Many nutritional interventions have been considered to enhance recovery from exercise. The body of published research supports the practice of ingesting nutrients to enhance performance for both endurance and resistance training athletes. There is also sound evidence which supports the value of post-exercise nutritional supplementation as a means of improving the recovery of intramuscular glycogen, providing a positive stimulation for acute changes in amino acid kinetics and improvement of the net PRO balance, as well as enhancing the overall adaptation to resistance training.
Maximization of muscle glycogen re-synthesis
Athletes who ingest 1.5 g CHO/kg body wt. within 30 minutes after exercise have been shown to experience a greater rate of muscle glycogen re-synthesis than when supplementation is delayed by two hours, largely due to a greater sensitivity of muscle to insulin . Additionally, both solid and liquid forms of CHO promote similar levels of glycogen re-synthesis [15, 62, 63]. Moreover, different forms of CHO have different effects on insulin levels, with fructose ingestion being associated with lower levels of glycogen re-synthesis than other forms of simple carbohydrates . It has been demonstrated that delaying CHO ingestion by as little as two hours can reduce the rate of muscle glycogen re-synthesis by 50% . If an athlete is glycogen-depleted after exercise, a CHO intake of 0.6 – 1.0 g CHO/kg/h during the first 30 minutes, and again every two hours for 4 – 6 hours, can adequately replace glycogen stores [65, 66]. Similarly, maximal glycogen re-synthesis rates have been achieved when 1.2 g CHO/kg/h is consumed every 15 – 30 minutes [65, 67]. Consequently, frequent feedings of CHO in high amounts over the 4 – 6 hours following exercise is recommended to ensure recovery of muscle and liver glycogen [15, 49]. Additional studies have also reported that maximal glycogen levels can be restored within 24 h if optimal levels of CHO are available (8 g CHO/kg/day), and the degree of glycogen depletion is not too severe . A CHO intake of 9 – 10 g CHO/kg/day is suggested for athletes who are completing intense exercise bouts on consecutive days .
Several studies have suggested that adding PRO to CHO supplementation after exercise may help to promote greater recovery of muscle glycogen and attenuate muscle damage. Ivy and colleagues  instructed cyclists to complete a 2.5 h bout of intense cycling before ingesting either a CHO + PRO + Fat (80 g CHO, 28 g PRO, 6 g Fat), low CHO (80 g CHO, 6 g fat), or a high CHO (108 g CHO, 6 g fat) supplement immediately after exercise, and 2 h post-exercise, to determine if the CHO + PRO + Fat combination promoted greater restoration of muscle glycogen. While glycogen replenishment did not differ between the two CHO conditions (low CHO [70.0 ± 4.0 mmol/kg/wet wt] and high CHO [75.5 ± 2.8 mmol/kg/wet wt]), muscle glycogen levels were significantly greater (p < 0.05) in the CHO + PRO + Fat treatment (88.8 ± 4.4 mmol/kg/wet wt). The authors concluded that a CHO + PRO + Fat supplement may be more effective because of its provocation of a more pronounced insulin response [66, 69, 70]. Similarly, studies by Berardi and Tarnopolsky [71, 72] utilized cyclists for the completion of exercise bouts of 60 – 90 min on separate occasions before ingesting CHO + PRO or CHO alone. Both authors concluded that ingestion of either CHO preparation resulted in greater restoration of muscle glycogen when compared to a placebo. Berardi , however, reported even greater glycogen levels when the CHO + PRO combination was consumed post-exercise. Furthermore, the availability of essential amino acids (EAA) following exercise, especially the branched-chain amino acids, have been reported to influence recovery by optimizing PRO re-synthesis as well as glycogen re-synthesis rates after exercise [61, 69, 70, 72–74]. As these studies suggest, the ingestion of CHO (1 – 1.5 g CHO/kg/day) within 30 minutes following the termination of an exercise bout promotes restoration of muscle glycogen, while the addition of PRO may have additional benefits in enhancing both muscle PRO and glycogen re-synthesis.
Acute changes in amino acid kinetics and protein balance
A single bout of resistance training modestly stimulates PRO synthesis, but also further stimulates PRO breakdown resulting in an overall negative PRO balance after exercise [75, 76]; an effect which shifts PRO balance more towards neutral as training status progresses . Infusion or ingestion of amino acids increases amino acid concentrations at rest or after resistance exercise . In addition, providing CHO in combination with amino acids immediately before or after exercise may further increase amino acid availability and post-exercise PRO synthesis [73, 78]. Consequently, increasing the concentration and availability of amino acids in the blood is an important consideration when attempting to promote increases in lean tissue and improve body composition with resistance training [77, 79].
Ingestion of a large dose of CHO (100 g) alone and within 1 h after resistance exercise causes marginal improvements in overall PRO synthesis while maintaining a negative net PRO balance . While no studies have found CHO to be detrimental, it is not the ideal nutrient (in isolation) to consume after resistance exercise. Its inclusion, however, is an important consideration regarding stimulation of glycogen re-synthesis and enhanced palatability [69, 72]. The EAAs, however, in dosages ranging from 6 – 40 grams have routinely been shown to play a primary role in promoting muscle PRO synthesis [74, 80], though adding CHO to them may enhance this effect [9, 81].
Regarding post-exercise timing, ingestion of amino acids after resistance exercise has been shown at many different time points to stimulate increases in muscle PRO synthesis, cause minimal changes in PRO breakdown and increase overall PRO balance [74, 75, 80]. Unfortunately, the optimal time point for supplementation has not yet been demonstrated. Similar changes have been found in studies that have administered amino acids alone, or with CHO, immediately, 1 h, 2 h and 3 h after exercise [9, 74, 79, 81]. Levenhagen et al.  found that after ingesting 10 g PRO + 8 g CHO + 3 g Fat either immediately or 3 h after 60 min of moderate-intensity exercise, leg muscle glucose uptake and whole body glucose utilization were elevated threefold and 44%, respectively. Leg muscle and whole-body PRO synthesis was increased threefold and 12%, respectively. Furthermore, Tipton and colleagues  supplemented participants with 35 g sucrose + 6 g EAAs immediately before, and immediately after, a single bout of resistance exercise. They reported significantly greater levels of PRO synthesis when the nutrients were ingested immediately before the exercise bout.
In summary, the optimal dosage and ratio of EAAs and CHO necessary to optimize protein balance is not currently known. Studies using similar techniques to measure protein kinetics during resistance exercise have used 6 g EAA only, 6 g EAA + 6 g non-essential amino acids, 12 g EAA only, 17.5 g whey PRO, 20 g casein PRO, 20 g whey PRO, 40 g mixed amino acid, and 40 g EAA only; all have noted similar increases in PRO synthesis and PRO balance [9, 73, 77]. While the ratio of CHO to PRO requires additional investigation, a often utilized practical approach is to consume a supplement containing CHO + PRO in a 3:1 or 4:1 ratio within 30 minutes following exercise, which translates to 1.2 – 1.5 g/kg of simple CHO (e.g., dextrose, sucrose) with 0.3 – 0.5 g/kg of a quality PRO containing EAA [73, 74, 83]. A summary of relevant findings is provided in Table 2 (Additional File 2).
Post-exercise supplementation for promotion of training adaptations
In an attempt to stimulate greater adaptations associated with resistance training researchers have investigated the impact of administering varying combinations of CHO and PRO after (1 – 3 h post-exercise) each exercise bout over the course of training [8, 10, 32, 84–91]. The collective findings of these studies support the rationale for post-exercise administration of CHO and PRO to facilitate greater improvements in strength and body composition. Additionally, PRO source may be an important consideration as studies have suggested that whey PRO may exhibit a faster kinetic digestive pattern when compared to casein PRO [92, 93]. Furthermore, this faster kinetic pattern for whey PRO is responsible for greater increases in PRO synthesis upon ingestion, with little to no impact over PRO breakdown. Casein PRO, on the other hand, releases its amino acids at a slower rate from the gut. This kinetic pattern results in little control over PRO synthesis, but a powerful attenuation of PRO breakdown. When both of these milk PRO sources are compared using area under the curve analysis, results suggest that casein may be responsible for a greater overall improvement in PRO balance when compared to whey [92, 93]. A summary of these studies is provided below in table 3 (Additional File 3), but the universal findings of these studies suggest that adding some combination of CHO (50 – 75 g) to a PRO source (20 – 75 g) while completing heavy resistance training facilitates an increase in the development of lean mass and overall improvements in body fat %.
Adding creatine to carbohydrate and protein
In addition to providing a combination of CHO + PRO after regular resistance training, researchers have also examined the impact of adding creatine monohydrate (Cr) in an attempt to facilitate greater training adaptations [84, 85, 88, 90]. Cr is a popular dietary supplement that has been heavily researched for its ability to increase performance and facilitate positive training adaptations [94, 95]. For example, Tarnopolsky et al.  had previously untrained male participants undergo resistance training for eight weeks while ingesting, in a double-blind fashion, either a Cr (10 g) + CHO (75 g) or PRO (10 g) + CHO (75 g) combination 30 min after exercise. Following assessment of changes in strength and muscle mass, the Cr + CHO group gained significantly more body mass (5.4% increase from baseline) when compared to the PRO + CHO group (2.4% increase). Changes in fat-free mass, muscle fiber area, 1 RM, and isokinetic strength improved in both groups, but were not different among groups. Another study had participants resistance train for 11 weeks while consuming daily one of the following: 1) 0.1 g Cr/kg/day + 1.5 g CHO/kg/day, 2) 0.1 g Cr/kg/day + 1.5 g whey PRO/kg/day), 3) 1.5 g/kg/d whey PRO only or 4) 1.5 g CHO/kg/day only. Supplementation in the first three groups resulted in greater increases in 1 RM strength and muscle hypertrophy when compared to CHO only, but no differences were found among the groups ingesting Cr in conjunction with either CHO or PRO .
In contrast, two published studies have suggested that the addition of Cr may be responsible for greater increases in muscle hypertrophy. The first study had participants complete heavy resistance training for 10 weeks while ingesting one of the following isoenergetic groups: 1) 1.5 g/kg/day of PRO only, 2) 0.75 g PRO/kg/day + 0.75 g CHO/kg/day, or 3) 0.1 g Cr/kg/day + 0.75 g PRO/kg/day + 0.75 g CHO/kg/day. Changes in strength and muscle hypertrophy were found to be greater in the Cr + CHO + PRO group when compared to the CHO + PRO group . Similarly, Kerksick and colleagues  had participants complete 12 weeks of resistance training while ingesting a blend of whey and casein PRO, with or without Cr. While all groups saw increases in strength and muscle mass, those groups ingesting Cr with the PRO blend experienced greater gains in body mass and fat-free mass. Though these findings are somewhat mixed, the available data does provide support that adding Cr to a post-exercise regimen of CHO and PRO may help to facilitate greater improvements in body composition during resistance training [84, 85, 88, 90].
Summary of post-exercise nutrient ingestion findings
• Post-exercise (within 30 minutes) consumption of CHO at high dosages (8 – 10 g CHO/kg/day) has been shown to stimulate muscle glycogen re-synthesis [15, 65].
• Adding PRO (0.2 g – 0.5 g PRO/kg/day) to CHO at a ratio of approximately 3: 1 (CHO: PRO) has been shown to stimulate glycogen re-synthesis to a greater extent .
• Post-exercise ingestion (immediately after through 3 hours post) of amino acids, primarily EAAs, have been shown to stimulate robust increases in muscle PRO synthesis [73, 74, 83]. The addition of CHO may increase PRO synthesis even more, while pre-exercise consumption may result in the best response of all .
• During prolonged resistance training, post-exercise consumption of CHO + PRO supplements in varying amounts have been shown to stimulate improvements in strength and body composition when compared to control, placebo, or CHO-only conditions [10, 87, 90].
• The addition of Cr (0.1 g Cr/kg/day) to a CHO + PRO supplement may facilitate even greater adaptations to resistance training [84, 88].