If we never ate, basal, or background, insulin would meet our needs just fine. But we do eat, and food makes our blood glucose go up, which is why many insulin users now use a basal–bolus insulin regimen, in which boluses of insulin are taken before each meal or substantial snack.
Bolus insulin doses should be tailored to the amount of carbohydrate in a meal or snack, because carbohydrate in food is what makes your blood glucose level rise. For the most part, carbohydrate is also the body’s preferred source of energy, so some is necessary in the diet. Carbohydrate is found in most starchy foods (such as breads, cereals, potatoes, rice, pasta, and beans) as well as fruits, juices, milk, sweets, and to a lesser extent, nonstarchy vegetables.
Fat’s direct impact on blood glucose levels is minimal. However, consuming large amounts of fat can produce a temporary state of insulin resistance, in which the body’s cells become resistant to the effects of insulin and take up glucose from the blood less readily. This temporary, fat-induced insulin resistance can cause a gradual blood glucose rise over many hours.
Protein’s effect on blood glucose is minimal when it is included as part of a complex meal. But when protein is consumed in the absence of carbohydrate, upward of 50% of the protein may be converted into glucose within a few hours, resulting in a moderate blood glucose rise.
However, since it is uncommon for most people to consume purely protein or excessive amounts of fat in all their meals, bolus insulin doses are generally calculated to offset the impact of carbohydrate on blood glucose levels.
Covering the carbohydrate
Appropriate bolus insulin dosing requires individualization and adjustment. For starters, different amounts of carbohydrate require different-size bolus doses. To determine what size bolus dose is needed, first count up the grams of carbohydrate you are about to eat, then use something called an “insulin-to-carbohydrate ratio.” This ratio specifies how many grams of carbohydrate are “covered” by each unit of rapid-acting insulin. For example, a 1-unit-per-10-grams-of-carbohydrate (1:10) ratio means that one unit of insulin covers 10 grams of carbohydrate. A 1:20 ratio means that each unit covers 20 grams.
Calculating a meal or snack bolus becomes simple when you know your insulin-to-carbohydrate ratio. Simply divide the number of grams of carbohydrate you plan to eat by the second number in your ratio. If each unit of insulin covers 10 grams of carbohydrate and you consume 65 grams of carbohydrate, you will need 6.5 units of insulin (65/10 = 6.5).
The beauty of knowing your insulin-to-carbohydrate ratio is that it gives you the flexibility to eat as much or as little carbohydrate as you choose while still maintaining good blood glucose control. However, it is common to have different insulin-to-carbohydrate ratios at different times of day due to changes in hormone levels (which affect insulin sensitivity), physical activity (which enhances insulin sensitivity) and the amount of basal insulin overlapping with the bolus. For most people, insulin sensitivity tends to be a bit lower in the morning than later in the day. For example, I require a 1:10 ratio at breakfast, 1:12 at lunch, and 1:15 at dinner and in the evening.