Study: Dietary Fat Raises Blood Glucose, Carbohydrate Lowers Blood Glucose

I saw this study in Dr. Greger’s book, How Not To Die. Take note of that date. We’ve known this for close to a century:

Dietary Factors That Influence The Dextrose Tolerance Test, J. Shirley Sweeney MD, Archives of Internal Medicine, December 1927

Young, healthy, male medical students were placed on a number of diets.

  • A fat-rich diet group consumed olive oil, butter, mayonnaise made with egg yolk, and cream.
  • A carbohydrate-rich group consumed sugar, candy, pastry, white bread, baked potatoes, syrup, bananas, rice, and oatmeal.

They ate the diet for 2 days. On the morning of the third day they were given a sugar challenge (dextrose at 1.75 grams per kilogram body weight). Those who ate the fat-rich diet showed a marked decrease in their tolerance for sugar, that is, when they were given sugar their blood glucose skyrocketed and stayed high for the duration of the 2-hour test. In comparison, those who ate the carbohydrate-rich diet showed an increased tolerance for sugar.


Compare the numbers in the last column (100 cubic centimeters equals 1 deciliter, 100 cc = 1 dl). Look how high they are in the fat group.

  • About the fat diet, the authors said, “In no instance did the blood sugar return to normal within two hours.”
  • About the carb diet, the authors said, “Those in the carbohydrate group are all strikingly within normal limits.”

They even switched the diets. Those who ate fat changed to eating carb and vice versa. Again:

The curves of all who had been placed on carbohydrate diets manifested a definite increase in their sugar tolerance.

Even within the same person … if they ate fat for 2 days, they had a hard time clearing glucose from their blood. But when they switched to carb, they had no problem, they cleared the glucose from the sugar challenge easily.

I’ve explained why this happens, how fat increases insulin resistance. Here’s what Dr. Greger says:

Fat in your bloodstream, either from your own fat stores or from your diet, can build up inside your muscle cells, where it can create toxic breakdown products and free radicals that block the insulin-signaling process. No matter how much insulin you produce, your fat-compromised muscle cels can’t effectively use it.

Researchers are now able to track the fat going from the blood into the muscles and watch insulin resistance rise. One hit of fat, and within 160 minutes, the absorption of glucose into your cells becomes compromised.

All the minutes and hours and days that your blood glucose remains high are minutes and hours and days that blood sugar is damaging your vessels, tissues, and organs.

Fat is not your friend.

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