Hi, I’m Claire’s husband, Dan. Wanted to ask you a question about loss of muscle mass when beginning keto. I’m a 54 year old, 5’8" male. I’m generally fit and exercise regularly (165 lb, 14.3% body fat, 5 days/week at Orange Theory Fitness, etc.). I started eating keto five weeks ago and have been tracking body composition daily, recording my results weekly. I’ve also been doing a 16/8 intermittent fasting protocol/time restricted eating. I’ve noticed that in those five weeks I have lost 6.1 lb, 0.7% body fat, 0.4% subcutaneous fat, and 1 lb of visceral fat - all good. But I’ve also lost 4 lb of muscle mass according to the scale. That’s a little alarming and I wondered if I’m doing anything wrong to cause that? It was my understanding that keto and intermittent fasting would maintain muscle while reducing fat. Appreciate your thoughts.
Losing muscle mass on Keto & 16/8 fasting
Hi Dan!
If you are using a scale as described below you’ll notice that they can be very inaccurate. It is essentially measuring the water in your system and back-calculating body fat.
If you stay on the same diet and are careful with hydration levels - accuracy goes up but, switch to keto and your body will hold a different amount of water - leading to inaccurate results.
If you’re tracking using a body composition scale then ignore it, not accurate at all. I have two. One puts me at 28% BF and the other at 16.8%
There’s no reason at all anyone would lose muscle on a well formulated keto diet.
Interested in this as well. I was under the impression that if you workout hard in the first month (while you’re not quite fat adapted), your body could possibly use glycogen stores in the MUSCLE to fuel itself with energy. It has to pull from somewhere, right? So if it doesn’t have carbs for fuel where does it tap into? It made sense to me but this is just something that I came across while doing research. Lots of misinformation out there that’s why this forum is helpful because it seems to have more knowledgeable people and less broscience. Again, chances are I’m completely wrong but I have heard that you should exercise with a moderate heart rate and don’t go crazy for the first month or until you’re fat adapted.
As you deplete your glycogen stores, you liver and kidneys will manufacture glucose through a process called gluconeogenesis and it doesn’t need carbs to do it.
Fortunately, those scales are notoriously inaccurate. But be sure to get enough protein, especially if you work out. We used to believe that excess protein turned to glucose in the bloodstream, but with further research, it is clear that is not a concern. The liver makes only the amount of glucose needed, and the body does other things with any excess protein. This means it’s better to err a bit on the side of too much protein—assuming you’re keeping carbohydrate low, of course—than risk not getting enough.
SFrustrating considering how much I paid for mine 
the one at home puts me at 23%, the other at the gym at 6% (WTF) so got a Dexa scan instead - 16%. Irrespective though our waist is slimmer, and I fit into the suit I wore for our engagement. Ditch the scales, grab a tape measure instead or get a dexa scan instead. 
♀
The bioimpedance scales are affected by hydration. If you go on a low carb/keto diet, you’ll lose hydration initially. That’ll affect the fat indication of the scale.
Similarly, measure in the morning (when you’re less hydrated) and in the evening (when you’re more hydrated). You’ll get vastly different numbers, even though there’s no way your muscle mass increased or decreased by such a large amount.