The around the individual’s workout session. It has been

      The Importance of Nutrient Timing in AthletesAbstractNutrient timing is a popular strategy which advocates the consumption of a combination of nutrients- primarily carbohydrates and proteins- around the individual’s workout session. It has been claimed that this approach can produce dramatic improvements in body composition. It has even been postulated that the timing of nutritional consumption may be more important than the absolute daily intake of nutrients. While some research has demonstrated that the timed ingestion of nutrients –majorly carbohydrates and proteins- may significantly affect the adaptive response to exercise, some show that the timing of nutrients hardly play a role if the target macronutrients are fulfilled by the end of the day. Several researchers have made reference to an anabolic “window of opportunity” whereby a limited time exists after training to optimize training-related muscular adaptations. In this paper, we identify the dietary guidelines for athletes and its physiological and psychological implications on beginners, intermediate and advanced lifters and if nutrient (carbohydrates and proteins) timing, in fact helps an individual maximize his potential. We will also discuss about a popular dieting strategy, Carb Backloading, in detail and review, if there are any additional benefits, to consuming carbs only post-workout.IntroductionOver the past few years, there have been a number of extensive research studies revolving around the concept of nutrient timing. This was based on the consumption of nutrients- carbohydrates and proteins- in and around a training session. The strategy is designed to maximize exercise-induced muscular adaptations and facilitate repair of damaged tissue 1. Theoretically, consuming the proper amount of nutrients around the training session not only helps in the restoration of damaged muscle tissues and replenishment of glycogen reserves, it also aids in better body composition and glycogen super-compensation, wherein after a bout of resistance training, the muscles are able to hold a greater amount of glycogen than they normally would be able to, hence super-compensating the muscle glycogen reserves and making it look fuller and bigger. Also, researchers have made reference to an anabolic “window of opportunity” whereby a limited time exists after training to optimize training-related muscular adaptations 5. However, this depends on a number of factors – the duration between training bouts, how well trained the athlete is – beginner, intermediate or advanced.Nutrient 1: ProteinsPre-exercise ingestion of proteinsMany studies have been conducted to explore the use of pre-exercise PRO and CHO ingestion in preventing acute exercise-induced muscle damage. Tipton et al. 15 reported that the ingestion of a mixture of essential amino acids and CHO before resistance exercise was more effective for the stimulation of post-exercise muscle protein synthesis than ingesting the same mixture immediately after exercise. This was attributed to the combination of increased amino acid levels at a time when blood flow is increased during exercise, thereby offering a greater stimulation of muscle protein synthesis by increasing amino acid delivery to the muscle. However, in a following study, the same study group was unable to demonstrate the same findings when examining the impact of 20 g of whey protein ingested before as opposed to 1 h after resistance-type exercise on muscle protein balance measured over a 4- to 5-h recovery period 16. This study, along with a few others, have concluded that rate of muscle protein synthesis is stimulated by ingestion of protein sources and the timing of protein ingestion is not important. 14, 16, 17, 18. Intra-exercise ingestion of proteinsThe ingestion of protein before or during exercise could be of even more benefit during the early stages of recovery from more intense exercise bouts and has been investigated in many short term studies 19.  In one study, participants completed 3 h of cycling @ 45 – 75% VO2max, followed by a time to exhaustion trial at 85% VO2max. During each session, participants consumed either a placebo, a 7.75% CHO solution, or a 7.75% CHO/1.94% PRO solution. While the CHO only group increased time to exhaustion (19.7 ± 4.6 min) versus the placebo (12.7 ± 3.1 min), the addition of PRO resulted in even greater performance (26.9 ± 4.5 min) 20. Another study by Saunders et al. analyzed the impact of a CHO + PRO combination for its ability to improve performance and minimize muscle damage 21. Cyclists exercised to exhaustion on two different occasions separated by 12 – 15 h. During exercise, the participants were given a CHO solution or a CHO+PRO solution. It was found that the participants who ingested the CHO+PRO solution had a much higher increase in performance and the muscle damage markers were significantly lower, suggesting the CHO + PRO supplement helped to attenuate the muscle damage associated with prolonged and exhaustive exercise. Similarly, few other short term studies showed a positive effects of consuming a CHO+PRO solution during exercise on attenuating muscle damage by lowering muscle damage markers such as creatine kinase 22, 23. However, in a long term study conducted for one week on elite cyclists by Mette et al., the supplementation of a PRO+CHO solution during exercise did not improve recovery or performance in elite cyclists despite high demands of daily exhaustive sessions during the one-week training camp. 24 Post-exercise ingestion of proteinsMost of the studies that have been done on nutrient timing are based on protein consumption after a training session and whether or not an “anabolic window” exists which could help an individual maximize his/her training-related muscular adaptations.  Some of the earlier studies which were done supported the idea of an anabolic window post training. For example, Esmarck et al. 25 provided evidence that consuming protein immediately post training enhanced muscular adaptations better than when the protein intake was delayed. Thirteen untrained elderly male volunteers were matched in pairs based on body composition and daily protein intake and divided into two groups: P0 and P2. P0received a protein supplement immediately post-exercise while P2 received the same supplement 2 hours following the exercise bout. After the end of the study period, they noticed that cross-sectional area (CSA) of the quadriceps femoris significantly increased in the P0 group while no significant increase was seen in P2, hence suggesting that delaying post-workout nutrient intake may impede muscular gains. In contrast to this, there have been numerous studies which have refuted the existence of the “window of opportunity” whereby a limited time exists after training to optimize training-related muscular adaptations. Verdijk et al. 26 didn’t notice any increase in skeletal muscle tissue in elderly men from consuming a post-exercise protein supplement. 26 elderly men were randomly assigned to a progressive, 12 week resistance training program with (protein group) or without (placebo group) protein provided before and immediately after each exercise session. After 12 weeks, no significant differences in muscle strength or hypertrophy were noted between groups indicating that timed nutrient supplementation does not enhance training related adaptations. Similarly, Hoffman et al. 27 conducted a study on 33 well- trained young men with protein supplementation given either in the morning and evening or immediately before and immediately after resistance exercise. At the end of the study, no significant between-group or absolute changes in body composition was observed. A lot of other studies have echoed the same sentiment when it came to timing of protein supplementation, be it in untrained individuals or trained athletes 28-31. Summary1. Pre-exercise/ intra-exercise intake of protein doesn’t have any additional benefits as compared to any other time of the day. However, addition of a PRO+CHO solution during prolonged endurance events might offset muscle damage.2. The post-exercise ‘anabolic window’ lasts a lot longer than what was originally hypothesized, indicating that ingesting a protein supplement immediately post exercise doesn’t have any additional benefits.Nutrient 2: CarbohydratesPre-exercise ingestion of carbohydratesGlycogen is considered to be quintessential for resistance training athletes, with 80% of ATP being derived from glycolysis. MacDougall et al. 7 demonstrated that a single set of elbow flexion at 80% of 1 repetition maximum performed to muscular failure caused a 12% reduction in mixed-muscle glycogen concentration, while three sets at this intensity resulted in a 24% decrease. Similarly, Robergs et al. 8 reported that six sets of 70% one repetition maximum (1 RM, I-70) and 35% 1 RM performed to muscular failure resulted in a 26.1% reduction of glycogen stores in the vastus lateralis while six sets at this intensity led to a 38% decrease, primarily resulting from glycogen depletion in type II fibers compared to type I fibers. As glycogen levels diminish, ATP production is hampered, exercise intensity decreases which results in suppression of the immune system 1. Research involving the ingestion of single high CHO feedings has also demonstrated the promotion of higher levels of muscle glycogen and an improvement of blood glucose maintenance (euglycemia) 1. This ensures the athlete is able to push harder during the training and the ATP production is enhanced via glycolysis. Also, glycogen availability also has been shown to mediate muscle protein breakdown. Lemon and Mullin 9 found that nitrogen losses more than doubled following a bout of exercise in a glycogen-depleted versus glycogen-loaded state. However, not all studies showed the same result. Hawley and Burke summarized several studies that administered some form of CHO within one hour prior to exercise: one study reported a decrease in performance, three studies reported an increase in performance and some studies reported no effect 10.  In light of these conditions, maintaining a higher muscle glycogen level before training appears to be beneficial to resistance training results.Intra-exercise ingestion of carbohydratesMost of the research involving the consumption of carbohydrates during workout have been performed on endurance athletes who train for longer periods of time (at least 90 minutes) or resistance training bouts spanning for very long hours. This is most likely because this is when glycogen stores will be significantly depleted because of the duration of the workouts, as opposed to a regular resistance training session in which glycogen can be depleted by about approximately 36-39% only 1. Carbohydrates during an endurance event maintain high rates of CHO oxidation which undergo glycolysis to provide ATP, offsets muscle damage and prevent hypoglycemia 11. A study conducted by Kulik et al. studied the effects of supplemental carbohydrate (CHO) ingestion on the performance of squats to exhaustion 12. In this study, the subjects were made to do sets of 5 reps to exhaustion at 85% of their 1RM.  Subjects consumed 0.3g.kgCHO.bodymass or a placebo of equal volume immediately before exercise and after every other completed set of squats. There was no significant statistical difference between groups in total sets, volume, work or the rate of perceived exertion. This study suggests that intra workout carbohydrates might not be as beneficial for an athlete performing resistance training. Post-exercise ingestion of carbohydratesIt is common lore that the post- exercise carbohydrates must have a substantial glycemic and insulinemic response in order to optimize recovery and there are studies which support this claim. One such study showed that glycogen storage was 2–3 times faster during four hours post-exercise resulting in greater glycogen storage at four hours rather than later in the day, suggesting that delaying the ingestion of a carbohydrates post-exercise will result in a reduced rate of muscle glycogen storage 4. However, there are numerous studies which suggest that consuming carbohydrates immediately post-workout may not aid in faster glycogen replenishment. In a study conducted by Parkin et al., 6 trained cyclists cycled at 70% of VO2 max for 2 hours followed by four 30 second sprints. Post exercise, all subjects were given 5 high glycemic index meals over a 24 hour period, the first three being fed immediately post workout and 3 being fed starting 2 hours after the training session. Muscle biopsies taken at 8 and 24 hours revealed that there was no difference in muscle glycogen or glucose-6-phosphate in either trial 13. These studies present conflicting opinions but taking all the relevant points into consideration, it becomes quite clear that accelerating glycogen resynthesis is important for endurance athletes when the duration between two events is less or for resistance training athletes who perform 2 training sessions in a day provided the same muscles are being worked in both the sessions. Summary1. Pre-exercise intake of carbohydrates may help with increased glycogen levels which translates to higher energy output in the training session. 2. Intra-exercise intake of carbohydrates are more beneficial for endurance/resistance training athletes who train for very long hours and are at risk of significant glycogen depletion.3. Post-exercise intake of carbohydrates help with glycogen resynthesis but the rate remains the same even at 24 hours, indicating that completing the CHO requirements before the next training session is optimal enough for athletes.Carbohydrate Backloading: A reviewCarbohydrate Backloading is a dieting strategy that was popularized by John Keifer. This strategy emphasizes on keeping carbs at an absolute minimum throughout the day and ingesting carbohydrates after the training session. Mechanism of carbohydrate Backloading:  It was hypothesized that carbohydrate backloading takes advantage of the non-insulin mediated uptake of glucose by the muscle tissues post-exercise, because of GLUT4 translocation, as opposed to in the morning, when insulin sensitivity in both muscle and fat tissue is generally higher 32.Similar to GLUT (Glucose Transporter) 1-3, GLUT4 and GLUT12 are a set of glucose transporters which are present in muscle and fat tissue. While GLUT 1-3 are exposed to the cell surface, GLUT4 are tucked below the surface within the cellular membranes. Due to this withdrawn nature of GLUT4, this only reacts to the presence of insulin by moving from the interior of the cell to its surface.  Thereby, this insulin-mediated transport of glucose transports high volumes of glucose in the cells containing these GLUTs (both muscle and fat tissue). 33, 34However, resistance training mimics the function of insulin in muscle cells and GLUT4 rises to the surface shuttling glucose into the muscle tissue. This non-insulin mediated uptake of glucose by the muscle tissues post-exercise is postulated to have increases in the skeletal muscle tissue without any significant increases in adipose tissue. 34Research on carbohydrate backloading: The most popular research to support the theory of CBL is a 6 month study. In this study, Sofer et al. 35 authors compared the effects of carbs eaten mostly at dinner (experimental group) vs. eaten throughout the day (control group) in a group of 78 Israeli police officers.  It was found that reductions in weight, body fat and waist circumference were greater in the evening-carb experimental group vs. the control group. In addition, glucose control, inflammation, blood lipids and satiety were improved to a greater degree in the evening-carb group.However, there are a few limitations to the design of the study. The subjects were fed a daily average protein intake of 0.66-0.76 g/kg, which is much less than what is consumed by resistance trainees aiming for a better body composition, which questions the applicability of the research to this population. Also, the experimental group lost an average 11.8 percent of their body weight in 6 months as compared to the control group who lost an average 10.9 percent, which isn’t statistically significant (<1%) over 6 months. Also, the experimental group started at a greater weight to begin with.There are other controlled studies 36, 37 which were conducted, but no changes in body composition or weight loss were observed, however they were conducted for very short durations (15 days and 18 days).SummaryCarbohydrate backloading doesn't provide any additional benefits in fat loss/ muscle gain when compared to a normal diet when calories and protein are equated.Though, in light of the limited evidence, it does seem that shifting caloric (and carbohydrate) intake to later in the day (around the training session) may provide a slight additional benefit with respect to body composition, better training performance and markers of health and disease.REFERENCES-1. 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