Caloric restriction is the most well-studied form of intermittent fasting.  I talked about how to approach caloric restriction and how it impacts disease and longevity in my previous post.  Many people practice caloric restriction to lose fat, but they find themselves losing lean muscle mass in the process.  It’s a no-brainer that if you eat less, you’ll eventually lose weight.  Several studies have shown that weight loss is possible and sustainable on a caloric restriction diet without negatively impacting the quality of life. ^1,2  

Figure 1. Some of the benefits associated with weight loss. Copyright © 2020 Fit With Heba, All rights reserved.

Weight loss is associated with better health outcomes including improvements in cognitive function, insulin sensitivity, blood pressure, stress, total cholesterol, LDL cholesterol and much more.^3,4 

Despite all the benefits associated with weight loss and caloric restriction, we can’t spot target fat, so we’ll inevitably lose muscle mass in the process.  For a while, it became the norm that if you wanted to lose fat, you had to sacrifice your muscle in the process.  Thanks to modern science and research, we can throw away that mentality and focus on the ways we can simultaneously lose weight and maintain muscle throughout caloric restriction.  Before that, we must understand why losing muscle is a bad idea in the first place.

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Why is Muscle Important?

Muscle plays a central role in protein metabolism by serving as the main supplier of amino acids which come from broken down protein from our food.  Blood amino acid supply is important because these amino acids get integrated into essential organs and tissues when needed.  During fasting, starvation, or low caloric intake, blood amino acid supply is low, so muscles supply the amino acids needed to maintain protein synthesis in essential organs and tissues.^5,6  Additionally, muscle amino acids serve as precursors for plasma glucose synthesis during fasting.  

Figure 2. Muscle protein gets broken down during fasting and caloric restriction to provide amino acids to vital organs and tissues for protein synthesis. Copyright © 2020 Fit With Heba, All rights reserved.

Altered muscle metabolism plays an important role in the initiation and prevention of many common conditions and chronic diseases.  Muscle mass is also very important for recovery from critical illness or severe trauma.  Additionally, muscle strength and function are central to the recovery process.

During stressed states such as cancer and traumatic injury, the body uses more amino acids from muscle protein breakdown than during starvation.⁷ Individuals with low muscle mass respond poorly to stress and recover more slowly.  For example, survival from a severe burn is lowest in individuals with reduced lean body mass.^8,9  Muscle mass can also predict the recurrence of many diseases.  For example, studies have shown that in cancer patients, a decrease in muscle protein supply increases the recurrence of cancer and decreases the survival rate.¹⁰  This is not to say this is strictly due to lean muscle mass. Individuals who accumulate lean muscle mass tend to lead healthier lifestyles, exercise more, and eat properly in comparison to the average person. All of these factors can positively impact injury recovery and survival rates.

How Diet Can Offset Muscle Mass Due to Caloric Restriction

The loss of muscle mass can occur during weight loss and can have several harmful consequences as I’ve already mentioned. These consequences may lessen or negate some of the benefits of weight loss in the long run. 

Protein seems to be the most important macronutrient during times of caloric restriction to replenish muscle protein.  Higher protein intake during periods of caloric restriction has been shown to help offset the lean muscle mass loss that is typically associated with higher carbohydrate diets.¹¹  After protein ingestion, amino acids stimulate increased rates of muscle protein synthesis and suppress muscle protein breakdown for several hours.¹²  This process promotes lean muscle accumulation and prevents lean muscle loss over time. 

Because muscle protein is being used up during fasting or caloric restriction, all the amino acids that come from food intake gets incorporated into muscle protein to replete what was lost.  Under normal conditions, the protein supplied to the muscles during a feeding window should cancel out the loss of muscle protein in the fasting state.

How Resistance Training Can Offset Muscle Loss Due to Caloric Restriction

Remember autophagy from my last blog post about caloric restriction?  It is responsible for getting rid of the damaged parts to stimulate the recovery and function of muscle.^13,14,15  Muscle can be easily damaged following physical activity, malnutrition, and aging.  Autophagy is increased during fasting, atrophy, and exercise, and impairment in autophagy results in the accumulation of dysfunctional proteins. ^16,17,18,19 

So far, we know that caloric restriction increases autophagy which in turn helps in muscle mass recovery, building, and maintenance.  Turns out, resistance training (RT) in combination with enough protein is also a great addition to a caloric restriction regime to increase autophagy and muscle building.

Figure 3. Resistance training with adequate amounts of protein is shown to offset muscle loss during fasting and caloric restriction. Copyright © 2020 Fit With Heba, All rights reserved.

One study found that a calorically restrictive diet composed of 55% carbohydrate, 15% protein, and 30% fat combined with 3 days RT per week for 12 and 24 weeks was able to prevent almost 100% muscle loss in obese elderly people.²⁰.  Another study found that with caloric restriction, consumption of a diet containing 2.4g protein/kg body weight/day was more effective than consumption of a diet containing 1.2g protein/kg body weight/day in promoting increases in lean muscle mass and loss of fat mass when combined with a high volume of resistance and anaerobic exercise in men.²¹ Studies on women are limited, considering men are more likely to practice RT. Despite this, there is no reason not to believe that RT would not work for women.

Conclusion

While we once thought that it was almost impossible to build or even maintain muscle mass on a caloric restriction diet, we now know that there are several ways to not only maintain but also gain lean muscle mass while simultaneously losing weight.  Caloric restriction will increase autophagy, leading to several health benefits.  With enough protein and resistance training, fat loss along with muscle building is very possible.  We shouldn’t only want to build muscle to look good, but we should also want to build it to remain healthy and make sure that we can deal with stress, injury, and disease to recover quicker.  

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References

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