Nutrition
Narrative Review
2019
Is an Energy Surplus Required to Maximize Skeletal Muscle Hypertrophy Associated With Resistance Training
By Gary John Slater, Brad P. Dieter, Damien J. Willoughby and Bill I. Campbell
Frontiers in Nutrition, 6, pp. 131
<h2>Abstract</h2>
<p>This narrative review examines the evidence regarding the role of <a href="/terms/caloric-surplus/" class="term-link" data-slug="caloric-surplus" title="energy surplus">energy surplus</a> in maximizing skeletal <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="muscle hypertrophy">muscle hypertrophy</a> induced by resistance training, with particular attention to the magnitude and composition of any energy excess required to optimize muscle protein accretion while minimizing concurrent fat mass gain. The fundamental relationship between energy availability and the anabolic machinery underlying myofibrillar <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="protein synthesis">protein synthesis</a> is explored, distinguishing between the thermodynamic requirements for depositing new muscle tissue and the anabolic signaling environment created by dietary energy availability. Evidence from <a href="/terms/nitrogen-balance/" class="term-link" data-slug="nitrogen-balance" title="nitrogen balance">nitrogen balance</a> studies, energy partition analyses, and body composition investigations suggests that modest energy surpluses on the order of 350-500 kcal/day above maintenance provide conditions conducive to maximal hypertrophy without disproportionate fat accretion — a strategy colloquially termed "lean bulking." Excessive energy surpluses beyond this range appear to primarily increase fat deposition rather than conferring additional anabolic benefit. Notably, untrained individuals, those carrying substantial excess body fat, and individuals returning to training after a <a href="/terms/detraining/" class="term-link" data-slug="detraining" title="detraining">detraining</a> period can achieve meaningful muscle hypertrophy even under energy-neutral or mildly hypocaloric conditions, a finding with significant practical implications for those unable or unwilling to tolerate weight gain during muscle-building phases [1].</p>
<h2>Introduction</h2>
<p>The question of whether — and how much — dietary <a href="/terms/caloric-surplus/" class="term-link" data-slug="caloric-surplus" title="energy surplus">energy surplus</a> is required to maximize resistance training-induced <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="muscle hypertrophy">muscle hypertrophy</a> is among the most practically consequential in applied sports nutrition. Practitioners routinely advise clients to consume caloric surpluses during dedicated "bulking" phases in the belief that <a href="/terms/concentric-contraction/" class="term-link" data-slug="concentric-contraction" title="positive">positive</a> energy balance creates an anabolic environment that maximizes muscle growth. Conversely, the desire to minimize concurrent fat mass gain during muscle accretion phases has led to the popularization of "lean bulk" or "clean bulk" strategies that advocate modest surpluses, in contrast to more aggressive "dirty bulk" approaches characterized by unrestricted caloric intake [1].</p>
<p>From a thermodynamic perspective, the deposition of new muscle tissue requires energy — skeletal muscle contains approximately 1300 kcal of energy per kilogram of wet mass. Given that even aggressive hypertrophy programs may add 1-2 kg of muscle per month under ideal conditions, the additional energy requirement for tissue synthesis is relatively modest (approximately 43-87 kcal/day above maintenance assuming perfect efficiency). In practice, however, the thermodynamic cost of muscle deposition represents only one consideration. Energy availability also influences anabolic hormone profiles (insulin, <a href="/terms/igf-1/" class="term-link" data-slug="igf-1" title="IGF-1">IGF-1</a>, testosterone), <a href="/terms/mtor/" class="term-link" data-slug="mtor" title="mTORC1">mTORC1</a> signaling activity, ribosomal biogenesis, and the overall anabolic environment in ways that may not be captured by simple thermodynamic calculations [2].</p>
<p>Critically, the relationship between energy surplus and muscle gain does not appear to be linear — there seems to be a ceiling beyond which additional calories no longer drive muscle protein accretion but instead primarily increase fat mass. Understanding where this ceiling lies and how it is influenced by training status, protein intake, and individual metabolic characteristics is essential for evidence-based program design [3].</p>
<p>This review evaluates the existing evidence to characterize the relationship between energy balance status, surplus magnitude, and skeletal muscle hypertrophy, and provides practical recommendations for individuals at varying stages of training experience.</p>
<h2>Energy Balance and <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="Muscle Hypertrophy">Muscle Hypertrophy</a></h2>
<h3>The Anabolic Environment of <a href="/terms/caloric-surplus/" class="term-link" data-slug="caloric-surplus" title="Energy Surplus">Energy Surplus</a></h3>
<p>Dietary energy intake above maintenance creates a hormonal and metabolic environment that is broadly permissive of anabolic processes. Insulin, secreted in response to carbohydrate and to a lesser extent protein ingestion, not only facilitates glucose disposal but also inhibits muscle protein breakdown (MPB) through suppression of FOXO transcription factors and the ubiquitin-proteasome pathway. Growth hormone and its primary mediator <a href="/terms/igf-1/" class="term-link" data-slug="igf-1" title="IGF-1">IGF-1</a> are also influenced by nutritional status, with both hormones showing attenuated secretion during prolonged energy deficits [1].</p>
<p><a href="/terms/mtor/" class="term-link" data-slug="mtor" title="mTORC1">mTORC1</a> — the central regulatory node of <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="muscle protein synthesis">muscle protein synthesis</a> — is sensitive to cellular energy status through AMPK, which is activated by low <a href="/terms/adenosine-triphosphate/" class="term-link" data-slug="adenosine-triphosphate" title="ATP">ATP</a>:AMP ratios characteristic of <a href="/terms/caloric-deficit/" class="term-link" data-slug="caloric-deficit" title="energy deficit">energy deficit</a>. AMPK phosphorylation of Raptor inhibits mTORC1 activity, providing a direct mechanistic link between energy availability and anabolic signaling. Conversely, energy-replete conditions with high ATP and insulin availability support robust mTORC1 activation in response to <a href="/terms/mechanical-tension/" class="term-link" data-slug="mechanical-tension" title="mechanical loading">mechanical loading</a> and amino acid availability [2].</p>
<h3><a href="/terms/body-recomposition/" class="term-link" data-slug="body-recomposition" title="Body Recomposition">Body Recomposition</a>: Muscle Gain During Energy Deficit</h3>
<p>Despite the theoretical favorability of energy surplus for anabolic processes, it is well established that certain population subgroups can achieve meaningful simultaneous muscle gain and fat loss — a phenomenon termed "body recomposition" — without requiring a caloric surplus. This capacity is most pronounced in three groups: untrained individuals (who respond to novel training stimuli with large anabolic responses even at maintenance or mild deficit); individuals with excess body fat reserves (who can mobilize stored adipose tissue to provide local energy for anabolism); and individuals returning to training after a <a href="/terms/detraining/" class="term-link" data-slug="detraining" title="detraining">detraining</a> period (who may re-capitalize on myonuclear memory mechanisms) [3].</p>
<p>In these populations, body fat stores serve as an endogenous energy source that can partially substitute for dietary energy surplus in supporting muscle protein accretion. This is not an unlimited capacity — as individuals approach lower body fat percentages and higher training advancement, the ability to recompose decreases substantially, and some degree of energy surplus becomes increasingly important for continued hypertrophy. The threshold between "recomposition-possible" and "surplus-required" states is not precisely defined but likely corresponds roughly to achieving a trained or advanced training status alongside a body fat percentage below approximately 15% in men and 25% in women [1].</p>
<h2>Magnitude of <a href="/terms/caloric-surplus/" class="term-link" data-slug="caloric-surplus" title="Energy Surplus">Energy Surplus</a></h2>
<h3>Evidence for Optimal Surplus Range</h3>
<p>The key question for practitioners managing <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="hypertrophy">hypertrophy</a>-focused programs is not whether surplus aids muscle gain, but how much surplus maximizes the muscle-to-fat gain ratio — the so-called "partitioning efficiency." The available evidence, while limited by methodological heterogeneity, consistently suggests that beyond a relatively modest surplus threshold, incremental energy does not proportionally increase the rate of muscle protein accretion but does increase fat deposition [1].</p>
<p>Energy partition analyses — which model the fraction of excess energy directed toward lean tissue versus fat mass during weight gain — indicate that even under favorable conditions (adequate protein, <a href="/terms/progressive-overload/" class="term-link" data-slug="progressive-overload" title="progressive resistance">progressive resistance</a> training), only approximately 20-30% of surplus energy above thermodynamic requirements is deposited as lean tissue, with the remainder stored as fat. This energetic inefficiency means that aggressive surpluses disproportionately accumulate fat relative to muscle, reducing the efficiency of a bulking phase and necessitating longer and more aggressive subsequent cutting phases to restore body composition [2].</p>
<p>Practical estimates based on observed rates of muscle hypertrophy in resistance-trained individuals suggest that the maximum achievable rate of muscle gain in a natural trainee is approximately 0.25-0.5 kg/week in novice trainees and falls progressively as training advancement increases — dropping to as low as 0.05-0.1 kg/week in advanced athletes. The energetic cost of depositing this mass, coupled with the metabolic adaptation overhead of supporting a larger anabolic machinery, suggests that surpluses of 350-500 kcal/day above maintenance are sufficient to support near-maximal muscle accretion without excessive concurrent fat gain [3].</p>
<h3>Consequences of Excessive Surplus</h3>
<p>Aggressive caloric surplus approaches — sometimes exceeding 1000-2000 kcal/day above maintenance — are common in recreational bodybuilding culture but are poorly supported by evidence as strategies to enhance muscle gain beyond the rate achievable with modest surpluses. The primary consequence of excessive surplus is accelerated fat mass accumulation, which carries several practical costs: it prolongs the subsequent <a href="/terms/caloric-deficit/" class="term-link" data-slug="caloric-deficit" title="caloric deficit">caloric deficit</a> required to reveal musculature, exposes the athlete to the health risks of excess adiposity, and may impair anabolic sensitivity through adipose tissue-mediated inflammatory and hormonal perturbations [1].</p>
<p>Furthermore, evidence from body composition research suggests that individuals with higher initial body fat percentages demonstrate poorer nutrient partitioning — a smaller fraction of surplus calories directed toward lean tissue — creating a progressive deterioration in efficiency as a bulking phase extends beyond optimal body composition ranges.</p>
<h2>Practical Recommendations</h2>
<h3>Individualized Surplus Targets</h3>
<p>The appropriate magnitude of <a href="/terms/caloric-surplus/" class="term-link" data-slug="caloric-surplus" title="caloric surplus">caloric surplus</a> for maximizing <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="hypertrophy">hypertrophy</a> while minimizing fat gain varies with training status and current body composition. The following evidence-informed guidelines are proposed [1]:</p>
<ul>
<li><strong>Novice trainees (0-1 year resistance training):</strong> A modest surplus of 250-500 kcal/day above maintenance provides anabolic conditions conducive to the rapid neural and hypertrophic adaptations characteristic of this phase. Novices can accrue muscle faster than intermediates and benefit most from surplus support.</li>
<li><strong>Intermediate trainees (1-3 years):</strong> A surplus of 350-500 kcal/day supports continued hypertrophy at the somewhat slower rates of intermediate development. Monthly body weight gains of 0.5-1.0% of total body mass serve as a practical monitoring target.</li>
<li><strong>Advanced trainees (3 years):</strong> Maximum monthly muscle gain rates are low, and aggressive surpluses primarily generate fat. Surplus of 200-350 kcal/day with frequent body composition monitoring is more appropriate.</li>
</ul>
<h3>Monitoring and Adjustment</h3>
<p>Weekly body weight tracking (averaging multiple morning readings to minimize fluctuation noise) and periodic body composition assessment (monthly DXA or weekly skinfold <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="if">if</a> accessible) enable progressive calibration of caloric intake toward desired rate of gain. If body weight is increasing faster than 1% per month in trained individuals, caloric intake should be modestly reduced; if weight is stable, a gradual upward adjustment is warranted [2].</p>
<h3>Protein Priority</h3>
<p>Regardless of energy surplus strategy, protein intake should remain at or above 1.6-2.2 g/kg/day to maximize the anabolic efficiency of available energy. Adequate protein intake is the single dietary variable most strongly supported by evidence as a determinant of muscle hypertrophy magnitude — even a well-calibrated energy surplus will produce suboptimal results if protein intake is insufficient [3].</p>
<h3>Avoiding the Dirty Bulk Trap</h3>
<p>The perceived convenience of unrestricted caloric intake during muscle-building phases is offset by the extended and metabolically stressful cutting phases required to restore body composition afterward. A disciplined lean-surplus approach, though less dramatic in week-to-week scale movements, produces comparable or superior long-term muscle gains with substantially less concurrent fat accumulation — improving overall training efficiency across annual <a href="/terms/periodization/" class="term-link" data-slug="periodization" title="periodization">periodization</a> cycles [1].</p>