In one sentence
What the scale says you weigh this morning, which is the sum of everything you are made of, including the water that came and went overnight.
Why it matters
Weight is the oldest and simplest body measure, and still one of the most useful, because it is the one number in the record that costs nothing to take every day. It is not useful as a single reading. Its value is as a series: the direction of the trend over weeks tells you whether what you eat and what you burn are in balance, and in Vitality Tracker that series is one of the two inputs the energy engine needs to estimate what you burn in a day.
What moves it
Water, mostly. Each gram of carbohydrate your muscles and liver store as glycogen is stored with about three grams of water (Fernández-Elías 2015), so a carbohydrate-rich day can add a kilogram that is not fat, and a low-carbohydrate day can drop one that is not fat either. Salt holds water; alcohol sheds it and then holds it; a hard session leaves muscles holding water for a day or two while they repair; a long flight, a hot day, a meal late at night and a missed bowel movement all show up on the scale. In women the second half of the menstrual cycle holds water. There is even a weekly rhythm: in 80 adults weighed daily for up to 330 days, weight was highest on Sunday and Monday and fell through the week, and the people who lost or kept their weight were the ones whose weekday compensation was strongest (Orsama 2014). A day-to-day move of 1 to 2 percent of body weight is water; fat changes at a few hundred grams a week.
How to measure it well
Same scale, same time, same way: in the morning, after the toilet, before food or drink, undressed or in the same light clothing. How to use a bioimpedance scale has the full routine. Weigh every day if you can, because a daily series with noise beats a weekly series without it.
How Vitality Tracker uses it
VT draws a 7-day trend through your mornings and reads that, not the latest number. The energy engine (TDEE) estimates your weight change by fitting a line through the whole window of days, never by subtracting two mornings, and discounts days you have marked with alcohol, illness or a missed bowel movement. Each scale keeps its own series: a new scale starts a new series, and VT never compares a reading from one scale with a reading from another. The day's weight is taken from the full scale reading when there is one.
How far to trust the number
Measured. A bathroom scale is accurate to about a hundred grams; what varies from day to day is you, not the scale. Trust the trend, question the morning.
Sources
- Fernández-Elías VE, Ortega JF, Nelson RK, Mora-Rodriguez R. Relationship between muscle water and glycogen recovery after prolonged exercise in the heat in humans. Eur J Appl Physiol 2015;115:1919–26. doi:10.1007/s00421-015-3175-z
- Orsama AL, Mattila E, Ermes M, van Gils M, Wansink B, Korhonen I. Weight rhythms: weight increases during weekends and decreases during weekdays. Obes Facts 2014;7:36–47. doi:10.1159/000356147
- Vitality Tracker, How Vitality Tracker estimates your energy expenditure, method note v1.0.