Elsevier

Nutrition

Volume 60, April 2019, Pages 136-146
Nutrition

Review article
Carbohydrate restriction: Friend or foe of resistance-based exercise performance?

https://doi.org/10.1016/j.nut.2018.09.026Get rights and content

Highlights

  • The metabolic glycolytic response to resistance exercise is diverse and likely most attributable to volume, duration, and intensity of effort.

  • Low muscle glycogen may not impair all resistance exercise performance, but increasing blood glucose before exercise may enhance higher-volume, longer-duration performance.

  • Carbohydrate-restricted hypocaloric diets are effective for reducing fat mass during resistance exercise, but carbohydrate-sufficient hypercaloric diets are likely optimal for inducing muscle hypertrophy.

Abstract

It is commonly accepted that adequate carbohydrate availability is necessary for optimal endurance performance. However, for strength- and physique-based athletes, sports nutrition research and recommendations have focused on protein ingestion, with far less attention given to carbohydrates. Varying resistance exercise protocols, such as differences in intensity, volume, and intraset rest prescriptions between strength-training and physique-training goals elicit different metabolic responses, which may necessitate different carbohydrate needs. The results of several acute and chronic training studies suggest that although severe carbohydrate restriction may not impair strength adaptations during a resistance training program, consuming an adequate amount of carbohydrate in the days leading up to testing may enhance maximal strength and strength–endurance performance. Although several molecular studies demonstrate no additive increases in postexercise mammalian target of rapamycin 1 phosphorylation with carbohydrate and protein compared with protein ingestion alone, the effects of chronic resistance training with carbohydrate restriction on muscle hypertrophy are conflicting and require further research to determine a minimal carbohydrate threshold necessary to optimize muscle hypertrophy. This review summarizes the current knowledge regarding carbohydrate availability and resistance training outcomes and poses new research questions that will better help guide carbohydrate recommendations for strength and physique athletes. In addition, given that success in physique sports is based on subjective appearance, and not objective physical performance, we also review the effects of subchronic carbohydrate ingestion during contest preparation on aesthetic appearance.

Introduction

Glucose availability is important for muscular performance lasting >30s, and dietary carbohydrates are now considered the most important of the three macronutrients for fueling endurance sports [1]. However, before the late 19th century, athletes, coaches, and even some scientists believed protein was the major source of energy for muscular activity [2]. From a historical perspective, Zuntz [3], [4], Frentzel and Reach [5], and Krogh and Lindhard [6] demonstrated through a series of landmark experiments that carbohydrates and lipids fuel exercise, with the varying proportions of the two macronutrients in the diet and the intensity of work determining the proportions in which they were oxidized. In 1924, researchers from Harvard Medical School conducted experiments with runners from the Boston Marathon to investigate the role of carbohydrates on exercise performance [7]. It was observed that blood glucose levels were reduced in several Boston Marathon runners who crossed the finish line, thus establishing a relationship between blood glucose, carbohydrate consumption, and performance [7]. In the following year, many of these same athletes were supplemented with a large amount of carbohydrates the day before the race, and they were allowed to eat sugary candy before and during the event [8]. Blood glucose concentrations after completion of the marathon were sampled by researchers and they found that by normalizing blood glucose concentrations (before and during running), symptoms of fatigue and stupor were prevented and mental focus was increased [8]. This was one of the first studies to establish a causal link between carbohydrate consumption and sport performance. In the 1960s, it was demonstrated that high-carbohydrate diets improved endurance performance and carbohydrate feedings during exercise delayed fatigue [9], [10], [11]. Finally, in the 1970s, research revealed that manipulation of dietary carbohydrate levels from a carbohydrate-free diet, mixed diet, and high-carbohydrate diet increased muscle glycogen content and consequentially increased the time of moderate exercise to exhaustion [12]. The supra-cited effects of manipulation of carbohydrate ingestion leading to increased muscle glycogen content are currently known as carbohydrate loading [13], [14]. Those studies were essential to paving the road for the carbohydrate saga in sports nutrition.

It is now fully appreciated that adequate carbohydrate ingestion is necessary for optimal endurance performance, with recommendations well established in sports nutrition societies [15]. However, for strength- and physique-based athletes, sports nutrition recommendations have focused more on amino acid and protein ingestion [16], with much less attention given to carbohydrate requirements. In fact, the role of dietary carbohydrates in strength training performance and adaptation are currently under debate, with some researchers questioning whether dietary carbohydrates impart any ergogenic or adaptive benefit to strength training at all [17]. For strength- and physique-based athletes, carbohydrates are consumed for the following four main purposes:

  • 1.

    To maintain high muscle glycogen levels to sustain muscle contractions during high-intensity strength training sessions.

  • 2.

    To enhance muscle recovery and adaptation (via increasing protein synthesis and suppressing protein breakdown) between exercise sessions [18].

  • 3.

    To enhance aesthetics acutely, especially for physique competitions, whereby carbohydrate loading increases muscle glycogen and intermuscular water content leading to a more “muscular” appearance [19].

  • 4.

    To improve body composition via reductions in fat mass; however, in this case, most individuals reduce carbohydrate consumption in an attempt to mobilize more fatty acids from the adipose tissue [20].

In all four situations, carbohydrates are important (either in high/adequate amounts or low amounts) for strength training and the physique athlete, and are discussed in detail after a brief explanation of carbohydrate metabolism.

Section snippets

Carbohydrate metabolism

Ingested carbohydrates are degraded by extensive enzymatic systems in the body, initially found in saliva (salivary amylase), then in the pancreatic juice (pancreatic amylase), and finally by intestinal microvilli enzymes (maltase, lactase and sucrase). After being digested, they are absorbed as glucose or fructose in the intestine, via glucose/fructose transporters [21]. In sports nutrition, the choice of a given source of carbohydrates is extremely important for several reasons, from

Muscle contractions and glucose metabolism

Muscle contraction is a process dependent on intramuscular adenosine triphosphate (ATP) being replenished by bioenergetic systems [23]. During efforts lasting 30 to 180s, the ATP required for muscle contractions is mainly powered through glycolysis [27]. Glycolysis is a set of 10 step reactions generating two or three ATPs, depending on whether glucose or glycogen is the initial substrate, respectively [27]. During anaerobic exercises, when available, glycogen is the preferred source of energy

Metabolic demands of strength/power athlete training and physique athlete training

Resistance-training variable prescriptions (volume, intensity, rest between sets, repetition tempo, etc.) vary widely based on the desired outcome. Consequentially, metabolic demand and substrate utilization also will differ between resistance-training prescriptions. During the yearly training plan, strength and power athletes often divide their training into periods of general preparation, defined in part by performing higher volumes and lower intensities to optimize work capacity and

Muscle glycogen availability and dietary carbohydrate affect resistance-based muscular performance

Maintaining muscle glycogen via adequate carbohydrate consumption (4–7 g/kg daily) is recommended to optimize both acute strength performance (i.e., a power lifter who must execute three 1 RM attempts for the squat, bench press, and deadlift with 15–30 min of rest between attempts during competition) and supporting high weekly volumes of resistance training [63]. From a mechanistic perspective, it would appear that maintaining glycogen is indeed important for performance in both acute and

Dietary carbohydrate and muscle glycogen affect postresistance exercise adaptations

Insulin and insulin growth factor 1 (IGF-1) have been shown to play a major role in skeletal muscle anabolism and potentially more importantly, restricting protein breakdown. The receptors of insulin (IR) and IGF-1 (IGF-1 R) have a 45% to 65% homology between their ligand-binding domains and 60% to 85% homology in their tyrosine kinase substrate recruitment domains. It has been suggested that their respective genes have evolved from an ancestral gene that has been highly conserved in the

Effects of carbohydrate manipulation on body composition outcomes

When an organism consumes less calories than expended for a consistent period (i.e., >24–48 h) weight loss from reductions in body tissue occur; however, a variety of factors that affect eating behavior and activity will influence both the rate as well as the composition of the weight lost [107]. Given the risk for cardiovascular and metabolic diseases associated with excess adiposity, even in individuals with body mass index <25 kg/m2 [108], and the importance of maintaining muscle mass to

Effects of carbohydrate manipulation on aesthetic outcomes in competitive physique athletes

Physique athletes (i.e., bodybuilding, physique, figure, and bikini divisions) are assessed based on their presentation of an aesthetic appearance, defined by appropriate levels of muscularity, bilateral symmetry, aesthetic balance between various muscle groups, and a low body fat [115]. Ergo, although two athletes of similar stature and muscular development may present with differing body fat percentages, the athlete with the lower measured body fat percentage may not always win. As such,

Conclusions and Future Perspectives

The metabolic response to resistance training is distinct from endurance training and differs from high-intensity interval training because of longer time under tension and more prolonged eccentric contractions [46]. Moreover, variances in resistance training variable prescription, such as the differences in volume, intensity, and interset rest between typical strength and hypertrophy prescriptions, results in varying metabolic responses. Research examining the effects of varying levels of

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