Of primary importance, we need to understand the limitations of bio-impedance measurements:
- It's an approximation based on an approximation
- The more outside the population they built their formulas on, the more inaccurate it is.
You'll find that your hydration levels can seriously affect the outcome, as well as whether you skin is dry or wet. The gist of it is that the more resistance the scale sees, the more body fat it registers. Being bloated with lots of water weight or stepping on the scale after your morning shower skews the results toward lean mass.
I don't know much about Withings scales, or what their track record is, but bio-impedance measurements are like trying to hit a bulls-eye throwing darts in the dark. Manufacturers do there best to make sure the dart hits the dartboard, but they can't account for everything.
Cheap ways of measuring progress:
- Flexible tape measure: grab one of these from a sewing store, and measure everything you want to track. As you add muscle and reduce fat you should see the measurements adjust accordingly.
- Mirror or monthly progress pics: compare with pictures of others in your activity range (examples).
Essentially, you should like what you see. If your abs are starting to have more definition, then you have lost fat mass. That doesn't happen as you gain fat.
There are a couple guidelines:
- If you are gaining more than 1/4" (1/2 cm) a week at the abdominal area you are gaining more fat than muscle. At or below that amount is a comfortable rate to increase muscle while keeping as much fat off as possible.
- Weight loss for cutting should be about 1 lb (1/2 kg) a week to make sure you are preserving as much muscle as possible.
- Weight gain for bulking should be about 3/4 lb (.34 kg) a week to make sure you aren't adding too much fat with the muscle.
Those guidelines are basically there to keep a reality check. Your body can only build muscle so fast. When you are obese (abs are covered by layers and layers of fat), you can lose weight faster and not have problems. When you are fairly lean, all you need to do is adjust your diet about 200-300 calories a week until you are within this rate of gain/loss.