You sound a lot more convincing when you actually know what you’re talking about.
When you were a kid, did your parents ever make you eat some random vegetable or meal that you hated because it was quote unquote “good for you?” And then you ask what about it is good for you, and they say “it just is?” Yeah, not very convincing, was it? Your kids, even into their teenage years, have an insatiable desire for knowledge. When you brush off their questions like that, you’re not actually teaching them anything.
“Is this food actually good for me, or are they just telling me that?” If you can’t properly explain nutrition to your kids, they’re not gonna take you seriously when you try to recommend food to them. This can be even worse when they get to their rebellious years (and they will). Not every meal time needs to be a TED Talk, but your kids rely on you to explain to them how the world works, and I can say with confidence that health class ain’t gonna teach them this stuff for you.
When you cook something for your kids, don’t just serve it to them wordlessly, tell them what’s in it. Break down the ingredients and let them know what each one is doing for them. Make it fun, like a Food Network show! If they understand the nutritional value of their foods, they’ll have a greater understanding for both that value and an appreciation of the knowledge of it as they get older. Besides regular nutrients, teach them about additional benefits, like increases in energy or improvements to mood. You may have to embellish a smidge, but hey, if it gets them to eat their spinach, a little white lie won’t hurt.
I’ll be the first to admit that explaining everything ever to a kid is exhausting, but if you can arm them with this knowledge now, you’ll be glad you did later when you don’t have to nag them to eat healthy.