Man Obsessed with his Scale

Time to comment on another Washington Post story. This one is about a man who bought a fancy digital scale that included a body fat measurement. He found that the body fat measurement varied from about 22% to 26%. Those are big swings and he was seeing within one day. Clearly something is going on here. He is discovered that the scale manufacturer only claimed to be good to +/- 1%. Further investigation revealed that the technique used bioelectric impedance analysis (BIA) is sensitive to the individual’s level of hydration.

This is a wonderful example of where you can do everyday science. A scientist never believes his instrument until he calibrates it. A true calibration would involve weighing something of know weight. I do not have a standard 100 kilogram mass, so I have to try to understand the quality of the scale with other techniques. When I weigh myself I try to control as many variables as I can. I weigh myself in the morning before I shower. I do this before I eat my breakfast and I have no clothes on, so I will not be affected by whether I ate a big breakfast or a small breakfast and whether I have have dressed in heavy winter clothes or light summer clothes. I wear size 13 shoes and trust me they weigh a couple of pounds. These are all variables that I am trying to control. I still see regular variations of a few pounds over the course of a week. I can see a three pound increase one day that vanishes the next day. These can be random variations or variables that you are not controlling like overeating at a party. John Walker has a website about his Hacker’s Diet that discusses how you can use averages to discover your real weight from these variations.

The variation in weight of two or three pounds for a 200 pound man like myself or the Washington Post contributor is quite a bit smaller than the 22-26% variation in body fat. So he should have asked himself if my body fat percentage went up by 4% that would mean he gained about 8 pounds of fat. If he only gained two pounds then somehow he converted 6 pounds of lean body mass to fat in one day. That seems very unlikely, so he could have learned that BIA was not very accurate. I do not know exactly how accurate the BIA is but I am beginning to learn that it is not very good.

The article had a sidebar on how accurate scales were. They bought four scales and found for the same person the weight varied from 187.4 to 189.4 This does not surprise me, although a single scale would probably reproduce its result if used repeatedly. Any of the scales should track your weight day to day better than that. I would be curious if they used all four each day whether they would track each other.

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