I received the following e-mail question: "I have type 1 diabetes and want to confirm with you on how to take insulin. I take 70/30 (NPH/Regular, premixed), 10 units in the morning and 8 units in the evening. i take my insulin at 6AM and 6PM and eat immediately. Is this the right way to do it?"
Although there's no single "right way" that will work for everyone, taking premixed NPH and Regular insulin twice daily seems unlikely to me to work well for someone who has been diagnosed with type 1 diabetes (T1D). I'd expect that other insulin programs would probably work better, although they would be more complicated to learn and to carry out. To make any of the adjustments that I discuss below would require frequent checking of your blood sugar level to see how the adjustment is working, and should be done only after discussion with your physician or diabetes nurse educator. By the way, the same adjustments I describe for your T1D would also apply to folks with type 2 diabetes (T2D) who are on insulin programs with or without oral diabetes medications.
First, Regular insulin works better if given about ½ hour before meals, not at mealtime, so changing the timing of your insulin injections to ½ hour before eating would probably allow the Regular component of your premixed insulin to work better to control the post-meal increase in blood glucose. If you want to take your shot right at mealtime, it'd be better to switch to the more-rapid-acting synthetic insulin analogs such as Humalog, Novolog, or Apidra, which work well if given at mealtime. Some of these analogs are also available in premixed insulin products.
Second, eating your meals at 6AM and 6PM may not be the most convenient times for you. The insulin shots doesn't need to be precisely 12 hours apart, so if these mealtimes need to be flexed (for example, eating dinner later than 6PM on some special occasions), it would not be any great problem to do so occasionally. And if your dining schedule would work better to eat at a different time on a routine basis (say, 6:30AM for breakfast and 7PM for dinner), it would be perfectly reasonable to plan to switch to times that are not exactly 12 hours apart.
Third, by using any premixed insulin product, you are giving up a lot of flexibility in dosing the two components of your program (the short-acting one (Regular), and the intermediate-duration one (NPH). If you are willing to change to a more complex program, you would be able to adjust doses of these two insulins independently, and allow more flexibility in meal size and meal timing, and probably get better diabetes control than you presently have. I must add that for people with T1D, it is rarely recommended for people to be on pre-mixed insulin programs; it is now the trend to encourage use of mealtime rapid-acting insulin, and one or two doses daily of long-acting insulins (Lantus or Levemir), or an insulin pump. The doses of mealtime insulin would be based on the carbohydrate content of the meal you're about to eat, so you would need to learn about "carb counting" to ensure you give an appropriate dose of mealtime insulin. The choice of long-acting insulin, and dosing, and whether to give once or twice daily, would be pretty much up to your physician or nurse educator.
One final point: the main goal of your insulin program is to control overall blood glucose levels, as measured by the A1C test, while avoiding low glucose (hypoglycemia) episodes. If your A1C is normal, and you are not having hypoglycemia, then switching to these more complex programs isn't really necessary, but might add more flexibility to your lifestyle. On the other hand, if your A1C is still high, or if you are having frequent hypoglycemic episodes on your present program, then it is extremely important to switch programs. And if your present physician is uncomfortable with recommending a more complex program for you, you should request a consultation with an experienced diabetes team to review your present program with you, and to make recommendations for changes.