Health at Every Size is the notion that all body types, regardless of shape, size, and BMI can be considered healthy. In fact, BMI aka Body Mass Index was never a determination of health at all! It was a system utilized by insurance companies to determine if someone is "overweight".
BMI does not account for the amount of lean body versus fat tissue. Someone could be a body builder with less than 10% body weight but be considered "overweight" or even obese on the BMI scale.
BMI is old news. As is the notion that you have to be thin to be healthy. Health is an internal equation; it's based on how well the heart functions, how nourished are the cells, how easily does blood flow through the veins, etc! These are not determined by body size. They are determined by lifestyle.
Here are my thoughts on health, body size, and whole-self nourishment:
You are the best and only expert on your body
Beauty and health are much more than a number on the scale
Dieting and deprivation can be harmful - instead, choose kindness and listen to the body's hunger cues
Your body is made for health and vitality. Trust it.