Updated August 06, 2015.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Welcome to my Six-Week Diet Turnaround. There are six lessons. I think it's best to take one week at a time, but you can choose a pace that's comfortable for you -- take longer or go faster. If you'd like a weekly reminder, sign up here: Six Weeks To a Healthy Diet

Did you miss the first lesson? No problem, it's right here: Start With the Stats.

Fat provides a rich texture and flavor. The foods that contain the largest amounts include meat, dairy, eggs, nuts and seeds.

Typical examples of cooking fats include olive oil, lard, canola oil, butter, margarine, and shortening. This week's lesson is all about fats.

Lesson Four - Healthy Fats

You need to eat fats -- good fats are necessary for a healthy body. But you also need to avoid some fats: the bad fats that raise your cholesterol and increase inflammation. This week, I'll teach you about dietary fats and give you some tips on getting more healthy fats (and fewer bad fats) into your diet.

First, a Little Chemistry

Fats are made up of individual molecules called fatty acids, which are chains of carbon atoms along with some oxygen and hydrogen atoms. The carbon atoms in the fatty acid molecules are linked by single or double bonds.

Fatty acids vary in length. Short-chain fatty acids have two to four carbon atoms; medium chain fatty acids have six to 12 carbons atoms, long fatty acids have 14 to 18 carbon atoms. A few fatty acids have more than 20 carbon atoms chains.

Fatty acids are either saturated or unsaturated.

Saturated fatty acids have no double bonds between any of the carbon atoms in the chain. Saturated fatty acids are solid at room temperature (think of the fat on red meat). Unsaturated fatty acids have one or more double bonds in the carbon chain. Monounsaturated fatty acids have one double bond, and polyunsaturated fatty acids have two or more.

Unsaturated fatty acids are sometimes named by the position of the double bonds in the carbon chain. The names omega-3, -6 or -9 refer to the locations of the first double bond in the three different fatty acid molecules.

The monounsaturated and polyunsaturated fatty acids are liquid at room temperature (think of vegetable oil).

Unsaturated fatty acids can have two different configurations of the hydrogen atoms that are located on either side of the double bonds. These are referred to as "cis" or "trans" configurations. Cis configurations have those hydrogen atoms both on the same side of the molecule. Cis configurations cause the molecule to look like it is bent. Trans configurations have those hydrogen atoms on opposite sides of the double bond, which gives the molecule a linear appearance, like saturated fats.

Fats (and cholesterol -- a type of fatty substance that is mostly made by your liver, but some comes from your diet) have a number of important functions, which include:

• Lubrication of body surfaces
• Components of cell membrane structures
• Formation of steroid hormones
• Energy storage
• Insulation from cold
• Carrying fat-soluble vitamins A, D, E, K

Good Fats and Bad Fats

Some fats are better for your health than others. The polyunsaturated and monounsaturated fatty acids are usually good, and most saturated fats are bad. The largest amounts of polyunsaturated fats are found in plants, such as in seeds, nuts and vegetable oils. Fish and seafood are also rich in polyunsaturated fats. Olive oil, canola oil, avocado, and nuts also contain monounsaturated fatty acids, which are good for your heart and blood vessels.

The bad fats included some types of saturated fats and trans fats. People who eat large amounts of saturated fats from red meats tend to have higher cholesterol levels than people who eat mostly plant-based foods. Plus they're at risk for inflammation and cardiovascular disease. Still, it isn't clear if all types of saturated fatty acids are bad.

It is clear, however, that most trans fats are bad. Most trans fats are formed when hydrogen is forced into liquid vegetable oils to make them semi-solid.

Some types of stick margarine contain large amounts of trans fats, and some highly processed foods have trans-fats. Some naturally occurring trans-fats are in dairy products; however, they don't seem to be as detrimental as the trans-fats that are created artificially.

Eating a healthy diet means you need to eat fewer trans fats and saturated fats and more of the polyunsaturated and monounsaturated fats. This week you're going to work on both.

The United States Department of Agriculture suggests that about 30 percent of your calories come from fats, but many people eat much more fat than that. You can start by choosing low-fat and non-fat foods at the grocery store and by choosing recipes that are low in fat, generally, you should:

• Cut back on creamy sauces and oily dressings.
• Avoid fried foods.
• Stay away from highly processed foods (or at least read the labels to choose the products with the least amount of total fat).
• Don't eat rich desserts that are high in sugar and fat.
• Use non-stick cookware and non-stick cooking spray instead of butter and oils.
• Choose baked chips and snacks that are lower in fat than regular chips.