Get your Body Mass Index (BMI) - old and new ways

• The original Body Mass Index (BMI) is defined as \( \frac{weight}{height^{2}} \), with weight measured in kilogram and height in meter.
  
• The BMI is an attempt to quantify the amount of tissue mass in an individual, and then categorize that person as underweight, normal weight, overweight, or obese.
  
• Recently, Prof.Nick Trefethen from University of Oxford proposed that \( height^{2.5} \) might be a better approximation to the actual sizes and shapes of healthy bodies.
  
• He propsed a new BMI formula: \( \frac{1.3 \times weight}{height^{2.5}} \).
  I developed a Shiny app that will calculate BMI and other useful information using the original and new formula based on the weight and height input from users.

• Typical online BMI calculator only tells users their BMI value and whether they are normal under/overweight, or obese.
• My application goes one step further, and provides valuable, actionable information - the minimum weight users need to lose/gain to reach the normal BMI range (18.5-25). This provides a target for their workout.
• For underweight users, it uses 18.5 as the target BMI, and uses users' height to calculate target weight and required weight gain.
  
• For overweight and obese users, it uses 24.99 as the target BMI and uses users' height to calculate target weight and required weight loss.

Suppose a user is 1.83 meters tall and weighs 92 kilograms. Let's go through the BMI calculation.

weight <- 92; height <- 1.83
BMI_original <- 92/1.83^2
round(BMI_original,digits=2)

[1] 27.47

BMI_new <- 1.3*92/1.83^2.5
round(BMI_new,digits=2)

[1] 26.4

It is clear that the new formula yields a smaller BMI - good news!?

The user's BMI falls within the overweight range (25 - 30). How much weight does he need to lose? Let's calculate the minimum weight loss to be within the upper limit of normal BMI - 25.

round(weight - 24.99*1.83^2,digits=2)

[1] 8.31

round(weight_loss_new<- weight - 24.99*1.83^2.5/1.3,digits=2)

[1] 4.91

My Shiny app provides all the information based on both formula - BMI, BMI category, and minimum weight loss required.