As expected, this is also false. It shouldn’t be a surprise to anyone – it just doesn’t make sense. Off of the top of your head name some important muscles: heart, diaphragm, right? These are necessary to our life. Without them, we don’t function. Why would your body go and target these areas BEFORE going and eating away at fat? After all, stored fat has more energy than muscle (over twice as much, actually) and when was the last time you called someone “starving” because they had only fat and no muscle on them? That’s right, you didn’t.
So why do we continue insisting that our bodies are somehow programmed to preferentially break down our muscles over our energy rich fat? I think Alloran on Fitocracy made the best analogy – “Why tear apart chairs, tables, bed frames, etc. to build a fire when there’s a pile of firewood just outside the door?”
But I wouldn’t call myself Bill Nye with a vagina is I didn’t throw some science at you.
During a two day fast you have a 5 fold increase in Growth Hormone. Growth hormone is what tells the body to conserve protein and therefore muscles. More GH, more protein conservation. GH is a jack of all trades, though, because this badass chick also promotes lipolysis. For those of you who need a Latin reminder, lipo = fat lysis = killing or destroying. GH kills fat and saves protein. I told you she was a badass chick.
Growth Hormone isn’t the only big bad chick proving that you burn fat during fasting. Glycerol (released when the body breaks down stored fat) and palmitic acid (also found in fats) are high in the plasma during the first 12-72 hours of fasting. In fact, they double.
But if there’s so much science – and common sense – showing that you don’t lose muscle preferentially, where did it come from? Turns out a few studies showed a decrease in lean body mass (muscle) during a fast. Don’t worry, new science helped to remedy what may have been a misunderstanding. Turns out that majority of the “weight loss” from muscle was a loss of glycogen and water. Glycogen, for you non-bio nerds, is the stored form of glucose. So most of the loss of “mass” from muscle was water and stored glucose. Additionally, these same studies showed that about 14% of the energy from a fast came from protein, whereas 85% came from stored fat.
So no, skipping breakfast, participating in IF, or even lying on the couch all day not eating because you’re sick and lazy won’t cause your body to eat your muscles away. It just doesn’t make sense.