A number of readers have commented on the practice of counting protein and fat when determining insulin boluses, a practice called TAG. There seems to be some confusion on the use of the TAG system in meal planning, so we asked the registered dieticians at Joslin to weigh in on the issue. Here’s what they had to say:
The TAG (Total Available Glucose) system is a meal planning tool that was in vogue in previous decades and may, in an altered form, be making a come back. This system identifies the amount of glucose available to the body not only from carbohydrates, but also from protein and fat.
The original TAG system suggested that each macronutrient (carbohydrate, fat, protein) in a food supplies a fixed percentage of glucose. One hundred percent of the carbohydrate in a food converts to glucose, as does 50 percent of the protein, while only 10 percent of the fat content converts to glucose.
Here’s an example. One cup of fat-free milk has 12 grams of carbohydrate, 0 grams of fat, and 8 grams of protein. So the TAG for fat-free milk would be 16 grams —all of the carbohydrate (12 grams) plus half of the protein (4 gram) + one-tenth of the fat (which in this case is zero). If you were dosing your insulin based on an insulin-to-carb ratio of one to eight, you would need to take two units of insulin for the milk instead of 1.5.
Due to the challenges of counting up all the components of individual foods, as well as research which indicated the limited influence of protein on glucose control, original TAG system fell out of favor.
The type and amount of carbohydrate in meals and snacks is generally an effective tool in managing blood glucose levels but it doesn’t account for all the response (variability) of blood glucose to food.
Today, we know that excessive fat intake also has a discernable effect on both the timing and amplitude of blood glucose levels. There is a small amount of insulin required in protein metabolism but it seems to result in much less available glucose than previously determined.
This means that the TAG system in its original form as outlined above may overestimate the amount of insulin you need if you had a high-protein, low-fat meal. It could also underestimate your requirements when you are eating a high-fat meal.
To determine the individual effect of foods on your blood glucose levels it is often valuable to check blood glucose pre-meal and two hours after the meal.