Intermittent Fasting and Muscle Loss: Does it Burn Muscle?

Today, a question from one Muscle Evo reader on the subject of intermittent fasting and muscle loss.

“I’m lifting weights three times a week,” he wrote. “Can I have a 24-hour fast twice a week? Or will I lose muscle because of this?”

Unless you’ve recently arrived from a parallel universe, there’s a good chance you’ve heard of intermittent fasting.

It involves alternating times where you don’t eat (the fasting window) with times where you do (the feeding window).

Whether you realize it or not, you already use a form of intermittent fasting (IF for short). That’s because you eat during the day and fast at night when you sleep.

The various IF protocols out there just make the feeding window shorter and the fasting window longer. And it’s the length of these fasting periods that has left some people concerned about muscle loss.

Can you fast and still have all your muscle mass remain intact?

Or are you going to lose muscle faster than Superman losing his powers whenever a chunk of kryptonite shows up?

Let’s take a closer look at what the science says about intermittent fasting and muscle loss.

Intermittent Fasting and Muscle Loss: The Science

Back in 2009, researchers looked at the effect of alternate day fasting in a group of 16 obese men and women [6].

As the name suggests, alternate day fasting involves a fast day followed by a feed day. On the fast day, you don’t go without food completely.

Rather, you eat lunch — 400 to 500 calories for women and 500 to 600 calories for men — between noon and 2pm. Then on feed day, you get to eat (within reason) whatever you want.

After eight weeks, the participants lost an average of 12 pounds in weight. In most diet studies, you expect to see a small amount of muscle loss. But not this time, as all of the lost weight came from fat.

That’s despite the fact that subjects taking part in the study made no special effort to maintain muscle mass.

Nobody lifted any weights.

Nor was there any emphasis on maintaining a high protein intake, which averaged just 80 grams per day on the feed days and 24 grams on the fast days (food intake during the study was reported here).

In 2011, Dr Krista Varady of the University of Illinois at Chicago compared the results from a number of studies on both intermittent and continuous calorie restriction [5].

Her conclusion reads as follows:

“From the studies reviewed here, it would appear as though a lower proportion of lean mass is lost in response to intermittent calorie restriction (90% weight lost as fat, 10% weight loss as fat free mass) when compared to daily calorie restriction (75% weight lost as fat, 25% weight loss as fat free mass).”

She does, however, issue a note of caution:

“The majority of daily calorie restriction trials implemented dual-energy X-ray absorptiometry (DXA) and magnetic resonance imaging (MRI), while the majority of intermittent CR trials employed bioelectrical impedance analysis. It is well known that DXA and MRI are vastly more accurate techniques for the assessment of fat mass and fat free mass when compared to bioelectrical impedance analysis. Thus, the different methods employed could create variability when comparing findings between diets.”

In other words, while the studies on intermittent fasting show a reduction in muscle loss compared to those using standard diets, the methods used to track changes in body composition have not been as accurate, which has the potential to skew the results.

But there’s another problem, and it’s that the review compared the findings from a completely different set of studies.

Variations in the amount of lost weight coming from muscle and fat could have had more to do with differences in the way each study was set up rather than any special “nutrient partitioning” effect that’s unique to intermittent fasting.

Since Dr Varady’s review was published in 2011, a couple of studies have been released that bring some interesting new data to the table.

The first comes from a research group based in Manchester, and is one of the few trials to directly compare intermittent with continuous calorie restriction [2].

A group of 107 overweight or obese young women was assigned to either a standard or an intermittent fasting diet, where they ate an average of 540 calories per day for two consecutive days each week.

After six months, there was no significant difference in the rate of fat loss between the two groups. The intermittent fasting group had lost an average of 14 pounds of fat compared to 12 pounds in the normal dieters.

More interesting still, especially if you’re worried about intermittent fasting and muscle loss, the proportion of lost weight that came from fat was exactly the same in both groups.

For every 10 pounds of weight lost, roughly 8 pounds came from fat. The remaining 2 pounds came from muscle.

Intermittent fasting was no better or no worse than continuous calorie restriction as a way to preserve muscle while dropping fat.

In one of the most recent trials to look at the effect of an alternate day fasting protocol on weight loss, a group of 32 obese subjects followed either a low- or high-fat diet for eight weeks [4].

The low-fat group got 25% of their total calorie intake from fat, compared to 45% in the high-fat group.

At the end of eight weeks, subjects in the high-fat group had lost 12 pounds of fat, compared to 9 pounds in those on the low-fat diet.

If you take a look at the figure below, which shows the change in body composition during the study, you’ll see that both groups actually registered a small gain in lean mass. And this was measured using DXA, which is more reliable than the methods used in many of the other studies on intermittent fasting.

ADF-HF = alternate day fasting high-fat group; ADF-LF = alternate day fasting low-fat group; BW = body weight; FM = fat mass; FFM = fat-free mass.

Again, none of the subjects did any sort of resistance training. Nor were they given a diet with a particularly high intake of protein, which was set at just 15% of total calories in both groups.

Does this mean that intermittent fasting can help you preserve muscle without the need for strength training or a higher protein intake?

Not exactly. The big limitation with all of the studies we’ve looked at is that they’ve used overweight and obese subjects with a relatively large amount of fat to lose.

Someone is classified as overweight when their body mass index (BMI) hits 25 and obese when it reaches 30. A BMI of 30 corresponds roughly with a body fat percentage of 40% in women and 30% in men [1, 3].

For obese individuals, muscle loss doesn’t tend to be much of a problem unless they severely restrict their calorie intake.

It’s only as you get leaner that the proportion of lost weight coming from muscle tissue starts to rise. That’s why we can’t extend the results from studies of overweight and obese participants to leaner subjects as well.

The less fat you have to lose, the more “attention to detail” you’ll need to pay to stuff like protein intake, strength training and the size of your calorie deficit. I’ve explained more in How to Lose Fat without Losing Muscle.

Intermittent Fasting and Muscle Loss: Summary

In summary, the research to date – most of which has been done on overweight and obese individuals – shows that most people don’t need to worry about intermittent fasting and muscle loss.

But given the fact that muscle protein breakdown only becomes a major issue when liver glycogen stores are extremely low, as well as the fact that amino acids are still being released into your blood for some time after a meal, I don’t think it’s something that leaner individuals need to worry about either.

My own experience with shorter fasts lasting 12-16 hours is that they’re no worse, nor significantly better, than regular diets when it comes to maintaining muscle mass while you drop fat. The main benefit of intermittent fasting is convenience and simplicity, rather than any massive difference in terms of results.

References
1. Gallagher D, Heymsfield SB, Heo M, Jebb SA, Murgatroyd PR, Sakamoto Y. (2000). Healthy percentage body fat ranges: an approach for developing guidelines based on body mass index. American Journal of Clinical Nutrition, 72, 694-701
2. Harvie MN, Pegington M, Mattson MP, Frystyk J, Dillon B, Evans G, Cuzick J, Jebb SA, Martin B, Cutler RG, Son TG, Maudsley S, Carlson OD, Egan JM, Flyvbjerg A, Howell A. (2011). The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. International Journal of Obesity, 35, 714-727
3. Kyle UG, Schutz Y, Dupertuis YM, Pichard C. (2003). Body composition interpretation: Contributions of the fat-free mass index and the body fat mass index. Nutrition, 19, 597-604
4. Klempel MC, Kroeger CM, Norkeviciute E, Goslawski M, Phillips SA, Varady KA. (2013). Benefit of a low-fat over high-fat diet on vascular health during alternate day fasting. Nutrition & Diabetes, 3, e71
5. Varady KA. (2011). Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss? Obesity Reviews, 12, e593-601
6. Varady KA, Bhutani S, Church EC, Klempel MC. (2009). Short-term modified alternate-day fasting: a novel dietary strategy for weight loss and cardioprotection in obese adults. American Journal of Clinical Nutrition, 90, 1138-1143