There are several widespread misconceptions about fat that might be hilarious if they didn't lead to the poor health of millions of people:
Eating fat will make you fat.
Saturated fat is bad for health.
Polyunsaturated fat is good for health.
Low fat diets are good for health.
These misconceptions are partly responsible for the obesity epidemic and an important contributor to the the declining health of millions for people around the world. Here's the real truth about fat:
Eat more fat to lose weight.
Excess carbohydrates (sugar and starches) will make you fat.
Saturated and monounsaturated fat are good for health.
Polyunsaturated fat should be restricted to about 4% of total calories.
Artificial trans-fats should be avoided completely.
Eat Fat Lose Fat
Eating more fat to lose weight seems like an oxymoron. But ironically, because of the satiating effect of fat, most people eat fewer calories in the long run when they eat a higher percentage of calories as fat. And eating fewer calories will lead to weight loss if your activity levels remain the same.
Eat Carbs Get Fat
When you eat more carbohydrates than you burn, your body converts the carbs to saturated fat and stores it in fatty tissue. Eating large amounts of refined carbohydrates, like sugar and white flour, causes a rapid rise in blood sugar levels. In healthy people,the pancreas then releases insulin to reduce blood sugar by storing small amounts as glycogen while large excesses are stored away as fat. Diets that are high in refined carbs often lead to a roller coaster ride with a blood sugar high followed by a blood sugar crash. With the blood sugar high, you may briefly feel happy and energetic. But when your blood sugar crashes you will likely feel tired, irritable, and hungry. If you then eat more refined carbs, you keep the roller coaster ride going. Over time, it can lead to obesity and insulin resistance and then type II diabetes.
Saturated versus Polyunsaturated Fat
Saturated fats have falsely been blamed for increasing risk to heart disease largely because they tend to raise low-density lipoprotein (LDL) "cholesterol" levels. Likewise, polyunsaturated fats are supposedly heart healthy because they reduce LDL "cholesterol" levels. But LDL levels are not a strong indicator of heart disease, and in fact oxidized LDL is a much stronger indicator. How does LDL get oxidized? Polyunsaturated fats are more prone to oxidation in processing, storage, and cooking, and when ingested, they end up in LDL. It's the oxidized polyunsaturated fats that greatly increase heart disease risk.
Saturated fats are mainly from animal foods that have been predominant in healthy human diets for hundreds of thousands and perhaps millions of years. The change to more animal foods may be largely responsible for increasing human brain size and our evolution away from apes. So, it doesn't make much common sense that foods that nutured our ancestors for so long are now suddenly bad for us.
In contrast, polyunsaturated fats were low in our ancestral diets and have increased dramatically in consumption the last 100 years with the large-scale production of cheap vegetable oils like soybean oil, corn oil, safflower oil, and sunflower oil. Also, entirely new to the human diet within the last 100 years, are artificial trans-fats, mainly in the form of partially hydrogenated vegetable oils. Trans fats appear to be even worse than polyunsaturated fats in causing health problems.
PUFA in Meat and Dairy