WASHINGTON - Michelle Obama on Thursday defended her efforts to implement healthier nutrition guidelines as she hosted elementary-school students for a homegrown lunch at the White House.

"It's up to us to make sure that these kids get the best food that they can get into their stomachs, because it's not just about nutrition. It's about their academic success," Obama said as she and the students prepared salads with produce from the White House garden.

She hosted the lunch amid significant pushback against national school-lunch standards.

Nationwide, some children also have used ThanksMichelle in social-media posts to vent their exasperation about the new foods - and sometimes smaller portions - served in their school cafeterias as a result of the new standards.

Most recently, the House Appropriations Committee started debating a bill that would provide a yearlong waiver for schools that are struggling to pay for food that meets the new standards. Obama and the White House have staunchly opposed the proposal.

"We simply can't afford to say, 'Oh, well, it's too hard, so let's not do it,' " she said. "And as this first lady will tell you, I'm going to fight until the bitter end to make sure that every kid in this country continues to have the best nutrition that they can have in our schools."

Not all feedback has been negative.

Twenty-five students from Washington, most of whom had helped plant the produce in the White House garden in April, accompanied the first lady Thursday for what was almost literally a farm-to-table lunch. Their elementary schools were invited as examples of Obama's healthier standards.

"All of these schools have gardens of their own," Obama said. "All of these schools, in this area, are finding ways to incorporate healthy foods into their lunches. It is doable."