Tanita BC-418MA Causes of errors in impedance measurements, Intra-day fluctuations Weight

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W Causes of errors in impedance measurements

BIA measures impedance and calculates body composition on the basis of this impedance. It is known, however, that impedance can vary due to changes in the quantity and distribution of body water, which accounts for approximately 60% of body weight, as well as changes in body temperature. Therefore, when measurements are being taken for research purposes or on a daily basis, uniform measurement conditions must be ensured.

Consideration should be given to two types of changes in impedance: inter-day changes and intra-day fluctuation (cyclic changes within a single day). In the 8-electrode method, full body measurements use the impedance between the arms and legs, while measurements for specific body parts use the impedance of those particular body parts.

The reasons for changes in impedance are different for the upper limbs and the lower limbs. Given that the impedance of the trunk is extremely low (a mere 5-10% of the impedance between hands and feet), changes involving the trunk can be almost wholly ignored when measuring the impedance between hands and feet, and the changes for the upper and lower limbs are synthesized.

When studying intra-day and inter-day changes with the 8-electrode method, it is thus necessary to determine the respective changes for the upper and lower limbs.

1) Intra-day fluctuations (cyclic activity within a single day)

Peripheral impedance ordinarily rises during sleep and falls during activity. This is believed to occur because extracellular fluid, typified by the blood, moves to the trunk during sleep, reducing the extracellular fluid in peripheral areas and thus increasing impedance, and because the extracellular fluid returns to peripheral areas during activity, causing a drop in impedance.

Intra-day activities during this cycle cause changes in body water volume and distribution due to the consumption of food and drink as well as exercise.

The charts on page 33 illustrate one example of intra-day activities. The changes in weight, impedance, and body fat percentage are shown on the respective vertical axes, while the measurement times are shown on the horizontal axes (plotted points indicate the variation from the average change).

Increases in body weight occurred following the consumption of food or drink, and a sharp decrease was seen between 20:00 and 22:00 because of a bath.

The impedance between the hands and feet, as mentioned earlier, synthesizes the changes in impedance of the upper limbs and the lower limbs.

During the two or three hours of digestion and absorption that followed breakfast and lunch, a decrease occurred in the impedance of both the upper and lower limbs, but the degree of change was larger in the lower limbs than in the upper limbs. This is because the impedance of the upper limbs temporarily increased due to the impact of specific dynamic action (SDA) from the meals but then turned downward as the body fluid mass increased. No such increase or decrease in impedance due to SDA was seen, though, in the lower limbs. After supper, the lower level of activity caused an increase due to lessened extracellular fluid in peripheral areas at the same time that upper and lower limb impedance fell due to digestion and absorption; as a result no clear trend could be seen.

Among the particularly large changes that occurred were decreases in impedance while taking baths and subsequent increases, increases and decreases attributable to exercise, and decreases after tasks such as carrying heavy objects that require use of the upper limbs. In the course of a single day, the largest intra-day change in the lower limbs was the decrease in impedance due to the flow of blood to the lower limbs (blood congestion). No clear trend was noted in impedance in the upper limbs over the course of a single day, but extremely large changes did occur as a result of certain activities in the daily routine.

Intra-day fluctuations (Weight)

1.5
1.0
							After supper	Before bathing
0.5				After lunch					Sleeping
		After breakfast
0.0								After bathing
kg	Rising
-0.5				Before lunch			Before supper
	Before breakfast
-1.0
-1.5	6	8	10	12	14	16	18	20	22
					Time

Intra-day fluctuations (Impedance value)

	60
		Rising
	40		Before breakfast
	20				Before lunch		Before supper
Ω	0	After							Before bathing
		breakfast								Sleeping
	-20				After lunch				After
									supper
	-40								After bathing
	-60	6	8	10	12	14	16	18	20	22
						Time

Between hand and foot Between both hands Between both legs

	Intra-day fluctuations(						%FAT)
3.0
2.0
	Before breakfast
1.0
Rising			Before lunch			After supper Before bathing
％ 0.0	After							Sleeping
	breakfast
			After
-1.0			lunch				Before
							supper
-2.0							After bathing
-3.0
6	8	10	12	14	16	18	20	22
				Time

Between hand and foot Between both hands Between both legs

Intra-day changes are thought to occur in unique cycles arising from differences in the rhythm of individual daily routines, occupations, and day-to-day activities.

Consequently, to obtain consistent measurement figures it is recommended that measurements be taken under the following conditions:

1)Measuring three hours or more after waking up, with normal day-to-day activities carried on during this time (simply sitting or traveling by car will keep impedance

high)

2) Measuring three hours or more after last eating (impedance tends to fall for 2 to 3 hours after a meal)

3)Measuring twelve or more hours after vigorous exercise (changes in impedance vary, depending on the type and intensity of the exercise done)

4)Urinating before the measurement

5)Measuring as far as possible at the same time every day in the case of repeated measurements (more dependable measurements can be obtained if body weight and impedance are both measured at fixed times)

Complying with the conditions above should result in very consistent measurements.

When developing this device, the following six conditions were established in deriving the regression formula:

1)No alcohol less than 12 hours prior to measurement

2)No vigorous exercise less than 12 hours prior to measurement

3)No excess food and drink on the day before measurement

4)No food and drink less than 3 hours prior to measurement

5)Urination immediately before measurement

6)No measurements during menstrual period (women)

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