New study: do refeeds help preserve muscle mass?

You’ve probably heard of this study over the past few months. I’ve mentioned it a few times myself, as the lead author, Bill Campbell is a friend. The study’s main conclusion is summarized in its title: “Intermittent Energy Restriction Attenuates the Loss of Fat Free Mass in Resistance Trained Individuals: A Randomized Controlled Trial.” In short, refeeds seem to help preserve your gains when dieting. The full text was just published, so let’s dig into the details, because as you may have noticed in my study reviews, the details often considerably change the take-home message.

The study design was simple and elegant: 2 groups of trained men and women would cut for 7 weeks with the same weekly average energy deficit and macros, either with a continuous daily 25% energy deficit or 5 days of a 35% energy deficit followed by 2 days of maintenance energy intake. During the 2 refeed days, all extra energy intake was instructed to come from carbs.

Before and after 7 weeks, their body composition and metabolic rate were measured. Strength was not measured, unfortunately.

The authors report the following as their conclusion: “A 2-day carbohydrate refeed preserves fat-free mass (FFM), dry fat-free mass, and resting metabolic rate (RMR) during energy restriction compared to continuous energy restriction in RT-individuals.” The amount of fat loss didn’t significantly differ between groups, neither did total training work volume.

That conclusion seems like a big win for refeeds, but it’s not strictly in line with the results. The authors don’t report between-group statistical significance tests, unfortunately, but the group x time interaction, which should tell us the same thing, was only significant for dry FFM, not for total FFM or RMR. The differences in effect size for the change in FFM and RMR were also quite trivial: 0.03 vs. 0.09 for FFM and 0.13 vs. 0.21 for RMR. So the proper conclusion from the results was quite different: Refeeds do not augment fat loss or reduce total fat-free mass loss, but they reduce the percentage of dry FFM loss.

Why would refeeds preserve dry mass? In other words, why would continuous dieting lead to more water loss? Probably because refeeds, especially those high in carbs, increase glycogen stores and glycogen attracts water into the muscles. Body composition was measured with ultrasound (BodyMetrix) and dry FFM was estimated using bio-electrical impedance (BIA; InBody). Ultrasound and BIA are not the most accurate devices to estimate someone’s body composition, in particular because they’re notoriously influenced by changes in hydration status. If you’ve worked with these devices before, you’ll know they can give you a wildly different body fat percentage before vs. after a workout, a big meal or a hot bath. Typically, these devices will register water as lean mass and thus think you have a better body composition when you’re more hydrated. The total body water and dry estimates aren’t very reliable in my view as a result of these effects.

In this study, the final body composition measurements were done 2 days after the study to avoid being confounded by the refeeds. However, this turned out to be right after the last refeed and the subjects didn’t train those final 2 days, so the stored intramuscular glycogen would still be there. Intramuscular glycogen is not like liver glycogen. It can only be used by the muscle and that normally only occurs during high intensity exercise.

In conclusion, it’s likely both groups experienced the same body composition change, but the measurements just turned out a little different for the refeed group because they just had their refeed and this changed their hydration level. This conclusion is very much in line with the overall literature, which finds that intermittent and continuous energy restriction with the same cumulative energy deficit have the same effect on our body composition.

This conclusion of confounded body composition measurements is also much more plausible in my view than the traditional explanation from refeed proponents that high carb intakes increase leptin levels and leptin helps counteract the effects of energy deficit. We have strong research showing you can’t trick your body as easily as that. As soon as you go back into energy deficit after a refeed, leptin levels come back down. To quote my online PT Course:

“…in so far as the increase in leptin levels is practically significant in the first place, leptin levels in response to energy balance adapt quickly, within 12 hours  [2]. Moreover, since leptin is produced by your fat cells, it shouldn’t be surprising that leptin levels are very strongly correlated to your total fat mass, not your acute energy balance. As such, leptin levels correspond with cumulative, not acute, energy balance, and to increase your leptin levels back to pre-weight loss values, you have to restore the whole diet period’s energy deficit and thus essentially gain back all the fat, or at least stored energy, you lost. It is not enough to restore energy balance: you must repay your whole energy debt.”

I’ve also heard the explanation that refeeds help you train harder because of the extra stored glycogen. This explanation also doesn’t sense. As you may have read in the last article on my site, in contrast to popular belief, glycogen stores are not a rate limiting factor for strength training performance. Correspondingly, in this study total training work output didn’t differ between groups and the authors rightfully concluded the refeeds therefore likely didn’t improve performance in the gym.

On a final note, I will say the overall results seemed better in the refeed group than the continuous dieting group, as you can see in the table below. However, the effect size differences were mostly trivial because the variance in the data was so large that we can’t say much about a kilogram more or less here and there. The reason for the huge variance and why we shouldn’t read much into non-significant differences is that adherence to the study protocol was outright horrendous. Of the 59 trainees recruited, only 27 finished the study(!) That’s an attrition rate of over 50%. Anything over 20% is generally considered very problematic, even in studies where they try to have obese individuals diet, so I wouldn’t put much stock in the self-reported diet adherence in this study.

Even of those that managed to complete the study, the continuous diet group didn’t achieve its target 25% energy deficit: they only got 21%.

Of note, the refeed group actually ended up consuming non-significantly fewer carbs than the continuous diet group, so they didn’t even do the refeeds as planned in the first place. I sympathize with the authors, as it’s very difficult to get young adults, especially college students, to follow a study per protocol for many weeks in a row, but unfortunately in this case it hugely affects the integrity of the study.

All in all, I’d say this study is in line with the overall literature that continuous and intermittent dieting result in very similar body composition outcomes. Whether you ‘refeed’ more in the weekend or not likely doesn’t affect your gains compared to dieting with the same energy intake each day and the same total weekly macronutrient intakes. I do think there’s great promise in calorie cycling, but for that to be effective I think the higher energy intakes have to be strategically aligned with training sessions and their post-workout elevations in protein synthesis (‘the anabolic windows’).

Study reference

Campbell et al. 2020. Intermittent Energy Restriction Attenuates the Loss of Fat Free Mass in Resistance Trained Individuals. A Randomized Controlled Trial. J. Funct. Morphol. Kinesiol. 2020, 5(1), 19; https://doi.org/10.3390/jfmk5010019