To most of us, fats are a four-letter word – the nutritional equivalent of the guy in the black hat. We tend to think of fats as the root of all dietary evil, responsible for the growing rates of obesity and type 2 diabetes. The truth is both happier and less black and white than that. Fats (note that it's a plural term) are both good and bad for you. They can help or harm your heart, and it's good to know the differences.

Fats are an important part of our diet. Our bodies can’t function properly if we avoid fats completely because fats provide important building blocks for many tissues in the body and brain (this is why no-fat diets can’t be sustained). However, eating too much fat — particularly too much of the wrong kinds of fat — is just as bad, as this can lead to significant health problems down the road, like heart attack and stroke. So it is important to become educated about the different kinds of fat and to know what a healthy low-fat diet looks like. (Hint: it does not mean loading up on carbohydrates and low-fat chips and cookies by the bag-full.)

Fats (note that it's a plural term) are both good and bad, heart-helping and heart damaging, and it's good to know the differences.

Today our take on fats is more balanced and more accurate than it was years ago, as researchers understand more clearly the different roles that fats play in our bodies. As opposed to the diet crazes of old, most experts now recommend eating a balanced diet, rich in proteins, complex carbs, and healthy fats, like omegas and unsaturated fatty acids, which our bodies need to function properly.

We hope to clear up some of the confusion that surrounds people's ideas about fat these days. We’ll start with a break-down of the types of fats you should consume for a healthier you, as well as those you should try to avoid or eat in moderation. We’ll also discuss how the fats in your body affect the health of both your body and mind.

What Do Fats Do? How Much Do You Need?

Fats provide your body with the energy that it needs to function properly. In fact, fats are the most energy-efficient form of fuel out there, with one gram of fat providing nine calories (contrast this to both carbohydrates and protein, where one gram only provides four calories). For this reason, fats are great for energy, but are also easy to consume too much of (and excess fat from food is converted to fat stores in the body, making us… well, fat).

Children need more fat than adults, because their bodies are still developing and require more fats for this purpose.

Another function of fat is to provide the fatty acids that the body uses for building blocks for certain tissues, for example, like those that insulate nerve cells. Fats are also necessary for the absorption of fat-soluble vitamins like A, D, E, and K, and, as we’ll see, play an important role in keeping cholesterol levels in check, which directly influences your heart health.

The NIH recommends that adults get between 20% and 35% of one’s daily calories from fat, depending on a person’s lifestyle and activity level. Children need more fat than adults, because their bodies are still developing and require more fats for this purpose. In fact, the NIH recommends that children under 2 be given whole milk rather than skim, and the USDA suggests that kids between the ages of 2 and 3 should get 30-35% of their total calories from fat. For children 4-18 years of age, between 25-35% of their energy should come from fat. But, as we’ll discuss shortly, for adults and kids alike, the type of fats that make up your diet is as important as the total amount that you consume. In general, adults should eat healthy fats like mono- and polyunsaturated fats (like omega-3s) rather than saturated and hydrogenated fats. Learning to find a balance between the good and the bad fats, as well as between fats, carbs, and protein, is an important skill, and one that gets easier with practice.

Fat In – and On – Your Body

Fat and Fats Are Not the Same
The first thing is to understand that fats in our diet and blood and being fat are not one and the same problem. People who are fat, who have a high body mass index (BMI) do often consume too much of the unhealthy forms of dietary fats in food and may often have high levels of blood fats (discussed below). But there are also many thin people with dangerously high levels of blood fats (also called lipids) because genetics plays a role in how our bodies process the fats we consume.

When it comes to fats we eat, some fats (saturated and trans or hydrogenated fats) are bad for us and some fats (unsaturated) are good for us. The same is true with fats within our bodies, our blood fats. Instead of just "cholesterol," we have "good" (HDL) and "bad" (LDL) cholesterol.

Big Thighs? Good News
It also matters where we are fat. Heavy thighs are not nearly the cardiovascular warning sign that belly fat is. The health community has discovered that the way fat cells are distributed can be a powerful marker of overall health, heart health, and even life expectancy.

Bad Fats

Trans Fats or Partially Hydrogenated Oils
In 2006, New York City's Mayor Michael Bloomberg famously banned trans fats from being used in foods in restaurants and fast-food chains in New York City. Vendors from McDonalds and KFC to more upscale eateries had to do some serious reformulation of their menus and rethinking of their frying practices.

What’s the big deal about trans fats? Fats are made up of carbon, hydrogen, and oxygen molecules in various ratios. The word trans is a chemical term that simply describes how the carbon bond is set up: it implies that the carbon atoms are arranged in a way that makes the molecule "straighter." Since trans fats are straighter, they make oils more solid, give food a heartier texture, and spoil less quickly than other fats. They can be used multiple times in deep frying, which is why fast-food chains love them so much.

. A note about packaged goods: if a serving of food has less than 0.5 grams of trans fat in it, it can be listed as 0 grams. So make sure to scan the product’s ingredients for the word "hydrogenated" if you want to be sure.

Trans fats are also known as partially hydrogenated fats, which until a few years ago was the more common name on food labels, and the reason that you never saw trans fats listed as an ingredient. However, the FDA changed this convention in 2006, so that now trans fats must be also listed on the label. But be aware that trans fats aren’t just in fast food and junk food, they’re also in snack foods, baked goods, candies, margarines and shortenings. They do occur naturally in some animal products, and it's still not clear whether the naturally-occurring trans fats are as dangerous for your health as the man-made version. (The term, "partially-hydrogenated" refers to the process by which the fats are processed to make them more solid: hydrogen atoms are added to the fat, which makes the molecule more stable.)

Why are trans fats bad for you? They’ve been shown to raise LDL ("bad") cholesterol (which we’ll discuss in more detail later), lower HDL ("good") cholesterol, clog arteries, and raise the risk of heart attack and stroke. They may also be linked to type 2 diabetes, and according to the American Diabetes Association, they are worse for your health than saturated fats. So eat trans fats — partially hydrogenated fats — in moderation. A note about packaged goods: if a serving of food has less than 0.5 grams of trans fat in it, it can be listed as 0 grams. So make sure to scan the product’s ingredients for the word "hydrogenated" if you want to be sure you are truly avoiding them.

The Other "Bad" Fat: Saturated fats
When a fat molecule is fully hydrogenated it is said to be saturated. Saturation is the reason that margarine, which is high in saturated fat, is solid at room temperature, as opposed to vegetable oil that is liquid. Saturated fats also occur naturally, and are found in animal products like butter, cheese, ice cream, and meat with high fat content.

Saturated fats present a risk because they are the number one culprits for raising the "bad" LDL cholesterol, which is a strong marker for coronary heart disease risk. One study found that saturated fat raises LDL levels by interfering with how LDL is processed and cleared away, so more LDL stays circulating in the body.

There’s some evidence that saturated fats also raise the heart healthy HDL cholesterol, which creates a bit of a paradox. But the risks of saturated fats seem to outweigh their benefits, and most experts would probably agree that replacing them with unsaturated fats is a much smarter and more effective way to go.

Good Fats

Unsaturated Fats: The Mono- and Poly- Team
If you’ve already guessed that unsaturated fats must be those which are not totally saturated by hydrogen atoms (in contrast to saturated fats), you’d be right. In an unsaturated fat, at least two carbon atoms are free double-bond with each other, making the fat less rigid than its saturated cousin. Monounsaturated means that there’s just one double bond between carbons, and polyunsaturated means that double bonds occur in more than one place in the molecule.

Monounsaturated fats (MUFA) include the "healthy oils" like olive oil, safflower oil, and soybean oil. Though these oils also have a lot of calories, eating them in moderation is generally considered very beneficial. One of the reasons is that a diet higher in monounsaturated fats has been shown to increase "good" HDL cholesterol, compared to a diet that includes more saturated or trans fats.

Men in one study who ate a diet rich in monounsaturated fatty acids, mainly coming from olive oil, safflower oil, and almonds, had better higher HDL levels and lower triglyceride (blood fat) levels than those who didn't, and their LDL (bad) cholesterol levels remained basically unchanged. So the benefit of unsaturated fats is that they increase HDL cholesterol without raising LDL, as with saturated fats.

For every 5% increase in polyunsaturated fats, the risk for CHD decreased by 10%, a big reduction.

Polyunsaturated oils, which include the much-praised omega-3 fatty acids, are considered to be extraordinarily healthy for a variety of reasons. Decades ago, researchers realized that certain groups of Eskimos had dramatically lower rates of coronary heart disease. Researchers soon realized that Eskimos' high intake of fish — and, therefore, of polyunsaturated fatty acids — was responsible.

Recent studies have validated this. One large review study showed that people who replace their saturated fats with polyunsaturated fats reduce their risk for coronary heart disease (CHD) by 19%. And the authors calculated for every 5% increase in polyunsaturated fats, the risk for CHD decreased by 10%, which is a big reduction. Other studies have found that polyunsaturated fats can significantly lower total cholesterol and LDL cholesterol. Some researchers have also reported that adding polyunsaturated fats in the form of salmon oil (which is rich in omega-3s) reduced both LDL cholesterol and triglycerides significantly, though it did not affect HDL cholesterol.

More on Omegas
Why do we need healthy fats like the omegas anyway? One reason is that our bodies can’t produce certain unsaturated fatty acids, which makes them "essential" nutrients. This is part of the reason that a totally fat-free diet cannot be sustained, and even a low-fat diet can be risky.

Certain fats like omega-3 fatty acids provide a laundry list of benefits to the body, like prevention of heart disease , stroke, and cancer to reducing cognitive decline to reducing inflammation and treating depression. These fats are absolutely crucial for the body — and brain — to function properly, so it is important that we consume the right amount of the omega fats, either through diet alone or with supplements, if necessary.

The essential omega-3 is alpha-linolenic acid (ALA), which is found in many vegetarian sources like soy, flax, canola oil (rapeseed), and walnuts, as well as some greens like spinach and kale. From ALA, the body can produce other omega-3s, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are the types of omegas found in fish, like salmon, tuna, and sardines. It is likely that the Mediterranean diet, which is high in fish, nuts, greens, and healthy oils, may exert its effects, at least in part, by way of the higher doses of omega fatty acids it offers.

The Lesser-Known Omegas
You may have seen supplements that say "Omega-3/6/9" in the health food store. The health benefits associated with omega-6 fatty acids are not quite as clear-cut as with omega-3s, but they do seem to exist. While some researchers have suggested that omega-6 may increase inflammation, there is not much, if any, evidence that this is the case.

Several studies have found that omega-6 fats prevent cardiovascular disease and lower cholesterol and the American Heart Association (AHA) recommends that people make the omega-6s a regular part of the diet. Linoleic acid is the main omega-6, accounting for 85-90% of omega-6 intake. According to the AHA, most people probably consume enough omega-6 fats in nuts, salad dressing, and vegetable oils like soybean, corn, safflower, and sunflower, so supplements are usually not necessary.

Omega-9s are less talked about because they are non-essential, meaning that we do not need to consume the fats because our bodies can make them out of other compounds. Omega-9s are thought to be important for cardiovascular health and keeping cholesterol levels in check. The main omega-9 is oleic acid, which is a compound in olive oil, and a monounsaturated fatty acid (in contrast to omegas-3 and -6, which are polyunsaturated).

Blood Fats: The Fats Inside Our Bodies

Cholesterol: The Good (HDL) and the Bad (LDL)
Now you know more about the fats you do and don't want to put into your body. It is equally useful to know a little about what fat does inside your body. Yes, fats are crucial to health, but clearly, they are also a danger.

Almost everyone has heard that cholesterol levels are important markers for our heart health, and that we need to pay attention to LDL, HDL, and total cholesterol. But why is one form of cholesterol good for us, and one bad?

The reason that LDL is thought to be "bad" is that it tends to allow cholesterol to build up on artery walls, leading to atherosclerosis (plaque build-up on the blood vessel walls), heart attack, and stroke.

The difference between HDL and LDL molecules is their relative densities: the actual name of HDL is high-density lipoprotein, and LDL stands for low-density lipoprotein. So, HDL and LDL are not actually cholesterol molecules themselves: instead, these two lipoprotein molecules help escort cholesterol molecules around the body, shunting them from place to place through the blood. The reason that LDL is thought to be "bad" is that it tends to allow cholesterol to build up on artery walls, leading to atherosclerosis (plaque build-up on the blood vessel walls), heart attack, and stroke.

On the other hand, experts believe that HDL lowers risk for heart disease because it directs cholesterol away from the arteries and towards the liver where it can then be removed from the body. This is why higher HDL levels are associated with decreased risk for heart attack and stroke, and lower HDL levels are linked to increased heart risk.

Cholesterol comes only from animal products (think high-fat foods like hamburgers, bacon, and cheese). The body actually makes about 75% of its own cholesterol, making the need to consume cholesterol virtually nonexistent.

The best way to lower LDL is to trim the fat (both in what you consume and by reducing excess body fat), exercise regularly as you are able, stop smoking, and eat cholesterol-lowering foods like whole grains. As mentioned, eating healthy fats like unsaturated fats, while avoiding saturated and trans fats is also an important way to help your body raise the HDL and lower LDL levels.

Blood Fats: Triglycerides
Triglycerides are another type of blood fat that we need to keep in check. These blood fats are a natural part of food metabolism, and the body stores energy from both fats and carbohydrates in triglyceride form. When energy is needed in between meals, the body uses hormones to signal the release of triglycerides from the fat cells so that they can be used for fuel.

High triglyceride levels are common in obesity and part of the metabolic syndrome, which includes high LDL levels, insulin resistance, and carrying too much weight around one’s waist.

When our energy intake becomes larger than our energy outflow (when we’re eating more calories than we’re burning), excess triglycerides can circulate in the blood, and are thought to contribute to the plaques that cause atherosclerosis (and, therefore, heart attack and stroke). High triglyceride levels are common in obesity and part of the metabolic syndrome, which includes high LDL levels, insulin resistance, and carrying too much weight around one’s waist. It is also a strong marker other conditions like diabetes.

Body Fat, Beer Bellies and the BMI

Aside from the types of fats flowing through our blood, the types of fat cells that make up our bodies also seem to have a significant effect on our overall health. More and more evidence suggests that abdominal fat, better known as belly fat, is much worse for our overall and heart health than fat that is evenly distributed throughout the body (subcutaneous fat).

The BMI is just a way of measuring the relationship between one’s weight and height, and it can be misleading in people who are extremely muscular or those with very little muscle mass.

Studies have found that even a few pounds’ worth of weight gain in the belly takes a significant toll on the blood vessels, which in turn takes a toll on the heart. Even more, abdominal fat is linked to insulin resistance, increased inflammation, and higher LDL and lower HDL levels. Researchers think that belly fat may influence cholesterol simply because of its location near the portal vein, which goes to the liver where blood fats including cholesterol are processed. Whatever the mechanism, if you have extra fat around your middle, it is generally a good idea to lose it if you can. Your heart will thank you.

There are several ways to measure a person’s body fat, and specifically his or her fat distribution. The body mass index (BMI) is the classic tool used to measure how much of a person’s body mass is made up of fat. The BMI is just a way of measuring the relationship between one’s weight and height, and it can be misleading in people who are extremely muscular or those with very little muscle mass.

Another, simpler measure is waist circumference, which, as the name suggests, simply measures the circumference of a person’s waist, generally at about the level of the belly button. An even better measure is the waist-to-hip ratio, which is the proportion of the waist circumference to hip circumference. The larger this number, the greater the cardiovascular risk, in both men and women. When used in combination with the BMI, these tools are good predictors of a person’s risk for various issues like heart failure, stroke, diabetes, and even certain kinds of cancer. For more information on measuring body fat and its role in disease risk, see the NIH’s website.

Make Fats Work for You

The bottom line is that not all fats are created equal. There are some fats that are extremely beneficial to our bodies (like those in the unsaturated family), some that are very harmful (like trans fats) and some that present such a mixed bag that it’s best to avoid them or eat them only sparingly (like saturated fats).

Omega-3s can help keep cholesterol levels in check, while also providing a variety of other health benefits. Hydrogenated oils (partially or totally saturated) are fats that should be avoided or eaten in moderation. When choosing fats to cook with, try to use the healthy oils like olive, canola, safflower, and sunflower oil, rather than animal products like butter or lard. Avoid eating at restaurants you suspect use the saturated fats in preparing food.

Moderation is clearly a recurring theme in many areas of health and medicine, and a central one in nutrition these days. Eating too few fats is just as detrimental as eating too many. Replacing fats with carbs is not a good way to lose weight and may actually make you fatter (as well as upping your risk for diabetes).

Healthy fats provide important building blocks for the tissues in our bodies. It’s important to understand what the body needs to function its best and to meet these needs as best you can. Listen to the current research, but as always, be sure to listen to your body, too. It’s surprisingly good at telling you what it needs more of, and when it’s had enough.