Back in Volume 4 of MASS Research Review, I wrote an article called “Losing Fat-Free Mass During Weight Loss: Bad Now, Bad Later.” As you might have inferred, it argued that you generally don’t want to lose a large amount of fat-free mass over the course of a weight loss diet. The article was a review of a study by Turicchi et al (2), in which researchers observed weight gain over the course of a 26-week maintenance period. This weight regain period was preceded by an active weight loss intervention, in which the 209 study participants lost ≥8% of their body weight. Broadly speaking, the results indicated that minimizing the loss of fat-free mass may attenuate hunger and appetite after weight loss, which could prevent weight regain. However, there was a pretty big limitation: these study participants weren’t doing a resistance training program during the weight loss phase. In theory, resistance training should be an effective strategy to minimize weight regain by attenuating the loss of fat-free mass, therefore avoiding elevations in appetite. That’s great, but we need empirical data to turn this from a hunch or an assumption into an evidence-based recommendation.
That’s where the presently reviewed study (1) by Martins et al comes into play. You might recall the name Martins, as I’ve cited this researcher’s work numerous times in MASS articles focusing on metabolic adaptation and weight regain. Rather than digging up every single MASS article with a Martins citation, I’ll simply refer readers to my long MacroFactor article about reverse dieting, which covers just about all of them. The presently reviewed study was a retrospective analysis that combined data from two cohorts completing two studies with “the same sequence of events and same methodology.” Altogether, Martins and colleagues had data from 141 premenopausal women with BMIs in the overweight category in a weight loss program. Participants were 20-41 years old, had normal glucose tolerance, were eumenorrheic, were nonsmokers, and were sedentary (no more than one regular exercise session per week).
Rather than aiming for a certain percentage of weight loss, each participant’s goal in the weight loss program was to get to a BMI of 25 or lower. Within the program, participants were randomly assigned to one of three groups: diet only, diet plus aerobic exercise (three times per week), or diet plus resistance exercise (three times per week). All participants were given an 800kcal/day diet (20-22% fat, 20-22% protein, and 56-58% carbohydrate) until their BMI dropped below 25. One year after the end of the weight loss program, participants completed a follow-up visit. During the follow-up period, participants were encouraged (but not required) to continue their assigned exercise program (if applicable) and attend regular meetings with a support group that provided some ongoing dietary education. Body composition was measured via DXA at baseline, the end of the weight loss program, and the one-year follow-up visit.
Body composition variables at baseline, after weight loss, and at the one-year follow-up visit are presented in Table 1. During the weight loss intervention, participants lost 12.1 ± 2.6 kg of total body mass, 11.3 ± 2.5 kg of fat mass, and 0.5 ± 1.6 kg of fat-free mass. Only 3.6% of the weight lost was fat-free mass (which corresponds with a reduction of 0.5kg from the mean fat-free mass value at baseline to the mean value after weight loss), with the remainder of weight loss attributable to reductions in fat mass. By the one-year follow-up period, participants had regained 51.3% of the weight that was originally lost, which amounted to 6.0kg of weight regain, which was virtually all fat mass. The inter-individual variability in weight regain was pretty remarkable – 95% of participants regained some weight, but the range of weight change at one year spanned from -3.6 to +20.3 kg.
The researchers did a regression analysis to assess bivariate relationships between a number of predictor variables and the outcome of interest (weight regain). This involved building several regression models with only one predictor variable at a time, rather than throwing all of the predictors into the same model. The results of these bivariate regression models are presented in Table 2. Only three predictors cleared the typical threshold for statistical significance (p < 0.05), and all three were related to fat-free mass: baseline fat-free mass, absolute loss of fat-free mass (in kg), and weight lost as fat-free mass (as a percentage). The R2 value tells us the percentage of variance in the outcome (weight regain) that is explained by the predictor variable. For all three, the R2 value is around 4%, give or take. However, we can’t simply add these up to estimate the cumulative effect of all three, because it’s a virtual certainty that they are highly correlated with one another. This means that they’re all explaining similar portions of variance in weight regain. With this in mind, we can conclude that the loss of fat-free mass explains roughly 4% of the variance in weight regain, give or take.
Looking at Table 2, you’ll also notice that the p-value for group is pretty low as well. This predictor variable pertains to the intervention group participants were assigned to (diet only, diet plus aerobic exercise, or diet plus resistance exercise). The percentage of weight lost as fat-free mass was highest in the diet only group (-9.7 ± 9.9%), lower in the diet plus aerobic exercise group (-2.2 ± 10.0%), and the diet plus resistance exercise group actually experienced an increase in fat-free mass during the weight loss period (+3.6 ± 12.8%). The researchers presented an additional regression model demonstrating that the percentage of weight lost as fat-free mass was still significantly predictive of weight regain after controlling for the effect of group, but I personally think the bivariate models in Table 2 are a bit more informative.
So, what have we learned here?
For starters, exercise is a good idea during weight loss, and resistance training is particularly good. I was honestly quite surprised by the small percentage of weight that was lost as fat-free mass, even in the diet only group, but the diet plus aerobic exercise group still fared better in terms of fat-free mass retention. It’s no surprise that the diet plus resistance training group fared even better, with a net gain of fat-free mass during the weight loss period (despite a highly restrictive diet of only 800kcal/day!). This study also provides confirmation for a point that has been made in a previous MASS article: the loss of fat-free mass appears to be predictive of weight regain, and that relationship appears to be mediated by hunger and appetite. Research suggests that the loss of fat-free mass is a key driver of hyperphagia (excess hunger) after weight loss, and that hyperphagia seems to persist until baseline levels of fat-free mass are restored (3). So, the best bet is to avoid losing fat-free mass in the first place.
Having said all of that, we shouldn’t get too carried away when discussing the magnitude of the effect in the presently reviewed study. The loss of fat-free mass was only predictive of around 4% of the variation in weight regain, and there was plenty of variation in weight regain, with some folks actually losing small amounts of weight during the one-year follow-up period, and some folks gaining up to 20.3kg. There’s no question that resistance training is a great strategy for maintaining fat-free mass during weight loss, and I recommend slightly higher protein intakes during weight loss diets for the exact same reason. However, fat-free mass losses explain only a tiny portion of an individual’s weight regain response. As noted in a previous MASS article, the key factors driving successful weight loss (and subsequent maintenance of that weight loss) include adherence to lifestyle modifications, eating behavior characteristics, satiety-promoting strategies, regular self-monitoring, and plenty of physical activity. Retention of fat-free mass can be helpful, but it’s only a small part of the puzzle.
Note: This article was published in partnership with MASS Research Review. Full versions of Research Spotlight breakdowns are originally published in MASS Research Review. Subscribe to MASS to get a monthly publication with breakdowns of recent exercise and nutrition studies.
References
- Martins C, Gower BA, Hunter GR. Association between Fat-Free Mass Loss after Diet and Exercise Interventions and Weight Regain in Women with Overweight. Med Sci Sports Exerc. 2022 Dec 1;54(12):2031–6.
- Turicchi J, O’Driscoll R, Finlayson G, Duarte C, Hopkins M, Martins N, et al. Associations Between The Proportion Of Fat-Free Mass Loss During Weight Loss, Changes In Appetite, And Subsequent Weight Change: Results From A Randomized 2-Stage Dietary Intervention Trial. Am J Clin Nutr. 2020 Mar 1;111(3):536–44.
- Dulloo AG, Miles-Chan JL, Schutz Y. Collateral Fattening In Body Composition Autoregulation: Its Determinants And Significance For Obesity Predisposition. Eur J Clin Nutr. 2018;72(5):657.
