Food provides the body with the energy to do work and the building blocks for growth and repair of body tissue, principally in the form of the macronutrients: protein, fats and carbohydrates. Carbohydrates are primarily an energy source, proteins are primarily building blocks forming structural and functional components of the body, and fats provide both. Fibre, water and micronutrients are critical to the effective utilisation of these macronutrients in the body, however these will not be addressed today. The term ‘calories’ describes the potential energy contained within macronutrients.
Resistance training is the primary focus in the strength-power disciplines of weightlifting and powerlifting. Such athletes rely on muscular strength, power and explosivity to lift maximal weight, usually for single or low volume repetitions. In competition, as athletes compete in specific bodyweight classes, strength to bodyweight ratio can be arguably as important as absolute strength.
Muscle strength is largely determined by muscle size and the ability to execute a rapid and coordinated muscle contraction (Cribb et al., 2007; Maughan and Gleeson, 2010). Resistance training exerts mechanical load stress on the body, providing a stimulus for adaptations that increase muscle strength – such as muscle growth and more efficient energy production (MacDougall et al., 1977; Kadi et al., 2004; Andersen et al., 2005).
Skeletal muscle is highly plastic in response to exercise stimuli. Muscle hypertrophy translating to enhanced performance can be seen within a matter of weeks of a new training regime, in both trained and untrained subjects (Kadi et al., 2004; Andersen et al., 2005; Cribb et al 2007). Increases in muscle mass are primarily driven by increases in the amount of muscle protein. Considering this, it is perhaps unsurprising that nutrient availability modulates the body’s adaptations in response to training, given that muscle growth alone requires energy and building blocks … i.e. the things provided by food. So, a basal level of energy (calories) and essential macronutrients appears necessary for strength performance.
Whilst this is clear, the relative importance of total calories versus specific macronutrient intake is unclear. Although there is an emphasis on consuming a high protein diet to support muscle hypertrophy within the strength-power athlete community, evidence for an optimal nutritional strategy for such athletes remains equivocal. Reflecting this, current nutritional guidelines for resistance training athletes differ between institutions. For example, the 2009 position stand from the American College of Sports Medicine (ACSM) recommends a daily protein intake of 1.2-1.7g/kg bodyweight (American Dietetic Association et al., 2009), whereas the 2018 position stand by the International Society for Sports Nutrition (ISSN) recommends protein intake of 1.4-2.0g/kg bodyweight, increasing to 3.0g/kg for those on a hypocaloric diet and looking to retain lean mass (Kerksick et al., 2018).
The reason for this lack of clarity is the variance in the findings of research on the matter. The reason why research findings differ? It is likely driven by differences in the subjects, exercise and nutritional interventions made in the different studies. Much research has been performed in untrained and overweight populations, and it might be expected that their response to training and dietary change may differ to that of an experienced strength-power athlete whose physiology has already undergone significant training adaptations (Kadi et al., 2004; Andersen et al., 2005; Willoughby et al., 2007; Kerksick et al., 2010; Kreider et al., 2011; Weisgarber et al., 2012; Herda et al., 2013; Morton et al., 2018).
So, what do we know about total calories (regardless of macronutrient composition) versus specific macronutrient intake for strength performance in experienced resistance trained athletes?
Before we begin, it is worth noting that we will not address nutrient intake during acute weight cutting phases where extreme calorie deficits, dehydration and other protocols may be undertaken preceding a competition to enable an athlete to compete in a certain bodyweight class.
Calorie versus macronutrient intake: strength performance
Many studies that have investigated the impact of diet on strength performance altered both macronutrient and total calorie intake (Hoffman et al., 2009; Bird et al., 2013; Antonio et al., 2015; Antonio et al., 2016a). This means it is not possible to conclude whether any observed performance effect is due to the change in total calories or specific macronutrients. However, it is interesting that the impact of increasing total macronutrient and calorie intake on strength performance was mixed … some showed that diet did not impact performance (Hoffman et al., 2009; Antonio et al., 2015, Antonio et al., 2016a), some showed it did (Hoffman et al., 2009; Bird et al., 2013). What this does indicate is that increasing calorie or macronutrient intake does not necessarily result in strength performance improvements … perhaps there is an upper limit to the body’s capacity to use food for training adaptations, which would be consistent with research that suggests an upper limit to the amount of protein that can be used for muscle building from any one meal (Moore et al., 2009; Atherton et al., 2010; Moore et al., 2012; Areta et al., 2013; Macnaughton et al., 2016; Schoenfeld and Aragon, 2018).
If calories are more significant to strength performance than macronutrient composition, there would be no impact on performance if macronutrient composition is manipulated whilst maintaining same total calorie intake. Looking at studies that have performed such manipulations, the picture is unclear (Burke et al., 2001; Hoffman et al., 2006; Kerksick et al., 2006; Cribb et al., 2007; Snijders et al., 2015). In each of these studies the participants undertook a new resistance training regime alongside any dietary change, and strength performance increased significantly regardless of diet; it is possible the impact of this on strength performance was large enough to mask an impact of diet composition, particularly considering the variability in absolute and relative strength performance between participants and across time.
In four of the five studies, the baseline diet of participants was high protein, within or above the recommended range of the ACSM and the ISSN for resistance trained athletes (>1.3g protein/kg bodyweight per day) (Burke et al., 2001; Kerksick et al., 2006; Cribb et al., 2007; American Dietetic Association et al., 2009; Snijders et al., 2015; Kerksick et al., 2018). Considering the hypothesis that there is an upper limit to the amount of dietary protein that can be utilised by the body for muscle protein synthesis at any one time, this may explain why no strength performance effect is seen when protein consumption was further elevated (Moore et al., 2009; Atherton et al., 2010; Moore et al., 2012; Areta et al., 2013; Macnaughton et al., 2016; Schoenfeld and Aragon, 2018). It is also possible that a reduction in the intake of carbohydrate as protein intake increased (to maintain the same total calorie intake) had a balancing negative impact on strength performance, as the energy to drive the explosive and powerful skeletal muscle contractions in resistance exercise is derived principally from carbohydrates via anaerobic respiration. Whilst the typical duration and intensity of a whole-body resistance training session is not expected to exhaust muscle glycogen stores, such sessions have been shown to reduce muscle glycogen stores by up to 40%, it is still significant (MacDougall et al., 1977; Tesch et al., 1986; Robergs et al., 1991).
Calorie versus macronutrient intake: body composition
Multiple studies have focussed on manipulating macronutrient intake and assessing the impact on body composition in resistance trained athletes. As strength-power athletes compete in weight class divisions, maximising strength to bodyweight is an important factor in performance. Athletes therefore aim to optimise their lean mass to total body weight ratio, and so body fat percentage.
Current research supports a role of protein in promoting addition of lean mass and reducing body fat percentage. In four of the five studies manipulating macronutrient intake without changing total caloric intake (Table 2), at least one marker of body composition (lean mass, fat mass and / or trained muscle size) significantly improved with higher protein intake (Burke et al., 2001; Kerksick et al., 2006; Cribb et al., 2007; Snijders et al., 2015). In the fifth study, although there was a trend to higher lean mass accretion in the higher protein group, it was not statistically significant (Hoffman et al., 2006).
Where total calorie intake was increased by increasing protein intake, there was no significant impact of the increase on body composition, despite the fact the calorie intake of this group exceeded that of the lower group by approximately 300-800kcal per day (Hoffman et al., 2009; Antonio et al., 2014; Antonio et al., 2015; Antonio et al., 2016a; Antonio et al., 2016b). This may indicate the athletes were able to utilise the additional energy, perhaps suggesting they were previously hypocaloric (Trexler, Smith-Ryan and Norton, 2014). It could also indicate the athletes were able to utilise the additional protein for the turnover of tissue other than muscle.
Conclusion
Current evidence remains equivocal as to the relative significance of total calories and macronutrient composition to strength performance in resistance trained athletes. Evidence does support a significant role for protein in the preservation and accretion of lean mass in response to resistance training. This is consistent with the current recommendation for resistance athletes to intake protein above that of the general population (American Dietetic Association et al., 2009; Kerksick et al., 2018). As muscle size is correlated with muscle strength (Cribb et al., 2007; Maughan and Gleeson, 2010, p15), it might be hypothesised that over time such an increase in lean mass will contribute to strength gains.
It is perhaps too simplistic to try and consider the body’s ‘energy’ and ‘building block’ requirements independently; any macronutrient can provide both, different macronutrients can be converted into common ‘building blocks’, and maximal activation of the mTORC1 growth pathway requires the presence of both energy and certain building blocks. As such, use of any macronutrient for energy or as a building block is likely dependent on the relative energy and synthetic needs of the body at a given point in time.
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