If there were still any doubt that the healthy foods movement has reached critical mass, it was dispelled Thursday when Walmart jumped on the bandwagon with promises to improve the nutritional content of food it sells in its superstores. The retail giant vowed to force its suppliers to reduce the sugar and sodium content of the food it sells and eliminate trans fats from the products on its shelves. The company also pledged to bring down the price of healthy foods so that they are on par with less healthy fare.