Body recomposition describes the process of reducing fat mass while simultaneously increasing lean muscle. Body recomposition changes have been well documented in the scientific literature, yet many people remain skeptic arguing that to lose fat mass, the body must remain in an energy deficit (i.e. eating fewer calories relative to maintenance), and to increase lean muscle, the body must be in an energy surplus (i.e. consuming more calories relative to maintenance). Ultimately, those that believe it is not possible to lose fat while increasing muscle, argue body recomposition defies the law of thermodynamics. This article will discuss how it is possible to achieve simultaneous improvements in lean muscle while losing fat, as well as provide scientific support for body recomposition in trained and untrained populations, both under overfeeding (i.e. caloric surplus) and underfeeding (i.e. caloric deficit) conditions.
How Is Body Recomposition Possible?
To understand how body recomposition is possible, it is relevant to review the first law of thermodynamics, a fundamental law in physics that describes the movement of heat and energy in the body [5, 3]. The first law of thermodynamics states that energy cannot be created or destroyed, only changed . For example, consuming more calories than maintenance calorie level (i.e. the calories consumed to maintain stable weight), changes the energy in the body to store more energy. Conversely, reducing the energy stored by the body results from consuming fewer calories relative to those consumed at maintenance level. To simplify further, if the body is in negative energy balance, the body will reduce its own energy and when the body is in positive energy balance, it will store energy.
It is, however, incorrect to assume that negative energy balance will result in lowering bodyweight. Weight change can still occur without changing the amount of energy stored or expended. For example, weight change can occur through fluctuations in water intake from reducing certain foods . As each gram of stored carbohydrate (glycogen) carries 3-4 grams of water, reductions in glycogen storage from reducing carbohydrates would lower total bodyweight, but without changing total energy balance . Therefore, energy balance and weight change should be viewed as separate components as it is possible to lose body mass without losing stored energy.
It has been proposed that body recomposition is possible through a process known as calorie partitioning, whereby calories can be independently used for either increasing muscle building or fat mass. This process, expressed as a P-ratio in research, indicates the portion of lean mass lost, or gained, for every unit of body weight change . Evidence of P-ratio dates back to the research of Dr. Gilbert Forbes in the 1980’s. In his research, Forbes found that overfeeding subjects by 1,195-1,1783 calories per day for 17-21 days resulted in 3.5-5.8 kg of bodyweight . The author found that overfeeding, expectably, induced weight gain, but surprisingly a substantial portion (51%) of the subjects weight gain came from lean body mass . Forbes found the average composition of the weight gain among subjects was 44% lean body mass 56% fat mass .
Body Recomposition: Overfeeding Plus Resistance Exercise
To better understand the body’s capacity for calorie partitioning and body recomposition under exercise conditions, a number of studies reveal improvements in lean body mass and simultaneous reductions in fat mass when individuals are either in a calorie surplus or deficit and engaging in resistance exercise.
Spillane and Willoughby (2016) randomised 21 resistance-trained males to either protein plus carbohydrate (HPC) group or a (HC) carbohydrate supplement group and placed them through an 8-week resistance exercise program . Both the HPC and HC groups were overfed 1,394 and 1,101 calories above their current daily caloric intake, equating to a total of 3,700-3,800 calories per day .
The HC group received a maltodextrin carbohydrate supplement of 312 grams (1,248 calorie) per day, while the HPC group received an equivalent 312 gram (1,248) supplement, but comprised of whey protein (94g protein, 196g carbohydrates, and 22g fat) .
Participants engaged in a 4-day per week resistance exercise program that was split into two upper body and two lower body sessions for 8-weeks . Within each exercise, participants completed 3 sets of 8-10 repetitions using 70 – 80% repetition maximum .
Results showed that the HC group had a P-ratio of 14% lean body mass and 86% fat mass, while the HPC group had a P-ratio of 59% lean body mass and 41% fat mass . These P-ratio’s translated in to a 2.28kg increase in lean body mass in the HPC group and a .25kg increase in lean body mass in the HC group . Findings from this study suggest that additional calories in excess of daily caloric intake could result in improved lean body mass when performing resistance exercise and protein intake is maintained at 2.4g/kg/day .
Another study by Antonio and colleague (2015), demonstrated the potential for body recomposition during an calorie surplus combined with progressive resistance exercise . In their study, the authors assigned 73 resistances trained men and women to either a “normal protein” (NP) or “high protein group” (HP) . The NP group was asked to maintain their current dietary habits, while the HP group was prescribed a 3g protein per kilogram per day . Participants in the NP group increased their daily calories by approximately 100 calories per day from about 2,016 to 2,119 calories, on average . Participants in the HP group increased their daily calories by about 370 calories per day from about 2,240 to 2,614 calories, on average .
Subjects kept a food diary using MyFitnessPal for dietary adherence and evaluation . The resistance exercise program was completed 5 days per week for 8-weeks . A split routine was utilised whereby participants worked different muscle groups on consecutive days . The authors used BodPod air displacement to evaluate body composition changes .
Results of this study showed that fat free mass, which encompasses muscle, increased similarly in both groups by an average of 1.5kg from baseline . However, participants in the NP group increased fat mass, on average, by 0.3kg from pre to post-study, while participants in the HP group lost fat mass, on average, by 1.6kg . Also, body fat percentage reduced in the NP group by 0.6%, while participants in the HP group 2.4% over the 8-week period . This would lend further support to the capability of body recomposition in an energy surplus when performing resistance exercise and protein intake is elevated .
Body Recomposition: Underfeeding Plus Resistance Exercise
Demling and DeSanti (2000) published a study in the Annals of Nutrition & Metabolism that compared the effects of combining 12-weeks of resistance exercise with a high protein diet against dieting alone on body composition change . In their study, 38 untrained, male, overweight (23 – 35% body fat) police officers were randomised to either a diet group (250 calorie deficit), a diet plus resistance exercise plus casein protein (1.5g/kg/day) group, or a diet plus resistance exercise plus whey protein (1.5g/kg/day) group .
Participants performed 30-35 minute resistance exercise workouts 4-days per week using resistance machines for all major muscle groups (i.e. legs, shoulders, back) . Subjects could also choose to perform aerobic./cardiovascular exercise on non-training days, however this was not required . Body composition was measured using skin fold callipers .
Following 12-weeks, weight loss was approximately 2.5kg in all three groups . Body fat in the diet alone group decreased from 27% to 25% . Body fat in the diet, exercise and casein protein group decrease from 26% to 18% . Finally, body fat in the diet, exercise and whey protein group decreased from 27% to 23% . Lean mass remained unchanged in the diet group, while individuals in both the casein and whey protein plus exercise groups increased their lean body mass by 4kg and 2kg from baseline . Findings from this study lend support to the potential for body composition change under relatively low energy deficits (150-200 daily calorie deficit) .
Additional evidence exists that demonstrate, even under severe calorie deficits, body recomposition is possible . A study in the American Journal of Clinical Nutrition in 2016 showed that even during marked energy deficit, body recomposition is still attainable . In that study, the authors placed 40 overweight (BMI > 25), young men, through a 4-week study where they were allocated to either a control group (CON) or protein group (PRO) . The CON group were placed in a 40% calorie deficit and prescribed 1.2 grams per kilogram of protein per day and a macronutrient composition of 15% protein, 50% carbohydrates, and 35% fat . The PRO group were placed in a 40% calorie deficit as well, but daily protein doubled to 2.4 grams per kilogram of protein per day and a macronutrient composition of 35% protein, 50% carbohydrates, and 15% fat .
Each week, participants in both groups completed two full-body resistance exercise circuits (10 repetitions for 3 sets at 80% of one-repetition maximum), two sessions of slow-intensity and high-intensity training on a stationary bike, one session of cycle ergometer, one session of plyometric body weight circuits, and given a step target of10,000 steps per day .
Participants in both groups were provided with all meals and beverages . Both groups received supplements, the PRO group receiving a whey protein supplement and the CON groups receiving a placebo drink . To increase daily protein intake, subjects received 3 or 4 dairy-based beverages each day . Dietary compliance was assessed daily weight monitoring, checklists and contact with subjects . Body composition was assessed using BodPod, DEXA, and bioelectrical impedance .
Following the 4-week intervention, lean body mass was unchanged in the CON group (0.1kg +/- 1.0kg), while lean body mass increased in the PRO group by 1.2kg +/- 1.0kg . Additionally, both groups decreased fat mass, however fat mass losses were greatest in the PRO group (-4.8kg +/- 1.6kg) compared with the CON group (-3.5kg +/- 1.4kg) . Results from this study indicate further the potential for body recomposition even under a large energy deficit.
Body recomposition, the simultaneous increase in muscle and reduction in fat mass, is well documented in the scientific literature under conditions where individuals are performing resistance exercise and consuming higher (>1.6g g/kg/day) intake of dietary protein. In accordance with the first law of thermodynamics, energy cannot be created or destroyed, only transformed from one form to another . Although it is true that the body needs to be in an energy deficit to lose bodily energy and in an energy surplus to store body energy, both energy and mass are independent components. As previously indicated, it is possible lose weight without losing bodily energy (i.e. water loss). Also, research shows that energy can be extracted and deposited separately into either lean body mass (i.e. muscle tissue) or fat mass.
Under non-exercising conditions, when subjects are overfed calories, research demonstrates that a large portion of weight gain comes from increased lean body mass and not solely fat mass . When exercising (i.e. resistance exercise), a number of studies show a simultaneous reduction in fat mass and increased lean body mass. This is evidenced both when individuals are kept in positive or negative energy balance. Provided an individual is engaging in progressive resistance exercise and protein intake is maintained at about 1.6 g/kg/day, or potentially even higher under certain circumstances (i.e. energy deficit), it appears the body has incentive for recomposition. Caution, however, would need to be applied when using a caloric surplus for body recomposition, as larger caloric surpluses (>15%) would negate fat loss efforts despite favouring lean body mass increases. Although, evidence by Antonio et al. (2015) indicate this may not be the case if caloric surplus is achieved via protein supplementation (i.e. whey protein) as compared with obtaining protein from whole foods.
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 Demling, R. H. and DeSanti, L. 2000. Effect of a hypocaloric diet, increased protein intake and resistance training on lean mass gains and fat mass loss in overweight police officers. Vol.44, No.1, pp.21-29. Annals of Nutrition and Metabolism.
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