Why Does Fat Increase Blood Glucose?
September 27, 2011 Joslin Communications
Has this ever happened to you?
— You eat a meal such as fettuccine alfredo with garlic bread and tiramisu for dessert.
— You take either the appropriate amount of insulin for the carbohydrate in the meal or your oral medications.
— You check 2 to 3 hours after eating and see a blood glucose reading that is in range.
So far, so good, right?
—Then you wake up the next morning with a very high number?
Ever wondered what causes this?
There are two reasons.
First, Fettuccine Alfredo, garlic bread, and tiramisu are, for the most part, a mixture of carbohydrate and fat. But it’s the fat in the meal that is contributing to the elevated readings.
Although carbohydrate is the nutrient that has the most immediate effect on blood glucose levels, fat is not glucose neutral. But only a small portion of the triglyceride (fat) molecule, called the glycerol backbone, can be used as glucose. This very small addition to the glucose pool can’t be the source of your high blood glucose readings. So if fat doesn’t directly raise blood glucose, what is it doing?
For many years scientists thought that fat was a metabolically inert substance. Fat on the body was considered dead weight, just extra blubber people carted around. Well it turns out that fat has been masquerading as the quiet shy guy in the back row, all the while packing a considerable metabolic punch.
A high-fat meal can increase the amount of free fatty acids (FFAs) in the blood. Both repeatedly elevated levels of FFAs, as found in chronic intake of high fat (especially high saturated fat) meals and obesity, are associated with both skeletal muscle and liver insulin resistance.
That resistance means that it will take more insulin—either made by your pancreas or from an injection—to move the glucose in the bloodstream into the cells. There is also evidence that FFAs may have a direct role in reducing the amount of insulin secreted by the beta cells in the pancreas, although an exam mechanism for this role is unknown.
Second, fat also changes the timing of the rise in blood glucose after a meal.
Unlike carbohydrate, which is digested fairly quickly, fat takes a long time to move through the gastrointestinal tract. It can take 4 to 6 hours and sometimes even longer to be fully metabolized. This can be a problem for someone taking insulin.
Fast acting insulins such as Novolog, Humalog or Apridra are active in the body for 3 to 4 hours. When you eat a high fat meal, the insulin may start working before a significant amount of glucose reaches the blood. The insulin often is finished working before the rest of the glucose makes an appearance.
That is why blood glucose numbers can look in range 2 hours post eating a high fat meal and look significantly above goal 5 hours later.
The Bottom Line:
An occasional fatty meal is fine, but eating large amounts, for example, a meal containing 40 or more grams of fat, especially if the fat is saturated (found in animal meats, etc.), can make it harder to control blood glucose levels.
You may have to alter the amount and timing of your insulin if you eat high fat meals.
For those people taking oral medications, doing some type of physical activity—for example, walking—after a high fat meal can help lower blood glucose.
