential amino acids make up to 45% of the total protein content, and 9.4%, 8.7% and 2.2% of the protein comes from leucine, lysine and methionine, respectively [27
]. Burd et al. [30
] compared the mean muscle protein synthesis response to the ingestion of 30 g of protein coming from skimmed milk or beef after a resistance training session. Skimmed milk (from which WP is obtained) was more rapidly digested and absorbed than beef, thereby resulting in a greater leucine availability, and a higher stimulation of muscle protein synthesis in the early phase after exercise (0–2 h), but not for the whole recovery period (0–5 h post-exercise) [30
]. Thus, these results suggest that milk/WP might be more effective than BP for the stimulation of muscle anabolism during the early postprandial stage, but that no overall differences are found between these protein sources. In line with these findings, our results support similar mid/long-term (≥4 weeks) benefits of WP or BP supplementation on LBM.
On the other hand, the present study shows that BP might be effective compared to NP for increasing LBM and lower-limb muscle strength, although the magnitude of the benefit was small (SMD <0.50 in both cases) and this result should be confirmed in further studies as sensitivity analyses raised concern about statistical significance. Furthermore, no consistent benefits were observed on physical performance, as reflected by the lack of differences for upper-limb muscle strength. Given the abovementioned anabolic potential of BP, which also resulted in a higher total daily protein intake than NP, we expected greater benefits of BP on LBM and physical performance compared to NP. One of the factors that might explain the small benefits obtained in these endpoints is that participants already consumed an ‘optimal’ quantity of protein in their diet (1.1 to 2.0 g/kg/day for the NP group, above the current international recommendations for the general adult population of 0.8 g/kg/day [31
]). Of note, a protein intake higher than 0.8 g/kg/day is recommended to promote skeletal muscle anabolism in physically active individuals (i.e., 1.0, 1.3, and 1.6 g/kg/day for individuals with minimal, moderate, and intense physical activity levels, respectively; [32
]) as well as in older people (>1.2 g/kg/day, [33
]) such as those included in the study of Daly et al. [17
]. However, a recent meta-analysis found no additional benefits on LBM when protein intake was greater than 1.6 g/kg/day [2
]. Thus, the daily protein intake of the participants in the studies analyzed here could be considered within the ‘optimal’ range even without protein supplementation. Greater benefits might perhaps have been observed in individuals with a baseline protein intake below the recommended levels.
Another purported benefit of supplementation with BP is the potential increase in iron intake, which might theoretically improve hematological parameters. In this regard, our results show a non-significant trend towards an increased iron intake with BP compared to NP. Two studies [20
] analyzed the specific changes in heme iron intake and both found increased values with BP—although only in the female participants in one study [20
]—compared to WP or NP. In line with these results, although we could not perform a meta-analysis of hematological parameters, preliminary evidence suggests that BP could provide benefits at hematological level, such as increased hematocrit levels in collegiate distance runners [20
] or enhanced iron (ferritin) deposits in master-age triathletes [22
]. Future research should confirm if BP might serve as an effective intervention to counteract hematological conditions such as anemia in individuals with deficient iron intake.
An interesting finding was that, when compared to WP, BP supplementation seemed to increase the total daily intake of fat. However, in sensitivity analysis, this effect did not reach statistical significance and no differences in total daily fat intake were observed between BP and NP. Thus, there is not yet clear evidence to support that BP per se significantly increases fat intake. Moreover, no differences have been reported in lipid profile (e.g., cholesterol levels) or in fat mass with BP compared to WP or NP. However, more research is needed in the field.
Some limitations must be noted, notably the small number of studies included, the small sample size of most studies or the lack of participants’ blinding to group allocation in some of them. The included studies provided BP in different forms (powder or lean beef), which might have affected digestibility/absorption rates and potentially, anabolic responses [34
]. However, although minced beef is more rapidly digested/absorbed than beef steak resulting in a greater amino acid availability, no differences have been reported in post-prandial muscle protein synthesis [35
]. Future research should confirm if providing BP in isolate form (powder) results in a greater anabolic response than lean beef. The type of exercise training intervention did also differ between studies, with some including resistance training and others using endurance training. In this regard, resistance training is more effective for promoting muscle protein synthesis and muscle mass accretion than endurance exercise [36
], and this might have acted as a confounding variable in our findings. However, protein supplementation can also provide benefits in individuals performing endurance exercise training, notably by avoiding muscle catabolism—especially during periods of negative energy intake—and through the enhancement of recovery and the promotion of exercise-induced adaptations [37
]. On the other hand, we combined studies with participants of wide age ranges, including individuals aged 60 to 90 years. In this regard, aging is associated with an impaired anabolic response to both aminoacidemia and acute physical exercise [38
], which might have acted as a confounding factor in our results. Interestingly, the study that included older adults was the one reporting the greatest increases in muscle strength [17
]. Thus, BP supplementation might be particularly useful for this population segment. However, statistical analyses showed no heterogeneity between studies for the outcomes that we analyzed.
On the other hand, the benefits of supplementing with BP—and of every diet in general—should be viewed under the context of its sustainability, that is, it should have a low environmental impact that contributes to food and nutrition security and to a healthy lifestyle for present and future generations [39
]. Diets that primarily contain animal-derived food sources are associated with higher greenhouse gas emissions, and indeed beef production/consumption is the food source that results in greater gas emissions (>70 kg CO2
]. By contrast, the production of dairy products (such as milk or WP), and especially of plant-based protein foods, is associated with lower moderate greenhouse gas emissions [41
]. Thus, a balance between environmental sustainability and optimal dietary intake (particularly regarding protein consumption) should be sought, and WP might be a more recommendable option in this regard.