LONDON — Marks & Spencer has become the first retailer in the United Kingdom to add vitamin D to its packaged bread line. The initiative will provide a minimum of 15% of the daily requirement of vitamin D in every two-slice serving. Marks & Spencer said the move reflects concerns that consumers are not getting the recommended daily amount of the vitamin, which helps to keep bones healthy. In recent years, nutritionists have suggested a broad range of health benefits may be associated with vitamin D intake. Numerous bakers in the United States have offered bread with vitamin D for several years.