Does intermittent fasting preserve muscle mass during weight loss? A new study claims it does. This is quite exciting, so I’m interrupting my planned schedule to give you the low down on these findings. I just got access to this paper and I’ll be writing this post as I read through it.
The study:
“Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss?”

Intermittent what?

Right off the bat, let me note that the title and abstract does not mention intermittent “fasting” anywhere. Instead, the author is using intermittent “calorie restriction” (CR) as a catch-all term for various intermittent fasting regimens. This includes intermittent fasting (IF) and alternate-day fasting (ADF). Below I’ve listed the studies for intermittent CR included in the review:
Halberg N, Henriksen M, Soderhamn N. Effect of intermittent fasting and refeeding on insulin action in healthy men. J Appl Physiol 2005; 99: 2128–2136.
Michalsen A, Riegert M, Ludtke R. Mediterranean diet or extended fasting’s influence on changing the intestinal microflora, immunoglobulin A secretion and clinical outcome in patients with rheumatoid arthritis and fibromyalgia: an observational study. BMC Complement Altern Med 2005; 5: 22.
Heilbronn LK, Smith SR, Martin CK, Anton SD, Ravussin E. Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. Am J Clin Nutr 2005; 81: 69–73.
Johnson JB, Summer W, Cutler RG. Alternate day calorie restriction improves clinical findings and reduces markers of oxidative stress and inflammation in overweight adults with moderate asthma. Free Radic Biol Med 2007; 42: 665–674.
Varady KA, Bhutani S, Church EC, Klempel MC. Short-term modified alternate-day fasting: a novel dietary strategy for weight loss and cardioprotection in obese adults. Am J Clin Nutr 2009; 90: 1138–1143.
Varady KA, Bhutani S, Klempel MC, Kroeger C. Effect of alternate day fasting combined with exercise on body composition parameters in obese adults. Unpublished data.
Varady KA, Allister CA, Hellerstein MK. Effect of alternate day fasting on lipid metabolism in obese humans. Unpublished data.
Note the one study that is not published yet and includes exercise. I’m very much looking forward to seeing that one. It would be interesting to see what kind of exercise regimen they are using. I’m hoping it’s weight training. So far there is a severe lack of intermittent-fasting-based studies that include exercise and not one that includes weight training. I’m talking long-term trials and not acute experimental trials like the one I talked about here: “Fasted Training Boosts Muscle Growth?”
The aforementioned studies all involve fasting, with time-intervals ranging from 18 to 36 hours. The modified ADF/CR-regimens involve one day of very low calorie intake, 300-800 kcal, followed by one day of ad libitum feeding (which means that participants were free to eat whatever they wanted).
So that’s that. Not strict fasting as in “0 kcal” in a few of those studies but the energy deficit during the fasting days was quite severe. For the sake of simplicity, I’m going to use intermittent fasting (IF) when I refer to the various fasting-based regimens used in the study review. It gets damn confusing with “intermittent CR” when people are used to thinking of it as intermittent fasting – which is exactly what it is, with the exception of the modified ADF-regimens that are super-low calories every other day.
The physiological effect is probably not much different as it pertains to modified fasting regimens versus strict fasting regimens. I think the benefits of intermittent fasting is likely mediated by the cyclical nature of the meal pattern itself; a period – the fast – spent in an energy deficient state, followed by a brief period of an acute energy surplus – the feeding phase. More on that some other time. For a brief review of popular approaches to intermittent fasting, you might want to have a look at this post:
“Brief Summary of Popular Approaches to Intermittent Fasting.”
Anyway, let’s commence with the review.

Intermittent Fasting vs Calorie Restriction for Weight Loss

This is a review, which basically means a “study of studies”, where the author, Varady, attempts to draw a conclusion from the collection of evidence available. Specifically, the author examined results from weight loss studies with intermittent fasting and compared those to weight loss studies with traditional calorie restriction. This sums up the objective of this review:
What has yet to be determined, however, is whether one of these interventions produces superior changes in body weight and body composition parameters when compared to the other. Accordingly, the objective of this review was to evaluate and compare the effects of daily CR versus intermittent CR on weight loss, fat mass loss, lean mass retention and visceral fat mass reduction, in overweight and obese adults.
The author then provides some basic data and background facts about the effects of weight loss on biomarkers, the various diets and their treatment periods, typical results, and so forth. Nothing too exciting. This little tidbit is worth mentioning:
When an individual loses weight by dietary restriction alone, approximately 75% of weight is lost as fat mass, and 25% of weight is lost as fat free mass (31). Fat free mass is an important predictor of basal metabolic rate (32). Thus, dietary interventions that preserve fat free mass at the expense of fat mass may be metabolically advantageous.
Keep in mind that this goes for your average study participant under typical conditions. The figure 75% fat / 25% muscle assumes no exercise, fairly high body fat percentage – compared to our standards anyway – and a suboptimal diet (i.e. protein intake around 15-20%).
Exercise helps to preserve muscle mass and it’s not uncommon for study participants to gain muscle mass while losing fat if weight training is included in the treatment period.
Secondly, body fat percentage also matters. Lean participants lose more muscle mass than obese participants on an equal energy deficit, because body fat by itself provides a muscle-sparing effect. No need to touch the emergency funds (muscle protein) if you have money in your savings account (adipose tissue).
Thirdly, a higher protein intake improves muscle retention during any given energy deficit, though arguably it becomes more important the leaner you are.
OK, basic common sense stuff. Are you bored to tears yet? Well, here’s something interesting:
We were also interested in comparing the ratio of fat mass to fat free mass lost with intermittent CR versus daily CR. From the studies reviewed here, it would appear as though a lower proportion of lean mass is lost in response to intermittent CR (90% weight lost as fat, 10% weight loss as fat free mass) (21–23) when compared to daily CR (75% weight lost as fat, 25% weight loss as fat free mass) (4,7–16).
So on a standard diet, the typical composition of weight loss is 75% fat / 25% muscle, while it appears to be 90% fat / 10% muscle on intermittent fasting based regimens. You don’t have to be a rocket scientist to understand that this is quite amazing if true.
Now, here’s where it get a little bit strange. I just talked about factors that affect muscle retention during weight loss: exercise, body fat and diet. It seems that the author doesn’t take any of these factors into account when he states he provides the figures 90% / 10% and 75% / 25%  for IF and CR respectively.
A few of the CR trials that were used to arrive at the 75% / 25%  figure included exercise, one included growth hormone administration and another one used a high-protein diet. Only one of the IF trials included exercise.
As you may understand, this is a major weakness when attempting to formulate a good estimate of the amount of weight lost and fat and muscle respectively. While the author is aware of this, he briefly skims over the issue (in the discussion), saying that results may vary depending on subject population, etc., but it is truly a major weakness of the paper and its conclusions:

For instance, findings between trials may have varied based on the different populations used for each study. The trials reviewed here differed greatly in terms of subject age range, inclusion of one or both sexes, and BMI classification. Thus, it is possible that the discrepancy in effects observed between studies could be partly due to the subject population employed. Results between studies may have also varied depending on the different feeding protocols utilized.

Strangely there is no mention of the exercise regimens that were used in a few of the CR-trials.
Furthermore, as if there aren’t enough confounders present as it is, there’s the issue of how body composition was measured. In order to assess and track changes in body composition during weight loss studies, researchers use different methods of measuring. The most basic method of tracking changes in body composition involves a pair of calipers in the hands of a professional, but this is not commonly used nowadays. Instead, body composition is usually measured by bioelectrical impedance analysis (BIA), dual-energy X-ray absorptiometry (DXA) and magnetic resonance imaging (MRI).
Without going into a lot of technical detail, there is a lot of variance in terms of how precise each one of these methods are. Though BIA is regarded as “good enough” to use in clinical trials, DXA and MRI tend to be a lot more accurate. And therein lies the problem; a larger percentage of the intermittent-fasting-based trials used BIA, while many of the CR-trials used DXA or MRI. If tracking methods differ between trials, it obviously skews the results.

Although the data are limited, preliminary findings indicate that individuals may find it easier to adhere to intermittent CR when compared to daily CR for short treatment durations (i.e. 12 weeks) (22). Whether or not this improved adherence to intermittent CR can help people with long-term weight maintenance is an important issue that warrants further research.

Well, that’s certainly something most of us can agree with. Pretty much all of my clients convert to intermittent fasting once they try it. I haven’t actually heard from anyone that went back to high-frequency meal patterns again.
Here’s the conclusion:
In sum, intermittent CR and daily CR diets appear to be equally as effective in decreasing body weight, fat mass, and potentially, visceral fat mass. However, intermittent restriction regimens may be superior to daily restriction regimens in that they help conserve lean mass at the expense of fat mass. These findings add to the growing body of evidence showing that intermittent CR may be implemented as another viable option for weight loss in overweight and obese populations.
Note the “may be”. As I pointed out earlier, the study review has weaknesses that makes it hard to treat it as concrete evidence of a muscle-sparing effect.

Personal Observations

One thing that always stood out to me in working with clients was the fact that they were able to keep all their muscle – even while dieting to very low body fat percentages. I’m talking about clients with years of weight training experience dieting to 5-6% body fat. They either gained strength and muscle mass or were able to maintain strength and muscle mass perfectly. I’m not talking about Average Joe Beginner Client with a blurry two-pack losing a few pounds while gaining muscle (AKA “newbie magic”). I’ve had the same experience myself. 
46-year old Dan was one of many clients from my latest client update that gained strength despite losing substantial amounts of body fat during his diet (-35 lbs in his case). I’ve had natural bodybuilding competitors setting personal records two weeks before the stage. Perhaps there is something to the muscle-sparing effects of intermittent fasting?
Numerous Leangains followers and success stories have also attested to the muscle-sparing effects of intermittent fasting. They have shared their experiences in the intermittent fasting thread on, in blog comments, in emails, on Twitter, my Facebook wall, forums, and other places. If this counts as any sort of evidence, it’s easy to find.
Obviously, it’s damn near impossible to say something for certain based on anecdotes. As I noted with the tracking methods in the review just discussed, there are often major confounders present.
Was it the switch to intermittent fasting that produced the favorable effect or was it the change in training and diet? Perhaps protein intake was increased.
Maybe the person in question ditched his high-rep pump’n’tone BS routine and replaced it with a sensible training routine like Reverse Pyramid.
All of this makes self-reporting and anecdotes dubious methods for drawing definite conclusions. But let us for one second entertain the thought of a muscle-sparing effect of intermittent fasting: the sheer amount of evidence, even if weak when viewed in isolation, suggests that there might be something to it.
This would then beg the question: why is intermittent fasting superior for preserving muscle mass during weight loss? What is the mechanism? That’s a topic for another time and another article.