Whether people want to admit to this or believe it or not is up to them, but...when you are in a discussion with someone, particularly if that discussion involves a sensitive topic, what we hear and how we interpret it is based on the fact that it first passes through something similar to a filter in our brains; a "decoder", if you will. That filter/decoder (for lack of a better word) is composed of past experiences with that topic or situation as well as our own personal thoughts and feelings attached to that particular topic or situation. It does actually influence not only what you hear, but how you interpret what is being said to you (motives, implied meaning, etc...). When communicating with someone else, especially on a sensitive issue such as body image and weight gain/loss, it's important to remember that often what you said isn't nearly as important as what the other person "heard" and keep that in mind when the reaction you get isn't in line with what you actually said. In such a situation, ask them to repeat back to you what they heard. If you can manage to do this, I'd be willing to bet that what is repeated back to you isn't exactly what you said OR how you meant it if the reaction is a negative one. Doing this also gives a chance to clarify and correct the other person's interpretation.
On the other hand, if you find yourself getting upset by something that another person is saying to you, it might also help to step back and ask yourself if you are interpreting what they said correctly and first ask for clarification before blowing an O-ring on them.
"I" messages are also less likely to elicit hostility than "you" messages. For example: "I think that your health is in danger or will be soon if some changes aren't made" vs. "You're eating yourself into an early grave if you don't stop stuffing yourself and lose some weight". See the difference? One is an expression of concern (and how can you reasonably get angry with someone for expressing concern about your health?), the other would most likely be interpreted as a personal attack requiring defense...the infamous "kaboom".
Often it's not what you say, guys and gals, it's how you say it that matters.