One of the three macronutrients that supply calories in the diet, fat has historically been the most misunderstood. A diet high in fat has long been associated with accumulated body fat, high cholesterol, and heart disease, ever since the “lipid hypothesis” in the 1950s linked these ailments to the consumption of saturated fat and cholesterol from meat and dairy products. We now know, however, that eating fat is not the problem; it’s the ratio between saturated and unsaturated fats in our diet that matters. In fact, fat is a nutrient our bodies can’t do without.

The Function of Fat

Dietary fat, found in plant and animal foods, is made up of chains of fatty acids attached to glycerol. The most energy-dense of the macronutrients, fat supplies 9 calories per gram (in contrast to 4 calories per gram for protein and carbohydrates). This is why even healthy foods high in fat, like olive oil or almonds, are often high in calories.

Though this energy density can make for a substantial calorie count, the nutritional benefits of fat can’t be measured. First, without dietary fat the fat-soluble vitamins A, D, E, and K couldn’t be absorbed in our intestines, and we need these micronutrients for everything from maintaining bone health to fighting disease.  Second, fat in the diet supplies our bodies with essential fatty acids like omega-3s that are known to reduce inflammation and therefore are key to preventing cardiovascular, neurological, and joint ailments.

Stored body fat, or adipose tissue, though not necessarily a consequence of fat consumption, also performs a number of important roles in the body. First, fat tissue is essential to fighting off disease. According to a 2004 Purdue University study, fat cells, or adipocytes, will manufacture hormone-like proteins in response to infections that help the immune system fight back. Second, body fat aids in the dilution of toxins in the bloodstream by temporarily storing these substances until the body can effectively remove them, either through metabolic processes or waste removal (excretion, urination).

Third, adipose tissue helps us to maintain body temperature, and it cushions, insulates, and protects our internal organs. Finally, stored body fat is the body’s back-up energy source. Once our primary energy source, stored glucose or glycogen, is exhausted, fat cells are broken down into their basic components of fatty acids and glycerol, and the glycerol is subsequently broken down by the liver into glucose for use as energy. It’s actually considered healthy, therefore, to carry small amounts of stored body fat.

Types of Dietary Fat

There are four types of fat found in the foods we eat: saturated fats, trans fats, monounsaturated fats, and polyunsaturated fats.

Saturated fats, found in animal products like meat, cheese, eggs, and butter as well as in palm oils like coconut oil, are known to elevate LDL (“bad”) and total cholesterol levels and therefore have been linked to coronary heart disease and stroke. While more recent studies have challenged this assumption, stating that the ratio of unsaturated fats to saturated fats is more indicative of disease risk than the consumption of any one kind of fat (and that trans fats are in fact  much more dangerous than saturated fats), it’s still a good idea to limit saturated fat intake. Aim for lean meats like poultry and lean cuts of beef and pork, and choose low-fat dairy wherever possible.

Trans fats have recently stepped into the limelight as the fat to avoid. Also known as “partially hydrogenated” oils, trans fats are unsaturated fats that have been saturated with hydrogen. Unsaturated fats in their natural state are quick to become rancid, so hydrogen is added to stabilize the molecules and extend shelf life in baked goods and other processed foods. Because these fats are not naturally occurring, they are not essential to the human diet and in fact have been found to not only raise LDL cholesterol levels but lower HDL (“good”) cholesterol levels. As such they should be avoided completely.

The unsaturated fats, considered the “good fats,” contain hydrogen-eliminating double bonds in their fatty-acid chains (saturated fats lack these double bonds and therefore their molecules become “saturated” with hydrogen atoms). They have been linked to everything from lowering heart-disease risk to reducing belly fat. The two kinds are monounsaturated and polyunsaturated. Monounsaturated fats, found in foods like vegetable oils, avocados, and nuts and seeds, help reduce LDL cholesterol levels and the risk of heart attack and stroke. They also aid in cell growth and development and provide antioxidants like Vitamin E.

Polyunsaturated fats, found in vegetable oils, fatty fish like salmon, and some nuts and seeds, also help lower cholesterol levels. Most importantly, they supply the essential omega-3 and omega-6 fatty acids—omega-3s from fish and flaxseed oils as well as walnuts and olive oil; omega-6s from other seed oils like corn, sunflower, and soybean. And while these nutrients are indeed essential, as the body can’t produce them on its own, emerging research indicates that like the ratio of unsaturated to saturated fats we consume, the ratio of omega-3s to omega-6s may be of utmost importance to our health.

According to Michael Pollan in In Defense of Food, Americans currently eat a 1:10 ratio of omega-3s to omega-6s when it should be closer to 1:3. In studies of populations with diets high in omega-3s relative to omega-6s, such as the Japanese and Greenland Eskimos (both of which eat of lot of fish), the rates of heart disease, stroke, and general mortality are significantly lower than ours. This may be because of the opposite physiological functions of these nutrients: omega-3s reduce inflammation, aid circulation, and promote healthy cell structure, while omega-6s are linked to increased inflammation, blood clotting, and the rigidity of cell walls as well as fat storage. As such, eating a diet higher in omega-3s and lower in omega-6s than we presently do is recommended.

Best Sources of Healthy Fats

The consumption of healthy fats is essential to anyone looking to improve cardiovascular, neurological, and joint health as well as reduce her risk of heart disease, stroke, and cancer. But there’s a bonus: thanks to various research suggesting a link between inflammation in the body and weight gain, a diet rich in unsaturated fats, particularly foods containing omega-3s, is recommended to those wishing to lose weight.

Listed are the best sources of mono and polyunsaturated fats. To reap the benefits of a diet rich in these nutrients, up to 30 percent of daily calories (600 calories based on a 2,000-calorie diet, or about 67 grams) should come from the following:

• Coldwater fish like salmon, mackerel, sardines, and anchovies
• Vegetable oils like olive, canola, flaxseed, and safflower
• Nuts like walnuts, peanuts, and almonds
• Seeds like pumpkin and sunflower
• Fatty tree fruits like olives and avocados
• Nut-based spreads like peanut butter
• Supplements like fish oil or flaxseed oil capsules

Note: this 30 percent allowance can include small quantities of saturated fats from foods like lean meats, eggs, and low-fat dairy. According to 2006 recommendations by the American Heart Association, up to 7 percent of total calories (140 calories based on a 2,000-calorie diet, or less than 16 grams) can come from saturated fats.