When should I worry about my weight?

Usually on this blog, I answer my own questions. But when an old friend tells me that my answer to his question was helpful to him and could be to others too, well, I’ll break format for that.

How do I know I’m healthy?

The question:

So when it comes to being fat/body positive, I've been pretty successful in applying that to other people, but I'm not sure how/whether to use my own weight fluctuations to assess my own health. How do I assess my diet/exercise level? Should I evaluate weight gain/loss? Are there measurable signs that my diet and exercise routine are working for me? At what point (if ever) does weight gain/loss become a concern?

There are at least two ways to understand your weight changes and health status: with your head and with your heart. We’ll explore both. Before we do, though, let’s assume that you are a disease-free, able-bodied person who eats a reasonable diet, i.e. you’re not getting 40% of you calories from, say, alcohol or donuts.