My weight scale (Tanita UM-070) provides body fat and total water level monitoring feature using Bioelectrical impedance analysis. As body Fat Percentage fluctuates in at the different times of the day due to hydration levels in the body, the manual provided with my weight scale suggests that the measurement should be taken at the same time of day and under the same conditions. Furthermore, it suggests a reading in the evening before the evening meal while hydration levels are more stable.

I tried to take the reading in the evening as advised. The problem here, however, is that there are noticeable fluctuations in readings ​​when observed on consecutive days. This implies that "the same conditions" I was told for are not remaining the same on consecutive days. So the reading fluctuates.

Now I give up getting the actual reading as it is hard to maintain the same conditions in the middle of the day.

I am conscious that the reading will be high in the morning since the body tends to be dehydrated after a long night’s sleep. But I think every morning the body will be under the same conditions. So I want to take the reading in the morning and I will be able to calculate actual changes from those apparent readings taken in the morning and observe whether my body fat is increasing or decreasing.

I want to know:

  1. Does the body remain in the same condition every morning?
  2. Is it possible to observe the actual change in body fat from the apparent reading taken in the morning?

1 Answer 1


I think the first thing you need to recognise is that these scales are far from perfect, the technology firstly is at best an approximation, I won't go into too much detail but it measures the impedance (signal frequency shift and resistance) of a small electrical current between your two feet i.e. the current passes up one leg then down the other.

The first approximation these scales make is that your lower half of your body contains the same level of body fat than your top half, this is obviously strongly biased as genetics and gender play a HUGE role in where you store your body fat e.g. generally men tend to hold subcutaneous fat around their abdomen more than women and women tend to hold near their thighs, generally.

Second assumption is the impedance shift is solely due to the difference in densities of muscle and fat, the truth of the matter is a large number of factors play a role in the impedance of the human body and only a fraction of that is fat a muscle density differences. One of the fundamental aspects of circuit theory is that electricity will take the path of least resistance you can see this if you consider a simple parallel resistance circuit


You can see the total resistance is strongly biased by small resistance, your skin happens to be the best conductor of electricity in your body due to the high water content and the presence of significant ions like K+ and Na+, so small changes in things like how hot your skin is and how much you are sweating and largely skew the total resistance and frequency (equivalency not shown here for simplicity) of the resultant signal that the scale measures adding large uncertainty to the measurements. TL;DR The scale is pretty inaccurate

Finally, your body is not a machine, meaning at any one time you are going through high and low productions of hormones, metabolism cycles etc. So taking a snapshot of your body over a day wont really tell you much about the composition of your body, even things like what kind of food you ate the day before, whether you have urinated/defecated recently will obviously change your weight as well, and as you mentioned dehydration (how hot the night was, how well you sleep can affect this too).

If you want to truly capture how your body is changing you need to treat the scale and body as inaccurate, so my advice would be measure a) as you have said at the same time of the day, I normally measure in the morning right after I've gone the bathroom before I eat (fasted is the most consistent) and b) Often i.e. 3-5 times a week and keep a speadsheet and take an average over the week and then a month, depending where your at in your fitness you will tend to change in a period of a week- month not days

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