Why do we need fats? 

Fat plays a good cop/bad cop role in the food world. Fat is calorie-dense, containing 9 kcal/g opposed to the 4 kcal/gram that carbohydrates and protein contain. Therefore, consuming too much can cause you to take in an excess amount of calories. Fatty foods also slow stomach emptying, so high-fat foods can cause gastrointestinal distress. That being said, fats do have a role to play in maintaining our health. Fat is required for the absorption of important fat-soluble vitamins, like vitamins A,D,E,K, and their transport in the body. These vitamins are essential for the maintainence of healthy bones, skin, vision, and some act as antioxidants, helping to prevent disease. Fats are also part of what makes eating enjoyable, they provide flavour and keep you feeling satisfied. You should aim for 20-35 % of your total calorie intake to come from fats. 

However, not all fats are created equal. Here is a breakdown of which ones to enjoy and which ones to avoid.

Trans fats

Most people know that trans fats are like dietary kryptonite, but few actually know why. Because trans fats are the result of chemical alteration, the body recognizes them as foreign, and the immune system attacks them. This is a highly active and inflammatory proccess, in which immune cells engulf the fat to try and destroy it, which creates the dangerous fatty plaque in arteries that are responsible for heart disease. In June 2015, the FDA identified trans fats as "not recognized as safe" and have implemented a plan to remove them from the food system completely within the next three years.

The scoop on saturated

Saturated fats come primarily from animal sources such as meat and poultry, and dairy products like cheese, whole milk, and cream. Saturated fats get a bad rap because they are linked to the development of heart disease.

Which is better butter or margarine?

Butter
Butter consists mainly of saturated fat (about 60%) and contains some monounsaturated and a small amount of polyunsaturated fat. Definitely an ingredient to be used in moderation, but adds great flavour when cooking and baking. 

Hard Stick Margarine
To make vegetable-oil based margarine solid it undergoes a process called hydrogenation. Hydrogenation leads to the production of trans fat which has been shown to have many negative impacts on our health. Because of this hard stick margarine is not a healthier alternative to saturated fats like butter, and should be avoided altogether.

Soft Margarine
Nowadays you can purchase soft margarine, such as Becel, which is non-hydrogenated, making it free of trans fats and safe to consume. Margarine contains less saturated fat than butter and is higher in heart-healthy unsaturated fats, making it a good alternative for use as a spread on toast, or a top a baked potato. However, for use in baking I would suggest sticking to butter. It yields better flavour, texture, and lets be honest those cookies aren't good for you anyway. Just enjoy one, freeze or give away the rest, and move on.

Understanding unsaturated fats

Unsaturated fats, otherwise known as "good" fats, are mostly derived from plant sources. The two types found in foods are monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs). 
The two unsaturated fatty acids to focus on in particular are omega-3 and omega-6 polyunsaturated fatty acids, as they are essential to body function, and can only be obtained through the diet.
PUFA Food Sources:  canola oil, flaxseed, walnuts, and fatty fish such as salmon and mackerel.
MUFA Food Sources: olive oil, peanut oil, avocados, nuts, seeds.

What do they mean by "good" fat?

There are two types of proteins that move fat within the body, low denisity lipoproteins (LDL) are mainly responsible for delivering fat from the diet to cells in the body. Alternatively, high density liproteins (HDL), scavenge cholesterol from cells and return it to the liver to be used in the manufacturing of bile, hormones, and Vitamin D. Both monounsaturated and polyunsaturated  fats have be shown to lower levels of LDL without affecting HDL levels.
The greatest effects on blood lipid levels have been shown to come from the fatty acids DHA and EPA that are derived from omega-3 polyunsaturated fats. In addition to reducing the amount of lipids in the blood, these compounds have been shown to prevent blood clots, decrease blood pressure, and reduce the inflammatory response that causes the formation of fatty plaques. The body can only produce DHA and EPA in very small amounts, so the best way to obtain them is through fish and seafood, like salmon, trout, and mackerel. At least 2 servings/week of fish is suggested to ensure adequate intake and to help in the prevention of heart disease.

what about coconut oil?

Chemically speaking, coconut oil is the exception to the rule in the fat world. Although it is derived from a plant source, it is a saturated fat, and is solid at room temperature. This is good news for vegans or those with sensitivities to dairy, as it makes a good substitute to butter in cooking and baking. Coconut oil has been shown to increase levels of "good" HDL cholesterol, but does not have the same LDL lowering effect that unsaturated fats have. Therefore, coconut oil, like butter, should be used sparingly and not in place of unsaturated vegetable oils. Try it spread on toast with honey, or melted over popcorn.