DEAR DR. BLONZ: I appreciated your recent column on blood glucose and the hemoglobin A1C test. I have a follow-up question based on videos I have seen on social media: People are using wearable patch sensors to record their blood sugar levels after eating. They sample various foods, report the effects, then recommend foods to eat or avoid based on the results.
Do these patches provide credible information to use when making food choices? -- I.T.S., Chicago
DEAR I.T.S.: These wearable devices, called continuous glucose monitors, use small sensors to record blood glucose every few minutes. To understand the implications of these readings when looking at individual foods, let us consider some factors that can influence such measures.
Digestion involves the breaking down of complex proteins, fats and carbohydrates to their constituent parts using digestive enzymes, all of which must take place before availability for absorption. Substances that don't or can't break down, such as dietary fiber, pass to the large intestine for eventual elimination. The total transit time of a meal through the entire GI tract can be about 30 to 40 hours, but it can take even longer if there is constipation. Such estimates also assume that one eats at their regular mealtime and that all systems work as they should.
In the case of carbohydrates, the absorbable element is primarily glucose, which gets actively transported through the intestinal wall. Glucose will travel to the liver, where small amounts might get used, but most gets dumped into the blood for use elsewhere. It is that level of glucose that those sensors are recording.
How long does it take the absorbable elements of ingested food to get to the areas of the small intestine where absorption occurs? About half of a mixed meal (containing protein, fat and carbohydrates) will be out of the stomach in 2 1/2 to 3 hours. Carbohydrate digestion begins in the mouth with the starch-digesting enzyme amylase, and carbs require the least amount of stomach time. A total-carb (or carb-heavy) meal can end up in the small intestine in about 30 minutes, with the glucose from simple sugars or easily broken-down carbohydrates appearing in the blood soon after that.
Fats and proteins delay and parse out the emptying of the stomach, so when carbohydrate foods are part of a mixed meal -- that is, when simple sugars are not the first things swallowed -- the temporal effect on blood glucose will be less dramatic.
This helps explain why a low- or no-fat sweet food, such as fat-free ice cream, produces a more rapid rise in blood glucose than its whole-milk cousin. Another factor is the size of the meal: Smaller meals, or those with less sugar, result in smaller net amounts of glucose being absorbed and diluted into the blood.
Knowing how an individual food impacts your blood sugar level can be helpful for foods eaten on an empty stomach, but it has limited usefulness for mixed meals. This does not cancel out any potential utility of these sensors, but it does provide some context.
One can keep a log after various meals to learn how various foods impact blood glucose, then track how diet and lifestyle changes affect those dynamics. Certainly, it is something to discuss with your health professional.
Send questions to: "On Nutrition," Ed Blonz, c/o Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.