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How Dietary Fat Affects Blood Glucose

Nina Ghamrawi, MS, RD, CDE
December 26, 2023
December 26, 2023

When you think about what foods to limit to control your blood sugars, one particular group foods usually comes to mind: carbohydrates. Out of the three major macronutrients in the human diet — carbohydrates, proteins, and fats — carbohydrates have the greatest effect on blood glucose levels. But as a recent study makes clear, fats can also have a significant effect on blood glucose levels — both positive and negative.

There is a lot of research that suggests that controlling blood sugars is not just about reducing carbohydrate intake. Even protein and fat in very small amounts can raise blood glucose. But what we are now learning is that how the body processes carbohydrates may be different when dietary fat is present.  

What Happens

We know that when carbohydrate is ingested, our body's natural response is to make the hormone insulin to attach to the glucose molecules in the blood and absorb them through passageways in the cells. Researchers believe that eating too much of certain types of fats together with carbohydrates may reduce the body's ability to absorb sugar into the tissues and remove it from the blood. This may happen because the ingested fat blocks the passageways that insulin and glucose use to get out of the blood. As a result, glucose may circulate in the blood for longer, keeping the blood glucose elevated for a longer period of time.

Quality Matters

Studies in people without Diabetes show that saturated fats cause more significant insulin resistance than monounsaturated and polyunsaturated fats (1, 2). By design, the high fat dinners in this study were predominantly saturated fat. Another study that grouped together multiple research results and studied over 4000 people also looked at fat quality. The researchers found that some fats raised blood glucose levels less than others— and that substituting some fats for carbohydrates could also be better for blood glucose.

When participants switched out 5% of the calories in their diet from carbohydrates to saturated fat, HbA1c levels barely changed, but when the same 5% of calories were switched from carbohydrate to a healthier, monounsaturated, fat, A1c fell by an average of 0.09%. And it fell by 0.11% when carbohydrate was replaced by polyunsaturated fat.

Since saturated fat had the least beneficial effect of blood glucose control, the researchers also looked at the effect of switching 5% of calories from saturated fat to either monounsaturated or polyunsaturated fat. Here the effects were even more dramatic. HbA1c fell by an average of 0.12% when saturated fat was replaced with monounsaturated fat, and it fell by 0.15% when saturated fat was replaced with polyunsaturated fat.

The Bottom Line

Eating carbohydrates together with certain fats can either help or hurt your blood sugars. Eating saturated fat together with carbohydrate seems to keep your blood glucose higher for a longer duration. Monounsaturated fats, however can help lower your blood glucose more steadily, and polyunsaturated fats appear to have the best effects on blood glucose goals.

What Can I Do?

So when you are planning meals, instead of adding creams, cheese, butter or bacon for flavor or texture, try adding more avocado, olives, nuts, nut butters, and seeds to meals for a healthier, and potentially more carbohydrate-controlled meal option. Meanwhile, stay away from high fat types and cuts of  meat, like animal skins, organs, intestines and feet. Eating seafood like sardines, oysters, mussels or clams, or fish that is baked, grilled, smoked or sautéed will be healthier and lower in saturated fat.

  • An occasional fatty meal is fine, but eating large amounts, for example, a meal containing 30 or more grams of fat, especially if the fat is saturated, can make it harder to control blood glucose levels.
  • You may have to alter the amount, type 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.

More Research is Still Needed

The evidence from many studies showing that dietary fat can cause postprandial hyperglycemia in some individuals with type 1 diabetes highlights that our current approach of focusing on just carbohydrates when managing Diabetes may not be enough for more detailed and intensive diabetes management. In not completely contrary studies, diets that are extremely high fat (70% fat and only 5 % carbohydrate) have been shown to be actually beneficial for the blood sugars (see Ketogenic Diets). Further studies are needed to develop alternative insulin dosing methods for higher-fat meals, and to define new nutritional approaches for minimizing hyperglycemia induced by dietary fat. Further investigations will also be needed to determine the impact of foods enriched in types of unsaturated fat on glycemic control in individuals with type 1 diabetes.

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