On the contrary, they are very high in carbs. Sort of like in the hospital, the last thing an airline wants on its hands is a hypoglycemic passenger. I normally just order the regular meal and decline the bread, dessert, and other items loaded with carbs. There are almost always both meat and fish options in First Class/Business class and Business-first classes and airlines will customize meal and time of service on request. I also bring backup chopped walnuts on board and emergency candies to be able to deal with almost any delay or other situation at the airports or in-flight.
In coach the options are far more limited but there are still options. It is also wise to check with airlines as many passengers do not realize that airlines like United, especially on international flights have 10 different special meals to choose from and some may be better than their diabetic option for a diabetic.
United Airlines Special Meals