Bread, 100% beef, lettuce, cheese, sounds quite healthy.
Here are the ingredients in a McDonald's bun (from their US website):
Big Mac® Bun:
Enriched flour (bleached wheat flour, malted barley flour, niacin, reduced iron, thiamin mononitrate, riboflavin, folic acid), water, high fructose corn syrup, yeast, soybean oil, canola oil, partially hydrogenated soybean oil, contains 2% or less of each of the following: sesame seed, salt, wheat gluten, calcium sulfate, ammonium sulfate, monocalcium phosphate, ammonium chloride, calcium carbonate, baking soda, soy flour, dough conditioners (may contain one or more of the following: distilled monoglycerides, DATEM, ascorbic acid, azodicarbonamide, enzymes, ethoxylated mono- and diglycerides, sodium stearoyl lactylate, guar gum, mono-and diglycerides, calcium peroxide), calcium propionate & sodium propionate (preservatives), soy lecithin. CONTAINS: WHEAT AND SOY
It sounds like no bread I've ever known!