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Your cells use saturated fats, which are the most stable of the fats, to make about 45 percent of the cell membranes in the brain and liver, and about 35 percent in heart and muscle cells.¹⁹ Yes, saturated fat is the dominant fat in your brain, so don’t demonize it! Energy-producing cells will hold their level of saturated fat at about this level no matter what type of fat you eat. The only type of tissue that meaningfully changes its composition of saturated fat is adipose tissue—aka your muffin top. When you eat more saturated fats, the cells in adipose tissue will change their makeup to contain more saturated fat and less unstable fats without changing in size. This is fantastic, as stable fats make for fewer free radicals.

Think of saturated fat as the stable waxy bricks building the “walls” for your cells. The problem is that your cell membranes have to flex in order to make energy and receive chemical signals, and those nice stable saturated fat “bricks” don’t bend. So while it’s fine to go ahead and eat butter and other forms of saturated fat, it’s also important to eat other types of fats. And those include the next most stable group of fats, monounsaturated fats. These fats—found in food sources like olive oil, avocados, and some nuts—are more flexible than saturated fats. You can think of them as the gel-like “mortar” that supports your saturated fat bricks in the cell wall. Your cell membranes are made up of about 20 percent monounsaturated fat.