Gluten-free option: use an all-purpose gluten-free flour in place of the whole wheat (I like Trader Joe’s), 1/8 tsp xanthan gum added to dry ingredients, and 2 tsp apple cider vinegar and 2 tsp olive oil added to wet.  I don’t usually cook with oil but this small amount seems to help make these muffins moist and not seem like they are gluten-free (let’s be real here – a lot of g/f baked goods are just bad).  You can leave it out, the muffins will be fine, but I prefer to use it.