Take opinion out of it.
Consult a BMI chart.
Whatever the BMI chart says is a healthy weight, that is what you maintain. It isn’t about physical appearance, it is about weight. It isn’t about if a BMI chart is 100% perfect science. It is about if a BMI chart is “good enough” to provide a reasonable, healthy weight. The answer is clearly yes.
You select a healthy weight. Maintain that weight.
That way, it is no longer about your personal opinion. It is no longer about the opinion of a mother a friend or anyone else. It isn’t even about the opinion of a medical professional. You simply point to the BMI chart and say “This is a healthy weight, everyone else can f*** off with their opinions. The normal weight provided by BMI is healthy enough for me.”
