To answer the question directly, yes, you can definitely lose muscle tissue. Your muscles are constantly in a state of MPS (muscle protein synthesis) and MPB (muscle protein breakdown), and it’s the difference between the two (the net nitrogen balance) which determines whether you’re gaining or losing muscle, overall. This ‘losing tone’ stuff is just folk biology.
The same thing is true for your fat cells, by the way — you’re constantly storing and releasing fat within your body, and the balance shifts throughout the day based on your acute diet, exercise, etc.
As for losing weight, if you’re in bed and not moving significantly, your diet will determine the balance between fat and protein loss. Look up protein-sparing modified fasts (PSMF); they’re essentially low-calorie diets that are extremely high in protein (and low in carbs). The protein that’s being spared is your lean body mass (meaning muscles, etc). If you’re on something like this, you’re mostly losing fat, barring diabetes or other unusual conditions.
However, if your diet is low in protein (and thus higher in carbs), you’re going to be losing muscle mass (along with some water weight). Your MPB is going to be slightly higher than usual, because of your lack of activity, while your MPS will be especially low (since you’re neither stimulating the signaling pathways for anabolism through exercise, nor supplying the amino acids to your muscles so they have anything to build with). With this diet/exercise profile, you’re more likely to be increasing your fat mass, even as you lose overall weight.
That depends on the time frame and exercise type:
- Go for a long run or bike ride, and weigh yourself at the end; you’ll lose some water weight, which may show up on the scale.
- Starve yourself, and you’ll lose fat and muscle.
- Do lots of cardio, and you may lose a disproportionate amount of muscle compared to the fat you lose. Long-distance runners look so stringy.
- Adopt a low-carb diet and start doing a variety of resistance exercises, and you’ll gradually lose fat and gain muscle.
Most people don’t gain freakish amounts of muscle without abusing steroids or doing other weird stuff, but most people can develop more strength and look better.
Filed in: Aerobic Exercise » When you lose weight are you only losing fat?