In terms of eating healthily, you need to practice what you preach. Do you have unhealthy foods in your house? If so, why? If, for example, you eat ice cream several days a week, why is it a bad move on her part to do so? What is health, and why should we care about it? This is a multifaceted issue, and teaching by example is better than words.
Self-esteem should not come from how one 'looks', rather from who one is, because looks change constantly, and are often not under our control. Maybe you feel differently, but I would not concentrate on her looks at all, especially at the age of 8. If you must give her dietary advice, do so only from the standpoint of health and self-determination.
If I were in your situation, I would start introducing her to critical thinking skills. What is behind the judgements she hears/perceives? Is it important/true to one's worldview, or unimportant and therefore discardable? In my worldview, comments on the appearance of others are not to be treated as important (in other words, the comment puts value on the superficial and is, therefore, not to be taken seriously) and that the opinion is fundamentally irrelevant (opinion being different from the individual.) Avoid global judgements ("That boy is not nice") in favor of accurate ones ("That boy should not have said something that might hurt your feelings.")
If you choose to do this, you have to be there as well, because she will learn from you, and if you value physical appearance highly, she will learn that your words to the contrary are untrue.
Watch what she watches on television or other media. If there is a strong focus on physical appearance, discuss this in terms of marketing and how this informs us on deeper levels. Start the radical process of critical thinking now.
People tend to focus on negatives. Read about resilience. Start teaching her to think in terms of positive attributes, abilities, and actions rather than negative ones.*
People will face criticism in every aspect of their lives. One needs to be able to focus and prioritize on what is good (e.g. kindness, honesty, tact, etc.), what is important (you can fill this in as this varies with culture, but I would place integrity and self-efficacy very high on any list), and what is under our control (self-control, delaying gratification to achieve goals, etc.).
Finally, make sure you notice and praise her abilities and strengths, and avoid empty praise (e.g. "You're special/You can be whatever you want to be.")
*Although this may sound like psychobabble, keeping a "success journal" with her might be a good idea if she doesn't have much self confidence. How often do we ask ourselves in a meaningful manner, "What did I do right today?" Too often we start and end with, "It was a good/bad day". If you're worried that this will give her a swollen ego, don't. Life will poke holes in it constantly. It's how we deal with these setbacks that matters in terms of self-respect, which is really what self-esteem is about.
Building Resilience in Children