Weight loss notes. Planning and keeping track.

I wanted to get an idea of how long it will take to reach my body fat percentage goal of 8% (I’m a male; females shouldn’t go this low.) So I wrote a small script.

We’ll need the following constant, an approximation of how many calories there is in a kilogram of body fat (i.e., adipose tissue).

kcal_per_kg <-  7700

Then you provide approximations for your average calorie expenditure and intake. It’s probably a good idea to use a fitness tracking device for this, just be aware the calorie measurements aren’t that precise. I use an approximation of calories out equal to 3400. That is quite a lot, but it’s what Fitbit tells me I use on a weekday, so that’s what I’ll be going for. I’m assuming 3400 / 2 = 1700 calories in on average, as I’m doing alternate day fasting.

This is my input data on August 27 2022.

input_aug27 <- list(
  kcal_in = 1700, # The expected daily calorie intake, averaged across days.
  kcal_out = 3400, # I used Fitbit for this. 
  weight = 71.1, # Current weight in kg.
  fat = 8.6, #11.1, # Fat mass in kg. Calculated using a Withings scale.
  height = 180, # In centimeters. Can be used for BMI calculation.
  pct_goal = 0.08 # Goal in terms of pct body fat. This goal is semi-random.
)

I can use this input data do derive several interesting quantities.

#' Calculate derived quantities from a `weight_loss` object.
#' 
#' @param input An object of class `weight_loss`.
#' @return A list of derived quantities.

derived = function(input) {
  fat_pct = input$fat / input$weight
  rest_kg = input$weight - input$fat
  weight_goal = rest_kg / (1 - input$pct_goal)
  weight_diff = input$weight - weight_goal
  deficit = input$kcal_out - input$kcal_in
  days_needed = ceiling(weight_diff * kcal_per_kg / deficit)
  loss_per_day = deficit / kcal_per_kg    
  list(
    fat_pct = fat_pct, 
    weight_goal = weight_goal,
    fat_goal = weight_goal - rest_kg,
    kg_left = weight_diff, 
    days_needed = days_needed,
    loss_per_day = loss_per_day)
}

In my case I obtain:

knitr::kable(tibble::as_tibble(derived(input_aug27)), digits = 2)

So I have $15$ days of alternate day fasting left until I reach my goal, which happens to be $5.43$ kilograms of body fat. Moreover, I expect to lose $0.22$ kilograms of body fat each day.

input_aug28 <- input_aug27
input_aug28$weight <- 69.1
input_aug28$fat <- 8.7
knitr::kable(tibble::as_tibble(derived(input_aug28)), digits = 2)