This advice like others is overly simplified and although its intentions are good it does not always paint the right picture about how you should be eating. Let me follow this up by saying that no food is inherently good or bad...meaning within the context of a sensible diet whilst reaching set macronutrient goals you can any food without adverse outcomes on fat loss and health. For example the nefarious trans fat can be enjoyed in moderation with no affect on health, and the exalted Omega-3 fats can be detrimental if you have to much. For this reason I would like to provide some basic information about different kinds of dietary fat you hear about and follow that up with my thoughts on dietary fat and weight loss/ health.
First things first, all fats are composed of fatty acids. The properties of fats are dependent upon the specific fatty acids that constitute it. Fatty acids are chains of carbon, and hydrogen atoms with a carboxylic acid group at one end and a triacylglycerol molecule at the other.
Without getting to technical learning the basic chemistry can be helpful to understand some of the common fatty acids. A saturated fat is named this way because every carbon on the fatty acid is bound with a hydrogen atom...thus the molecule is fully saturated. In other fatty acids the carbon atoms may have a double bonds between themselves (creating a degree of unsaturation) aka monounsaturated fatty acids. If the fatty acids contain two or more double bonds between carbons they are referred to as polyunsaturated fatty acids. Unsaturated fatty acids with a trans isomer are commonly called trans fat and have received a lot of bad press.
Above are a few examples of fatty acids
Fats are an important part of our nutrition. Among other things dietary fat provides energy, is needed to regulate the fat soluble vitamins, provide essential fatty acids, and is important to maintain a good hormonal environment. The general nutrition world usually refers to polyunsaturated and monounsaturated fats as the "good" fats because of their ability to regulate cholesterol levels and because the polyunsaturates in our diet are essential. Saturated and trans fat are referred to as the "bad" fats. Each fat has unique affects after digestion but the terms good and bad are limited.
Essential fatty acids are fatty acids that the body cannot synthesize. They are required for good health and must be obtained through our diet. The two essential fatty acids are alpha-linolenic (an omega-3 fatty acid, and Linoleic acid (an omega 6 fatty acid) and besides from being necessary to control inflammation must be gotten dietarily or we could not survive. Just getting these Essential fatty acids is not enough for optimal health, these EFA's consumed in the appropriate proportion (supposedly 4:1 or less omega-6 or omega-3) purportedly will have a positive impact on numerous health events like heart disease, cancer, depression and other mood disorders. A typical American diet is full of omega 6 fatty acids and because these fatty acids compete with each other omega 3 absorption is compromised. The American diet may provide a ratio of 10:1 or more and this is not favorable. This is mostly because Americans eat so much processed food.
Saturated fat a "bad" fat has been villainized by the scientific community as an artherogenic agent (causes heart disease). Researchers believed that consumption of saturated fat elevated cholesterol levels which promoted cardiovascular disease. This research was horribly limited and we can fairly confidently say that for most people their is limited association between saturated fat and heart disease. Some studies have even shown saturated fat to have a protective effect on coronary artery disease . This goes against what many have had ingrained in their mind, but I suspect within the next couple of years their will be major breakthroughs in cholesterol and saturated fat research alleviating this fat of its "bad" moniker.
Of the most clear cut fats that can be delegated "bad" is transfat. Trans fatty acids are formed during partial hydrogenation of vegetable oils. "In the past decade new metabolic studies have provided unequivocal evidence that partial hydrogenation of vegetable oils creates fatty acids that adversely affect plasma lipid concentrations relative to natural oils." So basically trans fats are unnatural and detrimental to health. It is important to note that a natural occurring trans fatty acid (vaccenic acid) may actually be health promoting. If you are interested further in the subject I recommend you visit Alan Aragon's website and subscribe to his research review he provides a unbiased look at trans fat and its health affects.
Now that some of the common terminology has been sorted out I want to discuss dietary fats role in dieting and health. Some researchers have come to believe that our ancestral diet consisted of more fat then we once thought. It is very reasonable to believe that increasing the percentage of fat in your diet could be a good thing as far as health and weight loss go: assuming the increase in fat does not accompany an increase in calories. Fat has nine calories a gram as opposed to protein and carbohydrate which have four cals/g respectively and for that reason it is still important to maintain a sensible caloric intake. Further more fat as a macronutrient tends to be much more satiating the carbohydrate...that is a nice fatty meal will fill you up longer than a carbohydrate meal. For this reason dieters should not be afraid to include high fat foods such as full fat dairy, eggs, fish, and nuts etc. into their diet. Some people fear that the dietary cholesterol that can be associated with fatty foods is dangerous, yet their is another weak correlation here. Dietary cholesterol and blood cholesterol are weakly associated... some people even believe that cholesterol levels and heart disease are poorly associated...I have even seen some studies that show high cholesterol foods such as eggs improving the heart health of people.
As a closing statement I would like to say that fats are not simply good or bad. Within the context of different diets they can have very different effects. It is important for dieters and health enthusiasts to remember that the fat you eat does not make you fat. Furthermore within a sensible caloric intake dietary fat excluding trans fat should not have a negligible impact on your health. As a dieter it is important to remember that taste is importance for adherence, and adherence is important for results. Do not shy away from natural foods, and in order to better your omega 6:3 ratio eat processed food in moderation. Some fat sources I love to eat are eggs, guacamole, red meat, fish, seafood, all nuts, and many more. Don't be afraid to reintroduce some of these natural fatty foods back into your diet today and pass on all that processed junk!