When you swap fats (like butter or oil) for carbs (like bread or sugar), your blood fat levels (triglycerides) tend to go up.

Almost everyone who switched to a low-fat, high-carb diet saw their blood fat levels go up—only one person didn’t, which means this effect happens in nearly all people.

When people eat less fat and more carbs, their blood fat levels (triglycerides) go up a lot—even if their cholesterol stays the same—and high blood fat is linked to heart disease.

Eating a diet low in fat and high in carbs makes the body less able to clear fat from the blood, causing triglyceride levels to rise sharply.

If you swap out butter and fatty meats for foods like olive oil and avocados instead of swapping them for bread or sugar, your blood fat levels drop more — and that’s especially true if you already have health risks like high cholesterol or insulin resistance.

People with metabolic syndrome who follow intermittent fasting see a tiny but real drop in their triglyceride levels, which means their body is handling fat better.