Using the cut command to isolate a number and assign it as a variable

I have a file which contains a line of patient data (ex. 120lb, 14yr, 5.5ft, etc.) and I need an efficient way to manipulate the individual pieces of data from the command-line.

For example, if I wanted to increase the weight of a patient by 2kg, right now what I'm doing is very inefficient and requires me to read each file before.

cat patient.dat| tr '150kg' '152g' > patient.dat| cat patient.dat

I would like to assign each piece of information to a variable and have tried to do something like:

cat patient.dat| weight="$(cut -d" " -f1)"| echo "$weight" 

but am getting a blank line. Eventually, I would like to take this $weight and add/subtract from it and change that number in the file.

I also tried changing syntax with the "" and $ but have been getting the same result. Thanks in advance for any input/help.