The Diet Bore.

You probably know a diet bore: there's at least one in every office, every group, and at every get-together. It's almost always female - men lose weight too but don't seem to feel the same compulsion to convert the entire world. Blame it on our innate female need to change everyone else.
The diet bore is the one who knows the caloric count of every morsel you eat, and makes sure you know it too. She can expound, at length, on the relative merits of sugar, salt, protein and carbohydrates. She actually knows the difference (and explains it ad nauseum) between mono and unsaturated fats, transfats, and essential fats. She knows what's good for you and what terrible things will happen if you actually eat what's on your plate.
She's the one who makes you cringe in a restaurant as she meticulously quizzes the poor waitress about how everything is prepared and cooked. She demands special substitutions and omissions and then complains that her meal is bland. She carries salt and sugar substitutes in her tote along with her trusty food value books and a calculator to loudly total the calories and carbs she (and you) has consumed.
She causes more of us to fall of our diets than Ronald McDonald and Colonel Sanders combined because she makes the whole concept of losing weight so damned boring that we don't want anything to do with it.
As we happily pig out on our spaghetti and meat balls (with garlic toast), we can take comfort in noting that the diet bore, despite the breadth of her knowledge and her too public weight control efforts, is always a little heavier than she should be.
Maybe she bores herself too?