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YCMV (Your Carb-Counting May Vary)
These issues include glycemic index/glycemic load, the effects of ripeness, food processing, and food treatment, the effects of proteins and fats, servings per sitting, food combinations, and other assorted variations on the theme.
By now, most of us are familiar with the concept of "glycemic index (GI)". For those who aren't, it's the observation that certain foods will cause one's blood glucose to rise quickly, peak high, and drop quickly after consumption, and others will encourage a steady rise to a lower number, with a more gradual tail-off. Comparing a high-GI food to a low-GI food is like comparing Apidra to Lantus. Glycemic index, however, is not the whole story. The action of a particular high-GI food may be exacerbated by "freeing" the available carbs from their fibrous matrix (for example, mashing a baked potato before eating it), or mitigated by a consuming a smaller serving size, or by eating that food together with low-GI foods or non-carb foods.
Issues closely-associated with glycemic index include the ripeness of fruits, whether or not to account for the fiber content and sugar-alcohol content of foods, and how cooking and freezing may affect high-fiber grains, starches, and legumes. As fruits ripen, their relative sugar content increases, which will change how quickly those carbs affect your blood glucose. I've heard varying takes on how, or whether, to account for fiber and sugar alcohols in one's carb count, ranging from "ignore them" to "count them equally". I haven't seen anything regarding different approaches to soluble (clears the arteries) fiber and insoluble (clears the digestive tract) fiber.
Cooking fruits and vegetables may include chemical processes that change complex carbohydrates into simpler ones, or physical processes that cause large fibrous structures to break down. Either can make the carbohydrates in the food easier to digest, and therefore higher GI, than those same foods uncooked. Freezing boiled foods (such as prepared legumes) may have the same effect -- it's a technique we use in historical costuming to soften up stiff linen. While RDs have told me that freezing does not affect the (in)digestibility of nutritional fiber or the availability of nutritional carbohydrates (so it should not affect your glycemic response), my own experience is that cooked beans that have been frozen and reheated are much more likely to spike me than the same cooked beans before they were frozen. Chalk it up to YDMV -- yours may or may not behave the same way.
Back in high school biology class, we were taught that all macronutrients -- proteins and fats as well as carbs -- are broken down into simple carbohydrates (aka glucose) before they can be used by the body. This suggests that all food -- not just carbs -- should have an effect on one's blood glucose levels, and may need to be accounted for in one's insulin regime as well as one's caloric intake. I've read that some doctors suggest not accounting for proteins (or fats?) in calculating one's insulin dose; others suggest dosing for one gram carbohydrate per four grams protein. I'm sure I have the numbers off here (I'm not a doctor, nor am I on insulin), but if you were accounting for protein in this manner and you normally took one unit insulin for every 14 g carb, then if you ate two large eggs at 7 g protein each, you would need to add one-quarter unit insulin (14 g protein times 1/4 times 1 unit insulin/14 g carb) to account for the protein in those eggs.
Then there's the issue of how to account for fats. Not only do they break down into carbs, but fats can delay the postprandial glucose curve -- the so-called "pizza effect" -- and you may need to adjust the timing of your meal bolus (boluses?) to keep your numbers within range over a four- or five-hour range. Others of us respond nonlinearly to changes in the total number of calories, or servings, of a particular food consumed at one time -- some of us can eat one slice of bread at the time with little trouble, but give us two slices, and not only do we spike more than twice as high, but we stay there more than twice as long.)
Add to that time of day, time of month (especially for women!), and how any food acts in combination with one or more other foods. One standard slice (weighed out to the gram) of a favorite bread can have drastically different effects if it's eaten at breakfast or at dinner, with scrambled egg whites or with skinless chicken breast, and whether you spent today's (or yesterday's) gym session doing weights, cardio, or both.
With all these variables, it's a wonder that carb counting works as well as it does, and that we are able to estimate our consumption and our insulin needs as closely as we do.
Michelle Kowalski, a writer, editor and photography hobbiest living in Phoenix, was diagnosed with Type 2 diabetes in February 2005. In January 2008, as part of her quest to start on an insulin pump, Michelle learned that she actually has type 1 diabetes. (Read More)
Nicole Purcell lists having type 1 diabetes last when she's asked to provide information about herself - because that's where it belongs. (Read More)