Nutrition of lettuce gone to seed
Even lettuce at the correct stage of maturity does not have a lot of nutritional content. According to Fruit and Veggies Matter More six leaves of red leaf lettuce (102 g) contains 15 calories, 25 mg sodium, 2 grams carbohydrates, 1 gram protein, 150% daily value of vitamin A, 6% daily value of vitamin C, 4% daily value of calcium and 6% daily value of iron. http://www.fruitsandveggiesmorematters.org/red-leaf-lettuce-nutrition-selection-storage
Unfortunately I was not able to find specific nutritional information for red leaf lettuce once it goes to seed. It is possible that the nutritional content changes as it goes to seed but more importantly once lettuce goes to seed the leaves typically become very bitter. Often too bitter to eat.
Thank you very much for your very comprehensive response to my question. As long as the leaves remain not withered and still not too bitter, I will continue to use them. (Probably will not be for long!) Perhaps I should plant some new plants right now!!