Flexible dieting has gone through many waves of discussion, and confusion over the last decade. However, the highest-level nutrition and performance educators are starting to see the merit and test its accuracy for benefit. As recent as 2021, an extensive study conducted across resistance-trained individuals undergoing restricted (rigid) dieting protocol vs that of a flexible dieting protocol approach, and found some interesting results! 
What is ‘Flexible Dieting’?
Understanding the exact definition of ‘Flexible dieting’ is probably what has led this terminology to have such contrasting opinions in the health and fitness industry. Flexible dieting is defined as the moderate approach, thus meaning that foods are not necessarily restricted from the diet, they are consumed within a moderate amount in an aim to suit a calorie goal, usually in a calorie deficit.
Essentially it removes the “forbidden” stigma around foods and allows dieters to take an educated outlook on the make-up of foods from a macronutrient element. This fulfils certain macronutrient goals properly, which foods are rich in micronutrients too, and how satiating the foods are. It also removes the “on diet, off diet” element, puts the educated choices in the hands of the person and their goals.
What is “Rigid” dieting?
Rigid dieting prolongs the “forbidden” element. For some, it is easy to religiously implement if they have been doing it for a long time; however, it is probably the hardest form to commit to. When you remove all foods that are considered detrimental and implement a dichotomous element of restriction on foods, it sets up the opportunity for people to dramatically fluctuate between “off and on” mindsets, rather than moderate intake of a much wider range of foods.
This behaviour in the past of either on diet or off a diet has been proven to show an increase in “binging” behaviours, rebound weight gain and mental health concerns around negative mindsets towards foods and diet behaviours. 
Which form of dieting is effective?
Both. Both diets when applied to the main component of the goal of the diet, which is to be within a calorie deficit, work. However, one is more easily applied to lifestyle and sustainability towards the outcome, which is body composition changes. That form, based on the research at current, is the flexible dieting approach. In the study referenced at the beginning of this article, they covered both in comparison for results side by side with participants. A total participant cohort of 39 healthy and resistance-trained participants, mixed between male and female. Initially divided into 20 (flexible) and 19 (rigid).
The flexible group lost 9 participants to non-adherence and the rigid lost 7 to non-adherence. Leaving a total cohort of 11 (flexible) and 12 (rigid) analyzed for the entirety of the 20-week study. 10 weeks was the diet phase and 10 weeks was the post-diet phase. Participants were randomized across the two groups to avoid bias. A 20% calorie reduction was applied across the cohort, and the resting metabolic rate was assessed 5 times across the study; 5,10, 16, and 20 weeks.
The Weight Loss Findings…
In the two groups, there was an interesting insight across weight loss and that was, both groups lost an average/mean value approximately the same:
- In the flexible dieting group, their average weight loss across the population was 2.6kg for the duration of the 10-week diet phase study.
- In the rigid dieting group, their average weight loss across the population for the duration of the 10-week diet phase study was 3.0kg
An interesting insight on this was that in the post-diet phase, the flexible dieting group gained back 1.7kg, and the rigid diet group gained only 0.4kg. However, this is total body weight gained and loss, not taking into account fat mass, and fat-free mass. Which we will see some interesting results below for these two sections.
Fat-Free Mass: Diet-Phase and Post-Diet Phase
Flexible Dieting vs Rigid, and Fat-free mass outcomes…
Fat free mass is any mass of the body that isn’t contributed to total body fat, this is a muscle, water, organ weight, bone, and connective tissue mass measure.
The results across the groups in this measure were interesting, to say the least:
- During the diet phase, the flexible dieting group lost 0.3kg, and the rigid dieting group gained 0.2kg.
- During the post-diet phase, the flexible dieting group gained 1.7kg, while the rigid dieting group only gained 0.7kg.
What does this mean? This could potentially establish hesitancy to include a variety of foods back into the diet that allows for the foundation requirements of building muscle, and tissue. Hindering growth.
Fat Mass Diet-Phase and Post-Diet Phase
Fat mass is quite self-explanatory, this is the total mass estimate of fat mass deducted after the Fat-free mass is calculated within minimal deviation. This is the big kicker…
- In the Flexible dieting group, they lost 2.3kg of fat mass during the diet phase, and in the post-diet phase gained 0kg.
- In the Rigid dieting group, they lost 2kg of fat mass during the diet phase, however, in the post-diet phase gained back 1.1kg of fat mass
At a glance, after the study – the body fat % increase during the post-diet phase for the flexible dieting group was 0%, while the rigid dieting group had a body fat % increase of 1.3%.
What Can This Study on Flexible Dieting vs Rigid Dieting Tell Us?
At a break down of the data, as we have done above… the successor, in the end, turned out to be the flexible dieting group. Comparison of body weight loss was almost like during the diet phase, with only a 400g average being the difference between the two. The post-diet phase showed a greater increase of body weight in the flexible dieting group than that of the rigid group, which at a glance would likely deter people towards the flexible dieting method.
That is until it is broken down into the components that make up bodyweight – Fat-Free Mass and Fat Mass. Participants of the flexible dieting group lost weight, didn’t gain any fat mass back during the post dieting phase and increased their fat-free mass values!
Flexible dieting – from a holistic viewpoint, removes the “forbidden” mentality towards food and rather educates the dieter from a standpoint of values contained within foods. To make better-educated decisions, teaching intuitive eating possibilities long term, while establishing an environment that can remove the fear around certain foods. As seen in this cohort study, it is a more sustainable approach and this point was established in the data. This is the first study of its kind to establish these direct comparison values, in an already existing healthy resistance-trained population undergoing an acute weight loss intervention method. Looking forward to future studies in this area of diet and training, it could significantly benefit the body and mind of many population groups.
To find out more about Bill Campbell’s work, check out our recent interview with him on this subject, and his new research.
- Conlin, L.A., Aguilar, D.T., Rogers, G.E. et al.Flexible vs. rigid dieting in resistance-trained individuals seeking to optimize their physiques: A randomized controlled trial. J Int Soc Sports Nutr 18, 52 (2021). https://doi.org/10.1186/s12970-021-00452-2June 2021 DOI https://doi.org/10.1186/s12970-021-00452-2
- Patton GC, Selzer R, Coffey C, Carlin JB, Wolfe R. Onset of adolescent eating disorders: population-based cohort study over 3 years. BMJ.
- Hays NP, Roberts SB. Aspects of eating behaviours “disinhibition” and “restraint” are related to weight gain and BMI in women. Obesity (Silver Spring). 2008;16(1):52–8. https://doi.org/10.1038/oby.2007.12