The problemMy problem? My scale knows my weight gain and likely uses my weight to estimate my body fat percentage. If it would not use my weight to estimate body fat, why would it want to know my height (body mass index, BMI), age and sex? Part of the information on body fat likely comes from the currents send through my body by the scale, but another part from my weight or BMI. Thus my problem is, does the increase in body fat percentage tell me more than what I already knew, that I have gained 8 kg?
The experimentNow luckily, my fitness studio has another instrument to measure body fat. It is attached to a computer and also wants to know my weight. But here, you have to type in. Thus with this instrument you can experiment.
The instrument has a infra-red sensor that has to be placed on your biceps. To compute your body fat, and a colorful page full of other probably highly reliable information, it uses your weight, height, age and fitness.
We made one measurement stating my real current weight and one stating my weight at the beginning of the year. The difference in computed body fat percentage: 3,3%! Almost to similar to the increase estimated by my scale to be true.
The interpretationIt is possible, I gained some fat and my waist did increase some. It is also possible that nothing happened fatwise, other places look more muscular nowadays. I guess all I know is that I gained 8 kg.
I guess this does not surprise scientists and engineers working on these measurement devises. The German Wikipedia even mentions this effect qualitatively. But I am not sure if all users are aware of this problem and we can now put a number on it. If your fat gain is less than a third of your weight gain, the measurement is too uncertain to determine whether there really was a fat gain.
Post Scriptum. We forgot one experiment. I would love to know what the estimate for my body fat percentage would be if I tell the computer I am a young man. I expect some age discrimination by the multiple linear regression equations used.
Post Post Scriptum. Thanks to Caro of Stangenliebe (Pole art fitness Bonn) for the measurement.
UPDATE. I realised I could do the experiment with my age using my own scale. My Soehnle scale has two presets. I set person 1 to my age (rounded to 42) and person 2 to an Adonis of 18 years. In all four measurements the Adonis has exactly 3.7% fat less. If only my age group would not be so overweight, my readings would be a lot better.