The American College of Sports Medicine (ACSM) Position Stand, "Appropriate intervention strategies for weight loss and prevention of weight regain for adults", states there is little evidence to suggest that exercise-alone is as effective as energy restriction for promoting weight loss 1. Energy restriction-alone is not, however, a sustainable long-term solution for continued improvements in body composition and health, and instead it has been reported that greater than 50% of subjects that lose weight as a result of dietary restriction-alone eventually regain the weight,2. Chronic adjustments to exercise volume and intensity, on the other hand, are virtually limitless. In fact, exercise, in the absence of energy restriction, improves cardiovascular fitness and body composition in a dose-dependent fashion; specifically, decreasing body fat and increasing, or at least preventing significant loss of metabolically active lean body tissue 3.

Another strategy that is gaining support for treating overweight and obesity, either when combined with energy restriction or its absence, is the manipulation of macronutrient composition; more precisely, reducing carbohydrate (CHO) and increasing protein (PRO) intake to improve body composition and blood lipids 456. Krieger et al. 4, for example, concluded that, independent of energy intake, low-carbohydrate/high-protein diets elicit a metabolic advantage; significantly reducing body mass (BM), fat mass, percent body fat and retaining significantly more fat-free mass than diets consisting of greater than 42% of energy from CHO and ≤ 1.05 g/kg/d PRO, respectively. Similarly, Krauss et al. 7, which was further explored by Feinman and Volek 8, showed that in the absence of energy restriction, a reduction in CHO and concomitant increase in PRO and dietary fat (FAT) resulted in significant improvements in BM, total cholesterol, triglycerides, and total cholesterol-to-high-density lipoprotein ratio compared to a diet consisting of 54% CHO, 16% PRO and 30% FAT. Increased satiety and thermogenesis are also commonly reported in response to a high-protein versus normative-protein (~15% of total energy from PRO) diet 9.

Furthermore, it has been reported that the use of 1–2 macronutrient- and micronutrient-containing meal replacements, per day, reduces BM and improves a variety of disease risk factors in overweight and obese populations 10. Such results, however, have been found to be significantly and positively related solely to the hypocaloric state characteristic of said meal replacement interventions 10. Nevertheless, a major advantage to the use of a portion-controlled, nutrient-dense food supplement appears to be behavioral in nature – their use minimizes drastic behavior modification and simplifies decision-making 11, resulting in greater long-term compliance 10. Thus, the purpose to our minimal nutrition intervention, controlled design was to assess if provision of a high-protein/low-carbohydrate and low-fat (~52% PRO, 33% CHO, 15% FAT) nutrient-dense food supplement (Full Strength^®, Phillips Performance Nutrition, LLC, Golden, CO) would, in the absence of energy restriction, alter ad libitum macronutrient intake (behavioral effect) and impose any added metabolic advantage [or detriment] to the physiological effects of 10 weeks of combined aerobic and resistance training in previously sedentary, overweight men and women.

As hypothesized, consumption of the food supplement (FS) significantly increased protein (PRO) and fiber intake in EXFS; PRO rising from 15.9% (86.3 ± 8.5 g/d; ~1.02 g/kg/d) of total energy at baseline (PRE) to 28.3% (131.3 ± 4.0 g/d; ~1.58 g/kg/d) during weeks 3–10. Simultaneously, subjects in EXFS realized a significant decrease in carbohydrate (CHO), fat (FAT) and total energy. Specifically, CHO decreased from 48.3% (261.5 ± 20.9 g/d) to 41.1% (190.3 ± 10.5 g/d) of total energy; FAT, from 35.8% to 30.6%; and total energy, from 2166 (± 160.0) to 1854 (± 98) kcals/d [Table 3]. The reduction in total energy intake may be explained by the satiating effect of PRO and/or fiber 1819; though it is our conclusion the modest increase (+4 g/d) in dietary fiber, albeit statistically significant, was not a major contributing factor. Instead, the observed non-significant increase in PRO, during weeks 1–2, and the significant decrease in CHO, FAT and energy intake over that same time period in EXFS would seem to support the separate hypotheses that CHO or FAT reduction spontaneously reduce energy consumption 202122. Such a conclusion runs contrary to findings by Weigle et al. 23 that increasing PRO-alone elicits a spontaneous reduction in ad libitum energy intake, or that only PRO intake, within EXFS, was significantly different from both control (CON) and EX during weeks 3–10. Instead, it is proposed that both an increase in dietary PRO and a reduction in CHO are equally necessary 24; however, in accordance with our study design, we hypothesize that the decrease in CHO was predicated by the initial increase in PRO. Other possible explanations for the reduced energy intake are 1) the high viscosity of the nutritional shake 25, 2) added multivitamin and mineral supplementation 26, 3) reduced portion sizes 27, and/or 4) limiting variety and adding structure to the diet 28.

It has been well documented that, in the absence of dieting, exercise elicits only minor effects on total body mass (BM) despite significant improvements in cardiovascular fitness and strength 2930. King et al. (31 propose that "inter-individual variability," or behavioral and metabolic compensatory events in response to exercise-induced increases in energy expenditure, may largely explain non-significant changes in BM from exercise-only interventions. In agreement, we found little individual variability in BM (92.9% of EXFS subjects' ΔBM occurred within +1.01 and -3.85 kg; 92.9% of EX subjects' ΔBM occurred within +2.39 and -2.17 kg). Individual responses for ad libitum energy intake (Figure 2) did, however, provide evidence of what appears to be an apparent trend toward compensatory increase in energy intake in 57.1% (n = 8) of subjects in EX but only14.3% (n = 2) of subjects in EXFS. Consequently, average energy intake was significantly reduced within EXFS (Table 3); supportive of the FS provoking a satiating effect. Additionally, Lofgren et al. 32 state that even modest changes in BM (<5%), in response to reduced energy and CHO intake and increased physical activity, improves cardiovascular health as assessed by low-density lipoprotein (LDL) cholesterol. In agreement, reductions in LDL only reached significance in EXFS (ΔBM = -2.13%; p > 0.05); the only group within the current study that realized a significant reduction in energy and CHO intake (Tables 3 and 7).

Expectedly, significant improvements in strength, cardiovascular fitness and blood lipids were observed in both EX and EXFS (Tables 5, 6, 7). Of special note, however, the change in time-to-exhaustion (TTE), in EXFS, was significantly greater than both CON and EX (EXFS = +118.7 sec > EX = +66.8 sec > CON = +9.1 sec; Table 6), with all subjects in EXFS resulting in a minimum improvement in fatigue threshold of +45 sec (Figure 7). Speculatively, the 100% improvement rate in TTE, within EXFS, may be attributable to the rise in muscle mass (MM; +2.3%) and greater reduction in fat mass (FM) 33, stable blood glucose, hormonal or other physiological adaptation 24, specific macro- and/or micro-ingredients of the food supplement or improved recovery nutrition between exercise bouts 34, improved hydration due to twice daily liquid supplementation, or possibly that the EXFS group was not blinded to the intervention. Further research controlling for such variables is warranted.

Interestingly, whereas neither EX or EXFS realized a significant reduction in plasma triglycerides (-16.1% and -19.4%, respectively), only EXFS experienced significant reductions in total cholesterol (-23.6 mg/dL) and LDL (-15.9 mg/dL) (Table 7 and Figures 8 and 9). The non-significant change in triglycerides, within EXFS, is noteworthy because the %Δ (-19.4%) is, in fact, consistent with findings involving low-carbohydrate and/or energy-restricted diets 6732. One explanation for the non-significant change may simply be sample size-dependent, whereas it is also plausible that EXFS achieved neither a great enough absolute reduction in CHO and/or energy to elicit such a response 35. Layman and Walker 36, on the other hand, posit that both CHO must be below 150 g/d and PRO greater than 1.5 g/kg/d to elicit effective treatment against obesity and metabolic syndrome; only the latter was, in fact, achieved in EXFS (CHO = 190.3 ± 10.5 g/d; PRO ≈ 1.58 g/kg/d).

According to a recent meta-regression by Krieger et al. 4, the reduction in CHO to <41.4% of total energy and increase in PRO to >1.05 g/kg/d, observed within EXFS, can account for the 1.6 kg, 1.3%, 1.5 kg and 0.3 kg greater improvements in FM, percent body fat, BM and MM respectively, compared to EX [Note: The regression analysis by Krieger et al. stated an additional 0.60 kg of fat-free mass was associated with PRO intakes of >1.05 g/kg/d.]. Of particular value is that supplementation with FS reduced the variability in FM and MM responses to exercise, such that 100% of subjects in EXFS realized a significant improvement; a finding that would seem to support a metabolic advantage of low-carbohydrate/high-protein diet modification 37. However, changes in CHO and PRO alone cannot, in the current study, be viewed in lieu of modifications in dietary FAT. According to a prediction equation developed by Astrup et al. 22, 1.17 kg of the 1.8 kg of FM lost by EXFS can be accounted for by the 26.71% reduction in dietary FAT. Thus, it seems prudent that future research incorporate isocaloric manipulations of varying macronutrient contributions such that contributing factors and covariates become more evident.

If, instead, we assume 0.45 kg of FM is equivalent to an ~3500 kcal deficit, the -2.7 kg change in FM, within EXFS, could almost completely be accounted for by the -312 kcal/d (-14.4%) reduction in energy intake:

3500 k c a l s 0.45 k g F M × − 2.7 k g F M 70 d a y s = − 300 k c a l s / d MathType@MTEF@5@5@+=feaafiart1ev1aaatCvAUfKttLearuWrP9MDH5MBPbIqV92AaeXatLxBI9gBaebbnrfifHhDYfgasaacPC6xNi=xI8qiVKYPFjYdHaVhbbf9v8qqaqFr0xc9vqFj0dXdbba91qpepeI8k8fiI+fsY=rqGqVepae9pg0db9vqaiVgFr0xfr=xfr=xc9adbaqaaeGaciGaaiaabeqaaeqabiWaaaGcbaqcfa4aaSaaaeaacqaIZaWmcqaI1aqncqaIWaamcqaIWaamcqWGRbWAcqWGJbWycqWGHbqycqWGSbaBcqWGZbWCaeaacqaIWaamcqGGUaGlcqaI0aancqaI1aqncqWGRbWAcqWGNbWzcqWGgbGrcqWGnbqtaaGccqGHxdaTjuaGdaWcaaqaaiabgkHiTiabikdaYiabc6caUiabiEda3iabdUgaRjabdEgaNjabdAeagjabd2eanbqaaiabiEda3iabicdaWiabdsgaKjabdggaHjabdMha5jabdohaZbaakiabg2da9iabgkHiTiabiodaZiabicdaWiabicdaWiabdUgaRjabdogaJjabdggaHjabdYgaSjabdohaZjabc+caViabdsgaKbaa@60A9@

Such assumptions, simplified to "calories in versus calories out," would however fail to recognize the increased energy demands requisite for the +0.6 kg of MM observed within EXFS. Instead, a cumulative metabolic advantage, as postulated by Fine and Feinman 37 and reported by Scott and Devore 38, combined with the anabolic response to increased amino acid availability 2439, and potentially sustained thyroid hormone levels and reduced insulin response 6 are more probable mechanisms to explain the significant mean changes in EXFS body composition measures. Layman et al. 39, for example, suggests that a hypocaloric diet with carbohydrate-to-protein ratio (CHO:PRO, in g/d) of 1.5:1.0 or less would be more effective in altering body composition than the 3.5:1.0 ratio currently recommended 39. Consequently, these authors 39 reported decreases in FM (-22%) and no loss in lean body mass after 16 weeks of an energy-restricted diet composed of a CHO:PRO ratio of ~1.5:1.0, during an exercise program similar to that of the present study. Meckling and Sherfey 40 postulated similar conclusions in response to energy restriction and a 1:1 versus a 3:1 CHO:PRO ratio, with or without exercise, in overweight and obese women. It was reported that both a 1.5:1.0 diet-only and 0.96:1.00 diet + exercise treatment was more effective than traditional high carbohydrate, energy restriction-alone or with exercise. Our findings support these hypotheses and suggest that addition of the FS to the EXFS group's ad libitum diet lowered the CHO:PRO ratio from 3.03:1.00 to 1.46:1.00 for the 10-week training period and, as described by Wood et al. 21, spontaneously reduced ad libitum energy intake via an as of yet fully understood behavioral effect. This change may have accounted for the greater improvements and reduced variability of individual responses for FM and MM in EXFS, when compared to the 3.40:1.00 CHO:PRO ratio of EX. Another plausible explanation, though not directly assessed in the current study, may be that subjects in EXFS consumed one of the FS shakes as a breakfast meal; the addition of the second shake enabling for more frequent, protein-rich meals throughout the day. Such would be supportive of the hypothesis raised by Laymen 24; that, consuming a minimum of 30 g of PRO for breakfast is "the most critical meal" for supporting an anabolic environment as is consuming PRO every 5–6 hours.

In summary, in the absence of energy restriction or other dietary controls, provision of a commercially available high-protein/low-carbohydrate and fat, nutrient-dense food supplement (Full Strength^®; EXFS), consumed daily, during a 10-week combined aerobic and resistance training intervention: 1) elicited a behavioral effect in previously sedentary, overweight adults such that subjects' macronutrient profiles were significantly modified (protein increased; carbohydrate and fat decreased) and total energy intake decreased spontaneously; 2) physiological adaptations to exercise were improved; and lastly, 3) though the significant mean differences between the exercise-only and EXFS groups may not be impressive in the absolute, consumption of the food supplement reduced the variability of individual responses for fat mass, muscle mass and time-to-exhaustion – all three variables improving in 100% of subjects in EXFS. This is of particular interest because virtually all of the available information pertaining to the use of food supplement interventions in overweight and obese populations has involved both an energy-restricted diet and the methodological replacement of at least one traditional meal per day 10. It is recommended that future research explore the use of the food supplement under various isocaloric and controlled macronutrient ratios, as well as assess the potential impact of the added micronutrients and other ingredients, and whether or not meal timing and frequency are covariates to the improvements noted in body composition, fitness and cardiovascular health.

The authors declare that they have no competing interests.

CML: obtained funding, study design, intervention protocols, analysis and interpretation of data, and preparation of manuscript. JRM: study design, intervention protocols, acquisition of data, analysis and interpretation of data, and preparation of manuscript. SET: study design, intervention protocols, acquisition of data, analysis and interpretation of data, and preparation of manuscript. AAW: study design, intervention protocols, acquisition of data, analysis and interpretation of data, and preparation of manuscript. AES: study design, intervention protocols, acquisition of data, analysis and interpretation of data, and preparation of manuscript. VJD: intervention protocols, acquisition of data, analysis and interpretation of data, and preparation of manuscript. JTC: study design, intervention protocols, acquisition of data, analysis and interpretation of data, and preparation of manuscript. JRS: principal researcher, obtained funding, study design, intervention protocols, acquisition of data, analysis and interpretation of data, and preparation of manuscript. All authors read and approved the final manuscript.