When monkeys ate a high-fat diet, their bad cholesterol went up not because each particle got fuller of cholesterol, but because there were way more particles floating around in their blood.

When monkeys ate a lot of fat and cholesterol for just 6 weeks, their bad cholesterol levels went up, their good cholesterol didn't keep up, and their cholesterol particles didn't get bigger—just more numerous.

When monkeys ate a diet with moderate fat, it didn't matter if the fat came from palm oil, lard, or sunflower oil— their 'bad cholesterol' particles stayed the same in number, shape, and content.

In one type of monkey, healthy fats made the bad cholesterol particles bigger; in another type, they made them slightly smaller — showing the effect isn't the same in all animals.

High intake of dietary saturated fats elevates circulating low-density lipoprotein (LDL) cholesterol levels in humans.

The study found that what kind of fat is inside the bad cholesterol matters more for heart disease than how big or small the particles are.