Why the scales go up and down

The body stores carbohydrates as glycogen, together with three to four times as much water. That is why weight often drops quickly at the start of a diet and can rise quickly after a carbohydrate-rich meal.^[1]

An old but direct measurement shows the size of this: in 6 people on 800 kcal a day, 61% of the weight lost on a ketogenic diet was water, against 37% on a mixed diet.^[5] So quick results in the first week say little about body fat.

Salty food, more on the scale the next day: you hear this a lot, but the picture is less clear. In 32 healthy men under lab conditions, more salt raised neither body water nor weight.^[6] In a small study of 9 men, on the other hand, fluid outside the cells rose by about 2 litres when they went from little to normal salt.^[7]

So it depends on where you start. There is no reliable figure in kilos.

Gut content counts too. Healthy adults in a British study passed a median of 106 g of stool a day, and the trip through the gut took about 60 hours.^[8] More fibre raises stool weight, especially from cereals and vegetables.^[9] So a high-fibre week can briefly show more without any added fat.

In a study of 62 healthy women and 765 cycles, the feeling of fluid retention was strongest on the first day of the period.^[2] These were self-reports, not readings from scales.

An analysis of 4,657 measurements from 80 adults found higher values on Sunday and Monday than later in the week. The authors consider this normal.^[3]

Some medicines change weight. A review of 257 studies found an average of 1.5 to 2.6 kg more with single drugs for depression, psychosis, seizures or diabetes.^[10] Blood pressure drugs from the calcium channel blocker group led to swollen ankles in 10.7%, against 3.2% in the comparison group.^[11] Don't stop anything on your own, talk to your doctor.

For the pill, a Cochrane review found no large effect on weight, but the data are too thin to rule out any effect.^[12]

DGE gives about 1,410 ml a day from drinks for adults aged 25 to under 51. Needs are higher with heat, a lot of activity or fever.^[4]

Weighing alone does little: in a review of trials, daily was not better than weekly, and it worked better with the support of a programme.^[13] In adults who want to lose weight, regular weighing was not linked to more depression or anxiety.^[14]

That does not hold for everyone, though. In 69 young women, daily weighing brought more mood swings and less body satisfaction within two weeks.^[15] In teenage girls, frequent weighing was linked to later binge eating.^[16] If the number stresses you, weigh less often or not at all. If you have an eating disorder or are afraid of one, read When to get medical advice.

Compare means over weeks, not single days. Weighing is optional in the app, and you do not have to do it every day.