Most people these days are taught to count their carbohydrates and use that number as a basis for calculating their mealtime bolus. Many find, though, that counting only carbohydrates doesn't adequately predict the effect of food on their blood sugar. Fat and protein, if eaten in sufficient amounts, can make a significant difference.
Some sources believe that 58% of protein and 10% of fat are converted to glucose. Others believe these percentages to be excessive.
Although I feel that this is a gross misstatement of the actual metabolic process, the numbers seem to be useful as a practical tool for many. As always, you need to experiment to see how your body reacts and treat it accordingly.
Note: This document doesn't give any instruction on carbohydrate counting. For more information on carbohydrate counting, please see the following pieces: