Beyond strict meal planning: dietary flexibility won’t hurt body composition goals

Does a flexible or rigid diet result in greater weight loss and weight maintenance in young, healthy, resistance-trained (RT) participants?

Most diet plans are based on a specific macronutrient profile or type of food (keto, paleo, high-protein, low-fat, plant-based, vegan, etc.) alongside some specific food choice flexibility. However, people who struggle with weight loss may see improvements in dietary adherence by following meal plans provided by dietitians and nutrition coaches. Previous research suggests that providing meal plans and grocery lists to people in a weight loss program helps, and some data even suggests there is no further benefit when food is given to participants^[1]. One issue with meal plans, which have been identified as “rigid” in the current study, is that they may lead to low adherence due to participants missing out on desired foods. Rigid plans may also foster the idea that some foods are completely good or bad or cause people to have an “all-or-nothing” approach^[2] to a diet.

Psychological aspects play a role in weight loss success and long-term weight loss maintenance. The Three-Factor Eating Questionnaire (TFEQ) is a tool^[3] used by researchers to measure cognitive restraint, disinhibition, and hunger. It helps researchers understand the mental aspects behind dieting. Disinhibition^[4], or overeating for reasons other than hunger, is known to be associated with increased body weight, whereas the relationship between body weight and cognitive restraint varies in adults; with some data showing an inverse relationship^[5], while other data show no relationship^[6]. Thus, understanding how dieting affects the subjective feelings of eating can help determine what type of diet may be best for adherence.

For most people, the goal of a weight loss program is to lose fat mass while retaining lean mass. So, if a certain type of diet is better at attaining these goals, it can be deemed more beneficial. Previous research suggests that a diet high in protein may help retain lean mass^[7] during weight loss. Outside of protein, a diet with high adherence^[8] results in weight loss and the exact macronutrient ratio or diet type may not play a role^[9] except for individual preference, which can influence diet adherence. Furthermore, behavioral changes followed by lifestyle shifts^[10] are necessary for weight maintenance to occur after a diet. So, if one of the goals of a diet is learning how to choose foods, it may be beneficial to use a flexible diet approach.

The goal of the study under review was to compare flexible to rigid dieting over a 10-week period and a subsequent 10-week follow-up, measuring body composition, resting metabolic rate (RMR), and the TFEQ.

Adherence plays a major role in the success of diets, but it’s unclear how more rigid diet plans compare to diets with a bit more flexibility in terms of helping people meet their weight and physique goals. The aim of this study was to explore the efficacy of rigid versus flexible dieting in resistance-trained participants.

The study was not preregistered and used a randomized, parallel group design with two groups. The participants were a mix of healthy, young, resistance-trained men and women, with 19–24% body fat at baseline on average (breakdown by sex not reported).

The intervention lasted 20 weeks, beginning with a 10-week diet phase, followed by a 10-week post-diet phase. During the diet phase, participants were told to consume 25% fewer calories than the amount required for weight maintenance. Participants were matched according to fat mass and then randomized to a group. One group was assigned to a flexible diet (FLEX) and provided with macronutrients goals and an ebook to help them choose foods (The Beginner’s Guide to Macros by Sohee Lee). The other group followed a rigid diet (RIGID) for which participants were given a menu with a detailed meal plan. The meals consisted of fruits, vegetables, lean meats, and nuts, with some specific foods shown in Figure 1. Both diets were matched for protein intake (2 grams per kg of body weight) with the remaining calories equally allocated to carbohydrates and fats.

Figure 1: Foods consumed in the rigid diet plan

The researchers used a repeated measure design and measured body composition and resting metabolic rate, as well as administered the Three-Factor Eating Questionnaire four times throughout the study. The primary outcomes were body composition (fat-free mass, fat mass, body fat percentage) and the secondary outcomes were resting metabolic rate and the TFEQ. Dietary data was collected via self-report.

Resistance-trained men and women were randomly assigned to 10 weeks of a rigid diet with a menu to follow or a flexible diet that consisted of macronutrient goals along with an ebook on how to choose appropriate foods to meet their goals. Body composition, resting metabolic rate, and responses to the Three-Factor Eating Questionnaire were the main metrics. After 10 weeks of dieting, the participants were followed for an additional 10 weeks.

There were no differences between the two groups for any dietary intake macronutrient, except baseline total protein intake, which was about 30% higher in the FLEX group. However, after adjusting for body mass, the baseline relative protein intake was not significantly different, yet favored the FLEX group (1.8g/kg/bw vs 1.4 g/kg/bw). According to self-report, both groups slightly increased protein intake during the weight loss phase, and were within the ranges for optimal adaptations to resistance training^[11] and weight loss^[12], so the effects of the diets cannot be ascribed to differences in protein intake.

During the diet phase, self-reported caloric restriction was approximately 20% for both diet groups (-20% FLEX and -17.7% RIGID) indicating the prescribed deficit of 25% was likely difficult to adhere to. The authors found no significant differences between groups for the primary outcomes of fat-free mass, fat mass, or body fat percentage during the 10-week diet phase, with both groups losing about 3 kg of body weight. This data indicates that both dieting approaches were successful in causing body weight loss, which was predominantly from fat mass. Interestingly, the authors found that fat-free mass increased during the 10-week post-diet phase in the FLEX group (+1.7 kg), and declined in the RIGID group (-0.7 kg). However, these averages should be taken with a grain of salt, since there was a lot of variability in the results, as shown in Figure 2.

Figure 2: Individual participant changes in fat and fat-free mass in both phases of the study

The authors found no differences in TFEQ scores across the 20-week period, finding that there were no changes in cognitive restraint, disinhibition, and participant hunger, indicating that neither diet was superior or inferior in terms of the primary and secondary outcomes. Lastly, there were also no changes in resting metabolic rate, which is to be expected since the participants were consuming the same amount of calories, had similar protein intake, and were matched for body fat prior to being randomized to a group.

The authors told participants not to change their exercise routine during the intervention. When comparing groups, they found no differences in the amount of time participants spent on aerobic or resistance training exercise at any point in the study. Specifically, during the diet phase, the FLEX group completed 5 ± 1.1 hours/week of resistance training, while the RIGID group completed 4.6 ± 1.75 hours/week. During the post-diet phase, the FLEX group completed 4.5 ± 1.6 hours/week while the RIGID group completed 3.6 ± 1.725 hours/week. For aerobic training, the FLEX group completed approximately 42 ± 38 minutes/week during both the diet phase and post-diet phase, while the RIGID group completed 96 ± 72 during the diet phase and 65 ± 42 minutes/week during the post-diet phase.

During the diet phase, both the FLEX and RIGID groups lost about 3 kg, mostly from fat, by maintaining a 20% caloric deficit. There was no difference in cognitive restraint, disinhibition, and hunger in either group. During the post-diet phase, the FLEX group increased their lean mass, while the RIGID group experienced a slight decrease in lean mass.

People who use rigid dieting strategies, such as meal plans, sometimes show symptoms of eating disorders, mood disturbances, excessive concern with body size/shape^[13], increased incidences of overeating and binge eating, and higher levels of depression and anxiety^[2]. The current study is one of the first to compare flexible and rigid dieting with an intervention. The authors found no differences between groups or over time in dietary constraint, disinhibition, or hunger, indicating that eating behavior was not influenced by the interventions. The authors also found no differences in body composition between groups during the diet phase, which indicates either diet could be used to lose body fat.

The authors analyzed the data with a per-protocol analysis rather than an intention-to-treat^[14] (ITT) analysis because the main interest was in the effect of the intervention, rather than its prescription. This means that they only analyzed data from participants who followed the protocol. However, the problem with this approach is that it makes it hard to tell if the measured effect is due to the intervention itself or selection bias. If the authors would have taken an ITT approach, it would have included all of the participants, thus increasing the participant number for analysis, as well as internal validity. Moreover, ITT does not disturb randomization and eliminates selection bias^[15], which is not guaranteed with a per-protocol analysis. Ultimately, results obtained in an ITT analysis more closely represent the real-life effectiveness of a diet approach. Even ignoring this issue, it should also be noted that the remaining participants did not technically adhere to the prescribed caloric deficit of 25%, suggesting that the diets were similarly difficult.

Another issue with the study is the authors did not investigate nor report the foods that the FLEX group ate. Given they collected data on food intake, they could have provided some examples of meal choices in order to facilitate a comparison to the RIGID group. There was also no data on non-exercise thermogenic activity, such as step count, so it’s unknown how that affected the overall results.

During the trial, the participants were told not to change their exercise program. The authors reported no differences between groups or across time for aerobic activity or resistance training. However, the authors did not indicate what statistical analysis they used. The FLEX group completed 25% more resistance training during the post-diet phase, which might contribute to the significant fat free mass differences found between groups (FLEX +1.7 kg; RIGID -0.7 kg). Another issue is the number of statistical tests performed, with 22 different outcomes reported. Since the authors did not adjust for multiple comparisons across the whole study, this could lead to false positives.

Finally, the authors did not perform a power analysis and compared multiple time-points across the study, so the study may have been underpowered to detect some changes, such as differences in exercise. There were also more dropouts in the FLEX group (45%) compared to the RIGID group (63%) and a high dropout rate for the study (41%), which could undermine the ability of randomization to equalize subject characteristics between groups.

The large dropout rate in this study suggests that both diets may have been too difficult to follow for a sizable proportion of the participants, which isn’t reflected in the results because the authors used a per-protocol analysis, which may have introduced selection bias. Also, although participants were instructed not to vary their exercise routines, the RIGID group performed more aerobic exercise compared to the FLEX group.

The primary results of the current study is that flexible dieting and rigid dieting can both result in body weight loss, with the majority of the losses coming from fat mass. The analysis indicates the two diets are neither superior or inferior to each other in terms of their effects on body composition, RMR, or TFEQ, even though the flexible diet was hypothesized to be beneficial.

However, there were some methodological problems with the study, making it difficult to draw firm conclusions. The drop-out rate was very high, which could lead to selection bias, especially since the authors chose only to analyze the participants who roughly stuck with the diet. Also, many outcomes were tested, which could have resulted in false positives.