Does Fasted Exercise Consume Muscle?

I recently had a conversation with someone I highly respect about whether it’s necessary to eat carbohydrates before exercising, or specifically hiking.

They believed that once ready glucose stores (glycogen) were consumed and you entered a state of ketosis, that the body would preferentially burn muscle instead of fat for fuel, however I don’t believe that is true.

I believe that the average 80kg, 25% body fat human, carries enough stored energy in the form of fat, to walk about 60 marathons (80kg x 25% x 2.2lbs x 3,500 calories/lb = approx 150,000 calories at 100 calories/mile = 1,500 miles or 60 marathons).

So I spent some time reading the scientific literature on the subject to try to work out which one of us was correct. (Sorry, I know, I’m sad like that, but this is a subject that really interests me.)

There are plenty of studies on this subject, here are just a selection:

“…data provided by scientific literature suggest a negligible or no effect of KD [ketogenic diets] on muscle mass with concomitant resistance training. KD may instead exert a protective effect against muscle mass loss during aging or during low calorie diets.”

“The aim of the present study was to compare the effects of 10 weeks resistance training in combination with either a regular diet (Ex) or a low carbohydrate, ketogenic diet (Lc+Ex) in overweight women on body weight and body composition… Resistance exercise in combination with a ketogenic diet may reduce body fat without significantly changing LBM [lean body mass, i.e. muscle]”

“We have demonstrated that using VLCKD [very low carbohydrate ketogenic diets] for a relatively short time period (i.e. 30 days) can decrease body weight and body fat without negative effects on strength performance in high level athletes.”

“Our results suggest that a KD [ketogenic diets] might be an alternative dietary approach to decrease fat mass and visceral adipose tissue without decreasing lean body mass”

I think it’s pretty clear that low carb ketogenic diets do not result in muscle loss, however we weren’t really talking about diets over the course of weeks, we were talking about exercising / hiking when glycogen stores were depleted, i.e. whilst fasted, so I looked for similar articles around Intermittent Fasting. Again there are many, here’s a sample:

“MEDLINE, CINAHL, PubMed and SportDiscus databases were searched for articles investigating IF [intermittent fasting], combined with resistance training that reported measures of body composition. Eight studies met the eligibility criteria. LBM [lean body mass] was generally maintained, while one study reported a significant increase in LBM [lean body mass].”

“Emerging findings suggest the metabolic switch from glucose to fatty acid-derived ketones represents an evolutionarily conserved trigger point that shifts metabolism from lipid/cholesterol synthesis and fat storage to mobilization of fat through fatty acid oxidation and fatty-acid derived ketones, which serve to preserve muscle mass and function.”

“…there appeared to be a protective effect of 5:2 fasting on LBM [lean body mass] as magnitude of weight loss increased…”

I think it’s also clear that intermittent fasting doesn’t result in loss of lean body mass / muscle. However again I realised we weren’t really talking about intermittent fasting diets either, but rather exercising whilst fasted, so I looked for studies on that subject:

“Exercising during a fasting state increases lipolysis in adipose tissue while also stimulating peripheral fat oxidation, resulting in increased fat utilization and weight loss.”

“These findings indicate that body composition changes associated with aerobic exercise in conjunction with a hypocaloric diet are similar regardless whether or not an individual is fasted prior to training.”

“Intra-group analysis for the effect of fasted and fed aerobic exercise revealed trivial to small effect sizes on body mass. The inter-group effect for the interventions on body mass was trivial. Intra-group effects were small for % body fat and trivial for lean mass in females, with trivial effects also found for the inter-groups analyses.”

Here results showed that there was no evidence of lean body mass reductions as a result of exercising whilst fasted.

In conclusion:

  • Ketogenic diets don’t cause loss in lean body mass,
  • neither does intermittent fasting,
  • and nor does exercising whilst fasted.

I think the science is pretty clear.

Fat is excess glucose that’s been stored for later use. Whereas protein in the from of muscles is what’s used to obtain food from our environment, i.e. it’s crucial for human survival.

From an evolutionary perspective, does it make any sense that when ready glucose (glycogen) levels are depleted, the body should cannibalise its own muscles for energy, thereby reducing our ability to acquire more food? Or does it make more sense that the body goes to the ready made glucose reserves we carry around, i.e. fat?

That’s literally why we have fat in our bodies, it’s energy stored for later use when glucose is needed. That’s what it’s for.

Personally I regularly go hiking without having breakfast, and don’t eat until the walk is over, and I have zero issues with a lack of energy whilst hiking. One of the reasons I do this is because I’m lazy: I don’t want to carry around a belly full of food, nor lunch in a backpack, that’s extra weight I don’t need to carry! I accept that this only works for a day hike mind, not camping overnight.

However I also accept that I generally eat a lower carbohydrate diet anyway, and also practice intermittent fasting, so am possibly more “fat adapted” than most.

It’s probably true that anyone on a typical western diet of cereal or toast for breakfast, sandwiches for lunch, and meat & 2 veg + desert for tea, has probably never been in ketosis (burnt fat for energy) in their lives.

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