Comparing slow and fast fat loss Original paper

In this randomized controlled trial in resistance-trained women, severe energy restriction and progressive energy restriction promoted similar changes in body composition.

The effects of progressive energy restriction (PER) and severe energy restriction (SER) on body composition (assessed via DXA), muscular strength (assessed via Smith machine squat and bench press 1-repetition maximum), and muscular power (assessed via countermovement jump height).

14 women (average age of 39, average BMI of 24) with at least 2 years of continuous resistance-training experience.

The participants were randomly assigned to one of two dietary interventions for 8 weeks:

• SER: A total energy intake of 25 kcal per kilogram of fat-free mass per day (kcal/kg FFM/d) and a macronutrient distribution of 2 grams of protein per kilogram of body weight per day (g/kg/d), 1 g/kg/d of fat, and the remaining calories allotted to carbohydrate.
• PER: A total energy intake of 40 kcal per kilogram of body weight per day and a macronutrient distribution of 2 g/kg/d of protein, 1 g/kg/d of fat, and the remaining calories allotted to carbohydrate for the first two weeks. During weeks 3 and 4, energy intake was reduced to 35 kcal/kg FFM/d, followed by 30 kcal/kg FFM/d in weeks 5 and 6, and 25 kcal/kg FFM/d in weeks 7 and 8. The reduction in energy intake occurred through reduced carbohydrate intake.

The participants used a smartphone app to record their daily macronutrient intake and report their dietary adherence. They also consulted with a sports nutritionist throughout the intervention.

All of the participants performed the same concurrent training program, outlined below. Cardiovascular exercise was performed on a cycle ergometer at an intensity of 65% of heart rate reserve.

Concurrent training protocol

Body composition was measured seven days after menstruation in both the preintervention and postintervention periods to avoid potential confounding due to hormone-induced fluctuations in body water. To further improve the accuracy of the body composition measurements, fat-free adipose tissue mass (FFAT) was removed from the calculation of DXA-derived FFM using an equation that assumes 85% of adipose tissue mass is from fat.

Changes in body composition did not differ between groups. Both groups lost a small amount of fat mass compared to baseline (−1.7 and −1.2 kg in PER and SER, respectively) and there was no significant change in FFM.

There were no significant changes in strength and power compared to baseline or differences between groups.

An energy deficit has been shown to reduce the rate of muscle protein synthesis (MPS) at rest and increase muscle protein breakdown,^[1][2][3] which promotes negative net protein balance and muscle loss.

Larger energy deficits are associated with more rapid weight loss.^[4][5] However, this potential benefit comes with the higher risk of losing fat-free mass (FFM),^[6][7] especially if the individual is already relatively lean at the start of the intervention.^[8]

Minimizing FFM loss during energy restriction is of the utmost importance in order to maximize improvements in cardiometabolic health, preserve muscular strength and physical function, and reduce the risk of weight regain.^[9]

According to a meta-analysis published in 2021, an energy deficit of 500 kcal per day can prevent resistance exercise-induced gains in FFM.^[10] In addition, larger energy deficits were associated with reductions in FFM.

Relationship between estimated energy deficit and change in fat-free mass

Adapted from Murphy and Koehler, 2021.

The shaded area on the sides of the regression line represents the 95% confidence interval for the regression. RT= resistance training, CON = without an energy deficit, ED = energy deficit.

One way to counteract the adverse physiological responses to energy restriction and minimize reductions in muscle mass with weight loss is to perform resistance exercise.^[11] It’s been observed that a single bout of resistance exercise during an energy deficit can restore the MPS rate to a value comparable to that observed in a state of energy balance.^[2]

The other main tool available to combat muscle loss during energy restriction is a high-protein diet (1.6–2.4 g/kg/d).^[12] In a group of physically active military personnel who restricted their energy intake by 40% for 21 days, consumption of a high-protein diet preserved the MPS response to a 20 gram serving of protein, which was not the case for the lower-protein diet (0.8 g/kg/d).^[13]

The combination of resistance exercise and a high-protein diet has been found to significantly improve body composition outcomes in several studies. In the previously mentioned study in physically active military personnel, total weight loss did not differ between groups after the 21-day intervention, but the high-protein diet group lost more fat mass and less FFM than the lower-protein diet group.^[13] Similarly, a 14-day study that had resistance-trained men restrict their energy intake by 40% reported similar reductions in fat mass between the high-protein and lower-protein diet groups (2.3 vs. 1.0 g/kg/d), but the lower-protein diet group lost significantly more FFM.^[14]

The combination of resistance exercise and a high-protein diet has not only been shown to help maintain FFM during weight loss but may also even increase FFM during weight loss in some populations. In a 4-week study in untrained young men with overweight or obesity who reduced their energy intake by 40%, a high-protein diet (2.4 g/kg/d) and a resistance exercise intervention increased lean mass by 1.2 kg, on average, whereas lean mass did not change in the lower-protein diet group (1.2 g/kg/d).^[15]

The design of the present study was optimal for determining whether the degree of energy restriction influenced changes in body composition in resistance-trained women, as it featured an intensive resistance exercise intervention and a high-protein diet (2.4–2.6 g/kg/d) to minimize reductions in FFM.

The degree of energy restriction was severe in SER at the start of the intervention. Meanwhile, energy restriction was conservative in PER and gradually ramped up over the course of the study, so it may seem reasonable to predict that total weight loss and FFM loss would have been greater in SER, as previous evidence in similar populations supports this hypothesis.

For example, in a study that had elite athletes consume high-protein, energy-restricted diets and perform resistance exercise, the participants in the slow-rate weight loss group (energy deficit of 19%) gained lean mass compared to the fast-rate weight loss group (energy deficit of 30%), while fat mass decreased to a similar extent between groups.^[16] Of note, the slow-rate group dieted for 3 weeks longer than the fast-rate group.

However, in contrast to how the aforementioned study was designed, the degree of energy restriction in the study under review was in fact not greater in SER than PER. While energy intake was less in SER than PER in weeks 1–2, there were no significant differences between groups between weeks 3–4 and 5–6, and energy intake was actually higher in SER than PER in weeks 7–8. Throughout the study, SER reportedly consumed 29.5 kcal/kcal/kg FFM/d, on average, thus they failed to reach the prescription of 25 kcal/kcal/kg FFM/d, a feat which was achieved by PER in weeks 7–8. These findings ultimately explain the lack of difference in weight loss between groups.

The suboptimal dietary adherence in SER is unfortunate, as this study was the first to examine the effect of differing degrees of energy restriction on changes in FFM in resistance-trained women. As such, it remains unclear whether continuous severe energy restriction increases the risk of FFM loss compared to a more conservative and gradual approach in resistance-trained women who perform resistance exercise and consume a high-protein diet.

Nonetheless, this study may still provide a snippet of useful insight. The results suggest that severe continuous energy restriction impairs the overall effectiveness of a fat loss intervention, at least in some people, due to poor dietary adherence. Other studies show varied results.

In contrast to the summarized study, a notable body of evidence indicates that a faster rate of weight loss (and thus more severe energy restriction) in people with overweight or obesity results in greater amounts of weight loss both in the short and long term,^[17][18][19] which suggests that adherence is generally not an issue with severe continuous energy restriction compared to more conservative interventions.

Furthermore, studies that compared the effects of different rates of weight loss in physically active participants with a healthy BMI have not reported issues with adherence in the group with a faster rate of weight loss.^[4][16][5] Thus, the suboptimal dietary adherence in SER seems to be a finding unique to the population in the summarized study.